Glossary (A-Z)


The mold half that mounts to the fixed side of the molding press. Sometimes referred to as the stationary or cavity side of the mold, the A-side does not contain ejector pins and therefore it generally creates the visual side of the part.

Automatic Mold

A mold that repeatedly goes through the entire molding cycle, including ejection, without operator assistance.


Composite material made by blending polymers or copolymers with other polymers or elastomers under selected conditions.


The mold half that mounts to the moving side of the molding press. Sometimes referred to as the movable or core side of the mold, the B-side houses the ejector pins that push the part out of the mold.


The part of the injection molding press where resin is melted prior to injection.


Visual blemish at the point of injection in the finished part.


A cylindrical protrusion within a part that is often designed to improve strength, aid in alignment or accept fasteners.

Bridge Tool

A temporary mold created to make production parts until a high-volume production mold is ready. The benefits of bridge tooling include: faster times to market, revisions from market feedback before final tool, and generation of revenue before investing in long-term tooling.


Acronym for “computer aided design.”

Carbon Black

A black pigment produced by the incomplete burning of natural gas or oil. It is widely used as a filler. Because it possesses useful ultraviolet (UV) protective properties, it is also often used in molding compounds intended for outside weathering applications.


A concave feature within the mold into which an opposing core enters when the mold is closed. Resin is injected into the void between the cavity and core to form the part.

Center Gated Mole

An injection mold in which the cavity is filled through a sprue or gate directly into the center of the part.


A beveled edge connecting two surfaces.

Clamp Force

The force required to hold the mold shut and prevent resin from escaping resin during injection.

Clamping Force

The pressure applied to the mold to keep it closed in opposition to the injection pressure of the molding material within the mold cavity (cavities) and the runner system.

Cold Slug

The first material to enter an injection mold. Called a “cold slug” because, in passing through the sprue orifice, it is cooled below the effective molding temperature.

Cold Slug Well

Space provided directly opposite the sprue opening in an injection mold to trap the cold slug.

Contoured Pins

Ejector pins with ends shaped to match a surface contour on the part.

Cooling Channels

Channels or passageways located within the body of a mold or inserted through which a cooling medium is circulated to control the temperature of the mold.


A convex feature within the mold that will enter an opposing cavity when the mold is closed. The void between the cavity and core is where the resin is injected and the part is formed.


The design of a mold in which the A-side forms the exterior of the part and the B-side forms the interior. The advantage to this approach is that the part will shrink onto the B-side where it can be ejected. If the inside and outside are drafted with equal and opposite draft, then the wall thickness will be uniform.


Removal of excess material from the cross section of a molded part to achieve a more uniform wall thickness.


Fine cracks which may extend in a network on or under the surface, or through a layer of plastic material.


The dimensional change with time of a material under load, following the initial instantaneous elastic deformation.

Cycle Time

The period, or elapsed time, between a certain point in one cycle and the same point in the next.


Weight per unit volume of a substance. Expressed in grams per cubic centimeter, pounds per cubic foot, etc.

Diaphragm Gate

Gate used in molding annular or tubular articles. The gate forms a solid web across the opening of the part.

Direction of Pull

Refers to the motion of a part surface relative to a mold.


The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mold.

Drying of Plastics

Many plastics are hygroscopic (absorb water), and need to be dried prior to injection molding to ensure good cosmetics and material characteristics.


The hardness of a material as measured by the Shore Durometer. It is measured on a numeric scale with numbers ranging from lower (i.e. softer) to higher (i.e. harder).

Edge Gate

An injection gate on the parting line of the mold. It typically leaves a vestige on the outside of the part and is sometimes referred to as a tab gate. Edge gates often need to manually trimmed.


The process of pushing a finished part out of a mold.

Ejector Pin

Also called an ejector sleeve; This is a blade, pin or sleeve which pushes a finished part off of a core or out of a cavity of a mold. It is attached to an ejector plate which can be actuated by the ejector rod(s) of the press, or by auxiliary hydraulic or air cylinders.


That property of a material by virtue of which it tends to return to its original size and shape.

Electric Discharge Machining (EDM)

A metal working process applicable to mold construction in which controlled sparking is used to erode away the work piece.

Family Mold

A mold capable of producing two or more different parts.

Fan Gate

A shallow gate somewhat wider than the runner from which it extends.


This term usually refers to thin fibers of glass which are used to reinforce materials. One-inch long fibers are occasionally used, but the more commonly used fiber lengths are 1/2″ and 1/4″, or less.


An additive to resins for the purpose of improving physical properties (such as impact resistance, hardness, dimensional stability, etc.), or to reduce cost of resin.


A concave easing of an interior corner of a part used to reduce stress concentration.

Flame Retardant

A resin formulated to inhibit or resist burning.


Extra plastic attached to a molding along the parting line; under most conditions it would be objectionable and must be removed before the parts are acceptable.

Flash Gate

Usually a long gate extending from a runner, which runs parallel to an edge of a molded part along the flash or parting line of the mold.

Flow Line

A mark on a molded piece made by the meeting of two flow fronts during molding. Also called “Weld Line.”

Flow Marks

Visible indications on the molded part that indicate the flow of plastic within the mold.


The location where the plastic enters the mold cavity to create the part. There is typically a visible vestige when the gate is removed.

Guide Pins

Devices that maintain proper alignment of the core and cavity as the mold closes; also called “Leader Pins.”


A triangular rib that strengthens areas such as a wall to a floor or a boss to a floor.


The degree of cloudiness in a plastics material.

Heat Distortion Point

The temperature at which a standard test bar deflects 0.010 in. under a stated load of either 66 or 264 psi.

Hot Tip Gate

An injection molding method that uses a heated gate on the A-side of the part to eliminate the need for a runner or sprue. The gate vestige will be a small sharp bump that can be trimmed if necessary.

Hot Stamping

Engraving operation for marking plastics in which roll leaf is stamped with heated metal dies onto the face of the plastics.

Hot / Heated Manifold Mold

An injection mold in which the portion of the mold that contains the runner system has its own heating elements which keep the molding material molten and ready for injection into the cavities.

Impact Strength

Molded plastics are usually given a value on an Izod scale. An Izod impact test is designed to determine the resistance of a plastics material to a shock loading. It involves the notching of a specimen, which is then placed in the jaws of the machine and struck with a weighted pendulum.

Injection Molding

A molding procedure wherein a heat-softened plastic material is forced from a cylinder into a cavity which gives the article the desired shape.


An integral part of a plastics molding consisting of metal or other material which may be molded into position or may be pressed into the molding after the molding is completed. Also a removable or interchangeable component of the mold.


Turbulent flow of resin from an undersized gate or thin section into a thicker mold section, as opposed to laminar flow of material progressing radially from a gate through the cavity.

Knit Lines

Visual indications in a finished part that are formed by the intersection of two plastic flow fronts. They are formed downstream through holes and between multiple gate locations. Also known as weld lines.

Living Hinge

Very thin section of plastic used to connect two parts and keep them together while allowing them to flex open and close. They require careful design and gate placement.

Medical Grade

Resin that may be suitable for use in select medical applications.

Melt Index

The amount, in grams, of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subjected to 2160 grams force in 10 minutes at 190°C.

Mold Release

A lubricant used to coat a mold cavity to prevent the molded piece from sticking to it, and thus to facilitate its removal from the mold.

Molding Shrinkage

The difference in dimensions, expressed in inches per inch, between a molded part and the mold cavity in which it was molded.

Multi-Cavity Mold

A mold with multiple cavities of the same part that is typically used to reduce piece-part pricing for higher volume parts.

Notch Sensitivity

The extent to which the sensitivity of a material to fracture is increased by the presence of a surface in homogeneity such as a notch, a sudden change in section, a crack or a scratch. Low notch sensitivity is usually associated with ductile materials and high notch sensitivity with brittle materials.


The tapered fitting on the end of the barrel of the molding press where the resin enters the sprue.


The practice of using increased pressure when filling a part to force more plastic into the mold. This is used to combat sink or fill problems, but may increase the likelihood of flash or cause the part to stick in to the mold.

Parting Time

The location where the mold halves come together. Typically a thin visible line is created on the part here.

Pinponit Gate

A restricted orifice through which molten resin flows into a mold cavity.

Post Gate

An injection method that injects plastic through an ejector pinhole. This gating technique leaves a gate post on the B-side of the part where it is often made less visible for cosmetic purposes. The gate posts are typically trimmed in a secondary operation. There is often gate blush opposite a post gate.

Prototype Mold

A simplified mold construction often made to run a limited number of parts. It is generally used to obtain information for the final part or mold design.


An edge or vertex that has been rounded.

Reinforced Resin

A material reinforced with special fillers (such as glass or carbon) to meet specific requirements.


A reinforcing member of a molded part.


A channel in the mold that directs the resin from the sprue to the gate.


The force between layers of resin as they slide against each other or against the surface of the mold. The resulting friction (shear) causes some heating of the resin.

Short Shot

The incomplete filling of a mold cavity which results in the production of an incomplete part.


The surfaces where the A-side and B-side of the mold contact.

Side Action

A sliding cam arrangement in the mold that allows for the molding of parts with undercuts.


Depressions in the surface of a molded part caused by local internal shrinkage after the gate seals. Sink is most common in thick sections of a part or where features such as ribs or bosses intersect a visual surface.


Visible streaks in the part that are typically caused by moisture in the resin.


The route the resin takes from the point where it enters the mold until it reaches the runner.

Steel Safe

A change to the part design that requires only the removal of metal to produce desired geometry. This change is typically most important when a part design is changed after the mold has been manufactured because then the mold can be modified rather than entirely re-machined.

Straight Pull Mold

A mold that does not require side actions.

Stripper Gate

A plate that strips a molded piece from core pins or core, the stripper-plate is set into operation by the opening of the mold.

Submarine Gate

A type of edge gate where the opening from the runner into the mold is located below the parting line or mold surface. With submarine gates, the part is broken from the runner system upon ejection from the mold.

Surface Finish

Finish of molded product. (e.g. glossy, matte, textured, etc.) Consider visual appeal vs. functionality.

Tab Gate

An injection gate on the parting line of the mold. It typically leaves a vestige on the outside of the part and is sometimes referred to as a “edge gate.” Edge gates often needs to manually trimmed.

Telescoping Shutoff

An area within a mold where metal slides along metal and often creates a hole in the part. Adequate draft angles are required on any related part surfaces.


A surface treatment applied to the mold to create texture on the parts. There are a variety of available textures, and draft requirements vary depending on the selected texture.


A portion of the part geometry that would prevent the part from being ejected from a straight-pull mold without a portion of the mold passing through (and destroying) the part.


In a mold, a shallow channel or minute hole cut in the cavity to allow air to escape as the material enters.


A visible mark created by the gate.


A void or bubble occurring in the center of a heavy thermoplastic part. The void is usually caused by excessive shrinkage.


The curving or bending of a part that typically occurs after ejection while the part cools.


Dimensional distortion in a plastic object after molding.

Weld Lines

Visual indications in a finished part that are formed by the intersection of two plastic flow fronts. They are formed downstream through holes, and between multiple gate locations. Also known as knit lines.