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pressure, and ultimately form the polymer melt into a desired shape .As the plastic
leaves the die, it is passed through calibrator dies or calendar rolls to control
cooling and dimensions. If dimensional control is not critical, the extrudate may
simply be pulled through a water bath or just air cooled prior to spooling, cut-offs, or
other post-processing as per product requirement.
Breaker Plate and Screen packs: Screens are made out stainless steel wires.
They filter contamination present in hot plastic melt and improve mixing. The
screens get clogged with foreign particles, which raise pressure and temperature
and can affect output and product dimensions. Thus they are required to be
cleaned or changed regularly. Screens are held by a breaker plate which forms the
seal between the extruder and die. Breaker plate is a thick metal plate with holes
drilled through it and built to resist high extrusion pressures which at times can
exceed 5000 psi (34 MPa). This screen and plate assembly also serves to create
back pressure in the barrel, which is required for uniform melting and proper mixing
of the polymer, and pressure generated can be controlled by varying screen pack
composition (the number of screens, their wire weave size and other parameters).
This assembly also does the function of converting "rotational memory" of the
molten plastic into "longitudinal memory".
Types of Extruders: The extruder is the heart of machine and its job is to
adequately disperse and distribute all ingredients in the polymer to provide a melt
with consistent temperature and pressure.
Depending on the job requirements, different types and sizes of extruders are used
Screw Extruder: Single screw and twin screw extruders are the most widely used
The extruder function is to coveys and melt the material. The material gets melted
by heat from heaters on barrel and more primarily by the frictional heat created
between the solid bed the extruder barrel.
Two types of mixing occurs in the extruder - distributive and dispersive
Dispersive mixing, which breaks up agglomerated particles into the smallest
particle size, requires high energy and shear stress.
Distributive mixing, which uniformly distributes ingredients throughout the mixture,
requires less energy. Different types screw designs have different mixing ability.
Single-screw Extruders : comprise of a heated barrel surrounding single (one)
rotating screws driven by a motor. The screw has a deep-channeled feed section
that transports solids from the feed throat and compresses them. A transition or
compression section where the channels become shallower and the polymer is
melted, and a shallow-channeled metering section where the melt is conveyed to
the adapter and die. Different polymers process best with different compression
ratios (the ratio of the feed zone depth to the metering zone depth). Single-screw
extruders are widely used for extruding film, sheet, and other applications

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