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Plastic Injection Moulding Jargon Buster

There is a wide range of injection moulding knowledge and experience that often comes with its own language. Therefore, we feel it is helpful to have a basic understanding of the main terms used in plastic injection moulding.


Below is our jargon buster, which explains some of the most commonly used words and phrases.


We hope you find it useful:


Injection Moulding Term.



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



The part of the moulding press where resin is melted.


Bridge Tool

A temporary low production tool mould. It is made for the purpose of making production parts until a high-volume production mould is ready.



The round protrusions on plastic parts and moulds, often designed for fasteners.



Air pockets that have formed in the material of the component. Bubbles may vary in size.



Concave in mould, which usually forms the outer surface of the moulded part; depending on number of such depressions, moulds are designated as a single cavity or multi-cavity.



The pressure applied to the mould to keep it closed during a cycle, usually expressed in tons.



The complete sequence of operations in a process to complete one set of mouldings. The cycle is taken at a point in the operation and ends when this point is again reached and moving platens of the clamp unit in the fully open position.



The degree of taper of a mould-cavity sidewall or the angle of clearance designed to facilitate removal of parts from a mould. Generally, all plastic components should be designed with draft where possible.


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



Electric Discharge Machining. Sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges. Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage.


Ejector Pins

Pins that are pushed into a mould cavity from the rear as the mould opens to force the finished part out of the mould. Also called knockout pins.


Family Mould

A multi-cavity mould where each of the cavities forms one of the component parts of an assembled finished part.



Any excess material that is formed with and attached to the component along a seam or mould parting line.


Flow Line

Marks visible on the finished items that indicate the direction of the flow of the melt into the mould.


Flow Marks

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



An orifice through which the melt enters the mould cavity.


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.


Hand Loads

For low production mould with undercut, steel fixture which create the undercut are manually removed from the mould during the part ejection process.



Refers to the portion of an automatic custom injection mould that keeps the slide in the forward position when the moulding machine is closed on the mould.


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.


Insert Moulding

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



Turbulent flow in the melt caused by an undersized gate or where a thin section rapidly becomes thicker.


Knit Lines

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


Living Hinge

A thin flexible hinge made from the same material as the two rigid pieces it connects. It is typically thinned to allow the rigid pieces to bend along the line of the hinge to allow them to open and close. They require careful design and gate placement. A typical application would be the top and bottom of a box.


Mould Frame

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


Multi Cavity Mould

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


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.



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-packing or under-packing results in less than optimum fill.


Parting Line

On a finished part, this line shows where the two mould halves met when they were closed.



The forcing one moulding material out of the plasticiser with another material prior to moulding a new material. Special purging compounds are used.



A thin support features (usually triangle shape) on a part that are used for strengthening wall sections and bosses.



The channel that connects the sprue with the gate for transferring the melt to the cavities.



Refers to when plastic enters into the mould and the melt is maintained by friction produced by speed and pressure. Too much shear can cause the plastic material to burn, too little can cause the material to freeze off causing short shot.


Short Shot

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


Shrink Rate

The dimensional differences between a moulded part and the actual mould dimensions.


Slide Action

A sliding mechanism in the mould designed for the moulding of parts with undercuts. The undercut-steel fixture is held in place during the injection process and then slides out of the way prior to ejection.


Single Cavity Mould

A mould having only one cavity and producing only one finished part per cycle.


Sink Marks

A shallow depression or dimple on the surface of a finished part created by shrinkage or low fill of the cavity.



Area of the custom plastic injection moulds that is used for creating undercuts. Required for automatic injection moulds



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


Steel (Metal) Safe

Part design change will only require when removing metal from mould instead of “weld-add” metal on the mould.


Straight Pull

No side actions.



A shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity.



A protuberance or indentation that impedes withdrawal from a two-piece rigid mould.



A mark created by trimming the injection moulded part gate.


Wall Thickness

The thickness of the cross section of the plastic part.



Dimensional distortion in a moulded object. Caused by internal stresses via un-even material flow, cooling, and compression



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.


Added: 24 Apr 2018 10:14

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