Main Spi Plastics Engineering Handbook of the Society of the Plastics Industry
Spi Plastics Engineering Handbook of the Society of the Plastics IndustryReinhold Publishing Company
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.
Most frequently terms
SPI_Plastics_Engineering_Handbook/91810_01.pdf 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