Injection Molding Glossary

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By Jeremy Losek, Mar 17th, 2023

Plastic Injection Molding Glossary

We are a contract manufacturer for custom plastic injection molding. Our customers come to us with their product designs, and we manufacture the injection mold tooling to make their plastic parts. We work with a wide range of customers, with varying levels of knowledge about plastic injection molding. Our customers range from large, multinational appliance manufacturers, all the way down to inventors with an idea for a new product. In fact, a number of our customers are entrepreneurs with an idea for a new product that’s never been invented before, or for improving an existing product – making a “better mouse trap.” Many have never worked with a plastic injection molder before and this is all new to them.

Since there is such a wide range of prior injection molding knowledge and experience, it is helpful to have a basic understanding of the main terms used in plastic injection molding so we can communicate with our customers about their projects.

The injection molding glossary below is provided to help define the most common terms used in the industry. You can also use the search box at the top of the table to search for a term.




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


The part of the molding press where resin is melted.


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

Bridge Tool 

A temporary low production tool mold. It is built for the purpose of making production parts until a high volume production mold is ready. 


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

Cavity or Die Cavity 

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


Color, Material, Finish 


The B-side of the injection mold tool that create the internal features of a part. 


The complete sequence of operations in a process to complete one set of moldings. 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. 

Design for Manufacturing (DFM) 

This review evaluates all design changes that need to be made and reviews the proposed molding strategy on all parts. 2D drawings are reviewed for characteristics to be controlled in production. CMF (color, material, finish) is finalized along with the production schedule. 


The degree of taper of a mold-cavity sidewall or the angle of clearance designed to facilitate removal of parts from a mold. 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 mold cavity from the rear as the mold opens to force the finished part out of the mold. Also called knockout pins. 

Engineering Change Order (ECO) 

Document which records and approves any changes made to the design, cost, or timing. 

Family Mold 

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

Filled Resins 

Filled resins combine materials such as glass and carbon with either thermoplastic or thermosetting polymers (most commonly nylon). These materials often have improved mechanical properties compared to unfilled. 


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

Flow Line 

Marks visible on the finished items that indicate where plastic flow occurs. 

Flow Marks 

Wavy surface appearances on a molded part caused by improper flow of the melt into the mold. 

Gas-assisted Molding 

A small amount of gas is blown into the mold during cooling to apply pressure. Typically used to mitigate the formation of sink marks. 


An orifice through which the molten plastic enters the mold 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 Load 

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


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

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. 

Insert Molding 

Insert molding is the process of molding 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. 

Mold Frame 

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

Moldflow or Moldflow Analysis 

Simulation software for plastic injection molding used in computer-aided engineering. 

Multi-Cavity Mold 

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

Over Molding or Multi-Shot 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. 


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

Parting Line 

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


The base plate on an injection molding machine onto which the mold plates are attached. 


The molecular designation of long repeated linked chains, which includes resins used in injection molding. 

Product Requirements Document (PRD) 

The PRD is an initial project document that defines the purpose and scope of the project and breaks it down into features. It also determines a timeline and identifies stakeholders. 

Production Approval 

Production approval is necessary to move from first articles to production parts. 


The forcing one molding material out of the nozzle tip with another material prior to molding a new material for cleansing purposes. Special purging compounds are used. 


Resin is the material being injected into the mold, and is the thermoplastic material that the final part will be made from. Examples of available resins can be found here: 


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 molten plastic to the cavities. 


Refers to when plastic enters into the mold 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 mold or cavities of the mold. Edges may appear melted. 

Shrink Rate 

The dimensional differences between a molded part and the actual mold dimensions. 

Single-Cavity Mold 

A mold 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 molds that is used for creating undercuts. Required for automatic injection molds 

Slide Action 

A sliding mechanism in the mold designed for the molding 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. 


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

Steel (Metal) Safe 

Part design change will only require to remove metal from mold instead of “weld-add” metal on the mold. 

Straight Pull 

No side actions. 

T0, T1, T2… Shots 

Production shoots the first parts and samples are sent to client for review and checked for key dimensions. Any necessary changes are recorded and corrected. 


Thermoplastics are plastics that become moldable at elevated temperatures, and re-solidify upon cooling. 


A feature or indentation that impedes withdrawal from a two-piece rigid mold. 


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


A mark created by trimming the injection molded part gate. 

Wall Thickness 

The thickness of the cross section of the plastic part. 


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


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. 

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