Plastic Injection Glossary


The negative side of the mold, sometimes called the cavity, which often forms the exterior of the plastic part.


The process of gradually cooling a plastic part after it has been heated to a temperature just below its softening point, making the material tougher.


Joining completed molded parts together to form a final product.

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Sometimes referred to as the core, this is the side of the mold containing the injectors; most often forms the interior of the plastic part.


The section of the molding machine containing the screw which melts resin pellets and injects the compressed material into the runner of the mold.


A protrusion on a plastic part or mold used to add strength to the piece or create alignment for fasteners. 


Gas pockets or vacuum voids which occasionally occur inside plastic during the injection molding process.

Burn Marks
Typically black or brown-colored discolorations on an edge or surface of a molded plastic component.
A raised area on the surface of a molding caused by the pressure of gases inside it
A visible exudation or efflorescence on the surface of a plastic part
Backing Plate
A plate used as a support for the cavity blocks, guide pins, or bushings
Allowing gasses to escape the mold early in the molding cycle. Also known as degassing.
Thermal decomposition through discoloration, distortion or localized destruction of the surface of the plastic part.
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Acronym for Computer Aided Design
The negative shape formed in a steel tool in which plastic is injected to create the mold.
A portioned out amount of material loaded by the screw into the mold during a single cycle.
The process of hardening of injection molded parts.
The time is takes for the plastic injection process to complete. One plastic part can sometimes take multiple cycles to complete.
Color Streaking
Also known as “discoloration”, occurs when a plastic part is a different color than intended.
A dry chalk-like appearance or deposit on the surface of a plastic part. Also known as Blooming.
Core Pin
Pin used to mold a hole.
Curing Temperature
Temperature at which a cast or molded part is cured.
Forming a plastic object by filling an open mold with a monomer solution, allowing it to polymerize.
Clamping Force
Common in injection molding, it's the process of applying pressure to the mold to keep it closed. This works in opposition to the fluid pressure of the molten material, which is compressed.
Cold Slug
The initial material of an injection mold. This name takes after the process, which involves the material passing through the spruce orifice while it's cooled below the operative molding temperature.
Compression Mold
A mold which opens upon the addition of material, shaping this material by heat and the pressure of closing.
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The degree of taper a side wall has to facilitate removal of plastic parts from a mold.
A measure of the hardness and resistance to marring in a material.
Also known as “color streaking”, occurs when a plastic part is a different color than intended.
Allowing gasses to escape the mold early in the molding cycle. Also known as breathing.
A permanent change in the physical properties of a plastic component.
A range of finishing techniques used to remove the excess material on a plastic molding. These techniques can range from filing, sanding, milling, tumbling, and more.
Diaphragm Gate
The gate used in molding annular or tubular articles which forms a solid web across the opening of the part.
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EDM (Electric Discharge Machining)
A type of precision machining process used to build tools for plastic injection molding; utilizes electrical charges to erode metal.
Ejector Pin
A pin used to push the plastic molding out of its cavity or mold.
Acronym for Enterprise Resource Planning; a software dedicated to managing manufacturing processes and automating administrative functions for greater efficiency.
The compacting and forcing of a plastic material through an orifice.
Process of enclosing an electronic component in an envelope of plastic. Done by immersing an object in casted resin and allowing the resin to polymerize.
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The final surface texture of a part.

Flow Lines
Discolored lines that show up on a prototype part. Flow lines usually happen when the injection speed is too low, causing the plastic to solidify too soon.
Flexural Strength
Ability of a plastic component to flex without permanent distortion or breaking.
Flow Marks
Wavy surface marks on a molded object caused by irregular or improper flow of the plastic material into the mold.
Extra attached plastic present on a molding that must be removed before the final product sendoff.
Additive applied to resins to improve their physical properties such as resistance or hardness.
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The opening in the mold where the molten plastic is injected into the final part.

The shine or luster of the surface of a plastic material.
Guide Pins
Devices which help maintain a plastic core's alignment with the cavity as mold closes.
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The level of resistance a plastic material has to compression or surface damage.
Hot-Runner Mold
Heated molds that melt plastic and send it to nozzles which fill the plastic part cavities. 
Hot Stamping
An engraving process where a heated die is used to apply pressure to the plastic part, transferring pigment onto it.
Level of cloudiness in a plastic material's finish.
Kinds of plastics that absorb moisture.
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Injection Molding
A manufacturing process where melted plastic is injected into a negative of the part being produced, known as a mold cavity. After the plastic cools, it becomes a solid of the positive component.
Impact Strength
The ability of plastic material to withstand shock loading. Molded plastics are usually given a value on an Izod scale.
Process of pressing a metal or other material into the molding upon the mold's completion. These components are considered removable or interchangeable to the mold.
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Apparatus which holds and guides the tool during the plastic injection machining process.
When molten plastic fails to stick to the mold surface due to the speed of the injection.
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Knit Lines
Discolored lines that show up on a plastic prototype part. See Flow Lines.
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Laminar Flow
Thermoplastic resin poured into a mold is accompanied by solidification of the layer in contact with the mold surface. This resin acts as an insulating tube through which material flows to fill the remainder of the cavity, and is essential to duplication of the mold surface.
A plastic material's ability to not fade upon exposure to sunlight or ultraviolet light.
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Molding Material
Types of plastic material used in injection molding. These materials can comprise of resin, filler, pigments, plasticizers, or other ingredients.   
Mold Release
A material used to prepare a part’s surface for easier ejection of a mold.
Melt Strength
The strength of the plastic while in the molten state.
Molding Cycle
In molding, the cycle time is the period or elapsed time between a certain point in one cycle and the same point in the next.
A simple compound which can react to form a polymer.
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The hollow nose attached to the end of the injection machine which transfers melted plastic into the mold.
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The alignment of the crystalline structure in plastic material so as to produce a uniform structure.
A group of unsaturated hydrocarbons named after the corresponding paraffins by the addition of “ene” or “ylene" to the stem (ie propylene)
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Pad Printing
The process where ink is taken from an etched design and deposited on a molded plastic part to create a design.
Parting Line

A line formed on the seam of the part where two molds meet.

A polymeric material which can be melted under heat and pressure.
A certain material incorporated in melted plastic to increase its flexibility when hardened.
Cleaning one type of material out of the cylinder of an injection molding machine by forcing it out with new material.
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Generic term for a chemical compound injected in to a mold.
The channel that connects the sprue and nozzle to the gate of the mold cavity.
Residence Time
Duration of time an injected material charge sits in the heating cylinder before its injection into the mold
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A device housed in a barrel which portions out resin pellets and melts them on before they are injected into the mold.
A single complete cycle of the screw in the barrel of the molding machine.
Shrink Rate
The amount of shrinkage that occurs during plastic cooling (Usually between .004” and .20”)
Opening feed that sends material from the nozzle to the runner.
Sink Mark
Depressions which form in certain areas of a plastic molded part. They’re usually caused when the cooling device is insufficient and doesn’t fully cure the product.
Surface Delamination
When thin layers on the surface of a plastic part appear and peel off piece by piece
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A recyclable polymeric material that can be heated and cooled repeatedly without changing the structure of the plastic.
The solid mold in which resin is injected to for the plastic part.
A material that will has undergone a chemical reaction through heat and pressure
Tie Bars
Bars which provide structural rigidity to the clamping mechanism of a press
Specified range of deviation in weight, measurements, etc., or deviation from standard weight or dimensions.
Thermal Stress Cracking(TSC)
The crazing and cracking of thermoplastic resins—the result of over-exposure to elevated temperatures.
Thermal Conductivity
The ableness of a material to conduct heat.
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Ultrasonic Welding
A process of welding that produces a molecular bond between compatible thermoplastic parts.
A protrusion or indentation that impedes withdrawal from a two-piece, rigid mold
Unit Mold
A mold designed for interchanging cavity parts quickly.
UV Stabilizer
A chemical compound when admixed with thermoplastic resin selectively absorbs UV rays.
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A shallow channel or minute hole cut in the mold cavity to allow air to escape as the material enters the mold
A void or bubble occurring in the center of a thermoplastic part usually caused by shrinkage.
Vertical Flash Ring
The clearance between the core and the vertical wall of the cavity in a positive or semi-positive mold.
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Uneven shrinkage in different parts of the molded component. This usually occurs when there’s non-uniform cooling of the plastic material.
Weld Lines
Lines that can appear on the surface of a molded part where the molten material has converged after splitting off in a mold.
Water Absorption
The ability of thermoplastic material to absorb water.
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Yield Value
Known as yield strength, it's the lowest stress a material undergoes plastic deformation. The material becomes elastic below this level and viscous above the level.
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