Page 268 - Plastics Engineering

Processing of Plastics 25 1

mixing along the screw. Since the pressure at the die is important, extruders
also have a valve after the breaker plate to provide the necessary control.

4.2.2 Mechanism of Flow
As the plastic moves along the screw, it melts by the following mechanism.
Initially a thin film of molten material is formed at the barrel wall. As the screw
rotates, it scrapes this film off and the molten plastic moves down the front face
of the screw flight. When it reaches the core of the screw it sweeps up again,
setting up a rotary movement in front of the leading edge of the screw flight.
Initially the screw flight contains solid granules but these tend to be swept into
the molten pool by the rotary movement. As the screw rotates, the material
passes further along the barrel and more and more solid material is swept into
the molten pool until eventually only melted material exists between the screw
flights.
As the screw rotates inside the barrel, the movement of the plastic along
the screw is dependent on whether or not it adheres to the screw and barrel.
In theory there are two extremes. In one case the material sticks to the screw
only and therefore the screw and material rotate as a solid cylinder inside
the barrel. This would result in zero output and is clearly undesirable. In the
second case the material slips on the screw and has a high resistance to rotation
inside the barrel. This results in a purely axial movement of the melt and is the
ideal situation. In practice the behaviour is somewhere between these limits
as the material adheres to both the screw and the barrel. The useful output
from the extruder is the result of a drag flow due to the interaction of the
rotating screw and stationary barrel. This is equivalent to the flow of a viscous
liquid between two parallel plates when one plate is stationary and the other is
moving. Superimposed on this is a flow due to the pressure gradient which is
built up along the screw. Since the high pressure is at the end of the extruder
the pressure flow will reduce the output. In addition, the clearance between
the screw flights and the barrel allows material to leak back along the screw
and effectively reduces the output. This leakage will be worse when the screw
becomes worn.
The external heating and cooling on the extruder also plays an important part
in the melting process. In high output extruders the material passes along the
barrel so quickly that sufficient heat for melting is generated by the shearing
action and the barrel heaters are not required. In these circumstances it is the
barrel cooling which is critical if excess heat is generated in the melt. In some
cases the screw may also be cooled. This is not intended to influence the melt
temperature but rather to reduce the frictional effect between the plastic and the
screw. In all extruders, barrel cooling is essential at the feed pocket to ensure
an unrestricted supply of feedstock.
The thermal state of the melt in the extruder is frequently compared with
two ideal thermodynamic states. One is where the process may be regarded as