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To help improve knowledge and understanding about KLINGSPOR, we have compiled a searchable glossary. KLINGSPOR's list of definitions includes both common and uncommon industry standard terms.

Plastic Injection Molding Glossary

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Abrasion Resistance
The ability of a material to withstand mechanical actions such as rubbing, scraping or erosion that tends progressively to remove material from its surface.
Acceptable Runner/Cavity Ratio
Runner system designed for high pressure drops to minimize material usage and increase frictional heating in the runner.
A substance compounded into a resin to enhance or improve certain characteristics.
Adhesive Assembly
The process of joining two or more plastic parts by means of an abrasive.
The process of or the results of exposure of plastics to natural or artificial environmental conditions for a prolonged period of time.
Air Burn
A patch or streak of brown or black material on the component caused by air or gases that have not been properly vented from the mold and have caused the material to overheat and burn.
A term used in the plastics industry to denote blends of polymers or copolymers with other polymers or elastomers. (i.e. ABS/Polycarbonate).
Ambient Temperature
The temperature of a medium surrounding an object. The term is often used to denote prevailing room temperature.
Devoid of crystallinity or stratification. Most plastics are amorphous at processing temperatures. Material assumes more random molecular structure when cooling.
The tendency of a material to react differently to stresses applied in different directions.
The process of relieving internal stresses of molded plastic articles by heating to a predetermined temperature, maintaining this temperature for a predetermined length of time and slowly cooling the articles.
An additive used to reduce degradation from oxygen attach at normal or elevated temperatures. Sources such as heat, age, chemicals, and/or stress may accelerate oxygen attack.
Additives used to prevent the negative effects of ozone on the resin materials.
Antistatic agent
An additive used to improve the electrical conductivity of the plastic part so that any charge can readily go to ground and not remain in the part. This makes the part less susceptible to the accumulation of electrostatic charges which attract and hold fine dirt or dust on the surface of the plastic article.
What the molded plastic article4 will be in its final form.
Artificially balanced runner system
Involves balancing a runner system by adjusting the pressure drop of a long, large diameter runner against a short, small diameter runner. Since pressure drop over the small diameter runner will be much more affected by heat loss than the large diameter runner, an artificially balanced runner system will work with a set range of molding conditions. The width of this range of molding conditions determines the stability of the molding.
Ash content
The solid residue remaining after a substance has been incinerated or heated to a temperature sufficient to drive off all combustible or volatile substances.
Aspect ratio
The ratio of total flow length to average wall thickness.
A very early stage in the reaction of certain thermosetting resins where the molecular weight is low and the resin is still soluble in some liquids and still fusible.
The abbreviation for American Society for Testing and Materials.


Back pressure
The resistance of the molten plastic material to forward flow. In molding, back pressure increases the temperature of the melt and contributes to better mixing of colors and homogeneity of the material. However, as back pressure increases do does cycle time.
When molten resin flows back out of the mold returning to the runners.
Backing plate
A plate used as a support for the mold cavity block, guide pines, bushings etc.
Balanced runner
A runner system designed to place all cavities at the same distance from the sprue.
The section of a molding machine that contains the feed screw. This section is also where resin heating and mixing occurs.
A resin or other material used to hold particles together. The binder is the continuous phase in a reinforced plastic which provides mechanical strength or ensures uniform consistency, solidification or adhesion to a surface coating. Typical binder materials include resin, glue, gum and casein.
Black Specks
A specific kind of inclusion/contamination often associated with heat-degraded materials.
Blast Finishing
The process of removing flash from molded objects and/or dulling their surfaces by impinging upon them with sufficient force to remove the flash.
An imperfection on the surface of a plastic article caused by a pocket of air or gas beneath the surface.
Blocking & Anti-Caking Agents
These additives are used to prevent the adhesion and agglomeration of ingredients within a resin compound.
An undesirable cloudy effect or whitish powdery deposit on the surface of a plastic article or to the surrounding environment caused by the exudation of an ingredient such as a lubricant, stabilizer pigment, plasticizer or other non-bonded component. This may also be known as “migration”.
Blow Molding
A method of fabrication in which a warm plastic 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 internal pressure. This process forms hollow articles such as bottles, tanks, etc.
Blowing and Foaming Agents
These are additives for plastics or rubbers that generate inert gases within the resin matrix when heated. The resulting part construction will contain a cellular structure.
Refers to the tendency of a plastic article to turn white or chalky in areas that are highly stressed.
A raised feature of a molded part designed to add strength, facilitate alignment during assembly or for attachment to another part.
Materials that are used to add smoother or brighter coatings or finishings.
Brittle Temperature
A measure for judging the relative merits of materials for low temperature flexing or impact, i.e. the temperature at which materials rupture by impact under specified conditions.
Broken Mold Marks
Defects in a part’s surface caused by mold damage.
Term describes an intermediate stage of a thermoset resin reaction where the material will soften when heated and swells in the presence of certain liquids, but may not completely fuse or dissolve. The resin is usually supplied in this uncured state.
Air or gas pockets that have formed in the material of the component. Bubbles may vary in size.
Bulk Factor
The ratio of the volume of any given mass of loose plastic material to the volume of the same mass of the material after molding.
Bulk-Molding Compounds (BMC)
bulk-molding compounds are used as a premix in composite manufacturing. A BMC consists of a mixture of resin, reinforcements, inert fillers and other additives which form a puuylike preformed shape, rope or steel.
Showing evidence of excessive heating during processing or use of a plastic, as evidence by blistering, discoloration, distortion or destruction of the surface.


The process of forming solid or hollow articles from fluid plastic mixtures or resins by pouring or injecting the fluid into a mold or against a substrate with little or no pressure, followed by solidification and removal of the formed part.
A depression or a set of matching depressions in a plastics-forming mold which forms the outer surfaces of the molded articles.
The amount of material used to load a mold at one time or during one cycle or the measurement or weight of said material.
Charpy Impact Test
A destructive test of impact resistance, consisting of placing a test sample in a horizontal position between two supports then applying a blow of known magnitude. If the specimen does not break, a new specimen is put in position and the magnitude is increased until the specimen breaks.
Chopped Strand
A type of fiber reinforcement consisting of strands of individual glass fibers which have been chopped into short pieces.
The part of an injection molding machine incorporating the platens that provide the force necessary to hold the mold closed during injection of the molten resin and open the mold to eject the molded part.
Clamping Area
The largest rated molding area an injection press can hold closed under full molding pressure.
Clamping Force
The force applied to the mold to keep it closed, in opposition to the fluid pressure of the compressed molding material within the mold cavity and the runner system.
Clamping Plate
A plate fitted to a mold and used to fasten the mold to a platen.
Additives used in resins to improve transparency or translucency.
Closed-loop Control
A system for monitoring and automatically adjusting injection molding process conditions such as temperature, pressure and time. The automatic changes keep part production within preset tolerances.
Cold Flow Lines
Imperfections with the part wall due to thickening or solidification of resin prior to full cavity fill.
Cold Molding
The process of compression molding involving shaping an unheated compound in a mold under pressure then heating the article to cure it.
Color concentrate (aka colorant, pigment)
A plastic compound which contains a high percentage of pigment to be blended in appropriate amounts with the base resin so that the correct final color is achieved.
A structural non-homogenous material consisting of a combination of materials. Typically, one of the materials is a strengthening agent, the other being a thermoset or thermoplastic resin.
A mixture of resin(s) and additives usually formed in a separate machine downstream from the primary reactor.
A process required to mix polymer(s) with all of the additives that are necessary to provide the end user with a finished grade with suitable properties.
Compression Molding
A method of molding in which the molding material, generally pre-heated, is placed in an open heated mold cavity. The mold is then closed by pressing down from the top of the mold, which forces the material into contact with all the mold areas.
Compressive Strength
The ability of a material to sustain a force in a direction opposite of tension.
A process of subjecting a material to standard environmental and/or a non-standard stress state prior to testing or use.
Constant pressure gradient
The pressure drop per unit of length. The constant pressure gradient principle says that the most efficient filling pattern is when the pressure gradient is constant along the flow path.
Continuous Service Temperature
The highest temperature at which a material can perform reliably in long term application as determined by the manufacturer.
Cooling Channels
Channels located within the body of a mold through which a cooling medium is circulated to control the mold surface temperature.
Cooling Time
The elapsed time required for the melt to reach it Vicat softening temperature.
A protrusion or a set of protrusions in a plastics forming mold which forms the inner surfaces of the molded articles.
Corona treatment
The impingement of AC power on a component to bombard it with free radicals thus improving the ability to bond to a surface.
Corrosion Resistance
A broad term applying to the ability of plastics to resist degradation in many environments.
Coupling Agents
Materials that are used to form a chemical bridge between the resin and an additive such as glass fiber or mineral fiber. By acting as an interface, bonding is enhanced.
A physical separation or tearing of a part.
A term referring to the trait of plastic which is: a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application and the degree of this deformation is dependent of the load duration.
Cryogenic Processes
The reduction of parts to very low temperatures usually associated with liquid nitrogen. This is commonly used to create assemblies or to de-flash or de-gate a part.
A homogenous solid having an orderly and repetitive three dimensional arrangement of its atoms.
A state of molecular structure in some resins attributed to the existence of solid crystals with a definite geometric form. Such structures are characterized by uniformity and compactness.
A term describing the final stage of the reaction where a thermoset material is relatively insoluble and infusible.
Cure Cycle
The time period at defined conditions to which a reacting thermosetting material is processed to reach a desired property level.
The process of changing properties of polymer into a more stable and usable condition. This is accomplished by the use of heat, radiation, or reaction with chemical additives.
Custom Molder
A firm specializing in the molding of items or components to the specifications of another firm which handles the sale or distribution of the item or incorporates the custom molded components in one of its own products.
A complete, repeating sequence of operation for injection molding a part.
Cycle time
The time required by an injection molding system to mold a part and return to its original position/state.


The momentary opening and closing of a mold during the early stages of the cycle to permit the escape of air or gas from the heated compound.
A deleterious change in the chemical structure, physical properties or appearance of a plastic caused by exposure to heat, light, oxygen, weathering or other external influence.
When the surface of a finished part separates. Strata or fish-scale type appearance may be visible where the layers may be separated.
Diaphragm Gate
Used in symmetrical cavity filling to reduce weld-line formations and improve filling rates.
Differential Cooling
Occurs when one area of the part cools at a different rate or when the mold surfaces are at different temperatures. Warping can result from differential cooling.
Dimensional Stability
The retention of the precise shape of the part.
Direct Gate
A sprue that feeds directly into the mold cavity.
Dispersion Aids
Flow alteration components placed at the entry point of an additive to aid in mixing or dispersing actions of a compounding process.
Dominant Flow Path
The path at the juncture of two confronting flows where the dominant flow will reverse the direction of the other.
A slight taper in a mold wall designed to facilitate removal of the molded object from the mold.
Draft Marks
A form of deep scratch or scratches on the surface of the component usually cased by the ejection of the part.
The extrudation or leakage of molten resin from nozzle or nozzle sprue bushing area while filling or shooting the mold.
The removal of moisture from the resin pellets by exposure to certain time and temperature. All hydroscopic material must be dried prior to molding.
An instrument used for measuring the hardness of a material.
A pause in the applied pressure to a mold during the injection cycle just before the mold is completely closed. This dwell allows any gases formed or present to escape from the molding material.


Edge Gate
The entrance to the part from the runner located on the parting line.
Ejection Pin
A rod, pin or sleeve that pushes a molded part off of a core or out of a mold cavity.
Ejection Pin Marks
Residual marks on the part caused by the profile of the ejection pin.
Ejector return pins
Projections that push the ejector assembly back as the mold closes. They are also called surface pins or return pins.
Ejector Rod
A bar that actuates the ejector assembly when the mold opens.
Elastic Memory
The characteristic of certain plastics evidenced by their tendency to revert to a previously existing shape or dimension.
The ability of a material to quickly recover its original dimensions after removal of a load that has caused deformation.
A rubber-like material which at room temperature can be stretched repeatedly to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.
Engineering plastics
A broad term covering plastics, with or without fillers or reinforcements, which have improved mechanical, chemical, and thermal properties over commodity grades of resins.
A filler material added to a plastic compound used to reduce the amount of resin required per unit value.
The product or material delivered from an extruder. For example, film pipe profiles.
The process of forming continuous shapes by forcing a molten plastic material through a die.


The manufacture of plastic products by appropriate operations. This includes plastics formed into molded parts, rods, tubes, sheeting, extrusion and other forms by methods including punching, cutting, drilling, tapping, fastening or by using other mechanical devices.
Family Mold
A mold that produces non-identical part simultaneously from multiple cavities.
Fan Gate
A gate used to help reduce stress concentrations in the gate area by spreading the opening over a wider area. Less warping of parts can usually be expected by the use of this type of gate.
Fatigue Strength
The maximum cyclic stress a material can withstand for a given number of cycles before failure occurs.
The packing of the cavity or cavities of the mold as required to give a complete part or parts that are free of flash and porosity.
Fill Pattern
The contours of the advance of the material as the cavity fills. See flow pattern.
Fill Pressure
The pressure required to fill the cavity.
Fill Time
The time required to fill the cavity. May also be known as injection.
A relatively inert substance added to a plastic compound to reduce its cost and/or to improve physical properties, particularly hardness, stiffness and impact strength.
The surface texture and appearance of a finished article.
Flame Retardant
To have the ability to resist combustion. A flame retardant plastic is considered to be one that will not continue to burn or glow after the source of ignition has been removed.
Flash Gate
A wide gate extending from a runner which runs parallel to an edge of a molded part along the parting line of a mold.
Any excess material that is formed with and attached to the component along a seam or mold parting line.
Flexural Modulus
The ratio, within the elastic limit, of the applied stress on a test specimen in flexure to the corresponding strain in the outermost elements of the specimen.
Flexural Strength
The maximum stress in the outer fiber at the moment of cracking or breaking. In the case of plastics, this value is usually higher than the tensile strength.
Flexural Strength yield
The measure of resistance of the material to fracture during bending.
Flow Balancing
Modifying flow paths, particularly runner sections, so that all flow paths within a mold fill in equal time and with equal pressure.
Flow Leader
A local increase in thickness to encourage flow in a particular direction.
Flow Marks
These are wavy surface appearance on a molded part caused by improper flow of the melt into the mold.
Flow Pattern
The contour that the melt takes sequentially as it fills the cavity.
Flow Rate
The volume of material passing a fixed point per unit of time.


The channel through which the molten resin flows from the runner into the cavity.
Gate Blush
A blemish or disturbance in the gate area of an injection molded article.
Gate Trim
A remnant of plastic left over from cutting the component from the runner or sprue, usually to be cut flush with the edge of the component.
Glass Fibers
Family of reinforcing material for reinforced plastics based on single filaments of glass.
Glass Transition
A change in an amorphous polymer from viscous to hard and relatively brittle.


A substance or mixture of substances added to a material to increase or control the curing reaction by taking part in it.
The resistance of a material to compress, indentation and scratching.
The cloudy appearance of a material caused by light scattered from within the specimen or from its surfaces.
Heat Stabilizers
These additives increase the ability of the material to withstand the negative effects of heat exposure. They are used to increase the overall service temperature of the material.
Hesitation Effect
This occurs in parts of varied thicknesses. The flow moves preferentially into a thicker area causing an adjacent thin area to freeze off while the thicker area fills. Gates should be positioned as far as possible from where the flow divides into thick and thin flow paths.
A plastic that results from the polymerization of a single monomer.
Hopper dryers
Auxiliary equipment that removes moisture from resin pellets.
Hopper loaders
Auxiliary equipment for automatically loading resin pellets into machine hopper.
Hot-runner mold
A mold in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner molds make parts that have no scrap. This may also be known as insulated runner or runnerless molding.
Ability to readily absorb moisture.


Impact modifiers
Additives used to enhance the material’s ability to withstand the force of impact.
Impact resistance
The resistance of plastic articles to fracture under stresses applied at high speeds.
Impact strength
The ability of a material with withstand shock loading.
The time required to fill a cavity or mold. May also be known as “fill time”.
Injection blow molding
A molding process during which the plastic parison to be blown is formed by injection molding.
Injection molding
The method of forming objects from granular or powdered plastics, most often of the thermoplastic type, in which the materials are fed from a hopper to a heated chamber in which it is softened, after which a ram or screw forces the material into a mold. Pressure is maintained until the mass has hardened sufficiently for removal from the mold.
Injection molding pressure
The pressure applied to the cross-sectional area of the molding cylinder.
Injection pressure
The pressure on the face of the injection screw or ram when injecting material into the mold, usually expressed in PSI.
A removable part of the mold imparting increased resistance to wear, heat transferability, or changeable part shape to that area of the mold.
Insert molding
The process of molding plastic around pre-formed metal inserts. This process is compatible with both thermoplastic and thermoset materials.
Insulated runner
A mold in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner mold make parts that have no scrap.
Izod impact strength
A measure of impact strength determined by the difference in energy of a swinging pendulum before and after it breaks a notched specimen held vertically as a cantilever beam.
Izod impact test
A test to determine impact strength of a sample by holding a sample bar at one end and breaking it by striking. Sample specimens can be notched or un-notched.


A turbulent flow in the melt caused by an undersized gate or where a thin section rapidly becomes thicker.
A tool for holding parts of an assembly during the manufacturing process.


A term sometimes used for thermal insulation value or coefficient of thermal conductivity.
Knit lines
The lines where melted material flows together to form a line or lines that may cause weakening or breaking of the component.
Knockout Pin
A pin that ejects a molded article from the mold.


Land Gate Area
A gate dimension parallel to the direction of melt flow.
Land Area
The area of surfaces of a mold which contact each other when the mold is closed.
Light, UV Stabilizers & Absorbers
These are additives that increase the ability of the material to withstand the negative effects of light and UV exposure, thus increasing the service life of the material.
Linear Mold Shrinkage
The difference between the size of the part and the size of the mold cavity.
Linear Thermal Expansion
The fractional change in length of a material for a unit chance in temperature.
Liquid Injection Molding (LIM)
The process that involves an integrated system for proportioning, mixing and dispensing two component liquid resin formulations and directly injecting the resultant mix into a mold which is clamped under pressure.
Low Temperature Flexibility
The ability of a plastic to be bent without fracture at reduced temperatures.
Internal lubricants promote resin flow without affecting the fusion properties of the compounds. External lubricants promote release from metals which aids in the smooth flow of melt over die surfaces.


Machine Shot Capacity
Refers to the maximum volume of thermoplastic resin which can be displaced or injected by the injection ram in a single stroke.
Mar Resistance
The resistance of glossy plastic surfaces to abrasive action.
Mechanical Property
Properties of plastics which are classified as mechanical include abrasion resistance, creep, ductility, friction resistance, elasticity hardness, impact resistance, stiffness and strength.
Melt Flow Rate
Measure of the molten viscosity of a polymer determined by the weight of polymer extruded through an orifice under specified conditions of pressure and temperature. Particular conditions are dependent upon the type of polymer being tested.
Melt Flow
The rate of extrusion of molten resin through a die of specified length and diameter. The conditions of the test, i.e. temperature and load, should be given. Frequently, however the manufacturer’s data lists only the value, not the condition.
Melt Index
The amount of a thermoplastic resin, measured in grams, which can be forced through a specified orifice within ten minutes when subjected to a specified force.
Melt Strength
The strength of the plastic while in the molten state. This is a pertinent factor in extrusion, blow molding, and drawing of molten resin from a die.
The tendency of a plastic article to revert in dimension to a size previously existing at some stage in its manufacture.
A term covering all processes by which plastics are coated with metal.
An undesirable cloudy effect or whitish powdery deposit on the surface of a plastic article caused by the exudation of a compounding ingredient such as a lubricant, stabilizer pigment, plasticizer, etc.
Mineral Reinforcements
Inorganic substances used as filler for plastics. Some common examples are: clay, mica and talc.
Derived from the Latin word meaning “small measure”, modulus is the ration of stress to strain in the linear region of the s-e curve.
Modulus in Compression
The ratio of compressive stress to strain within the elastic limits of the material.
Modulus in Elasticity
The ratio of stress to corresponding strain below the proportional limit of a material in tensile testing.
Modulus of Flexure
The ratio of the flexure stress to strain, within the elastic limits of the material.
Modulus of Resilience
The energy that can be absorbed per unit volume without creating a permanent distortion.
Moisture in Shear
The ratio of shear stress to strain within the elastic limits of the material.
Moisture Absorption
The pick-up of moisture from the environment by a material.

(n) A hollow form or matrix into which a plastic material is placed and which imparts to the material its final shape as a finished article.

(v) To impart shape to a plastic mass by means of a confining cavity or matrix.

Mold Changer
An automated device for removing one mold from a machine and replacing it with another mold.
Mold Frame
A series of steel plates which contain mold components, including cavities, cores, runner system, cooling system, ejection system etc.
Mold Release
In injection molding, mold release is a lubricant used to coat the surface of the mold to enhance ejection of the molded article or prevent it from sticking to the tool.
Mold Release Problems
Occur when excess use of mold release may leave parts too oily and weaken the material.
Mold Temperature
The temperature at which the mold is maintained. Often the most important benefit of raising mold temperature is that it allows a slower injection rate without the plastic getting too cold.
The characteristic of being easy to mold without rupturing or developing flaws due to movement of the polymer during gelation.
Molding Cycle
The period of time occupied by the complete sequence of operations on a molding press required for the production of one set of molded articles.
Molding Pressures
The pressures applied to the ram of an injection machine or press to force the softened plastic to completely fill the mold cavities.
Molding Sensitivity
The variability of the pressure to fill the cavity and temperature of the melt at the part as influenced by changes in injection time and barrel melt temperature.
Molding Window
The range of molding conditions under which a part can be successfully molded.
Mold Temperature Control Unit
This is auxiliary equipment used to control mold temperature. Some units can both heat and cool the mold. Others, called chillers, only cool the mold.
Molecular Weight
The sum of the atomic weights of all the atoms in a molecule. In plastics, especially, an average molecular weight is reported.
A relatively simple molecular structure, usually containing carbon and of low molecular weight, which can react to form a polymer by combination with itself or with other molecules and energy.
Moving Platen
The platen of an injection molding machine that is moved by a hydraulic ram or mechanical toggle.
Multi-cavity mold
a mold having two or more impressions for forming finished items in one machine cycle.
Multi-directional flow
when flow direction changes during filling resulting in orientation in different directions which can cause flow marks, stresses and warping.
Multiple cavity molds
a mold that produces more than one identical part with each cycle.
Multi-shot molding
the injection of two or three materials in sequence into a single mold during a single molding cycle. The injection molding machine is equipped with tow or three plasticators. Also see “co-injection”.


Naturally Balanced Runner System
When each succession of runner is identical to the runners in the same succession in all other flows in the mold.
Nest Plate
A retainer plate in the mold with a depressed area for cavity blocks.
A failure to completely fill the mold or cavities of the mold. Edges may appear melted. This is also known as “short shot”.
Non-return Valve
A screw tip that allows for material to flow in one direction and closes to prevent back flow and inject material into the mold. This may also be known as “check valve”.
Notch Sensitivity
The extent to which the sensitivity of a material to fracture is increased by the presence of a surface notch or sudden change in section.
A hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine which is designed to form a seal under pressure between the cylinder and the mold.
Nucleating Agent
An additive used in a polymer to increase crystallization rate by providing additional sites for crystal growth (i.e. talc). This results in faster cycle time.


These are used to add odor to material, usually for safety reasons.
Means not able to transmit light.
Orange Peel
On a molded part, this is a surface finish defect that is rough and splotchy. Usually this is caused by moisture in the mold cavity or poor heat transfer properties.
The arrangement of the molecules of the melt. If the molecules are orientated, they are aligned with each other; if non-oriented they are not in alignment. In general, orientated material shrinks more than non-oriented material.
Over Molding
A process in which a mold cavity is first partially filled with one plastic and then a second shot is injected to encapsulate the first shot.
Results because melt will fill the easiest flow path first and will continue to pack this area while material reaches the other areas. This is a cause of warping created by an unbalanced flow.
Oxygen Index
A flammability test based on the principle that a certain volumetric concentration of oxygen is necessary to maintain combustion of a specimen after it has been ignited.


The filling of the mold cavity or cavities as full as possible without causing undue stress on the molds or causing flash to appear on the finished parts. Over or under packing can result in less than optimum fill.
Part Picker
An auxiliary unit usually mounted on a fixed platen which reaches into the open mold to grab parts and remove them prior to the next molding cycle. The term robot may also be used. This unit is used when you do not want to drop parts from mold upon ejection.
Partially-balanced Runner System
A system composed of both naturally and artificially balanced runners.
Parting Line
A mark on the part indicating where the two halves of the mold met in closing.
An open blister.
Tablets or granules of uniform size, consisting of resins or mixtures of resins with compounding additives which have been prepared for molding operations by extrusion and chopping into short segments.
The degradation of plastics due to the action of light.
A plastic compound which contains a high percentage of pigment to be blended in appropriate amounts with the base resin so that the correct final color is achieved. See colorant, color concentrate.
Pinpoint gate
A restricted gate of 0.030” or less in diameter. This gate is common on hot-runner molds.
See “ram”.
A small crater in the surface of plastic that is considered an imperfection.
Plastic Deformation
A change in dimensions of an object under load that is not recovered when the load is removed.
A material that contains an essential ingredient of one or more organic polymeric substances of large molecular weight. It is solid in its finished state and at some stage, in its manufacture or processing into finished articles, can be shaped by flow.
The ability of a material to withstand continuous and permanent deformation by stresses exceeding the yield value of the material without rupture.
To render a material softer, more flexible and/or more moldable by the addition of a plasticizer.
A substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability, or extensibility.
The mounting plates of a press on which the mold halves are attached.
An objectionable coating gradually formed on metal surfaces of molds during processing of plastics due to extraction and deposition of some ingredient such as pigment, lubricant, stabilizer or plasticizer.
Poisson’s Ratio
The constant relating the changes in dimensions which occur when a material is stretched. It is obtained by dividing the change in width per unit length by the change in length per unit length.
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. If two or more monomers are involved, a copolymer is obtained.
Polymer Structure
A general term referring to the relative positions, arrangement in space and freedom of motion of atoms in a polymer molecule.
A chemical reaction in which the molecules of a simple substance (monomer) are linked together to form large molecules whose molecular weight is a multiple of that of the monomer.
A plastic pre-shaped part produced by injection molding systems in the first step of a two-stage injection molding and blow molding process used to produce bottles or containers. The preform is subsequently re-heated and stretch blown through a blow molding process into the final container shape.
Pressure Pads
Reinforcements of hardened steel distributed around the dead areas in the faces of a mold to help the land absorb the final pressure of closing without collapsing.
Processing Aids
Additives specifically used to improve the injection process.
Proportional Limit (Hooke’s Law)
The greatest stress which a material is capable of sustaining without deviation from proportionality of stress and strain.
Prototype Tool
A preliminary mold that the final mold will be built from.
Pulled Gate
An area where the part was connected to the sprue or runner that has been drawn out or stretched from the surface.
Extrusion or injection molding, purging is the cleaning of one color or type of material from the machine by forcing it out with the new color or material to be used in subsequent production, or with another compatible purging material.


The forward motion of the screw in the plasticators barrel that forces the melt into the mold cavity.
Reaction Injection Molding (RIM)
A process that involves the high pressure impingement mixing of two or more reactive liquid components and injecting them into a closed mold at low pressure.
Reagent Resistance
The ability of a plastic to withstand exposure to chemicals.
The term used to designate any polymer that is a basic material for plastics (synthetic resins).
Restricted Gate
A very small orifice between a runner and cavity in an injection mold. When the part is ejected, this gate readily breaks free of the runner system. Generally, the part drops through one chute and the runner system through another leading to a granulator and scrap reclaim system.
Retainer Plate
The plate on which demountable pieces, such as mold cavities, ejector pines, guide pins, and bushings are mounted during molding.
Retractable Cores
Used when molding parts in cavities not perpendicular to the direction in which the part is ejected from the mold. The cores are automatically pulled from the mold prior to the mold opening and reinserted when the mold closes again and prior to injection.
A reinforcing member of a molded part.
Ring Gate
Used on some cylindrical shapes, this gate encircles the core to permit the melt to first move around the core before filling the cavity.
An injection mold, a runner is the feed channel, usually of circular cross section, which connects the sprue with the cavity gate. The term is also used for the plastic piece formed in this channel.
Runner Balancing
A runner system which delivers the required amount of melt to each cavity with the correct pressure to finish filling all the cavities simultaneously at the correct temperature for the part.
Runner Design
Using the runner as a flow control device (positioning the gate and using the size of the runner to control the filling pattern within the cavity) in addition to getting the melt into the cavity.
Runner System
This term is sometimes used for the entire resin feeding system including sprues, runners, and gates in injection molding.
Runnerless Molding
Molding in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner molds make parts that have no scrap.


Any output of a mold that is not usable as the primary product.
Screw Travel
The distance the screw travels forward when filling the mold cavity.
Shear Rate
The rate at which a layer of melt slides over the layer below. Shear rate is velocity-related rather than force-related.
Shear Strength
The maximum load required to shear the specimen in such a manner that the moving portion has completely cleared the stationary portion. Sheets are distinguished from films in the plastics industry only according to their thickness. In general, sheets have thicknesses greater than .040”.
Shear Stress
The shearing force divided by the area. It is always a maximum at the outside of the flow channel. As it is force-related, it depends on the viscosity of the material, which in turn depends on the material and molding conditions. The maximum allowable stress level is usually taken as 1% of the tensile strength of the material. High shear stress is unimportant at gates and in sprues and runners.
Short Shot
A failure to completely fill the mold or cavities of the mold. Edges may appear melted. This may also be called “non-fill”.
One complete cycle of a molding machine.
The contraction upon cooling of all areas of the part. Shrinkage occurs less in disorientated material and more across chains of molecules than along their lengths. Lower pack areas have lower areas of orientation and shrinkage.
Shrinkage Allowance
The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.
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. See also “retractable cores”.
Sink Mark
An indentation on the surface of the part as a result of significant local change in a wall section. The mark will occur in the thicker area.
A projection in the mold used to form the geometry of the part, which is not in the direction of the closing of the mold and must be withdrawn before the part can be ejected.
Slip Agent
An additive used to provide lubrication during and immediately following the processing of plastics.
Substances with the ability to dissolve other substances.
Spiral Flow
A test performed by injection molding a sample into a spiral mold and is used to compare the processability of different resins.
Splay Marks
Skin or surface defects on molded parts caused by abnormal racing of the melt in the mold.
Split Ring Mold
A mod in which a split cavity block is assembled in a channel to permit the forming of undercuts in a molded piece. These parts are ejected from the mold and then separated from the piece.
The feed opening provided in injection molding between the nozzle and the cavity or runner system.
Sprue Bushing
A hardened steel insert in the mold that accepts the nozzle and provides an opening for transferring the melt.
Sprue Gate
A passageway through which melt flows from the nozzle to the mold cavity.
Sprue Lock
The portion of resin retained in the cold-slug well by an undercut. This lock is used to pull the sprue out of the bushing as the mold opens. The sprue lock itself is pushed out of the mold by an ejector pin.
An agent used in compounding some plastics to assist in maintaining the physical and chemical properties of the compounded materials at suitable values throughout the processing and service life of the material and/or the parts made from it.
Stack Molds
Two or more molds of a similar type that are positioned one behind the other to allow for additional parts to be manufactured during a cycle.
Stationary platen
The large front plate of an injection molding press to which the front plate of the mold is secured. This platen does not move during normal operation.
The force producing or tending to produce deformation in a body measured by the force applied per unit area.
Stress Cracking

There are three types of stress cracking and all of these types of stress cracking have the same end result; the splitting or fracturing of the molding.

  • Thermal stress cracking is caused by prolonged exposure of the part to elevated temperatures or sunlight.
  • Physical stress cracking occurs between crystalline and amorphous portions of the part when the part is under an internally or externally induced strain.
  • Chemical stress cracking occurs when a liquid or gas permeates the part’s surface.
Stress Crack
An external or internal crack in a plastic caused by tensile stresses less than that of its short-time mechanical strength. The development of such cracks is frequently accelerated by the environment to which the plastic is exposed.
Marks evident on the molded-part surfaces that indicate melt flow directions or impingement.
Strings of material due to poor gate cut off. See pulled gate.
Stripper Plate
A plate that strips a molded piece from core pins or force plugs. The stripper plate is set into operation by the opening of the mold.
Structural Foam Molding
The process of molding thermoplastic articles with a cellular core and integral solid skins in a single operation.
An entrance to the part from the runner located below the parting line. On ejection the part breaks away from the subgate.
When the pressure on the sprue is not held long enough for the melt to cool before the screw returns. Some of the melt in the cavities or runner system may expand back into the nozzle and cause sink marks on the finished product.


Tab Gate
A small removable tab about the same thickness as the molded item, but usually perpendicular to the part for easy removal.
Tensile Strength
The maximum tensile stress sustained by the specimen during a tension test.
Thermal Conductivity
The ability of a material to conduct heat.
Thermal Degradation
The deterioration of the material by heat, characterized by molecular scission.
Thermal Shut-off
When the material freezes causing blockage.
The rubber-like elasticity exhibited by a rigid plastic resulting from an increase in temperature.
The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. Differential pressure is then applied to make the sheet conform to the shape of a mold or die positioned below the frame.
Thermoplastic Elastomers
The family of polymers that resemble elastomers in that they can be repeatedly stretched without distortion of the unstressed part shape, but are true thermoplastics and thus do not require curing.
A material that will repeatedly soften when heated and harden when cooled.
A polymer that doesn’t melt when heated. Thermoset polymers “set” into a given shape when first made and afterwards do not flow or melt, but rather decompose upon heating. They are often highly cross-linked polymers, with properties similar to those of network covalent solids i.e. hard and strong.
Tie-bar Spacing
The space between the horizontal tie-bars on an injection molding machine. Basically, this measurement limits the size of molds that can be placed between the tie-bars and into the molding machine.
Time Scan
The effects of old and melt temperature and injection time changes on pressure, stress and temperature at the end of flow.
A type of clamping mechanism that exerts pressure by applying force on a knee joint. A toggle is used to close and exert pressure on a mold in a press.
The measure by which injection molding machines are typically categorized, representing the clamping force of the injection molding machine.
The term sometimes used to describe the mold in injection molding.
Transfer Molding
A process of forming articles by fusing a plastic material in a chamber then forcing the whole mass into a hot mold to solidify.
Transition Temperature
The temperature at which a polymer changed fro (or to) a viscous or rubbery condition (or from) a hard and relatively brittle one.
Tunnel Gate
See submarine gate.


Ultimate Elongation
The elongation at rupture in a tensile test.
Ultimate Strength
A term used to describe the maximum unit stress a material will withstand when subjected to an applied load in a compression, tension, flexural or shear test.
A protuberance or indentation that impedes withdrawal from a two-piece rigid mold.
The dominant flow of two confronting flows over the other. The lesser flow reverses direction giving poor surface appearance and structural strength. Underflow should be avoided by positioning gates so the flow fronts meet at the end of filling.
Unidirectional Flow Pattern
Plastic flowing in one direction with a straight flow front throughout the filling.
Uniform Cooling Time
When the cooling time is the same throughout the part to avoid warping.


Vacuum Forming
A process whereby a heated plastic sheet is drawn against a mold surface by evacuating the air between it and the mold.
Valve Gating
A type of gate where a pin is held in the gate or channel by spring tension. As the injection stroke moves forward, this gate compresses the plastic in the runner. When this pressure build-up is sufficient to overcome the spring tension, the pin is then pushed back (pulled) and the fast decompression of the melt fills the cavity at extremely high speed.
A shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity.
Vented Barrel
A special barrel unit with a vent port over the compression section of the screw to permit the escape of gases prior to injecting the melt into the mold. Often used when molding moisture-sensitive resins.
Vertical Flash Ring
The clearance between the force plug and the vertical wall of the cavity in a positive or semi- positive mold. Also includes the ring of excess melt which escapes from the cavity into this clearance space.
Vicat Softening Point
The temperature at which a flat-ended needle will penetrate a specimen under a specific load using a uniform rate of temperature rise.
Virgin Material
Any plastic compound or resin that has not been subjected to use or processing other than that required for its original manufacture.
This property, possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics such as elasticity, strength and form stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate and amount of loading.
The resistance to flow of a liquid.
An unfilled space within a solid material.


Distortion caused by non-uniform internal stresses.
Water Absorption
The amount of water absorbed by a plastic article when immersed in water for a stipulated period of time. All plastics will absorb moisture to some extent.
Weld Line
Where melted material flows together during molding to form a visible line or lines on a finished part that may cause weakening or breaking of the component.
Similar to stringing but smaller in size. These also may occur as slight flashing when the mold is over-packed or forced open slightly. Mold-parting-line wear or misalignment can also cause wisps.


Yield Point
Tensile testing, yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress.
Yield Strength
The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.
Young’s Modulus
The ratio of tensile stress to tensile strain below the proportional limit.