Glossary of Plastic Injection Molding Terms

Mold (Tool)

The entire assembly that gives shape to molten plastic. Called a “tool” in some regions. The core asset in injection molding.

Two-Plate Mold

The simplest and most common mold design. It has one parting line, with the runner system and part(s) on the same plane.

Three-Plate Mold

A more complex mold with two parting lines. One opens to eject the runner system, and a second opens to eject the part. Allows for gating away from the part edge.

Hot Runner Mold (Runnerless Mold)

A system where the runners are kept hot and molten inside a manifold block. No solid runner is ejected, reducing waste and cycle time.

Cold Runner Mold

A mold where the entire runner system cools and is ejected with the part. Includes Two-Plate and Three-Plate molds.

Family Mold

A single mold that produces multiple different parts that form a product assembly.

Insert Mold

A mold designed to form plastic around a pre-placed metal or other component (an “insert”).

Prototype Mold

A mold, often made from aluminum or mild steel, built for speed and low cost to produce prototype parts. Shorter lifespan than production molds.

Production Mold

A mold built from hardened steel (e.g., H13, S7) for high-volume, long-term production. High cost and long lead time.

Mold Base

A standardized, pre-manufactured frame that holds the custom-machined core, cavity, and all other components.

Cavity Plate (“A” Plate)

The half of the mold usually attached to the stationary platen of the injection machine. It contains the Cavity.

Core Plate (“B” Plate)

The half of the mold attached to the moving platen. It contains the Core and the ejection system.

Support Plate (Backing Plate)

A thick plate that sits behind the “B” plate to prevent it from deforming under high injection pressure.

Sprue Bushing (Sprue Bushing)

A hardened steel component with a tapered hole that connects the mold to the injection machine’s nozzle.

Leader Pins (Guide Pins)

Precision pins that ensure the correct alignment of the “A” and “B” halves as the mold closes.

Leader Pin Bushings

The hardened sleeves into which the leader pins slide.

Cavity

The hollow portion of the mold that forms the external shape of the plastic part.

Core

The male portion of the mold that forms the internal shape of the part (e.g., the inside of a bucket).

Parting Line (P.L.)

The line or plane where the two halves of the mold meet. Often visible as a faint witness line on the finished part.

Shut-off

A raised land or sealing surface where two mold components meet tightly to prevent plastic from leaking into another area. Critical for holes and complex features.

Draft

A slight taper (typically 1°-3°) applied to walls parallel to the mold opening direction. Essential for the part to eject cleanly.

Texture (Mold Texture)

A surface finish applied to the mold cavity (e.g., via EDM, laser etching, chemical etching) to produce a desired appearance on the part (e.g., leather, matte, gloss).

Polishing

The process of manually finishing the cavity/core surface to a specific gloss level (e.g., SPI A1 mirror finish) for part appearance and release.

Sprue

The primary channel that delivers molten plastic from the machine nozzle to the mold.

Runner

The channel system that distributes plastic from the sprue to the various part cavities.

Gate

The small, controlled entrance from the runner into the part cavity. Its design and location are critical.

Cold Slug Well

A cavity at the end of the sprue to trap the initial, slightly cooled shot of plastic (“cold slug”) before it enters the part.

Manifold (Hot Runner)

A heated block containing internal channels that distribute molten plastic to the nozzles.

Hot Nozzle (Hot Runner Nozzle)

The heated tip that directly gates into the part cavity in a hot runner system.

Gate Vestige

A small, often raised mark left on the part after the gate is severed.

Ejector Housing

The assembly of plates (ejector plates) that hold the ejector pins and are pushed forward by the machine’s ejector rod.

Ejector Pins

The most common ejection component. Steel pins that push the part off the core.

Ejector Sleeves

Hollow pins used to eject features that surround a core pin (e.g., a boss around a hole).

Blade Ejectors

Thin, blade-like pins used to eject ribs or other thin features.

Stripper Plate

A plate that encircles the core, used to eject large, box-like parts or parts that cannot have pin marks.

Return Pins (Ejector Return Pins)

Pins that ensure the ejection system is pushed back to its starting position as the mold closes.

Early Ejector Return (Screw)

A screw or pin that mechanically ensures the ejector system is retracted before the mold closes completely.

Undercut

Any part feature that prevents its direct ejection from the mold, requiring a special mechanism.

Lifter

An angled component that moves with the ejection system to form and then release an internal undercut.

Slide (Cambered Slide)

A component that moves perpendicular to the mold opening direction to form an external undercut. It is driven by an angle pin as the mold opens.

Hydraulic Cylinder (for Slides)

A hydraulic actuator used to power complex or long-stroke slides instead of an angle pin.

Insert

A removable piece of steel that forms a specific, often high-wear, detail in the mold. Allows for easy replacement.

Core Pin

A small pin used to form holes or standing features (bosses) in the part.

Cooling Channels (Water Lines)

Passages drilled through the mold plates to circulate a coolant (water or oil) and control the mold temperature.

Baffle

A deflector inserted into a deep cooling channel to direct water flow for more efficient cooling.

Bubbler

A tube that directs coolant deep into a core pin to cool an area that is difficult to reach.

Heat Exchanger

A device used to control the temperature of the coolant.

Vent

A very shallow channel (0.015-0.03 mm) cut into the mold to allow trapped air to escape during injection. Prevents burns and short shots.

Mold Flow Analysis

Computer simulation used to predict how plastic will fill the mold, allowing for optimization of gating, cooling, and the prevention of defects.

Shot

One complete cycle of the injection molding machine, from clamp close to part ejection.

Tool Trial / Sampling

The initial test run of the mold in an injection molding machine to debug it and produce first samples (T1 Samples).

Mold Maintenance

The regular process of cleaning, inspecting, and repairing the mold to ensure consistent part quality and long tool life.