Main Spi Plastics Engineering Handbook of the Society of the Plastics Industry

Spi Plastics Engineering Handbook of the Society of the Plastics Industry

In preparing this Fifth Edition of the SPI Plastics Engineering Handbook, we have built upon a base of information that has grown along with the plastics industry over the past 45 years. As a work whose contents are largely contributed, the Handbook owes a great deal to the authors and reviewers whose time and efforts went into its publication. They all have our sincere appreciation. In compiling the new work, much text from the previous edition has been retained, with the necessary revisions and updates provided by current industry experts. We therefore thank all the contributors and reviewers who participated in the Fourth Edition as well. Acknowledgment is due once again to the many suppliers of plastic materials and processing equipment who have generously offered literature and illustrations. We have provided credit for those contributions throughout the handbook.Thanks are also in order to the Society of the Plastics Industry, which supported the publication of the Handbook and supplied us with association literature, data, standards, and other information contained in this volume. I would also like to give special thanks to Joel Frados, editor of the Fourth Edition and my mentor in the plastics publishing business, to Me1 Friedman and Ed Galli of Edge11 Communications for their help in gathering information, to Stephanie Seber for her long hours of manuscript preparation, to Emil Davidson for his inspiration and assistance, and to Tina Berins for her constant encouragement.
Categories: Technique
Year: 1954
Edition: 1st edition
Publisher: Reinhold Publishing Company
Language: english
Pages: 869
File: RAR, 113.74 MB
Download (rar, 113.74 MB)

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1 

Over the years, the plastics industry has built 
up a language and a terminology of its own. In 
this chapter, the most commonly used words 
and expressions are classified and defined. Def- 
initions of other terms can also be found in the 
text, and can be located by means of the index 
at the back of the book. 

In many instances, the words being defined 
are peculiar to the plastics industry and the way 
in which it manufactures its products. In other 
cases, the expression used by the industry may 
derive from words commonly used in other 
branches of manufacturing (e.g., the concept 
of forging plastics from metalworking termi- 
nology); as applied to plastics, however, these 
definitions may differ from common usage. 

Readers also are referred to the special glos- 
saries included in the chapters on extrusion and 
injection molding. 

A-stage-An early stage in the reaction of cer- 
tain thermosetting resins, in which the ma- 
terial is still soluble in certain liquids and fu- 
sible. Sometimes referred to as resol. See 
B-stage and C-stage. 

ablative plastic-Description applied to a ma- 
terial that absorbs heat (while part of it is 
being consumed by heat) through a decom- 
position process known as pyrolysis, which 
takes place in the near surface layer exposed 
to heat. This mechanism essentially provides 
thermal protection of the subsurface mate- 
rials and components by sacrificing the sur- 
face layer. 

accelerator-A chemical additive that in- 
creases the speed of a chemical reaction. 

accumulator-An auxiliary cylinder and pis- 
ton (plunger) mounted on injection molding 
or blow molding machines to provide fast de- 
livery of plasticated melt. The accumulator 
cylinder is filled during the time between 
“shots” with melted plastic coming from the 
main (primary) extruder. 

activation-The process of inducing radioac- 
tivity in a specimen by bombardment with 
neutrons or other types of radiation. 

additive-Substance compounded into a resin 
to modify its characteristics (i.e., antistats, 
stabilizers, plasticizers, flame retardants, 
etc.). 

adiabatic-An adjective used to describe a 
process or transformation in which no heat is 
added to or allowed to escape from the sys- 
tem under consideration. It is used, some- 
what incorrectly, to describe a mode of ex- 
trusion in which no external heat is added to 
the extruder although heat may be removed 
by cooling to keep the output temperature of 
the melt passing through the extruder con- 
stant. The heat input in such a process is de- 
veloped by the screw as its mechanical en- 
ergy is converted to thermal energy. 

aging-The process of exposing plastics to an 
adverse environment (i.e., heat, chemicals, 
light) for an interval of time, to determine the 
effect on properties. 

air ring-A circular manifold used to distri- 
bute an even flow of the cooling medium, air, 
onto a hollow tubular film (bubble) passing 
through the center of the ring. In extrusion 
blown films, the air cooling provides uni- 
form thickness. 

1 



2 SPI PLASTICS ENGINEERING HANDBOOK 

air-slip forming-A variation of snap-back 
forming in which the male mold is enclosed 
in a box in such a way that when the mold 
moves forward toward the hot plastic, air is 
trapped between the mold and the plastic 
sheet. As the mold advances, the plastic is 
kept away from it by the air cushion formed 
as described above, until the full travel of the 
mold is reached, at which point a vacuum is 
applied, destroying the cushion and forming 
the part against the plug. 

air vent-See vent. 
ambient temperature-Temperature of the 

amorphous-Having no crystalline structure. 
anchorage-Part of the insert that is molded 

inside of the plastic and held fast by the 
shrinkage of the plastic. 

anisotropy-The tendency of a material to 
react differently to stresses applied in differ- 
ent directions. 

anneal-To heat an article to a predetermined 
temperature and slowly cool it to relieve 
stresses. Annealing is employed on parts 
produced from both metals and plastics. (An- 
nealing of molded or machined parts may be 
done dry as in an oven or wet as in a heated 
tank of mineral oil.) 

antiblocking agent-A substance added to 
plastic resin to prevent adhesion between 
touching layers of film caused by pressure, 
heat, and contact during fabrication and stor- 
age. 

antistatic agent-A substance that can be ap- 
plied to the surface of a plastic article, or in- 
corporated in the plastic from which the ar- 
ticle is to be made. Its function is to render 
the surface of the plastic article less suscep- 
tible to accumulation of electrostatic charges 
which attract and hold fine dirt or dust on the 
surface of the plastic article. 

aramid fiber-Lightweight, high strength 
polymeric fibers used as ballistic armor and 
as reinforcements for plastics. 

arc resistance-Time required for a given 
electrical current to render the surface of a 
material conductive because of carbonization 
by the arc flames. Ref.: Standard Method of 
Test for High-Voltage, Low-Current Arc Re- 

medium surrounding an object. 

sistance of Solid Electrical Insulating Mate- 
rials (ASTM Designation: D 495). 

atactic-Description applied to a random ar- 
rangement of units along a polymer chain. 
See isotactic. 

autoclave-A closed vessel for conducting ? 
chemical reaction or other operation under 
pressure and heat. 

autoclave molding-Procedure used in rein- 
forced plastic molding, in which, after lay- 
up, the entire assembly is placed in a steam 
autoclave at 50 to 100 psi. Additional pres- 
sure achieves higher reinforcement loadings 
and improved removal of air. (Modification 
of pressure bag method.) 

automatic mold-A mold for injection, 
compression, or transfer molding that re- 
peatedly goes through the entire molding 
cycle, including ejection, without human as- 
sistance. 

average molecular weight-Term used to in- 
dicate the chain length of the most typical 
chain in a given plastic. Molecular weight of 
polymers is determined by measuring the 
viscosity of the material in solution at a spe- 
cific temperature. The value is independent 
of specific chain length and falls between 
weight average and number average molec- 
ular weight. 

B-stage-An intermediate stage in the reaction 
of a thermosetting resin in which the material 
softens when heated and swells in contact 
with certain liquids but does not entirely fuse 
or dissolve. Resins in thermosetting molding 
compounds usually are in this stage. See also 
A-stage and C-stage. 

back pressure-Resistance of a material, be- 
cause of its viscosity, to continue flow when 
mold is closing. 

back-pressure-relief port-An opening from 
an extrusion die for the escape of excess ma- 
terial. 

back taper-Reverse draft used in a mold to 
prevent the molded article from drawing 
freely. See undercut. 

backing plate-In injection molding equip- 
ment, a heavy steel plate used as a support 
for the cavity blocks, guide pins, busings, 



GLOSSARY 3 

etc. In blow molding equipment, it is the steel 
plate on which the cavities (i.e., the bottle 
molds) are mounted. 

baffle-A device used to restrict or divert the 
passage of fluid through a pipe line or chan- 
nel. In hydraulic systems, the device, which 
often consists of a disc with a small central 
perforation, restricts the flow of hydraulic 
fluid in a high pressure line. A common lo- 
cation for the disc is in a joint in the line. 
When applied to molds, the term is indica- 
tive of a plug or similar device located in a 
stream or water channel in the mold and de- 
signed to divert and restrict the blow to a de- 
sired path. 

bag molding-A method of applying pressure 
during bonding or reinforced plastics mold- 
ing in which a flexible cover, usually in con- 
nection with a rigid die or mold, exerts pres- 
sure on the material being molded through 
the application of air pressure or the drawing 
of a vacuum. 

Bakelite-A proprietary name for phenolic and 
other plastics materials, often used indis- 
criminately to describe any phenolic molding 
material or molding. The name is derived 
from that of Dr. Leo Hendrik Baekeland 
(1863-1944), a Belgian who developed phe- 
nolic resins in the early 1900s. 

Banbury-An apparatus for compounding ma- 
terials composed of a pair of contra-rotating 
rotors that masticate the materials to form a 
homogeneous blend. This is an internal-type 
mixer that produces excellent mixing. 

band heater-Electrical heating units fitted to 
extruder barrels, adaptors, dies, nozzles, 
etc., utilized for heating the polymer to a de- 
sired temperature. 

barrel-The tubular portion of an extruder in 
which the conveying screw rotates. 

barrier plastics-A general term applied to a 
group of lightweight, transparent, and im- 
pact-resistant plastics, usually rigid copoly- 
mers of high acrylonitrile content. The bar- 
rier plastics generally are characterized by 
gas, aroma, and flavor barrier characteristics 
approaching those of metal and glass. 

beta gauge-A thickness-measuring device 
used for sheeting or extruded parts. The de- 

vice operates by beta radiation being emitted 
on one side of the part and a detector placed 
on the opposite side. When a part is passed 
through the beam, some of the beta radiation 
is absorbed, providing an indication of the 
part's thickness. 

biaxial orientation-The process of stretching 
a hot plastic film or other article in two di- 
rections under conditions resulting in molec- 
ular orientation in two directions. 

binder-A component of an adhesive compo- 
sition that is primarily responsible for the ad- 
hesive forces that hold two bodies together. 
See extender, filler, and matrix. 

biscuit-See preform. 
blanking-The cutting of flat sheet stock to 

shape by striking it sharply with a punch 
while it is supported on a mating die. Punch 
presses are used. Also called die cutting 
(which see). 

bleed-(1) To give up color when in contact 
with water or a solvent. (2) Undesired move- 
ment of certain materials in a plastic (e.g., 
plasticizers in vinyl) to the surface of the fin- 
ished article or into an adjacent material; also 
called migration. (3) An escape passage at 
the parting line of a mold, like a vent but 
deeper, that allows material to escape or 
bleed out. 

blind hole-Hole that is not drilled entirely 
through. 

blister-Undesirable rounded elevation of the 
surface of a plastic, whose boundaries may 
be indefinitely outlined, somewhat resem- 
bling in shape a blister on the human skin. A 
blister may burst and become flattened. 

blocking-An adhesion between touching lay- 
ers of plastic, such as that which may de- 
velop under pressure during storage or use. 

bloom-( 1) A noncontinuous surface coating 
on plastic products that comes from ingredi- 
ents such as plasticizers, lubricants, anti- 
static agents, etc., which are incorporated 
into the plastic resin. It is not always visible. 
Bloom is the result of ingredients coming out 
of solution in the plastic and migrating to its 
surface. (2) Also, term used to describe an 
increase in diameter of the parison as it comes 



4 SPI PLASTICS ENGINEERING HANDBOOK 

from the extruder die(s) in the blow molding 
process. 

blow molding-A method of fabrication in 
which a warm plastic parison (hollow tube) 
is placed between the two halves of a mold 
(cavity) and forced to assume the shape of 
that mold cavity by use of air pressure. Pres- 
surized air is introduced through the inside 
of the parison and thereby forces the plastic 
against the surface of the mold that defines 
the shape of the product. 

blow pin-Part of the tooling used to form hol- 
low objects or containers by the blow mold- 
ing process. It is a tubular tool through which 
air pressure is introduced into the parison to 
create the air pressure necessary to form the 
parison into the shape of the mold. In some 
blow molding systems, it is a part of, or an 
extension of, the core pin. 

blow pressure-The air pressure required to 
form the parison into the shape of the mold 
cavity in a blow molding operation. 

blow rate-The speed or rate at which the air 
enters or the time required for air to enter the 
parison during the blow molding cycle. 

blowing agent-A substance that alone or in 
conjunction with other substances is capable 
of producing a cellular structure in a plastic 
mass. 

blown film extrusion-Technique for making 
film by extruding the plastic through a cir- 
cular die, followed by expansion (by the 
pressure of internal air admitted through the 
center of the mandrel), cooling, and collaps- 
ing of the bubble. 

blown tubing-A thermoplastic film produced 
by extruding a tube, applying a slight inter- 
nal pressure to the tube to expand it while 
still molten, and subsequent cooling to set the 
tube. The tube is then flattened through 
guides and wound up flat on rolls. The size 
of blown tubing is determined by its flat 
width in inches as wound, rather than by the 
diameter (which is used for rigid tubing). 

blow-up ratio-In blow molding, the ratio of 
the diameter of the product (usually its great- 
est diameter) to the diameter of the parison 
from which the product is formed. In blown 
film extrusion, the ratio between the diame- 
ter of the final film tube and the diameter of 

bolster-Space or filler in a mold. 
boss-Projection on a plastic part designed to 

add strength, to facilitate alignment during 
assembly, to provide for fastenings, etc. 

bottom blow-A specific type of blow molding 
technique in which hollow articles are formed 
by injecting the blowing air into the parison 
from the bottom of the mold (as opposed to 
introducing the blowing air at a container 
opening). 

bottom plate-Part of the mold that contains 
the heel radius and the push-up. 

branched-Chemistry term refemng to a con- 
~ figuration having side chains attached to the 

original chain (in a direction different from 
that of the original chain) in the molecular 
structure of a polymer. 

breakdown voltage-The voltage required, 
under specific conditions, to cause the failure 
of an insulating material. See dielectric 
strength and arc  resistance. 

breaker plate-A metal plate installed across 
the flow of the stock between the end of an 
extruder screw and the die, with openings 
through it such as holes or slots. It usually is 
used to support a screen pack. 

breathing-The opening and closing of a mold 
to allow gases to escape early in the molding 
cycle. Also called degassing. When refemng 
to plastic sheeting, breathing indicates 
permeability to air. 

brine11 hardness-Similar to Rockwell hard- 
ness (which see). 

bubble-A spherical, internal void or globule 
of air or other gas trapped within a plastic. 
See void. 

bubbler-A device inserted into a mold force, 
cavity. or core that allows water to flow deep 
inside the hole into which it is inserted and 
to discharge through the open end of the hole. 
Uniform cooling of molds and of isolated 
mold sections can be achieved in this man- 
ner. 

bulk density-The density of a molding ma- 
terial in loose form (granular, nodular, etc.) 
expressed as a ratio of weight to volume 
(e.g., g/cm3 cubed or lb/ft3 cubed). 

bulk factor-Ratio of the volume of loose 
molding powder to the volume of the same 
weight of resin after molding. 

the die orifice. bulk molding compound (BMC)-Thenno- 



GLOSSARY 5 

setting resins mixed with stranded reinforce- 
ment, fillers, etc., into a viscous compound 
for injection or compression molding. 

burned-Showing evidence of thermal decom- 
position through some discoloration, distor- 
tion, or localized destruction of the surface 
of the plastic. 

burning rate-A term describing the tendency 
of plastics articles to bum at given tempera- 
tures. 

burst strength-The internal pressure required 
to break a pipe or a fitting. This pressure will 
vary with the rate of buildup of the pressure 
and the time during which the pressure is 
held. 

butt fusion-A method of joining pipe, sheet, 
or other similar forms of a thermoplastic resin 
wherein the ends of the two pieces to be 
joined are heated to the molten state and then 
rapidly pressed together. 

C-stage-The final stage in the reaction of cer- 
tain thermosetting resins, in which the ma- 
terial is relatively insoluble and unfusible. 
The resin in a fully cured thermosetting 
molding is in this stage. Sometimes referred 
to as resite. See A-stage and B-stage. 

calcium carbonate (CaC03)-A filler and ex- 
tender used in thermoplastics. It occurs nat- 
urally in the form of minerals such as calcite, 
chalk, limestone, marble, and whiting. 

calender-To prepare sheets of material by 
pressure between two or more counter-rotat- 
ing rolls. Also, the machine performing this 
operation. 

carbon black-A black pigment produced by 
the incomplete burning of natural gas or oil. 
It is widely used as a filler, particularly in the 
rubber industry. Because it possesses useful 
ultraviolet protective properties, it is also 
much used in molding compounds intended 
for outside weathering applications. 

carbon fiber-Fibers produced by pyrolysis of 
an organic precursor fiber in an inert atmo- 
sphere at temperatures higher than 1800°F. 
The material is used as reinforcement for 
lightweight, high strength, and high stiffness 
structures. The high stiffness and the high 
strength of fibers depend on the degree of 
preferred orientation. 

cartridge heater-Cylindrical-bodied, electri- 

cal heater for providing heat for injection, 
compression, and transfer molds, injection 
nozzles, runnerless mold systems, hot 
stamping dies, sealing, etc. 

case harden-To harden the surface of a piece 
of steel to a relatively shallow depth. 

cast-(1) To form a “plastic” object by pour- 
ing a fluid monomer-polymer solution into 
an open mold where it finishes polymerizing. 
(2) Forming plastic film and sheet by depos- 
iting liquid resin, either molten or in solution 
or dispersion, onto a chilled surface. 

cast film-A film made by depositing a layer 
of liquid plastic onto a surface and stabilizing 
this form by evaporation of solvent, by fus- 
ing after deposition, or by allowing a melt to 
cool. Cast films usually are made from so- 
lutions or dispersions. 

casting-The finished product of a casting op- 
eration; it should not be used for molding. 

catalyst-A substance that markedly speeds up 
the cure of a compound when added in minor 
quantity as compared to the amounts of pri- 
mary reactants. See hardener, inhibitor, 
and promoter. 

cavity-Portion of a mold that usually forms 
the outer surface of the molded part. De- 
pending on the number of such depressions, 
molds are designated as a single-cavity or 
multi-cavity. See core. 

cavity retainer plate-Plates in a mold that 
hold the cavities and forces. These plates are 
at the mold parting line and usually contain 
the guide pins and bushings. Also called 
force retainer plate. 

cell-A single cavity formed by gaseous dis- 
placement in a plastic material. See cellular 
plastic. 

cellular plastic-A plastic whose density is de- 
creased substantially by the presence of nu- 
merous cells disposed throughout its mass. 
See cell and foamed plastics. 

center gated mold-An injection or transfer 
mold wherein the cavity is filled with mold- 
ing material directly into the center of the 
part. 

centipoise-A unit of viscosity, conveniently 
and approximately defined as the viscosity of 
water at room temperature. 

centrifugal casting-A method of forming 
thermoplastic resins in which the granular 



6 SPI PLASTICS ENGINEERING HANDBOOK 

resin is placed in a rotatable container, heated 
to a molten condition by the transfer of heat 
through the walls of the container, and ro- 
tated so that the centrifugal force induced will 
force the molten resin to conform to the con- 
figuration of the interior surface of the con- 
tainer. It is used to fabricate large-diameter 
pipes and similar cylindrical items. 

chalking-Dry , chalklike appearance or de- 
posit on the surface of a plastic. See haze and 
bloom. 

charge-The measurement or the weight of 
material used to load a mold at one time or 
during one cycle. 

chill roll-A cored roll, usually temperature- 
controlled with circulating water, that cools 
the web before winding. For chill roll (cast) 
film, the surface of the roll is highly pol- 
ished. In extrusion coating, either a polished 
or a matte surface may be used, depending 
on the surface desired on the finished coat- 
ing. 

chiller-A self-contained system comprised of 
a refrigeration unit and a coolant circulation 
mechanism consisting of a reservoir and a 
pump. Chillers maintain the optimum heat 
balance in thermoplastic processing by con- 
stantly recirculating chilled cooling fluids to 
injection molds, extruder chill rolls, etc. 

chromium plating-An electrolytic process 
that deposits a hard film of chromium metal 
onto working surfaces of other metals where 
resistance to corrosion, abrasion, and/or ero- 
sion is needed. 

C.I.L. (flow test)-A method of determining 
the rheology or flow properties of thermo- 
plastic resins developed by Canadian Indus- 
tries Ltd. In this test, the amount of the mol- 
ten resin that is forced through a specified 
size orifice per unit of time when a specified, 
variable force is applied gives a relative in- 
dication of the flow properties of various res- 
ins. 

clamping area-The largest rated molding area 
an injection or transfer press can hold closed 
under full molding pressure. 

clamping force-In injection molding and in 
transfer molding, the pressure applied to the 
mold to keep it closed, in opposition to the 
fluid pressure of the compressed molding 

material, within the mold cavity (cavities) 
and the runner system. In blow molding, the 
pressure exerted on the two mold halves (by 
the locking mechanism of the blowing table) 
to keep the mold closed during formation of 
the container. Normally, this pressure or 
force is expressed in tons. 

clamping plate-A plate fitted to a mold and 
used to fasten the mold to a molding ma- 
chine. 

clamshell molding-A variation of blow 
molding and thermoforming in which two 
preheated sheets of plastic are clamped be- 
tween halves of a split mold. Each sheet is 
drawn into the individual mold cavity by 
vacuum in the cavity and air injection be- 
tween the sheets. 

closed cell foam-Cellular plastic in which in- 
dividual cells are completely sealed off from 
adjacent cells. 

coefficient of expansion-The fractional 
change in dimension (sometimes volume) 
specified, of a material for a unit change in 
temperature. Values for plastics range from 
0.01 to 0.2 mil/inch, "C. Ref.: Standard 
Method of Test for Coefficient of Linear 
Thermal Expansion of Plastics (ASTM Des- 
ignation: D 696). 

coextrusion-Process of combining two or 
more layers of extrudate to produce a multi- 
ple-layer product in a single step. 

coinjection-Technique of injecting two ma- 
terials into a single mold from two plasticat- 
ing cylinders, either simultaneously or in se- 
quence. 

cold-cure foams-See high-resiliency flexible 
foams. 

cold drawing-Technique for using standard 
metalworking equipment and systems for 
forming thermoplastic sheet (e.g., ABS) at 
room temperature. 

cold flow-See creep. 
cold molding-Procedure in which a compo- 

sition is shaped at room temperature and 
cured by subsequent baking. 

cold parison blow molding-Technique in 
which parisons are extruded or injection 
molded separately and then stored for sub- 
sequent transportation to the blow molding 
machine for blowing. 



GLOSSARY 7 

cold runner molding-Process in which sprue 
and runner system (the manifold section) is 
insulated from the rest of the mold and tem- 
perature-controlled to keep the plastic in the 
manifold fluid. This mold design eliminates 
scrap loss from sprues and runners. 

cold slug-The first material to enter an injec- 
tion mold, so named because in passing 
through the sprue orifice it is cooled below 
the effective molding temperature. 

cold slug well-Space provided directly oppo- 
site the sprue opening in an injection mold to 
trap the cold slug. 

cold stretch-A pulling operation with little or 
no heat, usually on extruded filaments to in- 
crease tensile properties. 

color concentrate-A measured amount of dye 
or pigment incorporated into a predetermined 
amount of plastic. This pigmented or colored 
plastic is then mixed into larger quantities of 
plastic material to be used for molding. The 
“concentrate” is added to the bulk of plastic 
in measured quantity in order to produce a 
precise, predetermined color in the finished 
articles to be molded. 

combination mold-See family mold. 
compound-The plastic material to be molded 

or blown into final form. This includes the 
resin itself, along with modifiers, pigments, 
antioxidants, lubricants, etc., needed to pro- 
cess the resin efficiently and to produce the 
desired properties in the finished article. 

compression mold-A technique whereby 
molding compound is introduced into an open 
mold and formed under heat and pressure. 

compression ratio-In an extruder screw, the 
ratio of volume available in the first flight at 
the hopper to that in the last flight at the end 
of the screw. 

compressive strength-Crushing load at the 
failure of a specimen divided by the original 
sectional area of the specimen. Ref.: Tenta- 
tive Method of Test for Compressive Prop- 
erties of Rigid Plastics (ASTM Designation 
D 695). 

concentrate-A measured amount of additive 
(e.g., dye, pigment, foaming agent, anti- 
static agent, flame retardant, glass reinforce- 
ment, etc.) that is incorporated into a prede- 
termined small amount of plastic. This (the 

concentrate) then can be mixed into larger 
quantities of plastic to achieve a desired color 
or end-property. 

condensation resin-A resin formed by a 
chemical reaction in which two or more mol- 
ecules combine with the separation of water 
or some other simple substance (e.g., the 
alkyd, phenol-aldehyde, and urea formalde- 
hyde resins). 

conditioning-The subjection of a material to 
a stipulated treatment so that it will respond 
in a uniform way to subsequent testing or 
processing. The term frequently is used to re- 
fer to the treatment given to specimens be- 
fore testing. (Standard ASTM test methods 
that include requirements for conditioning are 
indexed in the Index of ASTM Standards.) 

cooling channels-Channels or passageways 
located within the body of a mold through 
which a cooling medium can be circulated to 
control temperature on the mold surface. It 
also is possible to heat a mold by circulating 
steam, hot oil, or other heated fluid through 
the channels, as in the molding of the ther- 
mosetting and some thermoplastic materials. 

cooling fixture-Block of metal or wood hold- 
ing the shape of a molded piece that is used 
to maintain the proper shape or dimensional 
accuracy of a molding after it is removed 
from the mold until it is cool enough to retain 
its shape without further appreciable distor- 
tion. Also called shrink fixture. 

copolymer-See polymer. 
core-(1) Male element in die, which produces 

a hole or a recess in the part. (2) The portion 
of a complex mold that molds undercut parts. 
These cores usually are withdrawn to one side 
before the main sections of the mold open 
(and usually are called side cores). (3) A 
channel in a mold for circulation of a heat- 
transfer medium; also called force. (4) Por- 
tion of a mold that forms the interior surface 
of the part. See cavity. 

core pin-Pin used to mold a hole. 
core pin plate-Plate holding core pins. 
coring-The removal of excess material from 

the cross section of a molded part to attain a 
more uniform wall thickness. 

corona discharge-A method of rendering the 
surfaces of inert plastics such as polyethy- 



8 SPI PLASTICS ENGINEERING HANDBOOK 

lene more receptive to inks, adhesives, or 
coatings by subjecting the surfaces to an 
electrical discharge. A typical method is to 
pass a film over a grounded metal cylinder, 
above which a high voltage electrode is 
spaced to leave a small air gap. The corona 
discharge oxidizes the film, leading to the 
formation of polar groups. 

cratering-Depressions of coated plastic sur- 
faces caused by excessive lubricant. Crater- 
ing results when paint thins excessively and 
later ruptures, leaving pinholes and other 
voids. The use of less thinner in the coating 
can reduce or eliminate cratering, as can the 
use of less lubricant on the part. 

crazing-Fine cracks that may extend in a net- 
work on or under the surface or through a 
layer of plastic material. 

creep-The dimensional change with time of a 
material under load, following the initial in- 
stantaneous elastic deformation. Creep at 
room temperature is sometimes called cold 
flow. Ref. : Recommended Practices for 
Testing Long Time Creep and Stress-Relax- 
ation of Plastics under Tension or Compres- 
sion Loads at Various Temperatures (ASTM 
Designation: D 674). 

crosslinking-The establishment of chemical 
bonds between the molecular chains in poly- 
mers. Crosslinking can be accomplished by 
chemical reaction, vulcanization, degrada- 
tion, or radiation. 

crystallinity-A state of molecular structure in 
some resins that denotes uniformity and 
compactness of the molecular chains forming 
the polymer. It normally can be attributed to 
the formation of solid crystals having a def- 
inite geometric form. 

cure-To change the physical properties of a 
material by chemical reaction, which may be 
condensation, polymerization, or vulcaniza- 
tion; usually accomplished by the action of 
heat and catalysts, alone or in combination, 
with or without pressure. 

curing temperature-Temperature at which a 
cast, molded, or extruded product, a resin- 
impregnated reinforcing material, an adhe- 
sive, or other material is subjected to curing. 

curing time-In the molding of plastics, the 
interval of time between the instant of ces- 

sation of relative movement between the 
moving parts of a mold and the instant that 
pressure is released. Also called molding 
time. 

curtain coating-A method of coating that may 
be employed with low viscosity resins or so- 
lutions, suspensions, or emulsions of resins 
in which the substrate to be coated is passed 
through and perpendicular to a freely falling 
liquid “curtain” (or “waterfall”). The flow 
rate of the falling liquid and the linear speed 
of the substrate passing through the curtain 
are coordinated with the desired coating 
thickness. 

cycle-The complete, repeating sequence of 
operations in a process or part of a process. 
In molding, the cycle time is the period, or 
elapsed time, between a certain point in one 
cycle and the same point in the next. 

dancer roll-A roller used as a tension main- 
tenance device in the production of films and 
sheeting. 

daylight-Distance between the stationary 
platen and the moving platen on a molding 
press when the actuating system is fully re- 
tracted without ejector box and/or spacers. 

debossed-An indented or depressed design or 
lettering that is molded into an article so as 
to be below the main outside surface of that 
article. 

deflashing-Any technique or method that re- 
moves excess, unwanted material from a 
molded article. Specifically, the excess ma- 
terial is removed from those places on the ar- 
ticle where parting lines of the mold that 
formed the article may have caused the ex- 
cess material to be formed. 

deflection temperature-The temperature at 
which a specimen will deflect a given dis- 
tance at a given load under prescribed con- 
ditions of test. See ASTM D 648. Formerly 
called heat distortion. 

degassing-See breathing. 
degradation-A deleterious change in the 

chemical structure or physical properties of a 
plastic, caused by exposure to heat, light, or 
other agent. 

delamination-The splitting of a plastic ma- 
terial along the plane of its layers. Physical 



GLOSSARY 9 

separation or loss of bond between laminate 
plies. See laminated plastics. 

deliquescent-Capable of attracting moisture 
from the air. 

density-Weight per unit volume of a sub- 
stance, expressed in grams per cubic centi- 
meter, pounds per cubic foot, etc. 

desiccant-Substance that can be used for 
drying purposes because of its affinity for 
water. 

destaticization-Treatment of plastic materials 
that minimizes the effects of static electricity 
on the surface of articles. This treatment can 
be accomplished either by treating the sur- 
face with specific materials or by incorporat- 
ing materials into the molding compound. 
Minimizing the surface static electricity pre- 
vents dust and dirt from being attracted to 
and/or clinging io the article’s surface. 

diaphragm gate-Gate used in molding an- 
nular or tubular articles. The gate forms a 
solid web across the opening of the part. 

die adaptor-The part of an extrusion die that 
holds the die block. 

die cutting-( 1) Blanking. (2) Cutting shapes 
from sheet stock by striking it sharply with a 
shaped knife edge known as a steel rule die. 
Clicking and dinking are other names for die 
cutting of this kind. 

dielectric constant (permittivity or specific in- 
ductive capacity)-The ratio of the capaci- 
tance of an assembly of two electrodes sep- 
arated solely by a plastics insulating material 
to its capacitance when the electrodes are 
separated by air. (ASTM Designation: D 
150.) 

dielectric heating (electronics heating or RF 
heating)-The process of heating poor con- 
ductors of electricity by means of high-fre- 
quency (20 to 80 MHz) currents. Dielectric 
loss in the material is the basis. The process 
is used for sealing vinyl films, preheating 
thermoset molding compounds, and drying 
hygroscopic resins before processing. 

dielectric strength-The electric voltage gra- 
dient at which an insulating material is bro- 
ken down or “arced through,” in volts per 
mil of thickness. Ref. : Standard Methods of 
Test for Dielectric Breakdown Voltage and 
Dielectric Strength of Electrical Insulating 

Materials at Commercial Power Frequencies 
(ASTM Designation: D 149). 

differential thermal analysis (DTA)-An an- 
alytical method where the specimen is heated 
simultaneously with an inert material as a 
control, with each having its own tempera- 
ture sensing and recording apparatus. The 
curves generated show the weight losses of 
both materials under the same rates of heat- 
ing. 

diffusion-The movement of a material, such 
as a gas or liquid, in the body of a plastic. If 
the gas or liquid is absorbed on one side of a 
piece of plastic and given off on the other 
side, the phenomenon is called permeability. 
Diffusion and permeability are not due to 
holes or pores in the plastic but are caused 
and controlled by chemical mechanisms. 

dimensional stability-Ability of a plastic part 
to retain the precise shape in which it was 
molded, fabricated, or cast. 

dip coating-Applying a plastic coating by 
dipping the article to be coated into a tank of 
melted resin or plastisol, then chilling the ad- 
hering melt. 

disc gate-Mold gate having the same cross 
section as the mold runner. 

discoloration-Any change from the original 
color, often caused by overheating, light ex- 
posure, irradiation, or chemical attack. 

dispersion-Finely divided particles of a ma- 
terial in suspension in another substance. 

dissipation factor-A measure of electrical 
power lost in the form of heat when insulat- 
ing materials are used in alternating current 
circuits. 

doctor roll; doctor bar-A device for regulat- 
ing the amount of liquid material on the roll- 
ers of a spreader. 

double-shot molding-Means of producing 
two color parts andlor two different thermo- 
plastic materials by successive molding op- 
erations. 

dowel-Pin used to maintain alignment be- 
tween two or more parts of a mold. 

draft-The degree of taper of a side wall or the 
angle clearance designed to facilitate re- 
moval of parts from a mold. 

drape forming-Method of forming thermo- 
plastic sheet in which the sheet is clamped 



10 SPI PLASTICS ENGINEERING HANDBOOK 

into a movable frame, heated, and draped 
over high points of a male mold. Vacuum 
then is pulled to complete the forming oper- 
ation. 

draw down ratio-The ratio of the thickness 
of the die opening to the final thickness of 
the product. 

drawing-The process of stretching a thermo- 
plastic to reduce its cross-sectional area, thus 
creating a more orderly orientation of poly- 
mer chains with respect to each other. 

drive-The entire electrical and mechanical 
system used to supply mechanical energy to 
the input shaft of a gear reducer. This in- 
cludes the motor, constant or variable speed 
belt system, flexible couplings, starting 
equipment, etc. 

drooling-Leakage of resin from a nozzle or 
around the nozzle area during the injection 
step in injection molding or around the 
screen-pack during extrusion. 

dry blend-Term applied to a molding com- 
pound, containing all necessary ingredients, 
mixed in a way that produces a dry-free- 
flowing, particulate material. This term com- 
monly is used in connection with polyvinyl 
chloride molding compounds. 

dry coloring-Method commonly used by fab- 
ricators for coloring plastic by tumble-blend- 
ing uncolored particles of the plastic material 
with selected dyes and pigments. 

durometer hardness-The hardness of a ma- 
terial as measured by the Shore Durometer. 
Ref.: Tentative Method of Test for Indenta- 
tion Hardness of Rubber and Plastics by 
Means of a Durometer (ASTM Designation: 
D 2240). 

dwell-A pause in the application of pressure 
to a mold, made just before the mold is com- 
pletely closed, to allow the escape of gas 
from the molding material. 

dyes-Synthetic or natural organic chemicals 
that are soluble in most common solvents; 
characterized by good transparency, high 
tinctorial strength, and low specific gravity. 

EDM-See electric discharge machining. 
E-glass-A low-alkali borosilicate glass widely 

ejector pin (or ejector sleeve)-A rod, pin, or 
used in reinforcing plastics. 

sleeve that pushes a molding off a force or 
out of a cavity of a mold. It is attached to an 
ejector bar or plate that can be actuated by 
the ejector rod(s) of the press or by auxiliary 
hydraulic or air cylinders. 

ejector return pins-Projections that push the 
ejector assembly back as the mold closes; 
also called safety pins and position push- 
backs. 

elastic deformation-The part of the defor- 
mation of an object under load that is re- 
coverable when the load is removed. 

elasticity-That property of a material by vir- 
tue of which it tends to recover its original 
size and shape after deformation. If the strain 
is proportional to the applied stress, the ma- 
terial is said to exhibit Hookean or ideal elas- 
ticity. 

elastomer-A material that at room tempera- 
ture stretches under low stress to at least 
twice its length and snaps back to the original 
length upon release of stress. 

electric discharge machining (EDM)-A 
metalworking process applicable to mold 
construction in which controlled sparking is 
used to erode away the workpiece. 

electroforming-Moldmaking method where- 
by a thin layer of metal is deposited onto a 
pattern. Molten metal then may be sprayed 
on the back of the mold to increase its 
strength. 

electroplating-Deposition of metals on cer- 
tain plastics and molds for finish. 

elongation-The fractional increase in length 
of a material stressed in tension. 

embossing-Techniques used to create depres- 
sions of a specific pattern in plastics film and 
sheeting. Such embossing in the form of sur- 
face patterns can be achieved on molded parts 
by the treatment of the mold surface by pho- 
toengraving or another process. 

encapsulating-Enclosing an article (usually 
an electronic component or the like) in a 
closed envelope of plastic, by immersing the 
object in a casting resin and allowing the 
resin to polymerize or, if hot, to cool. See 
potting . 

environmental stress cracking (ESC)-The 
susceptibility of a thermoplastic article to 
crack or craze when stressed, in the presence 



GLOSSARY 1 1  

of surface-active agents or in other environ- 
ments. 

exotherm-( 1) The temperaturehime curve of 
a chemical reaction giving off heat, particu- 
larly the polymerization of casting resins. (2) 
The amount of heat given off. The term has 
not been standardized with respect to sample 
size, ambient temperature, degree of mixing, 
etc. 

expandable plastic-A plastic compound that 
can be made cellular during processing by 
chemical or thermal means. 

extender-A substance, generally having some 
adhesive action, added to a plastic composi- 
tion to reduce the amount of the primary resin 
required per unit area. See filler. 

extrudate-The product or material delivered 
by an extruder, such as film, pipe, the coat- 
ing on wire, etc. 

extruder-Basically, a machine that accepts 
solid particles (pellets or powder) or liquid 
(molten) feed, conveys it through a sur- 
rounding barrel by means of a rotating screw, 
and pumps it, under pressure, through an ori- 
fice. The nomenclature used encompasses the 
barrel, the screw, and other extruder ele- 
ments. The metering section of the screw is 
a relatively shallow portion of the screw at 
the discharge end with a constant depth and 
lead, and having a length of one or more turns 
of the flight. 

extrusion-Process of compacting and melting 
a plastic material and forcing it through an 
orifice in a continuous fashion. Material is 
conveyed through the heated machine barrel 
by a helical screw or screws, where it is 
heated and mixed to a homogeneous state and 
then forced through a die of the shape re- 
quired for the finished product. 

extrusion coating-Process in which a resin is 
coated on a substrate by extruding a thin film 
of molten resin and pressing it onto or into 
the substrates, or both, without the use of an 
adhesive. 

fabricate-To work a material into a finished 
form by machining, forming, or other oper- 
ation. 

family mold-A multicavity mold wherein 

each of the cavities forms one of the com- 
ponent parts of the assembled finished ob- 
ject. The term often is applied to molds 
wherein parts from different customers are 
grouped together in one mold for economy 
of production. Sometimes called a combina- 
tion mold. 

fan gate-A shallow gate somewhat wider than 
the runner from which it extends. 

feed section-First section or zone of an extm- 
der screw, which is fed from the hopper and 
conveys solids to the melting zone. 

fiber reinforcement-Thin fibers of glass, car- 
bon, metal, or synthetic resin incorporated 
into resin to increase strength. Forms include 
continuous, chopped, knitted, and woven. 

filament-Fiber of extreme length used in 
yams and other compositions. 

filament winding-Process in which roving or 
single strands of glass, metal, or other rein- 
forcement are wound in a predetermined pat- 
tern onto a suitable mandrel. The pattern is 
so designed as to give maximum strength in 
the directions required. The strands can be 
run from a creel through a resin bath before 
winding, or preimpregnated materials can be 
used. When the right number of layers have 
been applied, the wound mandrel is cured at 
room temperatures or in an oven. 

fill-and-wipe-Process whereby parts are 
molded with depressed designs, and after ap- 
plication of paint the surplus is wiped off, 
with paint remaining only in depressed areas. 
Sometimes called wipe-in. 

filler-Inert substance added to a plastic to 
make it less costly. However, fillers may also 
improve physical properties, particularly 
hardness, stiffness, dimensional stability, and 
impact strength. 

fillet-A rounded filling of the internal angle 
between two surfaces. 

film-Sheeting having a nominal thickness not 
greater than 0.010 inch. 

fines-Very small particles (usually under 200 
mesh) accompanying larger grains, usually 
of molding powder. 

fisheye-Small globular mass that has not 
blended completely into the surrounding ma- 
terial; particularly evident in a transparent or 
translucent material. 



12 SPI PLASTICS ENGINEERING HANDBOOK 

fixture-Means of holding a part during a ma- 
chining or other operation. 

flame retardant-A chemical compounded 
into a resin to make it fire-resistant. 

flame spraying-Method of applying a plastic 
coating in which finely powdered fragments 
of the plastic, together with suitable fluxes, 
are projected through a cone of flame onto a 
surface. 

flame treating-A method of rendering inert 
thermoplastic objects receptive to inks, lac- 
quers, paints, adhesives, etc., in which the 
object is bathed in an open flame to promote 
oxidation of the surface of the article. 

flash-Extra plastic attached to a molding along 
the parting line; under most conditions it 
would be objectionable and must be removed 
before the parts are acceptable. 

flash gate-Usually a long gate extending from 
a runner that runs parallel to an edge of a 
molded part along the flash or parting line of 
the mold. 

flash line-A raised line appearing on the sur- 
face of a molding and formed at the junction 
of mold faces. See parting line. 

flash mold-A mold in which the mold faces 
are perpendicular to the clamping action of 
the press, so that the higher the clamping 
force, the tighter the mold seam. 

flexible molds-Molds made of rubber or elas- 
tomeric plastics used for casting plastics. 
They can be stretched to remove cut pieces 
with undercuts. 

flexural modulus, psi-The ratio of stress to 
strain for a given material within its propor- 
tional limit under bending load conditions. 
(ASTM test methods D 790.) 

flexural strength-Ability of a material to flex 
without permanent distortion or breaking. 
Ref.: Standard Method of Test for Flexural 
Properties of Plastics (ASTM Designation: 
D 790). 

flight-The outer surface of the helical ridge of 
metal on an extrusion or injection molding 
screw. 

floating core-Mold member, free to move 
vertically, that fits over a lower plug or cav- 
ity, and into which an upper plug telescopes. 

floating platen-Movable platen(s) between 

the stationary platen and the actuated platen 
on a vertically operating compression press. 

flock-Short fibers of cotton, wood, or glass 
used as a filler for resins. 

flour-An organic filler. Such organic fillers as 
wood flour and shell flours are used as exten- 
ders and reinforcements. Wood flour is a 
finely ground product commonly made from 
soft woods, whereas shell flours are derived 
from peanut and rice hulls. 

flow-A qualitative description of the fluidity 
of a plastic material during the process of 
molding. 

flow line-A mark on a molded piece made by 
the meeting of two flow fronts during mold- 
ing. Also called the weld line. 

fluidized bed coating-A method of applying 
a coating of a thermoplastic resin to an article 
in which the heated article is immersed in a 
dense-phase fluidized bed of powdered resin 
and thereafter heated in an oven to provide a 
smooth, pinhole-free coating. 

foamed plastics-Plastics with internal voids 
or cells. The foam may be flexible or rigid, 
the cells closed or connected, and the density 
anything from slightly below that of the solid 
parent resin down to, in some cases, 1 lb/ft 
cubed, or less. 

foaming agents-Chemicals added to plastics 
and rubbers that generate inert gases on heat- 
ing, causing the resin to assume a cellular 
structure. 

foam-in-place-A type of foam deposition that 
requires that the foaming machine be brought 
to the work, which is “in place,” as opposed 
to bringing the work to the foaming machine. 

foil decorating-Printing method where me- 
tallic or pigmented designs are transferred to 
a plastic surface using heat and pressure. See 
hot-stamping. 

force-The portion of the mold that forms the 
inside of the molded part. Also called a core 
or a plunger. 

forging-See solid phase forming. 
forming-A process in which the shape of 

plastic pieces such as sheets, rods, or tubes 
is changed to a desired configuration. See 
also thermoforming. (Note: The use of the 
term “forming” in plastics technology does 



GLOSSARY 13 

not include such operations as molding, cast- 
ing, or extrusion, in which shapes or pieces 
are made from molding materials or liquids.) 

friction welding-A method of welding ther- 
moplastics materials whereby the heat nec- 
essary to soften the components is provided 
by friction. See spin welding and vibration 
welding. 

frost line-In the extrusion of polyethylene lay- 
flat film, a ring-shaped zone located at the 
point where the film reaches its final diame- 
ter. In this zone, the film has a “frosty” ap- 
pearance caused by the film’s temperature 
falling below the softening range of the resin. 

frothing-Technique for applying urethane 
foam in which blowing agents or tiny air 
bubbles are introduced under pressure into 
the liquid mixture of foam ingredients. 

fusion bonding-Process for joining plastic 
parts where mating surfaces are heated to 
melting and held together under pressure. 
Also known as hot plate welding. 

glass transition temperature-The tempera- 
ture at which an amorphous polymer changes 
from a hard, brittle (glassy) condition to a 
viscous, elastomeric form. Also called sec- 
ond-order transition, gamma transition, rub- 
ber transition, and rubbery transition. The 
word transformation also has been used in- 
stead of transition. 

gate-The short, usually restricted, section of 
the runner at the entrance to the cavity of an 
injection or transfer mold. 

gate mark-A surface discontinuity on the part 
caused by the presence of the mold orifice 
through which material enters the cavity. 

gauge-Thickness of plastic film or sheet. 
gel-(1) A semisolid system consisting of a 

network of solid aggregates, in which liquid 
is held. ( 2 )  The initial jellylike solid phase 
that develops during the formation of a resin 
from a liquid. Both types of gel have very 
low strengths and do not flow like a liquid; 
they are soft and flexible, and will rupture 
under their own weight unless supported ex- 
ternally. (3) Small globular mass not com- 
pletely blended into film or sheet, causing a 
defect. 

gelatin-(1) Formation of a gel. ( 2 )  In vinyl 
dispersions, formation of a gel in the early 
stages of fusion. 

glass finish-A material applied to the surface 
of a glass reinforcement to improve its effect 
upon the physical properties of the reinforced 
plastic. 

gloss-The shine or luster of the surface of a 
material. 

granular structure-Nonuniform appearance 
of finished plastic material due to retention 
of, or incomplete fusion of, particles of com- 
position, either within the mass or on the sur- 
face. 

granulator-Machine used for size reduction 
of plastic scrap for reuse. Also called grinder. 

grid-Channel-shaped mold-supporting mem- 
bers. 

grit blasting-A surface treatment of a mold 
in which steel grit or sand materials are blown 
on the walls of the cavity to produce a rough- 
ened surface. Air escape from mold is im- 
proved, and a special appearance of the 
molded article is often obtained by this 
method. 

guide pins-Devices that maintain proper 
alignment of force plug and cavity as mold 
closes. Also called leader pins. 

guide-pin bushing-A guiding bushing 
through which the leader pin moves. 

gussets-Inward folds on sides of collapsed, 
blown film to reduce width and produce bags 
with rectangular form. 

hardener-A substance or mixture of sub- 
stances added to a resin or adhesive to pro- 
mote or control the curing reaction by taking 
part in it. The term is also used to designate 
a substance added to control the degree of 
hardness of the cured film. See catalyst. 

hardness-The resistance of a plastic material 
to compression and indentation. Among the 
most important methods of testing this prop- 
erty are Brinell hardness, Rockwell hard- 
ness, and Shore hardness. 

haze-Cloudiness in plastic film. 
heat-distortion point-The temperature at 

which a standard test bar deflects 0.010 inch 
under a stated load of either 66 or 264 psi. 



14 SPI PLASTICS ENGINEERING HANDBOOK 

Ref.: Standard Method of Test for Deflection 
Temperature of Plastics under Load (ASTM 
Designation: D 648). 

heat forming-See thermoforming. 
heat gun-Electrically heated gun for soften- 

ing, curing, drying, preheating, and welding 
plastics, coatings, and compounds as well as 
shrinking of heat-shrinkable plastic tubing 
and plastic films. 

heat-sealing-A method of joining plastic films 
by simultaneous application of heat and pres- 
sure to areas in contact. Heat may be sup- 
plied conductively or dielectrically. 

heat-treating-Term used to cover annealing, 
hardening, tempering, etc. 

helix-See extruder. 
helix angle-The angle of a screw flight at its 

periphery relative to a plane perpendicular to 
the axis. 

high-frequency heating-The heating of ma- 
terials by dielectric loss in a high-frequency 
electrostatic field. The material is exposed 
between electrodes, and by absorption of en- 
ergy from the electrical field is heated quickly 
and uniformly throughout. 

high-pressure laminates-Laminates molded 
and cured at pressures not lower than 1000 
psi and more commonly in the range of 1200 
to 2000 psi. 

high-resiliency flexible foams-Urethane 
foams that offer low hysteresis and modulus; 
they can be made more flame retardant than 
rigid foams, and process at lower oven tem- 
peratures and with shorter molding cycles. In 
trade parlance, these foams have a sag factor 
of 2.7 and above (i.e., better cushioning). 
They can be produced “cold cure” (with no 
additional heat needed over that supplied by 
the exothermic reaction of the foaming pro- 
cess) or with heated molds and heat cures. 

hob-A master model used to sink the shape of 
a mold into a soft steel block. 

hobbing-A process of forming a mold by 
forcing a hob of the shape desired into a soft 
steel blank. 

homopolymer-The result of the polymeriza- 
tion of a single monomer, a polymer that 
consists of a single type or repeating unit. 

honeycomb-Manufactured product consisting 
of sheet metal or a resin-impregnated sheet 

material (paper, fibrous glass, etc.) that has 
been formed into hexagonal-shaped cells. 
Used as core material for sandwich construc- 
tions. 

hopper-Feed reservoir into which molding 
powder is loaded and from which it falls into 
a molding machine or extruder, sometimes 
through a metering device. 

hopper dryer-A combination feeding and 
drying device for extrusion and injection 
molding of thermoplastics. Hot air flows up- 
ward through the hopper containing the feed 
pellets. 

hopper loader-A curved pipe through which 
molding powders are pneumatically con- 
veyed from shipping drums to machine hop- 
pers. 

hot gas welding-A technique of joining ther- 
moplastic materials (usually sheet) whereby 
the materials are softened by a jet of hot air 
from a welding torch, and are joined together 
at the softened points. Generally a thin rod 
of the same material is used to fill and con- 
solidate the gap. 

hot plate welding-See fusion bonding. 
hot-runner mold-A thermoplastic injection 

mold in which the runners are insulated from 
the chilled cavities and remain hot so that the 
center of the runner never cools in normal 
cycle operation. Runners are not, as is the 
case usually, ejected with the molded pieces. 
Sometimes called insulated runner mold. 

hot-stamping-Operation for marking plastics 
in which roll leaf is stamped with heated 
metal dies onto the face of the plastics. 

hydraulic clamp-Device used in variety of 
molding and forming machines that consists 
basically of a high-speed, variable hydraulic 
pump, valving, a fast-acting cylinder, and a 
high-pressure cylinder. Cylinders can be sin- 
gle or combination units. The clamp closes 
the mold halves to form the part. 

hygroscopic-Readily absorbing and retaining 
environmental moisture. 

impact strength-( 1) The ability of a material 
to withstand shock loading. ( 2 )  The work 
done in fracturing, under shock loading, a 
specified test specimen in a specified man- 
ner. 



GLOSSARY 15 

impregnation-The process of thoroughly 
soaking a material such as wood, paper, or 
fabric with a synthetic resin so that the resin 
gets within the body of a material. 

impulse sealing-A heat-sealing technique in 
which a pulse of intense thermal energy is 
applied to the sealing area for a very short 
time, followed immediately by cooling. It 
usually is accomplished by using an RF 
heated metal bar that is cored for water cool- 
ing or is of such a mass that it will cool rap- 
idly at ambient temperatures. 

inhibitor-A substance that prevents or retards 
a chemical reaction. 

initiator-Peroxide used as source of free rad- 
icals. These substances are used in free rad- 
ical polymerizations, in curing thermosetting 
resins, as crosslinking agents for elastomers 
and polyethylene, and for polymer modifi- 
cation. 

injection mold-A mold into which a plasti- 
cated material is introduced from an exterior 
heating cylinder. 

injection molding-A molding procedure 
whereby a heat-softened plastic material is 
forced from a cylinder into a cavity that gives 
the article the desired shape. It is used with 
both thermoplastic and thermosetting mate- 
rials. 

inorganic pigments-Natural or synthetic me- 
tallic oxides, sulfides, and other salts, cal- 
cined during processing at 1200 to 2100°F. 
They are outstanding in producing heat and 
light stability, weather resistance, and migra- 
tion resistance. 

insert-An integral part of a plastic molding 
consisting of metal or other material that may 
be molded into position or may be pressed 
into the part after the molding is completed. 

insert molding-Process by which compo- 
nents such as fasteners, pins, studs, and ter- 
minals can be incorporated into a part as it is 
being molded. 

integral-skin foams-As applied to urethane 
or structural foams, designation for molded 
foams that develop their own integral surface 
skins. The surface skin is generally “solid,” 
as contrasted to the cellular construction in 
the interior of the part. 

iridescence-Loss of brilliance in metallized 

plastics and development of multicolor re- 
flectance. It is caused by cold flow of plastic 
or coating and from extra heat during vac- 
uum metallizing. 

irradiation (atomic)-As applied to plastics, 
term that refers to bombardment with a 
variety of subatomic particles, generally 
alpha-, beta-, or gamma-rays. Atomic irra- 
diation has been used to initiate polymeriza- 
tion and copolymerization of plastics and in 
some cases to bring about changes in the 
physical properties of a plastic material. 

isotactic-Description applied to a chain of un- 
symmetrical molecules combined head to 
tail, with their methyl groups occupying the 
same relative positions in space along the 
chain. 

jet molding-Processing technique character- 
ized by the fact that most of the heat is ap- 
plied to the material as it passes through a 
nozzle or jet, rather than in a heating cylinder 
as is done in conventional processes. 

jetting-Turbulent flow of resin from an un- 
dersized gate or thin section into a thicker 
mold section, as opposed to laminar flow of 
material progressing radially from a gate to 
the extremities of the cavity. 

jig-Means of holding a part and guiding the 
tool during machining or assembly opera- 
tion. 

joint-The location at which two adherends are 
held together with a layer of adhesive. 

kirksite-An alloy of aluminum and zinc used 
for the construction of blow molds; it imparts 
a high degree of heat conductivity to the 
mold. 

kiss-roll coating-Roll arrangement that car- 
ries a metered film of coating to the web; at 
the line of web contact, it is split with part 
remaining on the roll and the remainder of 
the coating adhering to the web. 

knife coating-A method of coating a sub- 
strate (usually paper or fabric) in which the 
substrate, in the form of a continuous mov- 
ing web, is coated with a material whose 
thickness is controlled by an adjustable knife 
or bar set at a suitable angle to the substrate. 

knit line-See weld lines. 



16 SPI PLASTICS ENGINEERING HANDBOOK 

knockout bar-A bar or plate in a knockout 
frame used to back up a row or rows of 
knockout pins. 

knockout pin-See ejector pin. 

L / D  ratio-The ratio of the length (L) to the 
diameter (D)  of an extruder screw or barrel. 

laminar flow-Flow of thermoplastic resins in 
a mold that is accompanied by solidification 
of the layer in contact with the mold surface, 
which acts as an insulating tube through 
which material flows to fill the remainder of 
the cavity. This type of flow is essential to 
duplication of the mold surface. 

laminated plastic (synthetic resin-bonded 
laminate, laminate)-A plastics material 
consisting of superimposed layers of a syn- 
thetic. resin-impregnated or -coated substrate 
(paper, glass mat, etc.) that have been 
bonded together, usually by means of heat 
and pressure, to form a single piece. 

land-(1) The horizontal bearing surface of a 
semipositive or flash mold by which excess 
material escapes. ( 2 )  The bearing surface 
along the top of the flights of a screw in a 
screw extruder. (3) The surface of an extru- 
sion die parallel to the direction of melt flow. 

lay-up-( 1) As used in reinforced plastics, the 
reinforcing material placed in position in the 
mold; also the resin-impregnated reinforce- 
ment. ( 2 )  The process of placing the rein- 
forcing material in position in the mold. 

leach-To extract a soluble component from a 
mixture by the process of percolation. 

light-resistance-The ability of a plastics ma- 
terial to resist fading after exposure to sun- 
light or ultraviolet light. Ref. : Tentative Rec- 
ommended Practice for Exposure of Plastics 
to Fluorescent Sunlamp (ASTM Designa- 
tion: D 1501). 

liquid injection molding KIM)-( 1) A pro- 
cess that involves an integrated system for 
proportioning, mixing, and dispensing two- 
component liquid resin formulations and di- 
rectly injecting the resultant mix into a mold 
that is clamped under pressure. It generally 
is used for the encapsulation of electrical and 
electronic devices. ( 2 )  Variation on reaction 
injection molding that involves mechanical 
mixing rather than the high-pressure im- 

pingement mixing used with reaction injec- 
tion molding. However, unlike mechanical 
mixing in other systems, the mixer here does 
not need to be flushed, as a special feed sys- 
tem automatically dilutes the residue in the 
mixer with part of the poly01 needed for the 
next shot, thereby keeping the ingredients 
from reacting. 

loading tray-A device in the form of a spe- 
cially designed tray that is used to load the 
charge of material or metal inserts simulta- 
neously into each cavity of a multicavity 
mold by the withdrawal of a sliding bottom 
from the tray. Also called charging tray. 

locating ring-A ring that serves to align the 
nozzle of an injection cylinder with the en- 
trance of the sprue bushing and the mold to 
the machine platen. 

loss factor-The product of the power factor 
and the dielectric constant. 

low-pressure laminates-In general, lami- 
nates molded and cured in the range of pres- 
sures from 400 psi down to and including 
pressures obtained by the mere contact of the 
plies. 

low-profile resins-Designation applied to 
special polyester resin systems for reinforced 
plastics. These systems are combinations of 
thermoset resins and thermoplastic resins 
used to minimize surface waviness in molded 
parts. 

lubricant-Additive to plastic resin to promote 
mixing and improve flow properties. 

lug-(I) A type of thread configuration, usu- 
ally thread segments disposed equidistantly 
around a bottle neck (finish). ( 2 )  A small in- 
dentation or raised portion on the surface of 
a product, provided as a means of indexing 
for operations such as multicolor decoration 
or labeling. 

luminescent pigments-Pigments that pro- 
duce striking effects in darkness or light. 
Forms include fluorescence and phospho- 
rescence. 

mandrel-(1) In blow molding, part of the 
tooling that forms the inside of the container 
neck and through which air is forced to form 
the hot parison to the shape of the molds. (2) 
In extrusion, the solid, cylindrical part of the 



GLOSSARY 17 

die that forms tubing or pipe. (3) In filament 
winding of reinforced plastic, the form (usu- 
ally cylindrical) around which the filaments 
are wound. 

manifold-Mainly with blow molding and 
sometimes with injection molding equip- 
ment, the distribution or piping system that 
takes the single channel flow output of the 
extruder or injection cylinder and divides it 
to feed several blow molding heads or injec- 
tion nozzles. 

masterbatch-A plastics compound that in- 
cludes a high concentration of an additive or 
additives. Masterbatches are designed for use 
in appropriate quantities with the basic resin 
or mix so that the corrxt end concentration 
is achieved. For example, color master- 
batches for a variety of plastics are used ex- 
tensively, as they provide a clean and con- 
venient method of obtaining accurate color 
shades. 

mat-A fabric or felt of glass or other rein- 
forcing fiber used in manufacturing plastic 
composite parts. 

material distribution-The variation in thick- 
ness of various parts of a product (i.e., body, 
wall, shoulder, heel, base, etc.). 

matched metal molding-Method of molding 
reinforced plastics between two close-fitted 
metal molds mounted in a press. 

material well-Space provided in a compres- 
sion mold to care for the bulk factor of the 
material load. 

matrix-The continuous phase of a composite 
material; the resin component in a reinforced 
plastics material. 

matte finish-A type of dull, nonreflective fin- 
ish. 

mechanically foamed plastic-A cellular 
plastic whose structure is produced by phys- 
ically incorporated gases. 

melt fracture-An instability in the melt flow 
through a die, starting at the entry to the die. 
It leads to surface irregularities on the fin- 
ished article such as a regular helix or irreg- 
ularly spaced ripples. 

melt index-The amount, in grams, of a ther- 
moplastic resin that can be forced through a 
0.0825 inch orifice when subjected to 2160 
grams force in 10 minutes at 190°C. Ref.: 

Tentative Method of Measuring Flow Rates 
of Thermoplastics by Extrusion Plastometer 
(ASTM Designation: D 1238). 

melt strength-The strength of a plastic while 
in the molten state. 

melting point-The temperature at which a 
resin changes from a solid to a liquid. 

memory-The tendency of a plastic article to 
return to a size and shape that existed during 
the manufacturing process. 

metallizing-Application of a thin coating of 
metal to a nonmetallic surface. It may be 
done by chemical deposition or by exposing 
the surface to vaporized metal in a vacuum 
chamber. 

metallic pigments-A class of pigments con- 
sisting of thin opaque aluminum flakes (made 
by a ball milling either a disintegrated alu- 
minum foil or a rough metal powder and then 
polishing to obtain a flat, brilliant surface on 
each particle) or copper alloy flakes (known 
as bronze pigments). Incorporated into plas- 
tics, they produce unusual silvery and other 
metal-like effects. 

metering screw-An extrusion screw that has 
a shallow constant depth, and a constant pitch 
section over, usually, the last three to four 
flights. 

metering section-A relatively shallow por- 
tion of an extruder screw at the discharge end 
with a constant depth and lead, and having a 
length of at least one or more turns of the 
flight. 

migration of plasticizer-Loss of plasticizer 
from an elastomeric plastic compound with 
subsequent absorption by an adjacent me- 
dium of lower plasticizer concentration. 

modulus of elasticity-The ratio of stress to 
strain in a material that is elastically de- 
formed. Ref.: Standard Method of Test for 
Flexural Properties of Plastics (ASTM Des- 
ignation: D 790). 

moisture vapor transmission-The rate at 
which water vapor permeates through a plas- 
tic film or wall at a specified temperature and 
relative humidity. Ref. : Standard Methods of 
Test for Water Vapor Transmission of Ma- 
terials in Sheet Form (ASTM Designation: E 
96). 

mold-A hollow form or cavity into which 



18 SPI PLASTICS ENGINEERING HANDBOOK 

molten plastic material is introduced to give 
the shape of the required component. The 
term generally refers to the whole assembly 
of elements that make up the section of the 
molding equipment in which the parts are 
formed. Also called tool or die. 

mold base-The assembly of all parts making 
up an injection mold, other than the cavity, 
core, and pins. 

mold insert (removable)-Part of a mold cav- 
ity or force that forms undercut or raised por- 
tions of a molded article. 

mold mark-Identifying symbol of the molder 
who produced the part; usually molded into 
an unobtrusive area. 

mold release-A lubricant used to coat a mold 
cavity to prevent the molded piece from 
sticking to it, and thus to facilitate its re- 
moval from the mold. Also called release 
agent. See parting agent. 

mold seam-A line formed by mold construc- 
tion such as removable members in cavity, 
cam slides, etc. (not to be confused with 
mold parting line). 

molding cycle-See cycle. 
molding material-Plastic material in varying 

stages of granulation, often comprising resin, 
filler, pigments, plasticizers, and other ingre- 
dients, ready for use in the molding opera- 
tion. Also called molding compound or pow- 
der. 

molding pressure-The pressure applied di- 
rectly or indirectly on the compound to allow 
the complete transformation to a solid dense 
part. 

molding shrinkage-The difference in dimen- 
sions, expressed in inches per inch, between 
a molding and the mold cavity in which it 
was molded, both the mold and the molding 
being at normal room temperature when 
measured. Also called mold shrinkage, 
shrinkage, and contraction. 

molecular weight-The sum of the atomic 
weights of all atoms forming a molecule. 

molecular weight distribution-A measure of 
the relative amounts of polymers with differ- 
ent molecular weights within a batch of ma- 
terial. This measure may be indicated by the 
ratio of the weight-average molecular weight 
to the number-average molecular weight. 

monomer-A relatively simple compound that 
can react to form a polymer (i.e., polymer- 
ize). 

movable platen-The moving platen of an in- 
jection or compression molding machine to 
which half of the mold is secured during op- 
eration. This platen is moved by either a hy- 
draulic ram or a toggle mechanism. 

multicavity mold-A mold with two or more 
mold impressions; i.e., a mold that produces 
more than one molding per molding cycle. 

multiple-flighted screw-A screw having more 
than one helical flight, such as double 
flighted, double lead, double thread, or two 
starts, triple flighted, etc. 

multiple-screw extruders-As contrasted to 
conventional single-screw extruders, these 
machines involve the use of two or four 
screws (conical or constant depth). Types in- 
clude machines with intermeshing counter- 
rotating screws and those with nonintermesh- 
ing counter-rotating screws. 

neck-in-In extrusion coating, the difference 
between the width of the extruded web as it 
leaves the die and the width of the coating on 
the substrate. 

needle blow-A specific blow molding tech- 
nique where the blowing air is injected into 
a hollow article through a sharpened hollow 
needle that pierces the parison. 

nip rolls-A pair of rolls on a blown film line 
that close the bubble and regulate the rate at 
which the film is pulled away from the extru- 
sion die. 

nitriding-A hardening process for ferrous al- 
loys used on extruder screws. 

notch sensitivity-The extent to which the 
sensitivity of a material to fracture is in- 
creased by the presence of a break in the ho- 
mogeneity of the surface, such as a notch, a 
sudden change in section, a crack, or a 
scratch. Low notch sensitivity is usually as- 
sociated with ductile materials, and high 
notch sensitivity with brittle materials. 

nozzle-The hollow cored metal nose screwed 
into the extrusion end of (a) the heating cyl- 
inder of an injection machine or (b) a transfer 
chamber where this is a separate structure. A 
nozzle is designed to form under pressure a 



GLOSSARY 19 

seal between the heating cylinder or the 
transfer chamber and the mold. The front end 
of a nozzle may be either flat or spherical in 
shape. 

nucleating agent-A chemical substance that 
provides sites for crystal formation in poly- 
mer melts. 

offset printing-A printing process in which 
the image to be printed first is applied to an 
intermediate camer such as a roll or plate and 
then is transferred to a plastic film or molded 
article. 

olefins-A group of unsaturated hydrocarbons 
of the general formula CnH2,,, and named 
after the corresponding paraffins by the ad- 
dition of “ene” to the stem. Examples are 
ethylene and propylene. 

one-shot molding-In the urethane foam field, 
term applied to a system whereby the iso- 
cyanate, polyol, catalyst, and other additives 
are mixed together directly, and a foam is 
produced immediately (as distinguished from 
prepolymer) . 

opaque-Description of a material or sub- 
stance that will not transmit light; opposite 
of transparent. Materials that are neither 
opaque nor transparent sometimes are de- 
scribed as semiopaque, but are more prop- 
erly classified as translucent. 

open-cell foam-A cellular plastic in which 
there is a predominance of interconnected 
cells. 

orange peel-Uneven leveling of coating on 
plastic surfaces, usually because of high vis- 
cosity. Simple spray gun adjustments and/or 
addition of high boiling solvent to coating for 
a wetter spray is helpful. 

organic pigments-Pigments characterized by 
good brightness and brilliance, which are di- 
vided into toners and lakes. Toners, in turn, 
are divided into insoluble organic toners and 
lake toners. The insoluble organic toners 
usually are free of salt-forming groups. Lake 
toners are practically pure, water-insoluble 
heavy metal salts of dyes without the fillers 
or substrates of ordinary lakes. Lakes, which 
are not as strong as lake toners, are water- 
insoluble heavy metal salts or other dye com- 

plexes precipitated upon or admixed with a 
base or filler. 

organosol-A suspension of a finely divided 
resin in a volatile organic liquid. The resin 
does not dissolve appreciably in the organic 
liquid at room temperature, but does so at el- 
evated temperatures. The liquid evaporates 
at the elevated temperature, and the residue 
on cooling is a homogeneous plastic mass. 
Plasticizers may be dissolved in the volatile 
liquid. 

orientation-The alignment of the crystalline 
structure in polymeric materials so as to pro- 
duce a highly uniform structure. It can be ac- 
complished by cold drawing or stretching 
during fabrication. 

orifice-The opening in the extruder die formed 
by the orifice bushing (ring) and mandrel. 

orifice bushing-The outer part of the die in 
an extruder head. 

outgassing-Devolatilization of plastics or ap- 
plied coatings during exposure to vacuum in 
vacuum metallizing. Resulting parts show 
voids or thin spots in plating with reduced 
and spotty brilliance. Additional drying prior 
to metallizing is helpful, but outgassing is in- 
herent in plastic materials and coatings in- 
gredients, including plasticizer and volatile 
components. 

parallels-The support spacers placed between 
the mold and the press platen or clamping 
plate. Also called risers. 

parison-A precursor to a blow-molded part 
created by extrusion or injection molding. 

parison programmer-A device that allows 
the extrusion of a parison that differs in 
thickness along its length in order to equalize 
the wall thickness of a blow-molded product. 
It can be done with a pneumatic or hydraulic 
device that activates the mandrel shaft and 
adjusts the mandrel position during parison 
extrusion (parison programmer, controller, or 
variator). It also can be done by varying the 
extrusion speed on accumulator-type blow 
molding machines or, in parison reheat sys- 
tems, by varying the amount of heat applied. 

parison swell-In extrusion blow molding, the 
ratio of the cross-sectional area of the parison 
to the cross-sectional area of the die opening. 



20 SPI PLASTICS ENGINEERING HANDBOOK 

part-In its proper literal meaning, a compo- 
nent of an assembly. However, the word is 
widely misused to designate any individual 
manufactured article, even when (like a cup, 
a comb, a doll) it is complete in itself, not 
part of anything. 

parting agent-A lubricant, often wax, used 
to coat a mold cavity to prevent the molded 
piece from sticking to it, and thus to facilitate 
its removal from the mold. Also called re- 
lease agent. 

parting line-Mark on a mold or casting where 
halves of a mold met in closing. 

pearlescent pigments-A class of pigments 
consisting of particles that are essentially 
transparent crystals of a high refractive in- 
dex. The optical effect is one of partial re- 
flection from the two sides of each flake. 
When reflections from parallel flakes rein- 
force each other, the result is a silvery luster. 
Possible effects range from brilliant high- 
lighting to moderate enhancement of the nor- 
mal surface gloss. 

permanence-Resistance of a plastic to appre- 
ciable changes in characteristics with time 
and environment. 

permeability-( 1) The passage or diffusion of 
a gas, vapor, liquid, or solid through a bar- 
rier without being physically or chemically 
affected. ( 2 )  The rate of such passage. 

pigment-A coloring agent mixed with plastic 
material prior to processing to provide a uni- 
form color. 

pill-See preform. 
pinch-off-A raised edge around a cavity in the 

mold that seals off the part and separates the 
excess material as the mold closes around the 
parison in the blow molding operation. 

pinhole-A very small hole in a plastic con- 
tainer, film, etc. 

pinpoint gate-A restricted orifice of 0.030 
inch or less in diameter through which mol- 
ten resin flows into a mold cavity. 

pipe-A hollow cylinder of a plastic material 
in which the wall thicknesses are usually 
small when compared to the diameter, and in 
which the inside and outside walls are essen- 
tially concentric. See tubing. 

pipe train-Term used in extrusion of pipe to 
denote the entire equipment assembly used to 

fabricate the pipe (e.g., extruder, die, cool- 
ing bath, haul-off, and cutter). 

pitch-The distance from any point on the 
flight of a screw line to the corresponding 
point on an adjacent flight, measured parallel 
to the axis of the screw line or threading. 

plastic-( 1) One of many high-polymeric sub- 
stances, including both natural and synthetic 
products, but excluding the rubbers. At some 
stage in its manufacture, every plastic is ca- 
pable of flowing, under heat and pressure if 
necessary, into the desired final shape. ( 2 )  
Made of plastic; capable of flow under pres- 
sure or tensile stress. 

plastic, rigid-A plastic with a stiffness or ap- 
parent modulus of elasticity greater than 
100,000 psi at 23°C. (ASTM D 747.) 

plastic, semirigid-A plastic with a stiffness 
or apparent modulus of elasticity between 
10,000 and 100,000 psi at 23°C.  (ASTM D 
747.) 

plastic deformation-A change in dimensions 
of an object under load that is not recovered 
when the load is removed; opposed to elastic 
deformation. 

plasticate-To soften by heating or kneading. 
Synonyms are plastify, flux, and (impre- 
cisely) plasticize. 

plasticity-A property of a plastic that allows 
the material to be deformed continuously and 
permanently without rupture upon the appli- 
cation of a force that exceeds the yield value 
of the material. 

plasticize-To soften a material and make it 
plastic or moldable, by means of either a 
plasticizer or the application of heat. 

plasticizer-A material incorporated in a plas- 
tic to increase its workability and its flexibil- 
ity or distensibility; normally used in ther- 
moplastics. The addition of the plasticizer 
may lower the melt viscosity, the tempera- 
ture of the glassy transition, or the elastic 
modulus of the plastic. 

plastics tooling-Tools (e.g., dies, jugs, fix- 
tures, etc.) for the metal forming trades con- 
structed of plastics, generally laminates or 
casting materials. 

plastify-See plasticate. 
plastigel-A plastisol exhibiting gel-like flow 

properties; one having an effective yield 
value. 



GLOSSARY 21 

plastisols-Mixtures of vinyl resins and plas- 
ticizers that can be molded, cast, or con- 
verted to continuous films by the application 
of heat. If the mixtures contain volatile thin- 
ners also, they are known as organosols. 

plate dispersion plug-See breaker plate. 
platens-The mounting plates of an injection 

or compression molding press, to which the 
entire mold assembly is bolted. 

plate-out-The undesirable deposition of ad- 
ditives onto machinery during processing. 

plug forming-A thermoforming process in 
which a plug or male mold is used to par- 
tially preform the part before forming is 
completed using vacuum or pressure. 

plunger-That part of a transfer or injection 
press that applies pressure on the unmelted 
plastic material to push it into the chamber, 
which in turn forces plastic melt at the front 
of the chamber out the nozzle. See ram. 

plunger machines-Injection molding ma- 
chine whose plasticating system consists of a 
piston-in-cylinder arrangement. The plunger, 
on each stroke, pushes unmelted plastic into 
the heating cylinder, forcing melt out through 
a nozzle on the opposite end. 

polishing roll(s)-A roll or series of rolls, with 
a highly polished chrome-plated surface, that 
are utilized to produce a smooth surface on 
sheet as it is extruded. 

polymer-A high-molecular-weight organic 
compound, natural or synthetic, whose struc- 
ture can be represented by a repeated small 
unit, the monomer (e.g., polyethylene, rub- 
ber, cellulose). Synthetic polymers are 
formed by addition or condensation poly- 
merization of monomers. If two or more dif- 
ferent monomers are involved, a copolymer 
is obtained. Some polymers are elastomers, 
some plastics. 

polymerization-A chemical reaction in which 
the molecules of a monomer are linked to- 
gether to form large molecules whose molec- 
ular weight is a multiple of that of the origi- 
nal substance. When two or more different 
monomers are involved, the process is called 
copolymerization or heteropolymerization. 

porosity-The existence in a plastic material of 
very small voids. 

porous molds-Molds that are made up of 
bonded or fused aggregate (powdered metal, 

coarse pellets, etc.) in such a manner that the 
resulting mass contains numerous open in- 
terstices of regular or irregular size, by means 
of which either air or liquids may pass 
through the mass of the mold. 

postcure-Operation whereby thermoset parts 
are subjected to elevated temperatures for a 
period of time after being removed from the 
mold to attain maximum property levels. 

postforming-The forming, bending, or shap- 
ing of fully cured, C-stage thermoset lami- 
nates that have been heated to make them 
flexible. On cooling, the formed laminate re- 
tains the contours and shape of the mold over 
which it has been formed. 

pot-Chamber to hold and heat molding ma- 
terial for a transfer mold. 

pot life-See working life. 
pot plunger-A plunger used to force softened 

molding material into the closed cavity of a 
transfer mold. 

potting-A procedure similar to encapsulating 
except that here steps are taken to ensure 
complete penetration of all the voids in the 
object before the resin polymerizes. 

powder molding-General term used to de- 
note several techniques for producing objects 
of varying sizes and shapes by melting plas- 
tic powder, usually against the inside of a 
mold. The techniques vary according to 
whether the molds are stationary (e.g., as in 
variations on slush molding techniques) or 
rotating (e.g., as in variations on rotational 
molding). 

preform-( 1 )  A compressed tablet or biscuit of 
plastic composition used for efficiency in 
handling and accuracy in weighing mate- 
rials. (2) To make plastic molding powder 
into pellets or tablets. 

preheating-The heating of a compound prior 
to molding or casting in order to facilitate the 
operation, reduce the cycle, and improve the 
product. 

preplastication-Technique of premelting in- 
jection molding powders in a separate cham- 
ber, then transferring the melt to the injection 
cylinder. The device used for preplastication 
commonly is known as a preplasticizer. 

prepolymer-A chemical structure intermedi- 
ate between that of the initial monomer or 
monomers and the final polymer or resin. 



22 SPI PLASTICS ENGINEERING HANDBOOK 

prepolymer molding-In the urethane foam 
field, term used for a system whereby a por- 
tion of the poly01 is pre-reacted with the iso- 
cyanate to form a liquid prepolymer in a vis- 
cosity range suitable for pumping or meter- 
ing. This component is supplied to end-users 
with a second premixed blend of additional 
polyol, catalyst, blowing agent, etc. When 
the two components are mixed together, 
foaming occurs. See one-shot molding. 

prepreg-A term generally used in reinforced 
plastics to mean the reinforcing material con- 
taining or combined with the full comple- 
ment of resin before molding. 

preprinting-In sheet thermoforming, the dis- 
torted printing of sheets before they are 
formed. During forming the print assumes its 
proper proportions. 

pressure forming-A thermoforming process 
wherein pressure is used to push the sheet to 
be formed against the mold surface, as op- 
posed to using a vacuum to suck the sheet flat 
against the mold. 

pressure pads-Reinforcements distributed 
around the dead areas in the faces of a mold 
to help the land absorb the final pressure of 
closing without collapsing. 

primer-A coating applied to a surface, prior 
to the application of an adhesive or lacquer, 
enamel, or the like, to improve adhesion or 
finishing. 

promoter-A chemical, itself a feeble cata- 
lyst, that greatly increases the activity of a 
given catalyst. 

prototype mold-A simplified mold construc- 
tion often made from a light metal casting 
alloy, an epoxy resin, or an RTV silicone 
rubber, in order to obtain information for the 
final mold andlor part design. 

pultrusion-Automated method for producing 
continuous reinforced plastic shapes by pull- 
ing preimpregnated reinforcing fibers through 
a heated die where the resin is cured. 

purging-Cleaning one color or type of mate- 
rial from the cylinder of an injection molding 
machine or extruder by forcing it out with a 
new color or material to be used in subse- 
quent production. Purging materials also are 
available. 

quench (thermoplastics)-A process of shock 
cooling thermoplastic materials from the 
molten state. 

quench bath-The cooling medium used to 
quench molten thermoplastic materials to the 
solid state. 

quench-tank extrusion-Process in which the 
extruded film is cooled in a quench-water 
bath. 

radio frequency (RF) preheating-A method 
of preheating molding materials to facilitate 
the molding operation and/or reduce the 
molding cycle. The frequencies most com- 
monly used are between 10 and 100 mc/sec. 

radio frequency welding-A method of weld- 
ing thermoplastics using a radio frequency 
field to apply the necessary heat. Also known 
as high frequency welding. 

ram-Rod or plunger that forces melted plastic 
through the barrel and into a mold. 

ram travel-Distance a ram moves when op- 
erating a complete molding cycle. 

reaction injection molding (RIM)-Process 
that involves the high-pressure impingement 
mixing of two (or more) reactive liquid com- 
ponents; after mixing, the liquid stream is in- 
jected into a closed mold at low pressure. The 
finished parts can be cellular or solid elasto- 
mers, with a wide range of hardness and 
modulus values. It is used especially with ur- 
ethanes. Variations include reinforced reac- 
tion injection molding or RRIM (where re- 
inforcements are injected along with the 
reacting chemicals and structural reaction in- 
jection molding) and SRIM (where a rein- 
forcing mat is placed in the mold before in- 
jection). 

reaming-A method used to trim and size plas- 
tic bottle finishes. A special rotating cutting 
tool trims the sealing surface smooth and 
simultaneously reams (bores) the bottle 
opening to the desired size. 

reciprocating screw injection molding-A 
combination injection and plasticizing unit in 
which an extrusion device with a reciprocat- 
ing screw is used to plasticize the material. 
Injection of material into a mold can take 
place by direct extrusion into the mold or by 
reciprocating the screw as an injection 



GLOSSARY 23 

plunger, or by a combination of the two 
methods, when the screw serves as an injec- 
tion plunger, this unit acts as a holding, mea- 
suring, and injection chamber. See injection 
molding. 

recycle-Material from flash, trimmings, 
scrap, rejects, etc., that can be ground up or 
repelletized and fed back into the processing 
machine. 

reinforced molding compound-A material 
reinforced with special fillers or fibers to meet 
specific requirements (glass, synthetic fibers, 
minerals, etc.). 

reinforced plastics-Plastics with some 
strength properties greatly superior to those 
of the base resin, resulting from the presence 
of high-strength fillers embedded in the com- 
position. The reinforcing fillers usually are 
fibers, fabrics, or mats made of fibers. 

reinforcement-A strong inert material bound 
into a plastic to improve its strength, stiff- 
ness, and impact resistance. Reinforcements 
usually are long fibers of glass, sisal, cotton, 
etc., in woven or nonwoven form. 

release agent-See mold release. 
resin-Any of a class of solid or semisolid or- 

ganic products of natural or synthetic origin, 
generally of high molecular weight with no 
definite melting point. Most resins are poly- 
mers. 

resin, liquid-An organic, polymeric liquid 
that, when converted to its final state for use, 
becomes a solid. 

resin pocket-An apparent accumulation of 
excess resin in a small localized section vis- 
ible on cut edges of molded surfaces. Also 
called resin segregation. 

resin transfer molding-A process whereby 
catalyzed resin is transferred into an enclosed 
mold into which a reinforcing mat has been 
placed. 

resistivity-The ability of a material to resist 
the passage of an electrical current, either 
through its bulk or on a surface. The unit of 
volume resistivity is the ohm-cm; that of sur- 
face resistivity is the ohm. 

restricted gate-A very small orifice between 
the runner and the cavity in an injection or 
transfer mold. When the piece is ejected, this 

gate breaks cleanly, simplifying separation of 
the runner from the piece. 

restrictor bar-An extension into the flow 
channel of an extrusion sheet die at its widest 
point to equalize pressure and produce a bal- 
anced flow. 

retainer plate-The plate on which demount- 
able pieces, such as mold cavities, ejector 
pins, guide pins, and bushings, are mounted 
during molding; usually drilled for steam or 
water. 

retarder-See inhibitor. 
reverse-roll coating-Coating that is preme- 

tered between rolls and then wiped off on the 
web. The amount of coating is controlled by 
the metering gap and also by the speed of 
rotation of the coating roll. 

rheology-The study of material flow under 
varying conditions of heat and pressure. 

rib-A reinforcing member of a fabricated or 
molded part. 

ring gate-An annular opening for introducing 
melt into a mold, used to make cylindrical 
parts. 

Rockwell hardness-A common method of 
testing material for resistance to indentation, 
in which a diamond or steel ball, under pres- 
sure, is used to pierce the test specimen. 
Ref. : Standard Method of Test for Rockwell 
Hardness of Plastics and Electrical Insulating 
Materials (ASTM Designation: D 785). 

roller coating-Method used to apply paints to 
raised designs or letters. 

roll mill-Two rolls placed in close relation- 
ship to one another, used to admix a plastic 
material with other substances. The rolls turn 
at different speeds to provide a shearing ac- 
tion to the materials being compounded. 

rotating spreader-A type of injection tor- 
pedo consisting of a finned torpedo that is ro- 
tated by a shaft extending through a tubular- 
cross-section injection ram behind it. 

rotational casting (or molding)-A method 
used to make hollow articles from thermo- 
plastic materials. Material is charged into a 
hollow mold capable of being rotated in one 
or two planes. The hot mold fuses the ma- 
terial into a gel after the rotation has caused 
it to cover all surfaces. The mold then is 
chilled and the product stripped out. 



24 SPI PLASTICS ENGINEERING HANDBOOK 

roving-A form of fibrous glass in which spun 
strands are woven into a tubular rope. The 
number of strands is variable, but 60 is usual. 
Chopped roving is commonly used in pre- 
forming. 

rubber-Any elastomer capable of rapid elas- 
tic recovery after being stretched to at least 
twice its length at temperatures from 0°F to 
150°F at any humidity. Specifically, Hevea 
or natural rubber is the standard of compari- 
son for elastomers. See also thermoplastic 
elastomers. 

runner (refers to mold)-In an injection or 
transfer mold, the channel that connects the 
sprue with the gate to the cavity. 

runner system (refers to plastic)-The term 
usually applied to all the material in the form 
of sprues, runners, and gates that lead ma- 
terial from the nozzle of an injection machine 
or the pot of a transfer mold to the mold cav- 
ity. 

rupture strength, psi-A quantity whose true 
value is the stress of a material at failure, 
based on the ruptured cross-sectional area it- 
self. 

S-glass-A magnesia-alumina-silica glass used 

SRIM-See reaction injection molding. 
sag-The extension locally (often near the die 

face) of the parison during extrusion by grav- 
itational forces. This causes necking-down of 
the parison. The term also refers to the flow 
of a molten sheet in a thermoforming opera- 
tion. 

sag streaks-Uneven plastic surface due to 
heavy coating application and poor flow-out. 
It can be eliminated by thinning the coating, 
adjusting the spray gun, or changing the 
stroke. 

sandwich constructions-Panels composed of 
a lightweight core material (honeycomb, 
foamed plastic, etc.) to which two relatively 
thin, dense, high-strength faces or skins are 
adhered. 

sandwich heating-A method of heating a 
thermoplastic sheet prior to forming in which 
both sides of the sheet are heated simulta- 
neously. 

scrap-Any product of a molding operation 

in high-strength reinforcements. 

that is not part of the primary product. In 
compression molding, this includes flash, 
culls, and runners, and the material is not 
reusable as a molding compound. Injection 
molding and extrusion scrap (runners, re- 
jected parts, sprues, etc.) usually can be re- 
ground and remolded. 

screen-A woven metal screen or equivalent 
device that is installed across the flow of 
stock between the tip of the screw and the die 
and supported by a breaker plate to strain out 
contaminants or to increase the back pres- 
sure, or both. 

screen changer-A device for replacing filter- 
ing screens without interrupting the extrusion 
process. 

screw plasticating injection molding-A 
technique in which the plastic is converted 
from pellets to a viscous melt by means of 
an extruder screw that is an integral part of 
the molding machine. Machines are either 
single stage (in which plastication and injec- 
tion are done in the same cylinder) or double 
stage (in which the material is plasticated in 
one cylinder and then fed to a second for in- 
jection into a mold). 

segregation-A separation of components in a 
molded article, usually denoted by wavy lines 
and color striations in thermoplastics. In 
thermosets, it usually means segregation of 
the resin and the filler on the surface. 

self-extinguishing-A term indicating that a 
material will stop burning after the source of 
flame is removed. 

self-reinforcing-A term describing the devel- 
opment of strength in liquid crystal polymers 
because of their internal structure. 

shear strength, psi-The stress at which a ma- 
terial fails under a shear loading condition. 
(ASTM test method D 732.) 

shear stress-The stress developing in a poly- 
mer melt when the layers in a cross section 
are gliding along each other or along the wall 
of the channel (in laminar flow). 

force 
area sheared 

shear stress = = psi 

sheet (thermoplastic)-A flat section of a ther- 
moplastic resin with the length considerably 
greater than the width and 10 mils or greater 
in thickness. 



GLOSSARY 25 

shelf life-The time that a molding compound 
can be stored without losing any of its phys- 
ical or molding properties. 

Shore hardness-A method of determining the 
hardness of a plastic material using a scel- 
roscope. This device consists of a small con- 
ical hammer fitted with a diamond point and 
acting in a glass tube. The hammer is made 
to strike the material under test, and the de- 
gree of rebound is noted on a graduated scale. 
Generally, the harder the material is, the 
greater the rebound will be. A single inden- 
tor, without hammer, can be used to obtain 
Shore A or Shore D durometer measure- 
ments. Ref.: Tentative Method of Test for 
Indentation Hardness of Rubber and Plastics 
by Means of a Durometer (ASTM Designa- 
tion: D 2240). 

short or short shot-A molded part produced 
when the mold has not been filled com- 
pletely. 

shot-The yield from one complete molding 
cycle, including cull, runner, and flash. 

shot capacity-The maximum weight of ma- 
terial that a machine can produce from one 
forward motion of the plunger or screw. 

shrink fixture-See cooling fixture. 
shrink wrapping-A technique of packaging 

in which the strains in a plastic film are re- 
leased by raising the temperature of the film, 
thus causing it to shrink over the package. 
These shrink characteristics are built into the 
film during its manufacture by stretching it 
under controlled temperatures to produce ori- 
entation of the molecules. Upon cooling, the 
film retains its stretched condition, but it re- 
verts toward its original dimensions when it 
is heated. 

shrinkage-See mold shrinkage. 
side coring or side draw pins-Projections 

used to core a hole in a direction other than 
the line of closing of a mold, which must be 
withdrawn before the part is ejected from the 
mold. 

silicone-Chemical derived from silica; used 
in molding as a release agent and a general 
lubricant. 

silk screen printing-Printing method that, in 
its basic form, involves laying a pattern of an 
insoluble material, in outline, on a finely 

woven fabric, so that when ink is drawn 
across it, the ink passes through the screen 
only in the desired areas. Also called screen 
process decorating. 

single cavity mold (injection)-An injection 
mold having only one cavity in the body of 
the mold, as opposed to a multiple cavity 
mold or family mold, which has numerous 
cavities. 

sink mark-A depression or dimple on the sur- 
face of an injection-molded part due to col- 
lapsing of the surface following local internal 
shrinkage after the gate seals. It also may be 
an incipient short shot. 

sintering-In forming articles from fusible 
powders (e.g., nylon), the process of holding 
the pressed-powder article at a temperature 
just below its melting point for about half an 
hour. Particles are fused (sintered) together, 
but the mass, as a whole, does not melt. 

sizing-The process of applying a material to 
a surface to fill pores and thus reduce the ab- 
sorption of the subsequently applied adhe- 
sive or coating or to otherwise modify the 
surface. Also, the surface treatment applied 
to glass fibers used in reinforced plastics, for 
improving the bond between glass and plas- 
tic. The material used sometimes is called 
size. 

slip additive-A modifier that acts as an inter- 
nal lubricant which exudes to the surface of 
the plastic during and immediately after pro- 
cessing. In other words, a nonvisible coating 
blooms to the surface to provide the neces- 
sary lubricity to reduce the coefficient of fric- 
tion and thereby improve slip characteristics. 

slip forming-Sheet-forming technique in 
which some of the plastic sheet material is 
allowed to slip through the mechanically op- 
erated clamping rings during a stretch-form- 
ing operation. 

slot extrusion-A method of extruding film 
sheet in which the molten thermoplastic 
compound is forced through a straight slot. 

slurry preforming-Method of preparing rein- 
forced plastics preforms by wet processing 
techniques similar to those used in the pulp 
molding industry. 

slush molding-Method for casting thermo- 
plastics, in which the resin in liquid form is 



26 SPI PLASTICS ENGINEERING HANDBOOK 

poured into a hot mold where a viscous skin 
forms. The excess slush is drained off, the 
mold is cooled, and the molding is stripped 
out. 

softening range-The range of temperature in 
which a plastic changes from a rigid to a soft 
state. Actual values will depend on the 
method of test. Sometimes erroneously re- 
ferred to as softening point. 

solid phase forming-Using metalworking 
techniques to form thermoplastics in a solid 
phase. Procedure begins with a plastic blank 
that