Page 89 - Engineering Plastics Handbook

Processing 63

To calculate the first normal stress difference for low shear rates,
using the cone-and-plate method [2], use

N = 2 F z
1 π 2
R
p
where N = first normal stress difference, MPa (psi)
1
F = normal force, N (lb)
z
R = radius, cm (in)
p
Extrusion

Most of this information is relevant to extruders and injection molding
machines. Extrusion is a continual process to make products such as film
and sheet, wire covering, pipe, and profiles. Essential components include
the hopper, barrel, cylinder, plasticating screw, thrust bearing, breaker
plate and screen pack, backpressure regulating valve, die adapter, die,
sizing, and haul-off equipment. Heater bands heated by built-in thermo-
couples are strategically located around the cylinder. A key word for the
plasticating screw function is uniformity: uniform (homogeneous) melt
mix and uniform melt flow through the die. Uniformity is achieved by
screw configuration (e.g., flight and zone designs, length, and L/D) and cir-
cumferential speed (rpm). Flight design goes a long way to product qual-
ity and process economics. Flight design includes flight height, width,
clearance and angle, number of flights, and distance between flights.
Breaker plate functions include increasing backpressure and changing
melt flow from rotational to axial flow prior to melt flow to the die. The
screw root diameter increases from the hopper end to the metering zone.
Channel depth is increased or decreased according to melt rheology.
Two standard specifications for extruder and injection molding machines
are L/D and the helix angle. Length L is the length of the flighted section,
which consists of the feed section, transition section, and metering section
for metering screws; diameter D is the outside diameter.
To calculate the helix angle [3],

t
φ= tan −1
π D
where φ= helix angle, rad
t = axial distance from center of one flight to center of next
flight, mm
D = outside diameter of screw, mm

In addition to metering screws, there are other designs including bar-
rier screws which enhance melting and two-stage screws with five zones,