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
Diameter of the barrel
described in inches. Term used to characterize the extruder.
Die
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.
Elasticity/Plasticity
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.
Elastomer
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.
Extrudate
The product or
material delivered from an extruder, for example, film, pipe
profiles.
Extruder
A machine for producing more or less continuous
lengths of plastics sections such as rods, sheets, tubes, and
profiles.
Extrusion
The process of foaming continuous shapes by forcing
a molten plastic material through a die.
Back
Next
