Plastic Injection 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 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.
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.
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.
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.
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.
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.