Plastic Injection Molding Glossary

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.

Injection Molding TermDefinition
AgingThe change of a material with time under defined environmental conditions, leading to improvement or deterioration of properties.
BarrelThe part of the molding press where resin is melted.
Bridge ToolA temporary low production tool mold. It is made for the purpose of making production parts until a high volume production mold is ready.
BossThe round protrusions on plastic parts and molds, often designed for fasteners.
BubblesAir pockets that have formed in the material of the component. Bubbles may vary in size.
CavityConcave in mold, which usually forms the outer surface of the molded part; depending on number of such depressions, molds are designated as a single cavity or multi-cavity.
CoreThe pressure applied to the mold to keep it closed during a cycle, usually expressed in tons.
CycleThe 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.
DraftThe 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.
DurometerThe hardness of a material as measured by the Shore Durometer.
EDMElectric 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 PinsPins 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.
Family MoldA multi-cavity mold where each of the cavities forms one of the component parts of an assembled finished part.
FlashAny excess material that is formed with and attached to the component along a seam or mold parting line.
Flow LineMarks visible on the finished items that indicate the direction of the flow of the melt into the mold.
Flow MarksWavy surface appearances on a molded part caused by improper flow of the melt into the mold.
GateAn orifice through which the melt enters the mold cavity.
Gate TrimRemnant 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 LoadFor low production mold with undercut, steel fixture which create the undercut are manually removed from the mold during the part ejection process.
HeelRefers 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 MoldA 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 MoldingInsert molding is the process of molding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials.
JettingTurbulent flow in the melt caused by an undersized gate or where a thin section rapidly becomes thicker.
Knit LinesWhere melted material flows together to form a line or lines that may cause weakening or breaking of the component.
Living HingeA 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 FrameA series of steel plates which contain mold components, including cavities, cores, runner system, cooling system, ejection system, etc.
Multi-Cavity MoldA mold having two or more impressions for forming finished items in one machine cycle.
Over MoldingA 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.
PackingThe 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 LineOn a finished part, this line shows where the two mold halves met when they were closed.
PurgingThe forcing one molding material out of the plasticator with another material prior to molding a new material. Special purging compounds are used.
RibsA thin support features (usually triangle shape) on a part that are used for strengthening wall sections and bosses.
RunnerThe channel that connects the sprue with the gate for transferring the melt to the cavities.
ShearRefers 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 ShotFailure to completely fill the mold or cavities of the mold. Edges may appear melted.
Shrink RateThe dimensional differences between a molded part and the actual mold dimensions.
Slide ActionA 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.
Single-Cavity MoldA mold having only one cavity and producing only one finished part per cycle.
Sink MarksA shallow depression or dimple on the surface of a finished part created by shrinkage or low fill of the cavity.
SlideArea of the custom plastic injection molds that is used for creating undercuts. Required for automatic injection molds
SprueThe feed opening provided in injection molding between the nozzle and cavity or runner system.
Steel (Metal) SafePart design change will only require to remove metal from mold instead of “weld-add” metal on the mold.
Straight PullNo side actions.
VentA shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity.
UndercutsA protuberance or indentation that impedes withdrawal from a two-piece rigid mold.
VestigeA mark created by trimming the injection molded part gate.
Wall ThicknessThe thickness of the cross section of the plastic part.
WarpDimensional distortion in a molded object. Caused by internal stresses via un-even material flow, cooling, and compression
Weld-LineWhere 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.