Plastic Injection Molding Glossary



A standard computer file format for exchanging CAD data, typically from AutoCAD programs. ACIS is an acronym that originally stood for “Andy, Charles and Ian's System.”


A substance such as a colorant or volumizer compounded into a resin to enhance or improve certain characteristics.


The process of joining parts by any of several methods.


Sometimes called the “cavity”, it is the half of the mold that usually creates the exterior of a cosmetic part. The A-side usually does not have moving parts built into it.


A component of the injection-molding machine wherein the resin pellets are melted, compressed and injected into the mold's runner system.

Bead Blasting

Using abrasives in a pressurized air blast to create a surface texture on the part.


Also known as a “chamfer”, it is a flat truncated corner.

Blueing Off

A method to check the accuracy of mold shut-off surfaces by putting a thin coating of Prussian Blue on one-half and checking the blue transfer to the half.


A cosmetic imperfection that is created where the resin is injected into the part, usually visible as a blotchy discoloration on the finished part at the site of the gate.


A raised stud feature that is used to engage fasteners or support features of other parts passing through them.


Sometimes called the “core,” it is the half of the mold where ejectors, side-action cams and other complex components are located. On a cosmetic part, the B-side usually creates the inside of the part.


A feature in the mold with an undercut. To eject the part, it must bend or stretch around the undercut.


Depression in mold, between A-side and B-side which usually forms the outer surface of the molded part; depending on number of such depressions, molds are designated as a single cavity or multi-cavity.


Also known as a “bevel,” it is a flat truncated corner.

Clamp Force

The force required to hold the mold shut so resin cannot escape during injection. Measured in tons.

Cold Slug

The first material to enter an injection mold. So called because in passing through the sprue orifice it is cooled below the effective molding temperature.


A portion of the mold that goes inside a cavity to form the interior of a hollow part. Cores are normally found on the B-side of a mold, thus, the B-side is sometimes called the core.

Core Pin

Similar to the core, core-pins are used to create holes and are often seperate pins that can be removed for replacement or resizing.


The dimensional change with time of a material under load, following the initial instantaneous elastic deformation. Creep at room temperature is sometimes called "Cold Flow".


Material remaining in a transfer chamber after mold has been filled. Unless there is a slight excess in the charge, the operator cannot be sure cavity is filled.


In compression molding, the line where the two halves of a mold come together. Also called "Flash Groove", "Pinch-off" and "Shut-off".

Cycle Time

The time it takes to make one part including the closing of the mold, the injection of the resin, the solidification of the part, the opening of the mold and the ejection of the part.

Direction of Pull

The direction the mold surfaces move when they are moving away from the part surfaces, either when the mold opens or when the part ejects.

Direct Gate

A sprue that feeds directly into the mold cavity.


A taper applied to the faces of the part that prevent them from being parallel to the motion of the mold opening. This keeps the part from being damaged due to the scraping as the part is ejected out of the mold.

Drying of Plastics

Many plastics absorb water and must be dried prior to injection molding to ensure good cosmetics and material characteristics.


A measure of a material's hardness as measured by the Shore Durometer.It is measured on a numeric scale ranging from lower (softer) to higher (harder).

Edge Gate

An injection method that uses a 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.


Electric Discharge Machining. A mold making method which can create taller, thinner ribs than milling, text on the top of ribs and square outside edges on parts.


The final stage of the injection-molding process where the completed part is pushed from the mold using pins or other mechanisms.

Ejector Pin

A rod or pin on the B-side of the mold which pushes the completed part out of the mold cavity. They leave small marks and are generally placed on the non-cosmetic side of the part.

Ejection Plate

The ejection plate pushes all of the eject pins at one time and can be controlled for multi-stage ejection or timed ejection.


Stands for “electro static discharge”, an electrical effect that may necessitate shielding in some applications. Some special grades of plastic are electrically conductive or dissipative and help prevent ESD.

Family Mold

A mold that produces non-identical parts simultaneously from multiple cavities.

Fan Gate

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


A rounded filling of the internal angle between two surfaces. Intended to improve the flow of material and eliminate mechanical stress concentrations on the finished part.


The surface texture and appearance of a finished article.


Means of holding a part during a machining or other operation.

Flame retardant

A resin formulated to resist burning or reduce the flammability of a material.


Thin excess material caused by resin leaking into the fine gap in the parting line of the mold.

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.

Flash Groove

See “Cut-Off”.


A qualitative description of the fluidity of a plastic material during the process of molding.

Flow Marks

Wavy surface appearance on a molded object caused by improper flow of the material into the mold. See “Splay Marks”.

Food Grade

Resins or mold release spray that are approved for use in the manufacture of parts that will contact food in their application.


The location where the plastic enters the part.

Gate Blush

A blemish or disturbance on the part found in the areag of the gate.


Glass Filled. This refers to a resin with glass fibers mixed into it. Glass filled resins are much stronger and more rigid than the corresponding unfilled resin, but also more brittle. In general, filled resins can be very susceptible to warp.


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

Hot Runner Mold

A mold in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner molds make parts that have no scrap.

Hot Tip Gate

A specialized gate that injects the resin into a face on the A-side of the mold. This type of gate doesn’t require a runner or sprue.


Initial Graphics Exchange Specification. A common vendor-neutral CAD file format.

Impact Strength

The ability of a material to withstand shock loading.

Injection Molding

The process of forcing molten resin through a mold into a cavity which producess a finished plastic part. Used with both thermoplastic and thermosetting materials.


a removable part of the mold imparting increased resistance to wear, heat transferability, or changeable part shape to that area of the mold.

Insert Molding

Insert molding is the process of molding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials.


Flow marks caused by 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 to the extremities of the cavity.

Knit Lines

Also known as “stitch lines” or “weld lines”. These are imperfections in the part where separated flows of cooling material meet and rejoin, often resulting in incomplete bonds and/or a visible line.

Knockout Pin

See “Ejector Pin”.

Living Hinge

Very thin section of plastic meant to be repeatedly bent. Connects two parts and keeps them together while allowing them to open and close.


Liquid injection molding. A process used in the molding of liquid silicone rubber. Not supported by MCA.

Medical Grade

Resin that may be suitable for use in certain 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.

Metal Safe

A change to the part design that requires only the removal of metal from the mold to produce the desired geometry. Typically most important when a part design is changed after the mold has been manufactured, as the mold can be modified through machining without the need to build up metal or entirely re-machined.

MUD (Base)

A Master Unit Die (MUD) tool is a set of premade bases that industry-wide standard inserts fit into and can be used like traditional injection molds.

Multi Cavity Mold

A mold where more than one cavity is cut into the mold to allow for multiple parts to be formed in one cycle. Typically, if a mold is called “multi-cavity”, the cavities are all the same part number. See also “family mold”.


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


Not able to transmit light.

Over Molding

A process in which a mold cavity is first partially filled with one plastic, then a second shot is injected to encapsulate the first shot.


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


A file format for exchanging CAD data.

Parting Line

Mark on the part indicating where the two halves of the mold met in closing


The ability of a material to withstand continuous and permanent deformation by stresses exceeding the yield value of the material without rupture.


The mounting plates of a press on which the mold halves are attached.

Post Gate

A specialized gate that uses a hole that an ejector pin passes through to inject resin into the mold cavity. This leaves a post vestige that usually needs to be trimmed.


An injection molding machine.


Prototypes are pre-manufacturing models of a part or assembly meant to test, form, fit, and/or function.

Prototype Mold

A simplified mold construction often made from a light metal casting alloy or from an epoxy resin in order to obtain information for the final mold and/or part design.



An indentation in the plastic part caused by the impact of the ejector pins.


Resin is another name for plastic and generally refers to the pelletized raw form of the pre-injected plastic.


A thin feature that adds strength while preserving consistant wall thickness. Common on plastic parts and used to add support to walls or bosses.

Rockwell Hardness

A common method of testing material for resistance to indentation in which a diamond or steel ball, under pressure, is used to pierce the test specimen.


A channel that resin passes through from the sprue to the gate/s. Typically, runners are parallel to, and contained within, the parting surfaces of the mold.


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

Shore Hardness

A method of determining the hardness of a plastic material using a scleroscope. This device consists of a small conical hammer fitted with a diamond point and acting in a glass tube. The hammer is made to strike the material under test and the degree of rebound is noted on a graduated scale. Generally the harder the material the greater will be the rebound.

Short Shot

A molded part produced when the mold has not been filled completely.


The yield from one complete molding cycle, including cull, runner, and flash.

Shot Capacity

The maximum weight of material which a machine can produce from one forward motion of the plunger or screw.


Mold cavities are built larger than the resultant parts to account for the tendancy of material to shrink when it is cooled and its molecules become less active. Shrink factors for materials are known and published by the maker of the various resins. Shrink contributes to many molding defects and issues but can be avoided if the parts are built properly. Due to shrink factor compensation, molds are designed for particular resins and might not perform as well for materials with greatly differing shrink factors.

Shut Off

See “Cut Off”.


When a mold contains a feature that requires a side-pulling action (usually a hole in a vertical wall), and there is enough volume expected to make a hand-loaded insert cost-prohibitive, then a slide is used to automatically place the feature in the tool upon the closing of the mold


Dimples or other distortion in the surface of the part as different areas of the part cool at different rates. Commonly seen on parts uneven wall thickness.

Specific Gravity

The density (mass per unit volume) of any material divided by that of water.


Splay is a defect similar to blisters that is caused by escaping gas or water and which affects the surface of the finished parts.


The first stage in the resin distribution system, where the resin enters the mold. The sprue is perpendicular to the parting faces of the mold and brings resin to the runners, which are typically in the parting surfaces of the mold.

Sprue Gate

A passageway through which molten resin flows from the nozzle to the mold cavity.


Standard for the Exchange of Product model data. It is a common format for exchanging CAD data.


Originally stood for “STereoLithography.” It is a common format for transmitting CAD data but is not suitable for injection molding.

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 as opposed to conventional edge gating where the opening is machined into the surface of the mold. With submarine gates, the part is broken from the runner system on ejection from the mold.

Surface Fininsh

Finish of molded product.

Tab Gate

A small removable tab of approximately the same thickness as the mold item, usually located perpendicular to the molded part. The tab is used as a site for edge gate location, usually on items with large flat areas.

Tensile Strength

The pulling stress, in psi, required to break a given specimen.


A specific type of surface treatment applied to some or all faces of the part. This treatment can range from a smooth, polished finish to a highly contoured pattern that can obscure surface imperfections and create a better looking or better feeling part.


A material that will repeatedly soften when heated and harden when cooled. i.e. styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, polypropylene, vinyls, nylons, and the various fluorocarbon materials.


A material that will undergo or has undergone a chemical reaction by the action of heat and pressure, catalysts, ultra-violet light, etc., leading to a relatively infusible state. Typical of the plastics in the thermosetting family are the aminos (melamine and urea), most polyesters, alkyds, epoxies, and phenolics.

Tie Bars

Bars which provide structural rigidity to the clamping mechanism of a press often used to guide platen movement.


A specified allowance for deviations in weighing, measuring, etc., or for deviations from the standard dimensions or weight.

Tunnle Gate

See “Submarine Gate”.


Undercuts are areas in a part that are inaccessible from the mold split. Generally these areas must be designed out or use an insert or slide.


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


After molding, the plastic runner system will remain connected to the part at the location of the gate/s. After the runner is trimmed off, a small imperfection called a “vestige” remains on the part.


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


A common term for the faces of a hollow part. Consistency in wall thickness is important.


Dimensional distortion in a plastic object after molding. Caused by different portions of the part cooling and shrinking at different rates. Uniform wall thickenss can decrease warp.

Weld Lines

See “Knit Lines”.


A type of CAD model consisting only of lines and curves, in 2D or 3D. Wirefame models are not suitable for rapid-injection molding.



Source: Standards & Practices of Plastics Molders, SPE, Engineering 360