Molding Glossary


Many vocabulary words used in injection molding are industry specific, therefore we have added this Glossary as a resource.

If you are looking for the definition of a specific word, please click on the first letter of the word below.  This will bring you to that letter in the Glossary listing.



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


These compounds are added to resins to improve the overall performance and appearance of finished products. A key trend in this area today is using additives that are made from organic materials such as eggshells, wood pulp, rice hulls or materials that improve the biodegradability of the plastic.

AQL Testing (Acceptance Quality Limits):

The method used to measure a production order sample to find out whether or not the product order has met the customers specifications.  Once this is complete, the customer is able to make an informed decision about their product.

Automatic Mold:

A mold for injection, compression or transfer molding that repeatedly goes through the entire molding cycle, including ejection, without the assistance of an operator.


Backing Plate:

A plate which supports the mold, pins and bushings in the injection machine.


The portion of the molding press where resin is melted.


This is a part defect which appears as a small bubble or blister on the surface of a part and it is generally created by gas or air bubbles.


Similar to blisters, gas pockets, or voids that have formed inside the plastic.



The machined shape within a mold which created the form of the plastic part.

Cellular Manufacturing:

The process of manufacturing which is a subsection of just-in-time manufacturing and lean manufacturing. The goal of cellular manufacturing is to move as quickly as possible, make a wide variety of similar products, while making as little waste as possible.


A pigment system usually in pelletized, powder or liquid form which is mixed with resin to produce the desired color.

Coordinate Measuring Machine:

A coordinate measuring machine (CMM) is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine. Probes may be mechanical, optical, laser, or white light.


A polymer derived from more than one type of monomer.


A protrusion or set of matching protrusions which form the inner surface of a plastic part. Often considered the “male” portion of the part.

Cycle Time:

Cycle time is the time a machine requires to produce one unit. Cycle time has three basic components which consist of the time to load the machine, the actual machine time and the unloading time.



This defect appears as a flaky surface layer on the part and is often caused by contamination or moisture in the resin pellets.


Mass per unit volume of a substance.

DFT (Demand Flow Technology):

A strategy for defining business processes in a flow, driven by customer demand.

DFMEA (Design Failure Mode and Effects Analysis):

The combination of two methodologies – Design for Manufacture (the design for ease of manufacture of the parts to form a product) and Design for Assembly (The design of the product for ease of assembly.)


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


One of several measures of the hardness of a material as well as the instrument used to measure hardness. Hardness may be defined as a material’s resistance to permanent indentation. Durometer is often used to refer to the measurement as well as the instrument used.


Ejector Pin/Sleeve:

A rod, pin or sleeve which pushes a molding off a core or out of a cavity of a mold.  This is attached to an ejector bar or 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 of which it tends to recover its original size and shape after deformation.


A material which at room temperature stretches under low stress to at least twice its length and snaps back to the original length upon release of stress.


Techniques used to create depressions of a specific pattern in plastics film and sheeting. Such embossing in the form of surface patterns on molded part by the treatment of the mold surface by photoengraving or other process.


Family Mold:

A multi-cavity mold wherein each of the cavities forms one of the component parts of the assembled finished object. The term often applied to molds wherein parts from different customers are grouped together in one mold for economy of production. This is sometimes referred to “Combination Mold.”

FEA (Finite Element Analysis):

A mathematical technique for analyzing stress, which breaks down a physical structure into substructures called “finite elements.” The finite elements and their interrelationships are converted into equation form and solved mathematically.

FIFO (First In First Out):

An inventory method which ensures the first items placed into inventory are the first used/sold from inventory.


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


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

Flexural Strength:

The ability of a material to flex without permanent distortion.

FMEA (Failure Mode and Effects Analysis):The method used to analyze potential reliability issues in the development cycle of the project to enhance the reliability through the design.



In injection and transfer molding, the orifice through which the melt enters the cavity.


The shine or luster of the surface of a material. See Surface Finish.



The resistance of a material to compression and indentation. Among the most important methods of testing this property are Brinell hardness, Rockwell hardness and Shore hardness.

Height Indicator:
This is used to accurately measure small distances and angles as well as amplify them to make them more obvious.

Hot Plate Welding:

Hot-plate welding consists of two stages: Two parts to be joined together are placed close to or in contact with heating modules that are first pre-heated to the temperature needed for assembly. Secondly, once the parts soften sufficiently they are removed from the heating module and quickly brought into contact for the controlled assembly phase.


Induction Welding:

A type of welding in which coalescence is achieved by heat derived from the work’s resistance to an induced electric current, either with or without applied pressure.

Injection Molding:

A molding procedure whereby a heat-softened plastic material is forced from a cylinder into a cavity which gives the article the desired shape. Used with both thermoplastic and thermo-setting materials.


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

Insert Molding:

Insert Molding combines metal and plastics, or multiple combinations of materials and components into a single unit. The process makes use of engineering plastics for improved wear resistance, tensile strength and weight reduction as well as using metallic materials for strength and conductivity.

ISO Certification:

The International Organization for Standardization (ISO) is an international standard-setting body composed of representatives from various national standards organizations. This organization promotes worldwide proprietary, industrial and commercial standards for business.



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.



Lean Manufacturing:

A systematic method for the elimination of waste within a manufacturing environment.


Matte Finish:

A type of dull. non-reflective finish. See Surface Finish.

Mold Flow Analysis:

Moldflow has two core products: Moldflow Adviser providing manufacturability guidance and directional feedback for standard part and mold design and Moldflow Insight which provides definitive results for flow, cooling, and warpage along with support for specialized molding processes. In addition, Autodesk produces Moldflow Design, Moldflow CAD Doctor, Moldflow Magics STL Expert, and Moldflow Structural Alliance that serve as connectivity tools for other CAD and CAE software.

Molding Shrinkage:

The difference in dimensions, expressed in inches per inch, between a molding and the mold cavity in which it was molded, both the mold and the molding being at normal room temperature when measured. Also called “Mold Shrinkage,” or “Shrinkage,” and “Contraction.’

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. Also called Release Agent.”

Movable Platen:

The moving platen of an injection or compression molding machine to which half of the mold is secured during operation. This platen is moved either by a hydraulic ram or a toggle mechanism.

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 hollow cored metal nose screwed into the extrusion end of (a) the heating cylinder of an injection machine, or (b) a transfer chamber where this is a separate structure.


Optical Comparator:

A device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected upon the screen, and the dimensions and geometry of the part are measured against prescribed limits.

Orange Peel:

A type of granular surface finish resembling the peel of an orange.


PFMEA (Process Failure Mode and Effects Analysis):

A structured analytical tool used by a business to identify and evaluate the potential failures of a given process.


A property of plastics which allows the material to be deformed continuously and permanently without rupture upon the application of a force that exceeds the yield value of the material.

Poka-Yoke System:

A technique for avoiding simple human errors in the workplace. A system designed to prevent errors made by workers while preforming a process.


A high-molecular-weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, the MER: e.g.. polyethylene. rubber, cellulose. Synthetic polymers are formed by addition or condensation polymerization of monomers. If two or more monomers are involved, a copolymer is obtained. Some polymers are elastomers, some plastics.


A chemical reaction in which the molecules of a monomer are linked together to form large molecules whose molecular weight is a multiple of that of the original substance. When two or more monomers are involved, the process is called copolymerization or heteropolymerization.

PPAP (Production Part Approval Process):

The outline of methods used for approval of production and service. The purpose of this process is to ensure parts comply with design specifications.

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.



Reciprocating Screw:

A combination injection and plasticizing unit in which an extrusion device with a reciprocating screw is used to plasticize the material. Injection of material into a mold can take place by direct extrusion into the mold, or by reciprocating the screw as an injection plunger, or by a combination of the two. When the screw serves as an injection plunger, this unit acts as a holding, measuring, and injection chamber.


Any of a class of solid or semi-solid organic products of natural or synthetic origin, generally of high molecular weight with no definite melting point. Most resins are polymers.


In an injection or transfer mold, the channel that connects the sprue with the gate to the cavity.

Runner System:

The term usually applied to all the material in the form of sprues, runners and gates which lead material from the nozzle of an injection machine or the pot of a transfer mold to the mold cavity.


Secondary Operations:

In many applications, additional manufacturing steps will be required after molding thermoplastic parts. Parts made from thermoplastic materials lend themselves to a number of ways of machining, assembly and finishing.


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

Sink Mark:

A depression or dimple on the surface of an injection molded part due to collapsing of the surface following local internal shrinkage after the gate seals. This may also be caused by a short shot.

Six Sigma:

A set of techniques determined at Management level intended to improve business processes by decreasing the chances that an error or defect will occur during production.


Any substance (usually a liquid) which dissolves other substances.


An instrument used to measure the intensity of wavelengths in a spectrum of light compared with the intensity of light from a standard source.

Spin Welding:

The process of fusing two objects together by forcing them together while one of the pair is spinning, until frictional heat melts the interface. Spinning is then stopped and pressure held until they are frozen together.


An ingredient used in the formulation of some plastics to assist in maintaining the physical and chemical properties of the compounded materials at their initial values throughout the processing and service life of the material.

Structured Light 3-D Scanner:

A 3D scanning device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.

Surface Finish:

The finish of a molded product.


TAKT Time:

The average time between the start of production of one unit and the start of production of the next unit, when this is set to match the rate by customer demand.


Cutting threads in the walls of a circular hole.

Tensile Strength:

The pulling stress required to break a given specimen. Area used in computing strength is usually the original, rather than the necked-down area.


A material that will repeatedly soften when heated and harden when cooled. Typical of the thermoplastic family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, polypropylene, vinyls, nylons, and the various fluorocarbon materials.  Capable of being repeatedly softened by heat and hardened by cooling.

Tie Bars:

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

TPM (Total Preventative Maintenance):

The application of preventative maintenance strategies in an organized and standardized method.


Ultrasonic Welding:

An industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials.

UV Stabilizer (Ultraviolet):

Any chemical compound which when mixed with a thermoplastic resin, selectively absorbs UV rays.



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

Vision Plastics:

Your Plastic Injection Molder, from Concept to Completion.



Dimensional distortion in a plastic object after molding.



Yield Value/Yield Strength:

The lowest stress at which a material undergoes plastic deformation. Below this stress, the material is elastic; above it, viscous.



A method that uses 5 key components to organize a work space for efficiency and effectiveness by identifying and storing the items used in the manufacturing process. Sort, Set in Order, Shine, Standardize, Sustain.