Injection Molding Materials

There are two types of materials (resins) which can be used in the injection molding industry; Thermoplastic and Thermoset.

A thermoplastic, or thermosoftening plastic, is a plastic material, a polymer, that becomes pliable or moldable above a specific temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight.[1]

thermoset also called a thermosetting plastic, is a plastic that is irreversibly cured of a soft solid or viscous liquid, prepolymer or resin.[1]


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Resin selection is a very critical step for the injection molding. There are many material suppliers offering quality materials for the injection molding.

They can formulate a plastic to meet the consumer needs properly. For example, the applications of the product play a key role in the choosing material properties. For High voltage connector, the resin has to provide chemical resistance, excellent tracking defense and heat& humidity resilience. For a Fluid Engineering Components, the resin has to have a good hydrolytic stability, excellent resistance to water and FDA approved. The formulated resin has the processability and consistent properties after molding process and in the final use.



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Yellowness Index: A measure of the color on the yellow scale.

Yield Point: In tensile testing, the yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress.

Yield Strength: The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.

Young’s Modulus: The ratio of tensile stress to tensile strain below the proportional limit


Warpage: Distortion caused by nonuniform internal stresses.

Water Absorption: The amount of water absorbed by a plastic article when immersed in water for a stipulated period of time. All plastics will absorb moisture to some extent.

Weld Line: Where melted material flows together during molding to form a visible line or lines on a finished part that may cause weakening or breaking of the component.

Wisps: Similar to stringing but smaller in size. These also may occur as slight flashing when the mold is over packed or forced open slightly. Mold-parting-line wear or misalignment can also cause wisps.


Vacuum Forming: A process whereby a heated plastic sheet is drawn against a mold surface by evacuating the air between it and the mold.

Valve Gating: A type of gate where a pin is held in the gate or channel by spring tension. As the injection stroke moves forward, this gate compresses the plastic in the runner. When this pressure build-up is sufficient to overcome the spring tension, the pin is then pushed back (pulled) and the fast decompression of the melt fills the cavity at extremely high speed.

Vent: A shallow channel or opening cut in the cavity to allow air or gases to escape as the melt fills the cavity.

Vented Barrel: Special barrel unit with a vent port over the compression section of the screw to permit the escape of gases prior to injecting melt into the mold. Often used when molding moisture-sensitive resins.

Vertical Flash Ring: The clearance between the force plug and the vertical wall of the cavity in a positive or semi-positive mold. Also the ring of the excess melt which escapes from the cavity into this clearance space.

Vicat Softening Point: The temperature at which a flat-ended needle will penetrate a specimen under a specific load using a uniform rate of temperature rise.

Virgin Material: Any plastic compound or resin that has not been subjected to use or processing other than that required by its original manufacturer.

Viscoelasticity: This property, possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics such as elasticity, strength, and form-stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate and amount of loading.

Viscosity: Resistance to flow of a liquid.

Void: An unfilled space within a solid material.


UL Temperature Index: The maximum temperature below which a material maintains its electrical and mechanical integrity over a reasonable period.

Ultimate Elongation: In a tensile test the elongation at rupture.

Ultimate Strength: Term used to describe the maximum unit stress a material will withstand when subjected to an applied load in a compression, tension, flexural, or shear test.

Undercut: A protuberance or indentation that impedes withdrawal from a two-piece rigid mold.

Underflow: The dominant flow of two confronting flows, over the other. The lesser flow reverses direction giving poor surface appearance and structural strength. Underflow should be avoided by positioning gates so that the flow fronts meet at the end of filling.

Unidirectional Flow Pattern: Plastic flowing in one direction with a straight flow front throughout filling.

Uniform Cooling Time: Cooling time the same throughout the part to avoid warping.