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Refractive Index
The deviation caused by the material when a light beam passes through a transparent specimen.

Resin that has been previously molded and ground into small pieces for reprocessing. Regrind is usually comprised of sprues, runners and unfilled parts.

An inert fibrous or non-fibrous material incorporated in a plastic to improve or modify mechanical or physical properties.

Reinforced Plastic
A plastic with high-strength fibers embedded in the polymer matrix, resulting in increased strength properties superior to those of the base resin.

Reinforced Plastic A plastic composition in which fibrous reinforcements are imbedded, with strength properties greatly superior to those of the base resin.

Resin (Synthetic)
The term is use to designate any polymer that is a basic material for plastics.

Rockwell hardness is the resistance of a material to indentation of a defined steel ball. Three scales exist (R, L and M) with different ball diameters and different levels of loading for testing materials of different hardness.

In an injection mold, the feed channel, usually of circular cross section, that connects the sprue with the cavity gate. The term is also used for the plastic piece formed in this channel.

Runner Balancing
Developing a runner system which delivers the required amount of melt to each cavity with the correct pressure to finish filling all the cavities simultaneously at the correct temperature for the part.

Runner Design
Using the runner as a flow control device (positioning the gate and using the size of the runner to control the filling pattern within the cavity) in addition to getting the melt into the cavity.

Runner System
This term is sometimes used for the entire resin feeding system, including sprues, runners and gates, in injection molding.

Any output of a mold that is not usable as the primary product.

A rotating auger that uses a helical thread and a varying channel depth to convey, melt, mix, and pump resin. Consists of a feed section, transition zone, metering section and in some cased a mixing zone/zones.

Shore hardness values are measured by using calibrated durometers: Shore A for softest and Shore D for harder materials. The material is penetrated with a steel rod of predefined dimensions - different for Shore A and for Shore D. The values show no correlation with other hardness measurement values.

Short Shot
The amount of resin injected into a mold cavity at a volume less than required to fill the mold.

One complete cycle of a molding machine.

The percentage of reduction in overall part dimensions. Shrinkage occurs during the cooling phase of the process.

Shrinkage Allowance
The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.

Shut-Off Nozzle
A device attached to the front of the molding machine barrel or hot manifold system. It opens to allow resin flow into the mold cavity. It closes after injection to help prevent drooling.

Single Cavity Mold
Produces one part with each cycle.

A depression on the part surface over a thick wall feature i.e., a rib, boss, thick sprue gate or change in wall section. Caused by the delayed cooling effects of the center of the thick area. When the hot center cools, it shrinks and pulls down the skin on the surface.

Sink Marks
An indentation on the surface of the part as a result of significant local change in wall section. The mark will occur in the thicker area.

Specific Gravity
The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73F).

Specific Heat
The amount of heat needed to raise one gram of material one ŪC in temperature. Equals "heat capacity" or "Cp."

Specific Volume
The volume of a unit of weight of a material; the reciprocal of density.

A substance used in the formulation of plastics to help maintaining the properties of the material during processing and service life.

Entrance to the part from the runner located below the parting line. On ejection the part breaks away from the subgate.

Break up of a full molding into component areas.

Surface Appearance
Distressed melt flow due to welding, melding, splay, etc. will appear as surface imperfections.

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