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Plastic Injection Molding Terminology

These terms are used in the injection molding process. Use these and our design guidelines to help produce a quality injection moldable design.



Shrink Rate





Short Shot


Hand Pull



Cam/Horn Pin

Ejector Pins




Core Outs

Line Of Draw

Side Action

Piece Price




Wall Thickness

Thin Wall Molding



Sink Marks




Stock Safe

Bulk Pack

Reverse Engineering


Part  Refers to customer’s custom plastic part to be made.

Tooling  Refers to the injection mold that is created to make plastic parts in the molding process

Shrink Rate  Refers to how much the plastic material will shrink after cooled. This percent of shrink is added to the part before the mold is designed. Every plastic material has its own shrink rate ranging from .001 per inch to as much as .060 per inch. Although most fall in between .004 and .021. It is important when selecting a range of plastic materials that you may want to test before building the injection mold. For example if you wanted to use ABS which is .006 per inch and Nylon which is .015 per inch you may want to build the injection mold to .010 or leave critical dimensions stock safe until a final material is selected.

Gate  Referring to where the plastic enters into the cavity of the mold. Types of gates include; sub, edge, fan, cashew and XME exclusive vestige free edge for perfect shear

Vestige  Material or witness of material protruding from gate area after gate runner has been removed from the injection molded part. This vestige is usually trimmed by the molding machine operator.

Shear  referred 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.

Runner  A channel cut into custom injection molds, in which plastic travels from the injection molding machine, through the sprue, through the runner and then through the gate ultimately filling the part.

Short Shot  Is the result of a plastic part not filling completely, including some or all of the details.

Sprue  Is perpendicular to the runner and is the channel that links the injection molding machine nozzle to the runner.

Hand Pull  Portion of the custom injection mold that is used for creating undercuts in plastic parts.

Slide  Portion of custom plastic injection molds that is used for creating undercuts. Required for automatic injection molds.

Gibbs  Portion of the custom injection mold that holds the slide down so the cam can actuate it.

CAM/HORN PIN  The cam commonly referred to as Horn pin is used to actuate the slide on an automatic injection mold.

Ejector Pins  Ejector Pins are used to push the part off the core half of the injection molds.

Undercuts  Referring to the portion of the designed component where a slide or hand pull is required to create holes, windows or clips that are not in the line of draw.

Core  Refers to side of the tool where the plastic part will stick to and is ejected from, also known as bottom half of the tool.

Cavity  Refers to the upper half of the injection mold usually the show surface of the finished product but is mainly concave. There is not as much standing core outs on this side of the tool. Therefore the part will generally not stick to it when the injection molding machine opens the mold.

Core Outs  Refers to the portion of a part that is gutted out in order to achieve uniform wall thickness. This portion of the part has no end use function other than lightening the part and reducing warp.

Line of draw  Line of draw is the direction in which the two custom injection mold halves will separate from the plastic part allowing it to be ejected without any obstructions from metal creating undercuts.

Side Action  Is the term used for slides and/or hand pulls used in the injection mold build process. If your design requires side action it will require a slide or hand pull to make the plastic part in order to run in the molding process.

Piece Price  Refers to the cost of each plastic part as produced in the injection molding process.

SLA  Stereo Lithography Apparatus. Using 3D printing technologies such as laser sintering and electron beam melting, “rapid prototyping” evolved into “rapid molding,” in which short run prototypes of actual finished molding components are made.

FDM  Fused Deposition Modeling. Using three dimensional printing technology where prototypes are made by fusing layers of actual ABS plastic together. Much like an inkjet printer except it lays Plastic instead of ink.

Operator  A person used to run a plastic injection molding machine. Mold may be a manual tool or product may require stacking so they do not fall on each other when ejected from the injection molding machine.

Wall Thickness  This refers to how thick the cross section of the plastic part is.

Thin Wall Molding  Thin wall molding is the molding of plastic parts with wall thicknesses .005 to .060thick.

Boss  This refers to round protrusions on plastic parts and molds.

Ribs  Refers to thin bladed features on a part that are used for strengthening wall sections and bosses. Also, used to minimize warp.

Sink Marks  Refers to areas of the molded part where it seems to be sunk in, due to un-uniformed wall sections, thick wall sections and rib/boss to thickness ratios being off.

Warp  Refers to area of a injection molded part that distorts during cooling or molding, causing undesired results in the finished product. Usually caused by un-uniform wall sections.

Draft  Refers to portion of injection molding part that has some sort of taper to it. Generally all plastic components should be designed with draft where possible.

Heel  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.

Stock Safe  Refers to the amount of metal left on the mold in order to tweak in a dimension. For example, if you have an inside diameter that is supposed to be .500 you may leave the mold at .505 in case you get excessive shrink. It is cheaper to remove metal than it is to add it.

Bulk Pack  Refers to shipping plastic parts in a box without any form of stacking. At the injection molding machine Parts will be dumped in a box and shipped.

Reverse Engineering  Refers to the process of taking an existing plastic part and creating a 3D solid model for duplicating in the injection molding process. Sometimes the design may include some custom changes from the OEM.[/vc_column_text][/vc_column][/vc_row]