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Density/Specific Gravity
Density and specific gravity are used interchangeably, which is formally incorrect. The difference is the following: density is mass per unit volume of a material at 73°F (23°C); specific gravity is the mass of a given volume of material at 73°F (23°C) divided by an equal volume of water at the same temperature. The conversion is: density = specific gravity X 0.99756. The often used English term "relative density" has the same meaning as "specific gravity.

Diameter of the barrel described in inches. Term used to characterize the extruder.

Metal attachment to the end of the extruder that gives the polymer melt its particular shape.

Diehead Pressure
Pressure required to force the melt through the die.

Differential Cooling
Occurs when one area of the part cools at a different rate or when the mold surfaces are at different temperatures. Warping results from differential cooling.

Downstream Equipment
Equipment in the process following the extruder. Generally consists of a sizing fixture, used for sizing and cooling of the extrudate; a puller, used to remove the extrudate from the extruder at a continuous rate; and a cutter.

Dropping a Parison
Extruding a parison of sufficient length to position it between the two mold halves to produce a part.

Edge Gate
Entrance to the part from the runner located on the parting line.

Ejector Pin
A rod, pin or sleeve which pushes a molding off of a core or out of a cavity of a mold.

The ability of a material to return to its original state after deformation: the yield point is not exceeded: elastic behavior. Plastics in general respond elastically. If a material’s yield point is exceeded when stressed, it does not return to its original state after removal of the stress: permanent deformation by plastic behavior. Plasticity is the inverse of elasticity. Another way of explaining this is the following. During the first part of the pulling process in the tensile test, both tensile stress and tensile strain continue to increase, and in proportion. When this takes place, the material acts like a spring, and is said to have elastic behavior. Some materials – such as methacrylates – will be broken when they have been strained only a small amount, and while still showing essentially elastic behavior. Other materials – such as polycarbonates – can be stretched many times their original length before they break. The latter have a yield point, and a corresponding yield stress.

A material which at room temperature can be stretched repeatedly to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.

End Of Flow
The melt is just touching the last part of the mold to be filled and the pressure at that point is zero.

Engineering Plastics
A broad term covering all plastics, with or without fillers or reinforcements, which have mechanical, chemical and thermal properties suitable for use, in construction, machine components and chemical processing equipment.

The product or material delivered from an extruder, for example, film, pipe profiles.

A machine for producing more or less continuous lengths of plastics sections such as rods, sheets, tubes, and profiles.

The process of foaming continuous shapes by forcing a molten plastic material through a die.

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