Glossary

Absolute Viscosity- The tangential force on unit area of either of two parallel planes at unit distance apart when the space between the planes is filled with the fluid in question and one of the planes moves with unit differential velocity in its own plane. The C.G.S. unit for absolute (or dynamic) viscosity is poise (dyne-sec./sq. cm.). Centipoise (0.01) is often used.

Accelerator - A substance that hastens a reaction, particularly one which speeds up the vulcanization of rubber. Also known as Promoter.

Accumulator- A term used mainly with reference to blow molding equipment which designates an auxiliary ream extruder which is used to provide extremely fast parison delivery. The accumulator cylinder is filled with plasticated melt coming from the extruder between parison deliveries or "shots" and is stored or "accumulated" until the plunger is required to deliver the next parison.

Acrylic Resin- A synthetic resin prepared from acrylic acid or from a derivative of acrylic acid.

Acrylonitrile- A monomer with the structure (CH2:CHCN). It is most useful in copolymers. Its copolymer with butadiene is nitrile rubber, and several copolymers with styrene exist that are tougher than polystyrene. It is also used as a synthetic fiber and as a chemical intermediate.

Acrylontrile Butadiene Styrene (abbreviated ABS)- Acrylonytrile and styrene liquids and butadiene gas are polymerized together in a variety of ratios to produce the family of ABS resins.

Adhesion Promoter- A coating which is applied to the substrate before it is extrusion coated with the plastic and which improves the adhesion of the plastic to the substrate.

Adhesive- A substance which applied as an intermediate is capable of holding materials together by surface attachment.

Backing Plate- In injection molding, a plate used as a support for the cavity blocks, guide pins, bushings, etc.

Baffle- A device used to restrict or divert the passage of fluid through a pipe line or channel. 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 location for the disc is in a joint in the line. When applied to molds, the term is indicative of a plug or similar device located in a steam or water channel in the mold and designed to divert and restrict the flow to a desired path.

Bag molding- A method of applying pressure during bonding or molding, in which a flexible cover, usually in connection with a rigid die or mold, exerts pressure on the material being molded, through the application of air pressure or drawing of a vacuum.

Bakelite- The proprietary name for phenolic and other plastics materials produced by Bakelite Limited, but often used indiscriminately to describe any phenolic molding material or molding. The name is derived from that of Dr. Leo Hendrik Baekeland (1863-1944), a Belgian who, through his work on synthesis of phenolic resins and their commercial development in the early 1900's, is generally considered to be the "father" of the plastic industry.

Benzene Ring- The basic structure of benzene, the most important aromatic chemical. It is an unsaturated, resonant 6-carbon ring having three double bonds. One or more of the 6 hydrogen atoms of benzene may be replaced by other atoms or groups.

Binder- In a reinforced plastic, the continuous phase which holds together the reinforcement.

Bleed- To give up color when in contact with water or a solvent; undesired movement of certain materials in a plastic (e.g. plasticizers in vinyl) to the surface of the finished article or into an adjacent material. Also called Migration.

Blister- A raised area on the surface of a molding caused by the pressure of gases inside it on its incompletely hardened surface.

Butadiene- A gas, insoluble in water but soluble in alcohol and ether, obtained from the cracking of petroleum, from coal tar benzene or from acetylene produced from coke and lime. It is widely used in the formation of copolymers with styrene, acrylonitrile, vinyl chloride and other monomeric substances, where it imparts flexibility to the subsequent moldings.

Butadiene Styrene Plastics- A synthetic resin derived from the copolymerization of butadiene gas and styrene liquids.

Buttress Thread- A type of threading in which the thread sides terminate abruptly in threading gradually tapering down to the neck finish. Designed to withstand maximum force in one direction only. Cross section of thread is triangular.

Butylene Plastics- Plastics based on resins made by the polymerization of butene or copolymerization by butene with one or more unsaturated compounds, the butene being in greatest amount by weight.

Calender- (v.) To prepare sheets of material by pressure between two or more counter-rotating rolls. (n.) - 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 properties. It is also used in polyethylene compounds intended for such applications as cold water piping and black agricultural sheet.

Cast- To form a "plastic" object by pouring a fluid monomer-polymer solution into an open mold where it finishes polymerizing. (2) Forming plastic film and sheet by pouring the liquid resin onto a moving belt or by precipitation in a chemical bath.

Catalyst- A substance which markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts of primary reactants.

Cavity- Depression in a mold made by casting, machining, hobbing, or a combination of these methods; depending on number of such depressions, molds are designated as Single-Cavity or Multi-Cavity.

Cellular Plastics- Plastics containing numerous small cavities (cells), interconnecting or not distributed throughout the mass.

Celluloid- A thermoplastics material made by the intimate blending of cellulose nitrate with camphor. Alcohol is normally employed as a volatile solvent to assist plasticization, and is subsequently removed.

Cellulose- A natural high polymeric carbohydrate found in most plants; the main constituent of dried woods, jute, flax, hemp, ramie, etc. Cotton is almost pure cellulose.

Chlorinated Polyvinyl Chloride Plastic- Plastics based on chlorinated polyvinyl chloride in which the chlorinated polyvinyl chloride is in the greatest amount of weight.

Chlorofluorocarbon Plastics- Plastics based on polymers made with monomers composed of chlorine, fluorine, and carbon only.

Cold molding- A procedure in which a composition is shaped at room temperature and cured by subsequent baking.

Cold Slug- The first material to enter an injection mold; so called because in passing through a sprue orifice it is cooled below the effective molding temperature.

Cold Slug Well- Space provided directly opposite the sprue opening in an injection mold to trap the cold slug.

Cold Stretch- Pulling operation, usually on extruded filaments, to improve tensile properties.

Compound- An intimate mixture of (a) polymers(s) with all the materials necessary for the finished product.

Compression mold- A mold which is open when the material is introduced and which shapes the material by heat and by the pressure of closing.

Compression molding- A technique of thermoset molding in which the molding compound (generally preheated) is placed in the open mold cavity, mold is closed, and heat and pressure (in the form of a downward moving ram) are applied until the material has cured.

Dash-pot- A device used in hydraulic systems for damping down vibration. It consists of a piston attached to the part to be damped and fitted into a vessel containing fluid or air. It absorbs shocks by reducing the rate of change in the momentum of moving parts of machinery.

Daylight Opening- Clearance between two platens of a press in the open position.

Deckle Rod- A small rod, or similar device, inserted at each end of the extrusion coating die which is used to adjust the length of the die opening.

Decomposition Product- The constituent elements or simpler compounds formed when a substance decays or decomposes.

Decorative Sheet- A laminated plastics sheet used for decorative purposes in which the color and/or surface pattern is an integral part of the sheet.

Deflashing- Covers the range of finishing techniques used to remove the flash (excess, unwanted material) on a plastic molding.

Ejector Pin (on Sleeve)- A pin or thin plate that is driven into a mold cavity from the rear as the mold opens, forcing out the finished pieces. Also Knockout Pin.

Ejector Return Pins- Projections that push the ejector assembly back as the mold closes; also called Surface Pins and Return Pins.

Ejector Rod- Bar that actuates the ejector assembly when mold is opened.

Elastic Deformation- The part of the deformation of an object under load which is recoverable when the load is removed.

Elastomer- A material which at room temperature stretches under low stress to at least twice its length and snaps back to the original length upon release of stress. See also Rubber.

Electroformed molds- A mold made by electroplating metal in the reverse pattern on the cavity. Molten steel may be then sprayed on the back of the mold to increase its strength.

Electronic Treating- A method of oxidizing a film of polyethylene to render it printable by passing the film between the electrodes and subjecting it to a high voltage corona discharge.

Electroplating- The deposition of a layer of metal on a base of metal or conducting surface by electrolysis.

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

Embossing- Techniques used to create depressions of a specific pattern in plastics film and sheeting.

Emulsion- A suspension of fine droplets of one liquid in another.

Ethylene Plastics- Plastics based on polymers of ethylene or copolymers of ethylene with other monomers, the ethylene being in greatest amount by mass.

Ethylene-vinyl Acetate- Copolymers from these two monomers form a new class of plastic materials. They retain many of the properties of polyethylene, but have considerably increased flexibility for their density - elongation and impact resistance are also increased.

Exotherm- The temperature/time curve of a chemical reaction giving off heat, particularly the polymerization of casting resins. (2) The amount of heat given off. The term has not been standardised with respect to sample size, ambient temperature, degree of mixing, etc.

Exothermic Reaction- A chemical reaction in which heat is evolved.

Extender- A substance generally having some adhesive action, added to a plastic composition to reduce the amount of the primary resin required per unit area.

Extrudate- The product or material delivered by an extruder, such as film, pipe, the coating on wire, etc.

Extrusion- The compacting of a plastic material and the forcing of it through an orifice in more or less continuous fashion.

Extrusion Coating- The 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.

Family mold (injection)- A multi-cavity mold where each of the cavities forms one of the component parts of the assembled finished object.

Fiber- This term usually refers to relatively short lengths of very small cross-sections of various materials. Fibers can be made by chopping filaments (converting). Staple fibers may be 1/2 to a few in length and usually 1 to 5 denier (1/2 to 1 in diameter in Marlex polyethylene).

Fiber Show- Strands or bundles of fibers that are not covered by resin and that are at or above the surface of a reinforced plastic.

Filament- A variety of fiber characterized by extreme length, which permits its use in yarn with little or no twist and usually without the spinning operation required for fibers.

Filament Winding- Roving or single strands of glass, metal, or other reinforcement are wound in a predetermined pattern onto a suitable mandrel. The pattern is so designed as to give maximum strength in the directions required. The strands can either 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 temperature or in an oven.

Fill Point- The level to which a container must be filled to furnish a designated quantity of the content.

Fill-and-wipe- Parts are molded with depressed designs; after application of paint, surplus is wiped off, leaving paint remaining only in the depressed areas.

Filler- A cheap, inert substance added to a plastic to make it less costly. Fillers may also improve physical properties, particularly hardness, stiffness, and impact strength. The particles are usually small, in contrast to those of reinforcements but there is some overlap between the function of the two.

Fillet- A rounded filling of the internal angle between two surfaces of a plastic molding.

Film- An optional term for sheeting having a nominal thickness not greater than 0.010 inch.

Fin- The web of material remaining in holes or openings in a molded part which must be removed in finishing.

Gate- In injection and transfer molding, the orifice through which the melt enters the cavity. Sometimes the gate has the same cross-section as the runner leading to it; often, it is severely restricted.

Gaylord- A large corrugated container usually sized to match the length and width dimensions of a pallet. Gaylord is actually a trade name that has become synonymous with this specific type of container.

Glass Thread- A type of threading in which the thread sides gradually taper down to the neck finish. Cross section of threads are semi-circular.

Glass Transition- The reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. NOTE - The glass transition generally occurs over a relatively narrow temperature region and is similar to the solidification of a liquid to a glassy state: it is not a phase transition. Not only do hardness and brittleness undergo rapid changes in this temperature region but other properties, such as thermal expansion and specific heat also change rapidly. This phenomenon has been called second order transition, rubber transition and rubbery transition. The word transformation has also been used instead of transition. Where more than one amorphous transition occurs in a polymer, the one associated with segmental motions of the polymer backbone chain or accompanied by the largest change in properties is usually considered to be the glass transition.

Hardener- A substance or mixture of substances added to plastic composition, or an adhesive to promote 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.

Head- The end section of a blow molding machine (in a general extruder) in which the melt is transformed into a hollow parison.

Head Space- The space between the fill level of a container and the sealing plane.

Heat Chamber- In injection molding, that part of the machine in which the cold feed is reduced to a hot melt. Also Heating Cylinder.

Heat Deflection Temperature- The temperature at which a standard test bar (ASTM D648) deflects 0.010 in. under a stated load of either 66 or 264 psi.

Heat Sealing- A method of joining plastic films by simultaneous application of heat and pressure to areas in contact. Heat may be supplied conductively or dielectrically.

Heat-Distortion Point- The temperature at which a standard test bar (ASTM D648) deflects 0.010 in. under a stated load of either 66 or 264 psi.

Hob- A master model in hardened steel used to sink the shape of a mold into a steel block.

Hobbing- Forming multiple mold cavities by forcing a hob into soft steel (for beryllium-copper) cavity blanks.

Homopolymer- A polymer, consisting of (neglecting the ends, branch junctions, and other minor irregularities) a single type of repeating unit.

Honeycomb- Manufactured product consisting of sheet metal or a resin impregnated sheet material (paper, fibrous, glass, etc.) which has been formed into hexagonal-shaped cells. Used as core material for sandwich constructions.

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 is usually accomplished by using an RF heated metal bar which is cored for water cooling or is of such a mass that it will cool rapidly at ambient temperatures.

Infra-red- Part of the electromagnetic spectrum between the visible light range and the radar range. Radiant heat is in this range, and infra-red heaters are used in sheet thermoforming.

Inhibitor- A substance that slows down chemical reaction. Inhibitors are sometimes used in certain types of monomers and resins to prolong storage life.

Injection Blow molding- A blow molding process in which the parison to be blown is formed by injection molding.

Injection mold- A mold into which a plasticated 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 relatively cool cavity which gives the article the desired shape.

Injection molding Cycle- The complete time cycle of operation utilized in injection molding of an object including injection; die close and die open time.

Injection Pressure- The pressure on the face of the injection ram at which molding material is injected into a mold. It is usually expressed in psi.

Injection Ram- The ram which applies pressure to the plunger in the process of injection molding or transfer molding.

Jacket- A tough sheath to protect an insulated wire or cable, or to permanently group two or more insulated wires or cables.

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

Jetting- Turbulent flow of resin from an undersize 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.

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

Kiss-roll Coating- This roll arrangement carries a metered film of coating to the web; at the line of web contact, it is split with part remaining on the roll, the remainder of the coating adhering to the web.

Knit Lines- See Weld Marks.

Knockout Pin- A device for knocking a cured piece from a mold. Also called Ejector Pin.

Laminar Flow- Laminar flow of thermoplastic resins in a mold is accompanied by solidification of the layer in contact with the mold surface that 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.

Laminate- A product made by bonding together two or more layers of material or materials.

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

Lattice Pattern- In reinforced plastics, a pattern of filament winding with a fixed arrangement of open voids.

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

Light-resistance- The ability of a plastics material to resist fading after exposure to sunlight or ultra-violet light. Nearly all plastics tend to darken under these conditions.

Limiting Oxygen Index- The concentration of oxygen required to maintain burning. See ASTM Procedure D2863-74.

Linear Molecule- A long chain molecule as contrasted to one having many side chains or branches.

Loss Factor- The product of the power factor and the dielectric constant.

Low Pressure Laminates- In general, laminates molded and cured in the range of pressures from 400 psi. down to and including pressures obtained by the mere contact of the plies.

Mat- A randomly distributed felt of glass fibers used in reinforced plastics lay-up molding.

Melamine Plastics- Thermosetting plastics made from melamine and formaldehyde resins.

Melt Flow- The flow rate obtained from extrusion of a molten resin through a die of specified length and diameter under prescribed conditions of time, temperature and load as set forth in ASTM D1238.

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 finished article like a regular helix or irregularly-spaced ripples.

Melt Index- The amount, in grams, of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subjected to 2160 grams. forced in 10 minutes at 190 degrees C.

Melt Strength- The strength of the plastic while in the molten state.

Melt Temperature- The temperature of the molten plastic just prior to entering the mold or extruded through the die.

Melting Point- The temperature at which solid and liquid forms of a substance are in equilibrium. In common usage the melting point is taken as the temperature at which the liquid first forms in a small sample as its temperature is increased gradually.

Meniscus- The free surface of a liquid in a container, for example, water in contact with air confined in a capillary tube.

Mer- The repeating structural unit in any high polymer.

Nest Plate- A retainer plate with a depressed area for cavity blocks used in injection molding.

Nonpolar- Having no concentrations of electrical charge on a molecular scale, thus, incapable of significant dielectric loss. Examples among resins are polystyrene and polyethylene.

Nonrigid Plastic- A non-rigid plastic is one which has a stiffness or apparent modulus of elasticity of not over 50,000 psi. at 25 degrees C when determined according to ASTM test procedure D747-43 T.

Notch Sensitivity- The extent to which the sensitivity of a material to fracture is increased by the presence of a surface in homogeneity such as a face notch, a sudden change in section, a crack, or a scratch. Low notch sensitivity is usually associated 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 cylinder of an injection machine or (b) a transfer chamber where this is a separate structure. A nozzle is designed to form under pressure a 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.

Offset- A printing technique in which ink is transferred from a bath onto the raised surface of the printing plate by rollers. Subsequently, the printing plates transfer the ink to the object to be printed.

Opaque- Descriptive of a material or substance which will not transmit light. Opposite of transparent. Materials which are neither opaque nor transparent are sometimes described as semi-opaque, but are more properly classified as translucent.

Overcoating- In extrusion coating, the practice of extruding a web beyond the edge of the substrate web.

Oxidation- The addition of oxygen to a compound or the reduction of hydrogen.

Palletizing- A process of producing pellets.

Parison- The hollow plastic tube from which a container, toy, etc. is blow molded.

Permeability- (1) The passage or diffusion of a gas, vapor, liquid, or solid through a barrier without physically or chemically affecting it. (2) The rate of such passage.

PH- An expression of the degree of acidity or alkalinity of a substance. Neutrality is pH -acid solutions being under 7 and alkaline solution over 7. pH meters are commercially available for accurate readings.

Phenolic Resin- A synthetic resin produced by the condensation of an aromatic alcohol with an aldehyde, particularly of phenol with formaldehyde. Phenolic resins from the basis of thermosetting molding materials, laminated sheet, and varnishes. They are also used as impregnating agents and as components of paint, varnishes, lacquers, and adhesives.

Pigment- Any colorant, usually an insoluble powdered substance used to produce a desired color of hue

Pneumatic- A system in which energy is transferred by compression, flow and expansion of air.

Poise- The unit of viscosity expressed as one dyne per second per square centimeter.

Polyamide- A polymer in which the structural units are linked by amide or thioamide groupings. Many polyamides are fiber-forming.

Polybutylene- A polymer prepared by the polymerization of butene as the sole monomer.

Polyester- A resin formed by the reaction between a dibasic acid and a dihydroxy alcohol, both organic. Modification with multi-functional acids and/or bases and some unsaturated reactants permit cross-linking to thermosetting resins. Polyesters modified with fatty acids are called Alkyds.

Polyethylene- A thermoplastic material composed by polymers of ethylene. It is normally a translucent, tough, waxy solid which is unaffected by water and by a large range of chemicals.

Polyimide Resins- A new group of resins recently introduced in the United States. The material is an aromatic polyimide made by reacting pyromellitic dianhydride with aromatic diamines. The polymer is characterized by the fact that it has rings of four carbon atoms tightly bound together, and the manufacturers claim that the new resin has greater resistance to heat than any other unfilled organic material yet discovered. Suggested applications include components for internal combustion engines.

Polyisobutylene- The polymerization product of isobutylene. It varies in consistency from a viscous liquid to a rubber-like solid with corresponding variation in molecular weight from 1,000 to 400,000.

Polyliner- A perforated longitudinally ribbed sleeve that fits inside the cylinder of an injection molding machine; used as a replacement for conventional injection cylinder torpedoes.

Polymer- A high-molecular-weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, the mer; e.g., polyethylene, rubber, cellulose. Synthetic polymers are formed by addition or condensation polymerization of monomers. If two or more 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 together to form large molecules whose molecular weight is a multiple of that of the original substance. When two or more monomers are involved, the process is called copolymerization or heteropolymerization. See also Degree of, Condensation, and Polymer.

Polymethyl Methacrylate- A thermoplastic material composed of polymers of methyl methacrylate. It is a transparent solid with exceptional optical properties and good resistance to water. It is obtainable in the form of sheets, granules, solutions, and emulsions. It is extensively used for aircraft domes, lighting, fixtures, decorative articles, etc.; it is also used in optical instruments and surgical appliances.

Polyolefin- A polymer prepared by the polymerization of an Olefin(s) as the sole Monomer(s).

Rayon- The generic term for fibers, staple, and continuous filament yarns composed of regenerated cellulose but also frequently used to describe fibers obtained from cellulose acetate or cellulose triacetate. Rayon fibers are similar in chemical structure to natural cellulose fibers (e.g., cotton) except that the synthetic fiber contains shorter polymer units. Most rayon is made by the viscose process.

Reciprocating Screw- An extruder system in which the screw when rotating is pushed backwards by the molten polymer which collects in front of the screw. When sufficient material has been collected, the screw moves forward and forces the material through the head and die at a high speed.

Recycled Plastic- A plastic prepared from used articles which have been cleaned and reground.

Reformulated Plastic- Recycled plastic that has been upgraded to alter or improve performance capability or to change characteristics through use of plasticizers, fillers, stabilizers, pigments, etc.

Regrind- Plastic that is re-introduced into the production stream.

Reinforced molding Compound- Compound supplied by raw material produced in the form of ready-to-use materials; as distinguished from premix.

Reinforced Plastics- A plastic with high strength fillers embedded in the composition, resulting in some mechanical properties superior to those of the base resin.

Reinforcement- A strong inert material bound into a plastic to improve its strength, stiffness, and impact resistance. Reinforcements are usually long fibers of glass, sisal, cotton, etc. - in woven or non-woven form. To be effective, the reinforcing material must form a strong adhesive bond with the resin.

Rigid Plastics- For purpose of general classification, a plastic that has a modulus of elasticity either in flexure or in tension greater than 100,000 psi at 23 degrees C and 50% relative humidity when tested in accordance with ASTM Methods D 747 or D 790 Test for stiffness of plastics.

Rigid PVC- Polyvinyl chloride or a polyvinyl chloride/acetate copolymer characterized by a relatively high degree of hardness; it may be formulated with or without a small percentage of plasticizer.

Rigid Resin- Having a modulus high enough to be of practical importance, e.g., 10,000 psi or greater.

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

Sealing Plane- The plane on the inside of a bottle cap along the sealing surface.

Sealing Surface- The surface of the finish of the container on which the closure forms the seal.

Shrink Wrapping- A technique of packaging in which the strains in a plastic film are released 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 orientation of the molecules. Upon cooling, the film retains its stretched condition, but reverts toward its original dimensions when it is heated. Shrink film gives good protection to the products packaged and has excellent clarity.

Shrinkage- Contraction of a container upon cooling.

SI Units- The International System of Units (Systems International) is a modernized version of the metric system established by international agreement. It provides a logical and interconnected framework for all measurements in science, industry and commerce. Officially abbreviated SI, the system is built upon a foundation of seven base units.

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

Silicone- One of the family of polymeric materials in which the recurring chemical group contains silicon and oxygen atoms as links in the main chain. At present these compounds are derived from silica (sand) and methyl chloride. The various forms obtainable are characterized by their resistance to heat. Silicones are used in the following applications: (a) Greases for lubrication. (b) Rubber-like sheeting for gaskets, etc. (c) Heat stable fluids and compounds for waterproofing, insulating, etc. (d) Thermosetting insulating varnishes and resins for both coating and laminating.

Tensile Bar (specimen)- A compression or injection molded specimen of specified dimensions which is used to determine the tensile properties of a material.

Tensile Strength- The pulling stress, in psi, required to break a given specimen. Area used in computing strength is usually the original, rather than the necked-down area.

Therimage- A trademark for a decorating process for plastic which transfers the image of a label or decoration to the object under the influence of heat and light pressure.

Thermal Conductivity- Ability of a material to conduct heat; physical constant for quantity of heat that passes through a unit cube of a substance in a unit of time when difference in temperature of two faces is 1 degree.

Thermal Degradation- Deterioration by heat.

Thermal Expansion Coefficient- The fractional change in length (sometimes volume, specified) of a material for a unit change in temperature. Values for plastics range from 0.01 to 0.2 mils/in., degrees C.

Thermal Stress Cracking (TSC)- Crazing and cracking of some thermoplastic resins which results from over-exposure to elevated temperatures.

Thermoforming- Any process of forming thermoplastic sheet which consists of heating the sheet and pulling it down onto a mold surface.

Thermoforms- The product which results from a thermoforming operation.

Thermoplastic- Capable of being repeatedly softened by heat and hardened by cooling (n.) - A material that will repeatedly soften when heated and harden when cooled. Typical of the thermoplastics family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, vinyls, nylons, and the various fluorocarbons materials.

Thermoset- A material that will undergo or has undergone a chemical reaction by the action of heat, catalysts, ultra-violet light, etc., leading to a relatively infusible state. Typical of the plastics in the thermosetting family are the aminos (melamine and urea), most polyesters, alkyds, epoxies, and phenolics.

Ultrasonic Sealing- A film sealing method in which sealing is accomplished through the application of vibratory mechanical pressure at ultrasonic frequencies (20 to 40 KC.). Electrical energy is converted to ultrasonic vibrations through the use of either a magnetostrictive or piezoelectric transducer. The vibratory pressures at the film interface in the sealing area develop localized heat losses which melt the plastic surfaces effecting the seal.

Ultraviolet- Zone of invisible radiation beyond the violet end of the spectrum of visible radiation. Since UV wavelengths are shorter than the visible, their photons have more energy, enough to initiate some chemical reactions and to degrade most plastics.

Undercut- (a.) Having a protuberance or indentation that impedes withdrawal from a two-piece, rigid mold. Flexible materials can be ejected intact even with slight undercuts. (n.) Any such protuberance or indentation; depends also on design of mold.

Urea Plastics- Plastics based on resins made by the condensation of urea and aldehydes.

UV Stabilizer- Any chemical compound which, when mixed with a thermoplastic resin, selectively absorbs UV rays.

Vacuum Forming- Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mold. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary molds.

Vacuum Metallising- Process in which surfaces are thinly coated with metal by exposing them to the vapor of metal that has been evaporated under vacuum ( one millionth of normal atmospheric pressure).

Virgin Material- A plastic material in the form of pellets, granules, powder, flock, or liquid that has not been subjected to use or processing other than that required for its initial manufacture

Vulcanization- The chemical reaction which induces extensive changes in the physical properties of a rubber and which is brought about by reacting the rubber with sulphur and/or other suitable agents. The changes in physical properties include decreased plastic flow, reduced surface tackiness, increased elasticity, much greater tensile strength, and considerably less solubility. More recently, certain thermoplastics, e.g., polyethylene, have been formulated to be vulcanisable. Cross-linking is encouraged, thereby giving resistance to deformation of flow above the melting point.

Weld Lines- A mark on a container caused by incomplete fusion of two streams of molten polymer. See Spider Lines.

Weld Mark- A mark on a molded plastic piece made by the meeting of two flow fronts during the molding operation.

Welding- Joining thermoplastic pieces by one of several heat-softening processes. In hot-gas welding, the material is heated by a jet of hot air or inert gas directed from a welding "torch" onto the area of contact of the surfaces which are being welded. Welding operations to which this method is applied normally require the use of a filler rod. In Spin-Welding the heat is generated by friction. Welding also includes heat sealing and the terms are synonymous in some foreign countries including Britain.

Wet Strength- The strength of paper when saturated with water, especially used in discussions of processes whereby the strength of paper is increased by the addition, in manufacture, of plastic resins. Also, the strength of an adhesive joint determined immediately after removal from a liquid in which it has been immersed under specified conditions of time, temperature and pressure.