EXTRUSION AND INJECTION MOLDING OF POLYMERS
Extrusion of polymers
Extrusion is a process of
manufacturing long products of constant cross-section (rods,
sheets, pipes, films, wire
insulation coating) forcing soften polymer through a die with an opening.
Polymer material in form
of pellets is fed into an extruder through a hopper. The material is then
conveyed forward by a feeding screw and forced through a die, converting to
continuous polymer product.
Heating elements, placed
over the barrel, soften and melt the polymer. The temperature of the material
is controlled by thermocouples.
The product going out of
the die is cooled by blown air or in water bath.
Extrusion of polymers (in
contrast to extrusion of metals) is continuous process lasting as long as raw
pellets are supplied.
Extrusion is used mainly
for Thermoplastics, but Elastomers and Thermosets are also may be extruded. In
this case cross-linking forms during heating and melting of the material in the
extruder.
The thermoplastic extruded
products may be further
formed by the Thermoforming
method.
A principal scheme of an
extruder is shown in the picture.
Injection molding of
polymers
Injection Molding is a
process in which molten polymer is forced under high pressure into a mold
cavity through an opening (sprue).
Polymer material in form
of pellets is fed into an Injection Molding machine through a hopper. The
material is then conveyed forward by a feeding screw and forced into a split
mold, filling its cavity through a feeding system with sprue gate and runners.
Injection Molding machine
is similar to Extruder. The main difference between the two machines is in
screw operation. In extruder screw rotates continuously providing output of
continuous long product (pipe, rod, sheet). Screw of injection molding machine
is called reciprocating screw since it not only rotates but also moves forward
and backward according to the steps of the molding cycle.
It acts as a ram in the
filling step when the molten polymer is injected into the mold and then it
retracts backward in the molding step.
Heating elements, placed
over the barrel, soften and melt the polymer.
The mold is equipped with
a cooling system providing controlled cooling and solidification of the
material.
The polymer is held in the
mold until solidification and then the mold opens and the part is removed from
the mold by ejector pins.
Injection Molding is used
mainly for Thermoplastics, but Elastomers and Thermosets are also may
be extruded. In
this case cross- linking occurs during heating and melting of
the material in the heated barrel.
A principal scheme of an
Injection Molding Machine is shown in the picture.
Injection Molding is
highly productive method providing high accuracy and control of shape of the
manufactured parts. The method is profitable in mass production of large number
of identical parts.
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