Plastic Injection Moulding Terms: Glossary

Plastic Injection Moulding Terms


Abrasion Resistance

 The ability of a material to withstand mechanical actions such as rubbing, scraping, or erosion, that tend progressively to remove material from its surface.

Acceptable runner/cavity ratio

Runner systems 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 adhesive.


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 mould 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 moulded plastic articles by heating to a predetermined temperature, maintaining this temperature for a predetermined length of time, and slowly cooling the articles.


Abbreviation for American National Standards Institute.


Additive used to reduce degradation from oxygen attack at normal or elevated temperatures. Sources such as heat, age, chemicals, and/or stress may accelerate oxygen attack.


These additives are used to prevent the negative effects of ozone on the resin materials.

Antistatic Agent

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.


The act of applying or putting to use. What the molded plastic article will be in its final form.

Artificially balanced runner system

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 moulding conditions. The width of this range of moulding conditions determines the stability of the moulding.

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

Ratio of total flow length to average wall thickness.


The process of joining parts by any of several methods.


This is 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.


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, so does cycle time.


Molten resin flows back out of the mould, returning to the runners.

Backing Plate

A plate used as a support for the mold cavity block, guide pins, bushings, etc.

Balanced Runner

A runner system designed to place all cavities at the same distance from the sprue.


The section of a moulding machine that contains the feed screw, also the section 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.

Biocides & Fungicides

Additives that are used to inhibit the growth and colonization of fungus, bacteria, and other pests.

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 & Anticaking Agents

These additives are used to prevent the adhesion and agglomeration of ingredients within a resin compound.

Bloom (also know as Migration)

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.

Blow Moulding

Method of fabrication in which a warm plastic hollow tube is placed between the two halves of a mould cavity and forced to assume the shape of that mould cavity by use of internal pressure. This process forms hollow articles such as bottles, tanks, etc.

Blowing & Foaming Agents

Additives for plastics or rubbers that generate inert gases within the resin matrix when heated. The resulting part construction will contain a cellular structure.


The tendency of a plastic article to turn white or chalky in areas that are highly stressed.


A raised feature of a moulded part designed to add strength, facilitate alignment during assembly or for attachment to another part.


The modification of the molecular structure of a polymer derived from the growth of a new polymer chain from an active site on an established chain, in a direction different from that of the original chain.

Breakdown Voltage

The voltage required, under specific conditions, to cause failure of an insulation material. See Dielectric Strength.

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

Part surface defects caused by mould damage.


This 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 Density

A measurement of mass per unit volume of materials (such as powders) that describes the effects of the particle packing density.

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 moulding.

Bulk-molding compounds (BMC)

Bulk-moulding 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 puttylike preformed shape, rope or sheet.


Showing evidence of excessive heating during processing or use of a plastic, as evidenced 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 mould or against a substrate with little or no pressure, followed by solidification and removal of the formed object.


A depression, or a set of matching depressions, in a plastics-forming mould which forms the outer surfaces of the moulded articles.


The amount of material used to load a mould at one time or during one cycle.


The measurement or weight of material necessary to fill a mould during one cycle.

Charpy Impact Test

A destructive test of impact resistance, consisting of placing a test coupon 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 moulding machine incorporating the platens that provides the force necessary to hold the mould closed during injection of the molten resin and open the mould to eject the moulded part.

Clamping Area

The largest rated moulding area an injection press can hold closed under full moulding pressure.

Clamping Force

The force applied to the mould to keep it closed, in opposition to the fluid pressure of the compressed moulding material within the mould cavity and the runner system.

Clamping Plate

A plate fitted to a mould and used to fasten the mould to a platen.

Clamping Pressure

The pressure applied to the mould to keep it closed during the moulding cycle.


Additive used in resins to improve transparency or translucency.

Closed-loop Control

System for monitoring and automatically adjusting injection moulding process conditions such as temperature, pressure and time. The automatic changes keep part production within preset tolerances.

Coefficient of Thermal Expansion (CTE)

The change in length of a material for a unit change in temperature, per unit of length.


Simultaneous or near simultaneous injection of multiple materials.

Cold Flow Lines

Imperfections within the part wall due to thickening or solidification of resin prior to full cavity fill.

Cold Moulding

The process of compression moulding involving shaping an unheated compound in a mould under pressure then heating the article to cure it.

Color Concentrate (also know as 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.

Colorant (also know as Color Concentrate, 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 forn the primary reactor.


The 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 Moulding

A method of moulding in which the moulding material, generally preheated, is placed in an open heated mould cavity, the mould is closed with a top force, pressure is applied to force the material into contact with all mould areas.

Compressive Strength

The ability of a material to sustain a force in a direction opposite of tension.


Subjecting a material to standard environmental and/or a non-standard stress state prior to testing or use.

Constant pressure gradient

Pressure drop per unit 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 defined by the manufacturer.

Controlled frictional heating

Runners designed to deliver a higher melt temperature to the cavity. This results in lower stress levels in the part without causing material degradation due to long exposure to elevated temperatures in the barrel.

Conversions (Commonly Used in Injection Moulding)

KPa x 0.145 = psi
MPa x 145 = psi
°C x 1.8 + 32 = °F
Liters/min x 0.2642 = Gal/min
Inches x 25.4 = mm
Flow rate = ((# of cavities) x (volume per cavity))/(injection time)

Cooling Channels

Channels located within the body of a mould through which a cooling medium is circulated to control the mould surface temperature.

Cooling time

The elapsed time required for the melt to reach its Vicat softening temperature.


The chemical reaction of two different monomers with each other, result in a unique new polymer.


A protrusion, or set of matching protrusions, in a plastics forming mould which forms the inner surfaces of the moulded articles.

Corona Treatment

Impingement of AC power on a component to bombard 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, usually die to oxidation.

Corrosion Resistance

A broad term applying to the ability of plastics to resist many environments.

Coupling Agents

A material that is 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 the part.


Defect in plastics articles characterized by distinct surface cracks or minute frost-like internal cracks, resulting from stresses within the article which exceed the tensile strength of the plastic.


Due to its viscoelastic nature, 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.


The formation of chemical links between the molecular chains in polymers. This process can be achieved by chemical reaction, vulcanization, and electron bombardment.

Cryogenic Processes

Reduction of parts to very low temperatures usually associated with liquid nitrogen. Commonly used to create assemblies or to deflash or degate a part.


A homogeneous 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.


Abbreviation for the Canadian Standards Association.


This term describes the final stage of the reaction where a thermoset material is relatively insoluble and infusible.

Cure Cycle

The time periods 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 Moulder

A firm specializing in the moulding of items or components to the specifications of another firm which handles the sale of distribution of the item, or incorporates the custom moulded components in one of its own products.

Cycle Time

The time required by an injection molding system to mold a part and return to its original position/state.


Complete, repeating sequence of operations for injection moulding a part.



The effect on a mass that causes decreasing amplitude.

Deflection Temperature

The measurement of temperature at which a specimen deflects to a set point under a defined load.


The momentary opening and closing of a mould 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.


Mass per unit volume of a substance.

Design Review

A review of a blueprint, of an application, to be moulded in a plastic material, with recommendations given for design, material, processing, tooling.

Diaphragm Gate

Used in symmetrical cavity filling to reduce weld-line formations and improve filling rates.

Dielectric Strength

The voltage that an insulating material can withstand before dielectric breakdown occurs.

Differential Cooling

Occurs when one area of the part cools at a different rate or when the mould surfaces are at different temperatures. Warping can result from differential cooling.

Dimensional Stability

Retention of the precise shape of the part.

Direct Gate

A sprue that feeds directly into the mould cavity.


Any change from the designated color of the material or component.

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

At the juncture of two confronting flows the dominant flow will reverse the direction of the other.


A Slight taper in a mould wall designed to facilitate removal of the moulded object from the mould.

Drag Marks

A form of deep scratch or scratches on the surface of the component usually caused 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 mould.


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 mould during the injection cycle just before the mould is completely closed. This dwell allows any gases formed or present to escape from the moulding material.


Edge gate

Entrance to the part from the runner located on the parting line.

Ejection Pin Marks

A residual mark on the part caused by the profile of the ejection pin.

Ejection Pin

A rod, pin or sleeve that pushes a moulded part off of a core or out of a cavity of a mould.

Ejector Return Pins

Projections that push the ejector assembly back as the mould closes. Also called surface pins or return pins.

Ejector Rod

A bar that actuates the ejector assembly when the mold opens.

Elastic Memory

A 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.


A triangle defined by at least three nodes, creating the basis for the finite element analysis.

Elongation, Break

The increase in distance between two gauge marks at the break point divided by the original distance between the marks. A zero value in the field indicates that it measured less than one.

Elongation, Yield

The increase in distance between two gauge marks at a yield point divided by the original distance between the marks. A zero value indicates that it measured less than one.

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 Mould

A mould that produces non-identical parts 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.

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 (also known as Injection)

Time required to fill the cavity or mould.


The packing of the cavity or cavities of the mould as required to give a complete part or parts that are free of flash and porosity.


 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.


In the classification of powdered or granular materials such as moulding compounds according to particle size, fines are the portion of the material composed of particles which are smaller than a specified size.


The surface texture and appearance of a finished article.

Finite element analysis

The solution of simultaneous equations for each element with resulting pressure, temperature and elapsed time at each node.

Flame Retardant

Having 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

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 mould 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, Yield

The measure of resistance of the material to fracture during bending.

Flexural Strength

The maximum stress in the outer fiber at the moment of crack or break. In the case of plastics, this value is usually higher than the tensile strength.

Flow Balancing

Modifying flow paths, particularly runner sections, so that all flow paths within a mould fill in equal time with equal pressure.

Flow Leader

Local increase in thickness to encourage flow in a particular direction.

Flow Marks

Wavy surface appearances on a moulded part caused by improper flow of the melt into the mould.

Flow Pattern

 The contour the melt takes sequentially as it fills the cavity.

Flow Rate

The volume of material passing a fixed point per unit time.

Flow Rate

The volume of material passing a fixed point per unit time.

Foaming Agent

Any substance which alone or in combination with other substances is capable of producing a cellular structure in a plastic mass.


The separation of a body, usually characterized as either brittle or ductile.

Freeze off

The temperature of the material is reduced to the point that it blocks an area it would fill if it were hotter.

Frictional heating

Heat generated by the friction of the chains of molecules slipping past each other or over a surface.


Gate Blush

A blemish or disturbance in the gate area of an injection molded article.

Gate Trim

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.


The channel through which the molten resin flows from the runner into the cavity.

Glass Fibers

A family of reinforcing materials for reinforced plastics based on single filaments of glass.

Glass transition

Change in an amorphous polymer from viscous to hard and relatively brittle.



A substance or mixture of substance added to a material to increase or control the curing reaction by taking part in it.


The resistance of a material to compression, 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

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.


Plastic that results from the polymerization of a single monomer.

Hopper Dryers

Auxiliary equipment that removes moisture from resin pellets.

Hopper Loader

Auxiliary equipment for automatically loading resin pellets into machine hopper.

Hot-Runner Mold (also known as Insulated Runner or Runnerless Molding)

See hot-runner mould. A mould in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner moulds make parts that have no scrap.


Readily absorbs moisture.


Impact Modifiers

Additive 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 to withstand shock loading.

Injection (also known as Fill Time)

Time required to fill the cavity or mold.

Injection Blow Molding

Blow moulding process by which the plastic parison to be blown is formed by injection moulding.

Injection Molding Pressure

The pressure applied to the cross-sectional area of the moulding cylinder.

Injection Molding

The method of forming objects from granular or powdered plastics, most often of the thermoplastic type, in which the materials is 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 Pressure

The pressure on the face of the injection screw or ram when injecting material into the mould, usually expressed in PSI.

Insert Moulding

Insert moulding is the process of moulding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials.


A removable part of the mould imparting increased resistance to wear, heat transferability, or changeable part shape to that area of the mould.

Insulated Runner (also known as Hot-Runner Mold or Runnerless Molding)

A mould in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner moulds make parts that have no scrap.


A line of equal pressure. Any point along an isobar sees the same cavity pressure as any other point along the same isobar.


A line of equal time. Any point along an isochrone is filled at the same time as any other point along the same isochrone.


A line of equal temperature. Any point along an isotherm is at the same temperature as any other point along the same isotherm.

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

Test to determine impact strength of a sample by holding a sample bar at one end and broken by striking. Sample specimen can be either notched or unnotched.

Izod, Notched, LT

The energy required to break specimens in which there is a v-notch to create an initial stress point but measured at low temperature (minus 40°C).

Izod, Notched, RT

The energy required to break specimens in which there is a v-notched to create an initial stress point.



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

Where melted material flows together to form a line or lines that may cause weakening or breaking of the component.

Knit Lines

Where melted material flows together to form a line or lines that may cause weakening or breaking of the component.

Knockout Pin

Where melted material flows together to form a line or lines that may cause weakening or breaking of the component.


Land (Gate Area)

Gate dimension parallel to the direction of melt flow.

Land Area

The area of surfaces of a mould which contact each other when the mould is closed.

Light, UV Stabilizers & Absorbers

These additives 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. Values given are often the average of a range.

Linear Thermal Expansion

The fractional change in length of a material for a unit change 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 mould which is clamped under pressure.

Low Temperature Flexibility

The ability of a plastic to be bent without fracture at reduced temperatures.


Internal lubricants, without affecting the fusion properties of a compound, promotes resin flow. External lubricants promote release from metals which aids in the smooth flow of melt over die surfaces.


Internal lubricants, without affecting the fusion properties of a compound, promotes resin flow. 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.


A concentration of a substance (an additive, pigment, filler, etc.) in a base polymer.

Material Safety Data Sheets

Documentation regarding the toxicity or hazards associated with contact with some substances. The manufacturer of the plastic prepares these data sheets.

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

A 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

Rate of extrusion of molten resin through a die of specified length and diameter. The conditions of the test (e.g. temperature and load) should be given. Frequently, however, the manufacturer’s data lists only the value, not the condition as well.

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. (ASTM D-1238)

Melt Strength

The strength of the plastic while in the molten state. This is a pertinent factor in extrusion, blow moulding 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.

Migration (also know as Bloom)

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, talc.

Minimum Specifications

The minimum values, usually of mechanical properties, that a compound must meet by Quality Assurance prior to shipment.

Modulus in Compression

The ratio of compressive stress to strain within elastic limits of the material.

Modulus in Flexure

The ratio of the flexure stress to strain, within elastic limits of the material.

Modulus in Shear

The ratio of shear stress to strain within elastic limits of the material.

Modulus of Elasticity

The ratio of stress to corresponding strain below the proportional limit of a material in tensile testing.

Modulus of Resilience

The energy that can be absorbed per unit volume without creating a permanent distortion.


Derived from the Latin world meaning “small measure”, modulus is the ratio of stress to strain in the linear region of the s-e curve.

Moisture Absorption

The pick-up of moisture from the environment by a material.

Mould (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.

Mould (v)

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

Mould Changer

An automated device for removing one mould from a machine and replacing it with another mould.

Mould Frame

A series of steel plates which contain mould components, including cavities, cores, runner system, cooling system, ejection system, etc.

Mould Release Problems

Excess use of mold release may leave parts oily and weaken the material.

Mould Release

In injection moulding, a lubricant used to coat the surface of the mould to enhance ejection of the moulded article or prevent it from sticking to the tool.

Mould Temperature

The temperature at which the mould is maintained. Often the most important benefit of raising mould temperature is that it allows a slower injection rate without the plastic getting too cold.


The characteristics of being easy to mould without rupturing or developing flaws due o movement of the polymer during gelation.

Moulding Cycle

The period of time occupied by the complete sequence of operations on a moulding press requisite for the production of one set of moulded articles.

Moulding Pressure

The pressure applied to the ram of an injection machine or press to force the softened plastic completely to fill the mould cavities.

Moulding 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.

Moulding window

The range of moulding conditions under which a part can be successfully moulded.

Mould-Temperature-Control Unit

Auxiliary equipment used to control mould temperature. Some units can both heat and cool the mould. Others, called chillers, only cool the mould.

Molecular Weight

The sum of the atomic weights of all atoms in a molecule. Especially in plastics an average molecular weight is reported.


The smallest unit quantity of matter which can exist by itself and retain all of the properties of the original substance.


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 moulding machine that is moved by a hydraulic ram or mechanical toggle.

Multi-Cavity Mould

A mould having two or more impressions for forming finished items in one machine cycle.

Multidirectional flow

Flow direction changes during filling resulting in orientation in different directions which can cause flow marks, stresses and warping.

Multiple cavity mould

Produces more that one identical part with each cycle.

Multi-Shot Moulding

The injection of two-or-three materials, in sequence, into a single mould during a single moulding cycle. The injection moulding machine is equipped with two-or-three plasticators. (See also co-injection)


Naturally balanced runner system

Each succession of runner is identical to the runners in the same succession in all other flows in the mould.

Nest Plate

A retainer plate in the mold with a depressed area for cavity blocks.

Non-Fill (also known as Short Shot)

Failure to completely fill the mould or cavities of the mould. Edges may appear melted.

Non-Return Valve

Screw tip that allows for material to flow in one direction and closes to prevent back flow and inject material into the mould (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.


Hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine designed to form a seal under pressure between the cylinder and the mould.

Nucleating Agent

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.


Odorants & Deodorants

 Odorants are used to add odor to materials, usually for safety reasons.


Not able to transmit light.

Orange Peel

A surface finish on a moulded part that is rough and splotchy. Usually caused by moisture in the mould 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-orientated they are not in alignment. In general, orientated material shrinks more than non-orientated material.



Over Moulding

A process in which a mould cavity is first partially filled with one plastic and then a second shot is injected to encapsulate the first shot.


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 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 mould cavity or cavities as full as possible without causing undue stress on the moulds or causing flash to appear on the finished parts. Over- or under-packing results in less than optimum fill.

Part Picker

An auxiliary unit usually mounted on fixed platen, which reaches into the open mould to grab parts and remove them prior to next moulding cycle. Also called a robot, the device is used when you do not want to drop parts from mould upon ejection.

Partially-balanced runner system

Composed of both naturally and artificially balanced runners.

Parting line

Mark on the part indicating where the two halves of the mould 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 moulding operations by extrusion and chopping into short segments.


Degradation of plastics due to the action of light.

Pigment (also know as Colorant, Color Concentrate)

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.

Pinpoint Gate

A restricted gate of 0.030 in or less in diameter, this gate is common on hot-runner moulds.


See ram.


An imperfection, a small crater in the surface of the plastic.

Plastic Deformation

A change in dimensions of an object under load that is not recovered when the load is removed.


A material that contains as an essential ingredient one or more organic polymeric substances of large molecular weight, 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 mouldable 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 mould halves are attached.


An objectionable coating gradually formed on metal surfaces of moulds during processing of plastics due to extraction and deposition of some ingredient such as pigment, lubricant, stabilizer or plasticizer.


Graphical representations of analysis results.

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.

Polymer Structure

A general term referring to the relative positions, arrangement in space, and freedom of motion of atoms in a polymer molecule.


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.


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 moulding systems in the first step of a two-stage injection moulding and blow moulding process used to produce bottles or containers. The preform is subsequently re-heated and stretch blown through a blow moulding process into the final container shape.

Pressure Pads

Reinforcements of hardened steel distributed around the dead areas in the faces of a mould to help the land absorb the final pressure of closing without collapsing.

Processing Aids

Additives specifically used to improve the injection process.

Proportional Limit

The greatest stress which a material is capable of sustaining without deviation from proportionality of stress and strain. (Hooke’s Law).

Prototype Tool

A preliminary mould built upon which the final mould will be based.

Pulled Gate

Area where the part was connected to the sprue or runner that has been drawn out or stretched from the surface.


In extrusion or injection moulding, 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.


Raised Ejector Site

Where the ejector site is either heightened or raised above the surface of the component.


The forward motion of the screw in the plasticator barrel that forces the melt into the mould cavity.

Reaction Injection Moulding (RIM)

A process that involves the high pressure impingement mixing of two or more reactive liquid components and injecting into a closed mould at low pressure.

Reagent Resistance

The ability of a plastic to withstand exposure to chemicals.

Recovery Time

 The length of time for the screw to rotate, create a shot, and return to original position.


Waste material such as sprues, runners, excess parison material and reject parts from injection moulding, blow moulding and extrusion, which has been reclaimed by shredding or granulating. Regrind is usually mixed with virgin compound at a predetermined percentage for remoulding.

Reinforced Plastic

A plastic composition in which fibrous reinforcements are imbedded, with strength properties greatly superior to those of the base resin.

Release agent

A material sprayed on the mould which facilitates removing the part.

Resin (Synthetic)

The term is use to designate any polymer that is a basic material for plastics.

Restricted Gate

A very small orifice between runner and cavity in an injection mould. 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 mould cavities, ejector pins, guide pins and bushings are mounted during moulding.

Retractable Cores

Used when moulding parts in cavities not perpendicular to the direction in which the part is ejected from the mould. The cores are automatically pulled from the mould prior to the mould opening and reinserted when the mould closes again and prior to injection.


A reinforcing member of a moulded 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.


Automated devices for removing parts upon ejection from an open mould rather than letting the parts drop. Also see parts picker. Robots also can perform secondary functions, such as inspection, degating, precise placement of parts on a conveyor, etc.

Rockwell Hardness

A measure of the surface hardness of a material. A value derived from the increase in depth of an impression as the load of a steel indenter is increased from a fixed minimum value to a higher value and then returned to the minimum value. The values are quoted with a letter prefix corresponding to a scale relating to a given combination of load and indenter.

Runner Balancing

Developing 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 moulding.


In an injection mould, 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.

Runnerless moulding (also known as Hot-Runner Mould or Insulated Runner)

A mould in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner moulds make parts that have no scrap.



Any output of a mould that is not usable as the primary product.

Screw Travel

The distance the screw travels forward when filling the mould 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. Sheet 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 moulding 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 (also known as Non-Fill)

Failure to completely fill the mould or cavities of the mould. Edges may appear melted.

Shot Capacity

Generally based on polystyrene, this is the maximum weight of plastic that can be displaced or injected by a single injection stroke. Generally expressed as ounces of polystyrene.


One complete cycle of a moulding machine.

Shrinkage Allowance

The dimensional allowance which must be made in moulds to compensate for shrinkage of the plastic compound on cooling.


Contraction upon cooling of all or areas of the part. Shrinkage occurs less is disorientated material and more across chains of molecules than along their lengths. Lower pack area have lower areas of orientation and shrinkage.

Side-Draw Pins

Projections used to core a hole in a direction other than the line of closing of a mould and which must be withdrawn before the part is ejected from the mould. See also Retractable Cores.

Sink Mark

An indentation on the surface of the part as a result of significant local change in wall section. The mark will occur in the thicker area.


Projection in the mould used to form the geometry of the part, which is not in the direction of the closing of the mould and must be withdrawn before the part can be ejected.

Slip Agent

Additive used to provide lubrication during and immediately following processing of plastics.


Substances with the ability to dissolve other substances.

Specific Gravity

The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73F).

Specific Volume

The volume of a unit of weight of a material; the reciprocal of density.

Spiral Flow

Test performed by injection moulding a sample into a spiral mould and used to compare the processability of different resins.

Splay Marks

Scan or surface defects on moulded part caused by abnormal racing of the melt in the mould.

Split-Ring Mould

A mould in which a split cavity block is assembled in a channel to permit the forming of undercuts in a moulded piece. These parts are ejected from the mould and then separated from the piece.

Sprue Bushing

A hardened-steel insert in the mould 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 mould 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 mould opens. The sprue lock itself is pushed out of the mould by an ejector pin.


The feed opening provided in injection moulding between the nozzle and cavity or runner system.


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 therefrom.

Stack Moulds

Two or more moulds 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 moulding press to which the front plate of the mould is secured. This platen does not move during normal operation.


The capacity of a material to resist elastic displacement under stress.


In tensile testing, the ratio of the elongation to the gage length of the test specimen, that is, the change in length per unit of original length.

Stress Concentrators

Abrupt changes in geometry of the part serve as the focus of high stresses. Various means can be devised to relieve the abruptness of the geometric changes and thus the stresses.

Stress Cracking

There are three types of stress cracking: 1. Thermal stress cracking is caused by prolonged exposure of the part to elevated temperatures or sunlight. 2. Physical stress cracking occurs between crystalline and amorphous portions of the part when the part is under an internally or externally induced strain. 3. Chemical stress cracking occurs when a liquid or gas permeates the partís surface. All of these types of stress cracking have the same end result: the splitting or fracturing of the moulding.

Stress Relaxation

The decay of stress at a constant strain.


The force producing or tending to produce deformation in a body measured by the force applied per unit area.


External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength. Note: The development of such cracks is frequently accelerated by the environment to which the plastic is exposed.

Stress-Strain Curve

The curve plotting the applied stress on a test specimen versus the corresponding strain. Stress can be applied through shear, compression, flexure, or tension.


Marks evident on the moulded-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 moulded piece from core pins or force plugs. The stripper plate is set into operation by the opening of the mould.

Structural Foam Moulding

The process of moulding thermoplastics articles with a cellular core and integral solid skins in a single operation.


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 sinks marks on the finished part.


Tab Gate

A small removable tab about the same thickness as the moulded item, but usually perpendicular to the part for easy removal.


Additives used to enhance the adhesiveness or bonding ability of a material.

Tensile Modulus

(Also called modulus of elasticity). The ratio of nominal stress to the corresponding strain below the proportional limit of a material.

Tensile Strength, Break

The maximum stress that a material can withstand without breaking when subjected to a stretching load.

Tensile Strength, Yield

The maximum stress that a material can withstand without yielding when subjected to a stretching load.

Tensile Strength

The maximum tensile stress sustained by the specimen during a tension test.

Thermal Conductivity

Ability of a material to conduct heat.

Thermal Degradation

Deterioration of the material by heat, characterized by molecular scission.

Thermal Shut-Off

Material freezes causing blockage.


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 tender it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mould 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.


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 moulding machine. Basically, this measurement limits the size of moulds that can be placed between the tie-bars and into the moulding machine.

Time scan

Effects of mould 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 mould in a press.


The measure by which injection moulding machines are typically categorized, representing the clamping force of the injection moulding machine.


In injection molding, the term sometimes used to describe the mold.

Transfer Moulding

A process of forming articles by fusing a plastic material in a chamber then forcing the whole mass into a hot mould to solidify.

Transition Temperature

The temperature at which a polymer changes from (or to) a viscous or rubbery condition (or from) a hard and relatively brittle one.

Tunnel Gate

See submarine gate.


UL Temperature Index

The maximum temperature below which a material maintains its electrical and mechanical integrity over a reasonable period.

Ultimate Elongation

In a tensile test the elongation at rupture.

Ultimate Strength

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 mould.


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 that the flow fronts meet at the end of filling.

Unidirectional Flow Pattern

Plastic flowing in one direction with a straight flow front throughout filling.

Uniform Cooling Time

Cooling time the same throughout the part to avoid warping.


Vacuum Forming

A process whereby a heated plastic sheet is drawn against a mould surface by evacuating the air between it and the mould.

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

Special barrel unit with a vent port over the compression section of the screw to permit escape of gases prior to injecting melt into mould. Often used when moulding 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 mould. Also 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.


Resistance to flow of a liquid.


An unfilled space within a solid material.



Distortion caused by nonuniform 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 moulding 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 mould is over packed or forced open slightly. Mould-parting-line wear or misalignment can also cause wisps.

Yellowness Index

A measure of the color on the yellow scale.

Yield Point

In 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.