Basics of Injection Molding Design

Boss
Refers to the round protrusions on plastic parts and molds (#2 in Figure 1 below)

Cavity
Refers to the upper half of the injection mold usually the show surface of the finished product but is mainly concave

Core
Refers to the side of the tool where the plastic part is injected from; also known as the bottom half of the tool

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

Draft
Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. Generally all plastic components should be designed with draft where possible

Gate
Refers to where the plastic enters into the cavity of the mold. The two types of gates are as follows:
1. Automatically Trimmed Gates: Gates that incorporate features in the tool to break or shear the gate as the molding tool is opened to eject the part
2. Manually Trimmed Gates: Gates that require an operator to separate parts from runners during a secondary operation

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

Hand Load
Aluminum or steel feature in a mold used to create undercuts in molded parts.  They are manually removed from the mold during the part ejection process.

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

Horn Pin
Pin used to actuate the slide on an automatic injection mold

Line of Draw
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

Ribs
Refers to thin bladed features on a part that are used for strengthening wall sections and bosses. Also, used to minimize warp (#3 in Figure 1 below)

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

Shear
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
The result of a plastic part not filling completely, including some or all of the details

Shrink Rate
Refers to how much the plastic material will shrink after cooled. This % 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"

Side Action
Term used for slides and/or hand pulls used in the injection mold build process

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

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

Sprue
Channel that links the injection molding machine nozzle to the runner

Steel 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

Thin Wall Molding
The molding of plastic parts with wall thicknesses .005" to .060" thick

Undercuts
Refers 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 (#1 in Figure 1 below)

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

Wall Thickness
Refers to how thick the cross section of the plastic part is

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