Injection Moulding Glossary


Abrasion Resistance

The ability of materials and structures to withstand abrasion.


A substance compounded into a resin to enhance or improve certain characteristics.


The process of, or the results of, exposure of plastics to natural or artificial environmental conditions for a prolonged period of time.


The act of applying or putting to use. A parts application is the job it will do in its finished form.



When molten plastic flows back out of the mould, returning to the runners.

Backing Plate

A plate that supports the mould cavity block.


A raised feature on a moulded part. A boss can help to add strength or enable attachment to another part.


Air pockets that can form in materials during production.



A cut out or depression within a mould into which molten plastic is fed, forming the part. The moulded part is then released from the mould cavity.


Under the influence of stress, materials can deform to varying degrees. The rate of this deformation is known as creep, and the levels differ depending on the materials individual structure and the type of external stress applied i.e. high/cold temperatures, weathering, light, abrasion.

The time it takes for an injection moulding machine to make a part and return to its original position.



A detrimental change to the chemical or mechanical properties, or the physical appearance of a part or material usually caused by a form of external stress.


Plastic density measures mass per unit volume of a material.


A change from the original colour of the part or material.


The process of removing moisture from virgin material pellets prior to moulding.


A device  used to measure the hardness of a part or material, known as shore hardness

(see also Shore Hardness)


Ejector Pins

Pins or rods that push a moulded part out of the mould as it opens. Certain materials, specifically harder ones such as nylon, often need assistance to release themselves from the mould. Ejector pins are one of the key considerations at the design stage of a tool making.

Elastic Memory

The ability of a material to return to its original shape/form.


The ability of a material to recover its dimensional form quickly after deformation from an external load.

Engineering Plastics

Engineering plastics are a group of plastics that offer better mechanical properties than widely used commodity plastics.


Fatigue Strength

Fatigue strength is the highest amount of stress a material can withstand for a given number of cycles before breakage or failure.

Fill Pressure

The pressure needed to fill the mould cavity with molten plastic.


The appearance and texture of the surface of a finished part.

Flame Retardant

The ability to resist combustion. A part which is flame retardant will not continue to burn after the ignition source has been removed.


Excess material that forms along a seam or parting line during the moulding process. This is usually trimmed off during the finishing stage of production.

Flow Rate

The volume of molten material passing through a fixed point of the injection moulding machine, per unit of time.



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



The Hopper is where the plastic material is poured into the machine before the injection moulding process can begin. The Hopper usually contains a dryer unit to keep moisture away from the plastic material.

Impact Resistance

The resistance of a material or part to breaking or failing when stress is  applied at high speeds

Impact Strength

A materials ability to withstand extreme load levels.


A removable part of a mould. This can provide a change of part shape to the specific area of the mould or offer increased wear resistance.


A moulding defect that can occur during the manufacturing process. Molten plastic fails to stick to the mould cavity surface due to the speed of the injection. This occurs when the melt temperature is too low and the molten plastic’s viscosity becomes too high, increasing the flow resistance into the mould cavity.  When the plastic solidifies, wavy folds of the jet stream show on the surface of the moulded part.


Knit Line

Occurs where two streams of material meet and are unable to knit/weld together.

Knockout Pin

(See Ejector Pins)


Low Temperature Flexibility

The ability of a material or part to resist fracture or failure at reduced temperatures.


External lubricants aid the release of materials from metals and moulds. Internal lubricants aid the flow of the material, without compromising the material  properties.



A concentrated mixture of pigments or additives added to virgin materials, and used to colour or enhance the performance of the material.

Material Safety Data Sheets

Safety Data Sheets contain information regarding the toxicity and hazards associated with a material.  These are required by the UK REACH regulation and allow risk assessments to be completed as per the Control of Substances Hazardous to Health (COSHH) regulations.

Mechanical Property

Aspects of the mechanical performance of a material including abrasion resistance, creep, strength, stiffness, hardness, elasticity and impact/impingement resistance.

Mould (see also Tool)

A hollow container used to give shape to molten material when it cools and hardens. Moulds are usually manufactured form steel or aluminium.

Moulding Sprue

A moulding sprue is created where molten plastic is fed into a mould cavity. The sprue can be removed automatically (if the design of the tool allows) or manually once moulded.

Multi cavity moulds

Multi cavity moulds allow for multiple parts to be produced from the same mould at the same time, allowing for higher volume, faster production rates.



The nozzle is the front part of the screw and barrel of an injection moulding machine.


Over Moulding

A multi-step process where two or more components are moulded over the top of each other. Typically a mould is partially filled with one plastic, then a second shot of a different material encapsulates the first shot.


Raw virgin material comes in the form of small plastic pellets. These are also known as nurdles. These are heated until they become molten, fed into moulds, then cooled and finished to make to the final part. Colour and additive masterbatches also start in pellet form.


A group of synthetic materials made from polymers that can be moulded while soft into required shapes and then set into forms of varying hardness.


A substance composed of repeating chains of molecules bonded together. Different materials offer different properties depending on the type of molecule and how they are bonded together.


A thermoplastic polymer in which polymer units are linked by urethane groups. A highly versatile material, polyurethane is widely used in a broad range of applications.


An early sample or model of a part or product from which later versions are formed. Built for initial testing, prototypes enable customers to check designs before committing to final tool manufacture.


The cleaning of one type of material/colour from an injection moulding machine by forcing it out with the next material/colour or a compatible purging material.


Reciprocating Screw

In injection moulding, material flows from the hopper onto a reciprocating screw. The screw, using its mechanical energy and auxiliary heaters, converts the material into a molten state. When there is a sufficient amount, the screw moves forward forcing the molten material into the mould cavity where the part is formed. This ais also known as a ram.


Regrind material has been processed at least once before. Created from excess material from previous production runs e.g. moulding sprues/flash/scrap, this material is ground or chopped and can be used for other production runs. It is a form of recycling.

Runner System

A term sometimes used for the entire injection moulding machine process including sprues, runners, and gates.

Shore Hardness

Shore Hardness measures the resistance a material has to indentation. The hardness of a material is measured used a device called a durometer. The device measures the depth of an indentation created in the material created by a given force on a standardised presser foot. The result is rated on scale known as a Shore Hardness scale or Durometer scale. A higher number on the scale  indicates greater resistance, so therefore a harder material. Softer material s are lower on the scale.

Whilst there are a several scales used for different materials, in engineering plastics, Shore A and Shore D are most commonly used.

The Shore A scale is usually used for softer, more flexible plastics, and the Shore D scale is predominantly used for harder materials


Refers to the volume of material injected into the mould cavity to form the part.

Short Shot

When the volume of material injected into the mould cavity falls short and does fully occupy the cavity, resulting in an incomplete part.


The contraction of a moulded component as its cools after going through the injection moulding process.

Sink Mark

Sink marks appear as indentations or depressions on a part usually resulting from shrinkage during the cooling period. Shrinkage causes the surface of the outer wall to be pulled inwards causing a sink mark.

Split Line

Where sections of a mould join together, there can be a fine line visible on the surface of the moulded part.


A measure of a materials resistance to bending upon the application of a force.


Tensile Strength

The maximum stress that a material can withstand while being stretched/pulled before breaking/failing.


A polymer material that becomes mouldable at elevated temperatures and then solidifies on cooling. Thermoplastic materials can be reheated, re moulded and cooled as required without causing chemical changes within the material.


A polymer that is formed by irreversibly hardening a soft or liquid prepolymer. Thermoset materials strengthen when heated, but cannot be reheated or remoulded due to the chemical changes that occur in the material upon initial heating.

Tool (see also Mould)

A term used to describe a mould. A hollow container used to give shape to molten material when it cools and hardens. Tools are usually manufactured form steel or aluminium.


Ultimate Elongation

The percentage change in length of a specimen from its original size to the point of rupture.

Ultimate Strength (see also Tensile Strength)

The maximum stress that a material can withstand while being stretched/pulled before breaking/failing.

UV Resistance



A shallow cut in the mould cavity to allow the release of gas/air as the molten plastic enters the cavity.

Virgin Material

A material in its original state before it has been processed. Virgin material is usually supplied in pellet form (see also Pellets).


The measure of a liquid’s resistance to flow.


An empty, unfilled space within a solid material. A void in injection moulding usually occurs during the cooling stage. As the centre cools slowly, the polymer can shrink more, pulling away from itself to create a void or bubble (see also Bubbles).



Deformation that occurs when there is uneven shrinkage in different parts of the moulded component.

Water Absorption

The amount of water absorbed by a material. Factors that affect water absorption can include type of plastic, temperature, length of exposure, additives used.

 Weld Line

(See Knit Line)


Yield Strength

The stress at which a specific amount of plastic deformation occurs. Yield strength is often used to determine the maximum load for a mechanical component, before failure would occur.

Youngs Modulus

Essentially the stiffness of a material, Youngs Modulus measures how easily a material can be bent or stretched.

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