Tooling & Injection Molding Glossary

Learn the terms used in the moldmaking and injection molding industry and their definitions in our glossary.


Blush A cosmetic defect caused by shear through the gate.

Bridge Tool An injection mold tool used to supply parts while the production tool is being built.

Core Pins/Inserts Pins or protrusions in the tool that are inserted to produce critical dimensions or features.

Draft The angle or taper provided on molding sections to facilitate the removal of the molded piece part from the mold.

Ejector Pins Steel pins strategically placed in the mold used to push or eject the finished piece part from the mold.

Family Tool One mold that shares two or more different parts in the core and cavity for injection molding. Click here to see a 1+1 Family Tool.

Fillet - is a rounding of an interior or exterior corner of a part design.

Flash A thin fin of material which forms at the parting line of the mold.

Flow Lines Flaws or visual blemishes that indicate material flow direction in the mold.

Galling A mold defect when the metal mold material creates friction upon itself from rubbing.

Gate An opening in the mold where the injection-molded material enters the mold to create the intended plastic part.

Hand Pick Out This is a manual slide. A hand pick out is used to produce details such as undercuts and recesses on low-volume runs to reduce overall tooling cost. Click here to see a video of hand pick outs being used in complex injection molding.

Hot Gate / Hot Tip This gate allows material to flow directly into the part with no runner. It does not need to be trimmed. This gate is usually used for multi-cavity tools with high-volume runs. There is a heater wrapped around the gate to keep the material fluid so it can fill a cavity. In a hot tip, the material is shot through a gate opening between .040. and .120.

Leader Pins Pins located in one half of the mold assembly, mated with bushings in the other half and used to align the two portions of the mold base.

Low-Volume Production Tool A tool that is built for volumes that do not require the additional costs or lead times of production tooling.

Low-Volume Injection Molding (LVIM) Creating functional parts from plastic in volumes in upwards to 40,000+. LVIM produces parts with quality and accuracy produced by normal production tooling, but with shorter lead times.

M.U.D. Set (Master Unit Die) Inserts which are quickly changed over by using a standard frame.

Nit Lines / Weld Lines Flaws or visible blemishes where two material flow fronts meet.

Overmolding (Insert Molding) The act of taking an object and placing it into a cavity of a mold and injection molding around this object to create a multiple material product. Click here for example of overmolding.

Parting Line A line or mark on the piece part indicating where the two halves of the mold meet.

Radii / Radius Rounded edges or surfaces that allow plastic parts to be created in the injection mold.

Rapid Tool An injection mold that is built quickly and cost effectively for low quantities. These spec material parts are used for functional testing and low volume production.

Runner A groove or channel cut into either one or both halves of the mold which carries the molding material to the cavities.

Shrink The action of the material fluctuating when it is heated up and cooled down into the mold.

Sink A shallow depression defect caused by thick wall sections. The material retains heat and shrinks more than normal due to irregular wall thickness. Click here for example of sink in injection molding.

Slides A portion of the mold which is made to travel at an angle to the normal movement of the molding machine, to produce recesses, undercuts and holes. Click here for example of slides in an injection mold.

Soft Tooling Usually a term meant for RTV Molding, but also used for aluminum tooling for plastic injection molding compared to steel. Although, aluminum has been proven as a reliable mold material for production injection molding.

Splay / Shear A cosmetic defect caused by moisture in the resin or injection molding machine screw rotation or the movement of material through a curved area.

Sprue The round, tapered material, which connects the loading chamber or nozzle with the runner system or the piece part.

Sprue Gate This gate goes directly into the part with no runner.

Tab Gate A gate that is directly off the runner into the part. A tab gate needs to be trimmed in a second operation after the plastic part is ejected.

Tunnel Gate A gate that allows the material to flow into the plastic part. But when the part is ejected, the opening mold shears off the gate. (No second operations).

Undercut A recess or depression in a molded article, located in such a position that it impedes or makes impossible the ejection of the part by the normal opening of the molding machine.

Vent A shallow groove or slot machined into the parting line of the mold which allows air and gases to escape from the cavity as it is being filled with the molding material.

Warping Dimensional distortion in a molded part caused by material shrinking in different ways.