Glossary of Terms

AGING – The change of a material with time under defined environmental conditions, leading to improvement or deterioration of properties.

ALLOY – Composite material made up by blending polymers or copolymers with other polymers or elastomers under selected conditions. e.g., styrene-acrylonitrile copolymer resins blended with butadiene-acrvlonitrile rubbers.

ANNEAL – (1) To heat a molded plastic article to a predetermined temperature and slowly cool it to relieve stresses. (2) To heat steel to a predetermined temperature above the critical range and slowly cool it, to relieve stresses and reduce harness. (Annealing of molded or machined parts may be done dry as in an oven or wet as in a heated tank of mineral oil.)

ANTISTATIC AGENT – Agents which, when added to the molding material or applied on the surface of the molded object, make it less conducting (thus hindering the fixation of dust).

AUTOMATIC MOLD – A mold for injection, compression or transfer molding that repeatedly goes through the entire molding cycle, including ejection, without human assistance.

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BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. See UNDERCUT.

BACKING PLATE – In mold construction, a plate used as a support for the cavity blocks, guide pins, bushings, etc.

BLEED – (1) To give up color when in contact with water or a solvent. (2) 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”. (3) An escape passage at the parting line of a mold, like a vent but deeper, which allows material to escape or bleed out.

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

BLOOM – A visible exudation or efflorescence on the surface of a plastic. Bloom can be caused by lubricant, plasticizer, etc.

BLOW MOLDING – A method of fabrication of thermoplastic materials in which a parison (hollow tube) is forced into the shape of the mold cavity by internal air pressure. BLUEING OFF — The checking of the accuracy of mold cutoff surfaces by putting a thin coating of Prussian Blue on one-half and checking the blue transfer to the other half.

BOSS – Projection on a plastic part designed to add strength, to facilitate alignment during assembly, to provide for fastenings, etc.

BREATHING – The opening and closing of a mold to allow gases to escape early in the molding cycle. Also called degassing.


BUBBLE – A spherical, internal void of air or other gas trapped within a plastic. See VOID.

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

BURNED – Showing evidence of thermal decomposition through some discoloration, distortion or localized destruction of the surface of the plastic.

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CARBON BLACK – A black pigment produced by the incomplete burning of natural gas or oil. Widely used as a filler, particularly in the rubber industry due to its useful ultraviolet protective properties. It is also much used in molding compounds intended for outside weathering applications.

CASE HARDEN – To harden surface of a piece of steel to a relatively shallow depth.

CAST – To form a “plastic” object by pouring a fluid monomer-polymer solution into an open mold where it finishes polymerizing.

CAVITY – Depression in mold, which usually forms the outer surface of the molded part; depending on number of such depressions, molds are designated as a single cavity or multi-cavity.

CENTER GATED MOLD – An injection or transfer mold wherein the cavity is filled with molding material through a sprue or gate directly into the center of the part.

CHALKING – Dry chalk-like appearance or deposit on the surface of a plastic. See HAZE and BLOOM.

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 erosion is needed.

CLAMPING PLATE – A plate fitted to a mold and used to fasten mold to a molding machine.

CLAMPING FORCE – In injection molding and in transfer molding, the pressure which is 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.


COLD SLUG – The first material to enter an injection mold. So called because in passing through the 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.


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 heated open mold cavity, mold is closed, under pressure (usually in an hydraulic press) causing the material to flow and completely fill the cavity, pressure being held until the material has cured.

COMPRESSION RATIO – In an extruder screw, the ratio of volume available in the first flight at the hopper to 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. 

CONCENTRICITY – Relationship of all inside dimensions to all outside dimensions usually as with diameter expressed in thousandths of an inch TIR (Total Indicator Reading)

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. May also be used for heating a mold by circulating steam, hot oil or other heated fluid through channels as in molding of the thermosetting and some thermoplastic materials.

COOLING FIXTURE – Block of metal or wood holding the shape of a molded piece which 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 distortion. Also called: ‘Shrink Fixture’.


CORE – (1) Male element in a die which produces a hole or recess in part. (2) Part of a complex mold that molds undercut parts. Cores are usually withdrawn to one side before the main sections of the mold opens. (Usually called Side Cores.) (3) A channel in a mold for circulation of a heat-transfer medium. Also called “Force.”

CORE PIN – Pin used to mold a hole.

CORING – (Molded part design) — The removal of excess material from the cross section of a molded part to attain a more uniform wall thickness.

CRAZING – Fine cracks which may extend in a network 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 instantaneous elastic deformation. Creep at room temperature is sometimes called “Cold Flow”.

CRYSTALLINITY – A state of molecular structure in some resins which denotes uniformity and compactness of the molecular chains forming the polymer. Normally can be attributed to the formation of solid crystals having a definite geometric form.

CURE – To change the physical properties of a material by chemical reaction, which may be condensation, polymerization, or vulcanization: usually accomplished by the action of heat and catalysts along with or in combination with or without pressure.

CURING TEMPERATURE – Temperature at which a cast, molded or extruded product, a resin-impregnated reinforcing material, or adhesive etc., is subjected to curing.

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.

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DAYLIGHT OPENING – Clearance between two platens of a press in the open position.

DIAPHRAGM GATE – Gate used in molding annular or tubular articles. Gate forms a solid web across the opening of the part.

DEFLASHING – Covers the range of finishing techniques used to remove the flash (excess, unwanted material) on a plastic molding such as filing, sanding, milling, tumbling, etc.


DEGRADATION – A deleterious change in the chemical structure of a plastic. See DETERIORATION.

DELAMINTATION – The splitting of a plastic material along the plane of its layers. Physical separation or loss of bond between laminate plies. See LAMINATE.

DENSITY – Weight per unit volume of a substance expressed in grams per cubic centimeter, pounds per cubic foot, etc.

DESSICANT – Substance which can be used for drying purposes because of its affinity for water.

DESTATICIZATION – Treating plastics materials to minimize their accumulation of static electricity, and subsequently the amount of dust picked up by the plastics because of such charges. See ANTISTATIC AGENTS.

DETERIORATION – A permanent change in the physical properties of a plastic evidenced by impairment of these properties.

DIELECTRIC – Insulating material. In radio frequency preheating, dielectric may refer specifically to the material which is being heated.

DIMENSIONAL STABILITY – Ability of a plastic part to retain the precise shape in which it was molded, fabricated or cast.

DISCOLORATION – Any change from the original color, often caused by overheating, light exposure irradiation or chemical attack.

DISHED – Showing a symmetrical distortion of a flat or curved section of a plastic object so that as normally viewed it appears concave, or more concave than intended. See—WARP.

DISPERSION – Finely divided particles of a material in suspension in another substance.

DOMED – Showing a symmetrical distortion of a flat or curved section of a plastic object, so that, as normally viewed, it appears convex, or more convex than intended. See—WARP.

DOWEL – Pin used to maintain alignment between two or more parts of a mold.

DRAFT – The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mold.

DRY COLORING – Method commonly used by fabricators for coloring plastic by tumble blending uncolored particles of the plastic material with selected dyes and pigments.

DUROMETER HARDNESS – The hardness of a material as measured by the Shore Durometer.

DWELL – A pause in the application of pressure to a mold made just before the mold is completely closed to allow the escape of gas from the molding material.

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EJECTOR PIN – Or ejector sleeve. A rod, pin or sleeve which pushes a molding off of a core or out of a cavity of a mold. It is attached to an ejector bar or plate which can be actuated by the ejector rod(s) of the press or by auxiliary hydraulic or air cylinders.

EJECTOR PIN RETAINER PLATE – Retainer into which ejector pins are assembled.

EJECTOR RETURN PINS – Projections that push the ejector assembly back as the mold closes: also called Safety Pin, and Position Pushbacks.

EJECTOR ROD & BAR – Bar that actuates the ejector assembly when mold is opened.

ELASTICITY – That property of a material by virtue of which it tends to recover its original size and shape after deformation.

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 RUBBER.

ELECTRIC DISCHARGE MACHINING (EDM) – A metal working process applicable to mold construction in which controlled sparking is used to erode away the work piece.

ELECTRONIC TREATING – A method of oxidizing a film of polyethylene to render it printable by passing the film between electrodes and subjecting it to a high voltage corona discharge.

ELECTROPLATING – Deposition of metals on certain plastics and molds for finish.

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. Such embossing in the form of surface patterns on molded part by the treatment of the mold surface by photoengraving or other 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 formation under the influence of certain chemicals or aging, or weather and stress.

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

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FABRICATE – To work a material into a finished form by machining, forming, or other operation.

FAMILY MOLD – A multi-cavity mold wherein each of the cavities forms one of the component parts of the assembled finished object. The term often applied to molds wherein parts from different customers are grouped together in one mold for economy of production. Sometimes called “Combination Mold.”

FAN GATE – A shallow gate somewhat wider than the runner from which it extends.

FIBER – This term usually refers to thin fibers of glass which are used to reinforce both thermoplastic and thermosetting materials. One-inch long fibers are occasionally used, but the more commonly used fiber lengths are 1/2″ and 1/4″ or less.

FILLER – An additive to resins for the purpose of improving physical properties (impact resistance, hardness, dimensional stability, etc.), or to reduce cost of resin.

FILLET – A rounded filling of the internal angle between two surfaces.

FINISH – To complete the secondary work on a molded part so that it is ready for use. Operations such as filing, deflashing, buffing, drilling, tapping, degating are commonly called finishing operations. See SURFACE FINISH.

FIXTURE – Means of holding a part during a machining or other operation.

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 which runs parallel to an edge of a molded part along the flash or parting line of the mold.

FLEXURAL STRENGTH – Ability of a material to flex without permanent distortion or breaking.

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 molding. Also called “Weld Line.”

FLOW MARKS – Wavy surface appearance on a molded object caused by improper flow of the material into the mold. See SPLAY MARKS.

FOIL DECORATING – Molding paper, textile, or plastic foils printed with compatible inks directly into a plastic part so that the foil is visible below the surface of the part as integral decoration.


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GATE – In injection and transfer molding, the orifice through which the melt enters the cavity.

GLOSS – The shine or luster of the surface of a material. See SURFACE FINISH.

GUIDE PINS – Devices that maintain proper alignment of core and cavity as mold closes. Also called “Leader Pins.”

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HARDNESS – The resistance of a material to compression and indentation. Among the most important methods of testing this property are Brinell hardness, Rockwell hardness and Shore hardness.

HAZE – The degree of cloudiness in a plastics material.

HEAT-DISTORTION POINT – The temperature at which a standard test bar deflects 0.010 in. under a stated load of either 66 or 264 psi.

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.

HOT/HEATED MANIFOLD MOLD – A thermoplastic injection mold wherein the portion of the mold that contains the runner system has its own heating elements that keep the molding material in a plastic state ready for injection into the cavities from which the manifold is insulated.

HOT-STAMPING – Engraving operation for marking plastics in which roll leaf is stamped with heated metal dies onto the face of the plastics.

HYGROSCOPIC – Tending to absorb moisture.

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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 manner. (3) Molded plastics are usually given a value on an izod scale. An Izod impact test is designed to determine the resistance of a plastics material to a shock loading, it involves the notching of a specimen, which is then placed in the jaws of the machine and struck with a weighted pendulum.

INJECTION MOLDING – A molding procedure whereby a heat-softened plastic material is forced from a cylinder into a cavity which gives the article the desired shape. Used with both thermoplastic and thermosetting materials.

INJECTION BLOW MOLDING – A blow molding process in which the parison to be blown is formed by injection molding.

INSERT – An integral part of a plastics molding consisting of metal or other material which may be molded into position or may be pressed into the molding after the molding is completed. Also a removable or interchangeable component of the mold.

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JETTING – Turbulent flow of resin from an undersized 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 operation.

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



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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.

LAMINATED PLASTICS (Synthetic resin-bonded laminate, laminate) – A plastics material consisting of superimposed layers of a synthetic resin-impregnated or coated filler which have been bonded together, usually by means of heat and pressure, to form a single piece.

LIGHT-RESISTANCE – The ability of a plastics material to resist fading after exposure to sunlight or ultraviolet light.

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MATTE FINISH – A type of dull non-reflective finish. See SURFACE FINISH.

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 force in 10 minutes at 190°C.

MELT STRENGTH – The strength of the plastic while in the molten state.

METERING SCREW – An extrusion screw which has a shallow constant depth and constant pitch section over, usually the last 3 to 4 flights.

MICA – Any of a group of mineral silicates crystallizing in monoclinic forms that readily separate into very thin leaves. Used as a filler for plastics molding materials.


MODULUS OF ELASTICITY – The ratio of stress to strain in a material that is elastically deformed.


MOLDING MATERIAL – Plastic material in varying stages of granulation often comprising resin, filler, pigments, plasticizers and other ingredients, ready for use in the molding operation. Also called “Molding Compound’ or ‘Powder.”

MOLDING PRESSURE – The pressure applied directly or indirectly on the compound to allow the complete transformation to a solid dense part.

MOLDING SHRINKAGE – The difference in dimensions, 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”, or “Shrinkage”, and “Contraction”.

MOLD RELEASE – A lubricant 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 “Release Agent.”

MOLD SEAM – A line formed by mold construction such as removable members in cavity, cam slides, etc. (Not to be confused with mold parting line).

MOLECULAR WEIGHT – The sum of the atomic masses of the elements forming the molecule.

MONOMER – A relatively simple compound which can react to form a polymer.

MOVABLE PLATEN – The moving platen of an injection or compression molding machine to which half of the mold is secured during operation. This platen is moved either by a hydraulic ram or a toggle mechanism.

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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 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.

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OLEFINS – A group of unsaturated hydrocarbons of the general formula CnH2n, and named after the corresponding paraffins by the addition of “ene” or “ylene to the stem. Examples are ethylene and propylene.

ORANGE PEEL – A type of granular surface finish resembling the peel of an orange.

ORIENTATION – The alignment of the crystalline structure in polymeric materials so as to produce a highly uniform structure. Can be accomplished by cold drawing or stretching during fabrication.

OUT-OF-ROUND – Non uniform radius or diameter.

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PARTING LINE – Mark on a molding or casting where halves of mold met in closing.


PERMANENT SET – The increase in length expressed in a percentage of the original length, by which an elastic material fails to return to original length after being stressed for a standard period of time.

PINPOINT GATE – A restricted orifice through which molten resin flows into a mold cavity. Also called “Restricted Gate”.

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 – Natural and synthetic material and chemicals that can be transformed into a solid, as either or both heat and pressure is applied.

PLASTIC DEFORMATION – A change in dimensions of an object under load that is not recovered when the load is removed: opposed to elastic deformation.

PLASTIC TOOLING – Tools e.g. dies, jigs, fixtures, etc., for the metal forming trades constructed of plastics, generally laminates or casting materials.

PLASTICITY – A property of plastics which allows the material to be deformed continuously and permanently without rupture upon the application of a force that exceeds the yield value of the material.

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

PLASTICIZER – A material incorporated in a plastic to increase its workability and its flexibility or distensibility normally used in thermoplastics.

PLASTISOLS – Mixtures of vinyl resins and plasticizers which can be molded: cast, or converted to continuous films by the application of heat.

PLATENS – The mounting plates of a press on which the mold halves are attached.

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 through the nozzle. See RAM.

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.

POTTING – Similar to ENCAPSULATING, except that steps are taken to insure complete penetration of all the voids in the object before the resin polymerizes.

PERFORM – A pill, tablet, or biscuit used in thermoset molding. Material measured by volume, the bulk factor of powder reduced by pressure all in the interest of efficiency and accuracy.

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

PREMIX – A molding compound prepared prior to and apart from the molding operations and containing all components required for molding: resin, reinforcement fillers, catalysts, release agents, and other compounds.

PRESSURE FORMING – A thermoforming process wherein pressure is used to push the sheet to be formed against the mold surface as opposed to using a vacuum to suck the sheet flat against the mold.

PROTOTYPE MOLD – A simplified mold construction often made from a light metal casting alloy or from an epoxy resin in order to obtain information for the final mold and/or part design.

PURGING – Cleaning one color or type of material from the cylinder of an injection molding machine or extruder by forcing it out with the new color or material to be used in subsequent production. Purging materials are also available.

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RAM – The press member that enters the cavity block and exerts pressure on the molding compound designated as the “top force” or “bottom force” by position in the assembly.

RAM TRAVEL – Distance ram moves when operating a complete molding cycle.

RECIPROCATING SCREW – A combination injection and plasticizing unit in which an extrusion device with a reciprocating 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 plunger, or by a combination of the two. When the screw serves as an injection plunger, this unit acts as a holding, measuring, and injection chamber.

REINFORCED MOLDING COMPOUND – A material reinforced with special fillers to meet specific requirements (glass, carbon, etc.)

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 nonwoven form.


RESIN – Any of a class of solid or semi-solid organic products of natural or synthetic origin, generally of high molecular weight with no definite melting point. Most resins are polymers.

RETAINER PLATE – The plate on which demountable pieces, such as mold cavities, ejector pins, guide pins, and bushings are mounted during molding: usually drilled for water lines.

RIB – A reinforcing member of a fabricated or molded part.

ROCKWELL HARDNESS – A common method of testing material for resistance to indentation in which a diamond or steel ball, under pressure, is used to pierce the test specimen.

RUBBER – An elastomer capable of rapid elastic recovery after being stretched to at least twice its length at temperatures from 0 to 150°F at any humidity.

RUNNER – In an injection or transfer mold, the channel that connects the sprue with the gate to the cavity.

RUNNER SYSTEM – The term usually applied to all the material in the form of sprues, runners and gates which lead material from the nozzle of an injection machine or the pot of a transfer mold to the mold cavity.

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SCREW PLASTICATING INJECTION MOLDING – A technique in which the plastic is converted from pellets to a viscose melt by means of an extruder screw which is an integral part of the molding machine. Machines are either single stage (in which plastication and injection 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 injection into a mold.

SEGREGATION – A separation of components in a molded article usually denoted by wavy lines and color striations in thermoplastics. In thermosets, usually meaning segregation of resin and filler on surface.

SELF-EXTINGUISHING – The term applied to a flammable material that will cease burning when the source of heat is removed. Usually expressed in time elapsed.

SEMI-AUTOMATIC MOLDING MACHINE – A molding machine in which only part of the operation is controlled by the direct action of a human. The automatic part of the operation is controlled by the machine according to a predetermined program.

SHELF LIFE – An expression to describe the time a molding compound can be stored without losing any of its original physical or molding properties.

SHORE HARDNESS – A method of determining the hardness of a plastic material using a scleroscope. This device consists of a small conical hammer fitted with a diamond point and acting in a glass tube. The hammer is made to strike the material under test and the degree of rebound is noted on a graduated scale. Generally the harder the material the greater will be the rebound.

SHORT SHOT – A molded part produced when the mold has not been filled completely.

SHOT – The yield from one complete molding cycle, including cull, runner, and flash.

SHOT CAPACITY – The maximum weight of material which a machine can produce from one forward motion of the plunger or screw.



SIDE CORING OR 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.

SILK SCREEN PRINTING – This printing method, 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, it is able to pass through the screen only in the desired areas. Also called “Screen Process Decorating”.

SILICONE – Chemical derived from silica used in molding as a release agent and general lubricant.

SINK MARK – A depression or dimple on the surface of an injection molded part due to collapsing of the surface following local internal shrinkage after the gate seals. May also be caused by a 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 1/2 hour. Particles are fused (sintered) together, but the mass as a whole does not melt.

SOLVENT – Any substance, usually a liquid, which dissolves other substances.

SPECIFIC GRAVITY – The density (mass per unit volume) of any material divided by that of water.

SPIDER GATE – Multi-gating of a part through a system of radial runners from the sprue.

SPIN WELDING – A process of fusing two objects together by forcing them together while one of the pair is spinning, until frictional heat melts the interface. Spinning is then stopped and pressure held until they are frozen together.

SPLAY MARKS – Lines found in part after molding, usually due to flow of material in mold.

SPRUE – Feed opening provided in the injection or transfer mold; also the slug formed at this hole.

SPRUE-BUSHING – A hardened steel insert in an injection mold which contains the tapered sprue hold and has a suitable seat for the nozzle of the injection cylinder.

SPRUE GATE – A passageway through which molten resin flows from the nozzle to the mold cavity.

SPRUE LOCK OR PULLER – In injection molding, a portion of the plastic composition which is held in the cold slug well by an undercut; used to pull the sprue out of the bushing as the mold is opened. The sprue lock itself is pushed out of the mold by an ejector pin. When the undercut occurs on the cavity block retainer plate, this pin is called the Sprue Ejector Pin.

STABILIZER – An ingredient used in the formulation of some plastics to assist in maintaining the physical and chemical properties of the compounded materials at their initial values throughout the processing and service life of the material.

STATIONARY PLATEN – The plate of an injection or compression molding machine to which the front plate of the mold is secured during operation. This platen does not move during normal operation.


STRIATION – A separation of colors resulting in a linear effect of color variation.

STRIPPER-PLATE – A plate that strips a molded piece from core pins or core, the stripper-plate is set into operation by the opening of the mold.

SUBMARINE GATE – A type of edge gate where the opening from the runner into the mold is located below the parting line or mold surface as opposed to conventional edge gating where the opening is machined into the surface of the mold. With submarine gates, the part is broken from the runner system on ejection from the mold.

SURFACE FINISH – Finish of molded product.

SURFACE TREATING – Any method of treating a material so as to alter the surface and render it receptive to inks, paints, lacquers, and adhesives such as chemical, flame, and electronic treating.

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TAB GATED – A small removable tab of approximately the same thickness as the mold item, usually located perpendicular to the molded part. The tab is used as a site for edge gate location, usually on items with large flat areas.

TAPPING – Cutting threads in the walls of a circular hole.

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.

THERMAL CONDUCTIVITY – Ability of a material to conduct heat; physical constant for quantity of heat that passes through unit cube of a substance in unit of time when difference in temperature of two faces is 1°.


THERMAL STRESS CRACKING (TSC) – Crazing and cracking of some thermoplastic resins which results from over-exposure to elevated temperatures.

THERMOPLASTIC – (a.) 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 thermoplastic family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, polypropylene, vinyls, nylons, and the various fluorocarbon materials.

THERMOSET – A material that will undergo or has undergone a chemical reaction by the action of heat and pressure, 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.

THREAD PLUG OR RING OR CORE – A part of a mold that shapes a thread and must be unscrewed from the finished piece.

TIE BARS – Bars which provide structural rigidity to the clamping mechanism of a press often used to guide platen movement.

TOGGLE ACTION – A mechanism which exerts pressure developed by the application of force on a knee joint. It is used as a method of closing presses and also serves to apply pressure at the same time.

TOLERANCE – A specified allowance for deviations in weighing, measuring, etc., or for deviations from the standard dimensions or weight.


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ULTRASONIC SEALING OR BONDING – A 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 interface in the sealing area develop localized heat losses which melt the plastic surfaces affecting the seal.

ULTRASONIC INSERTION – The inserting of a metal insert into a thermoplastic part by the application of vibratory mechanical pressure at ultrasonic frequencies.

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

UNIT MOLD – Mold designed for quick changing interchangeable cavity parts.

UV STABALIZER (ULTRAVIOLET) – Any chemical compound which, when admixed with a thermoplastic resin, selectively absorbs UV rays.

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VACUUM METALIZING – 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.)

VENT – In a mold, a shallow channel or minute hole cut in the cavity to allow air to escape as the material enters.

VERTICAL FLASH RING – The clearance between the core and the vertical wall of the cavity in a positive or semi-positive mold; also the ring of excess material which escapes from the cavity into this clearance space.

VOID – A void or bubble occurring in the center of a heavy thermoplastic part usually caused by excessive shrinkage.

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WARPAGE – Dimensional distortion in a plastic object after molding.

WATER ABSORPTION – The ability of a thermoplastic material to absorb water from an environment.




WELDING – Joining thermoplastic pieces by one of several heat-softening processes: Butt fusion, spin welding, ultrasonic, and hot gas are several methods.

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YIELD VALUE – (Yield Strength) The lowest stress at which a material undergoes plastic deformation. Below this stress, the material is elastic; above it, viscous.

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