The feed section usually has a constant-diameter
core that has the smallest diameter, the largest channel depth, and the largest
cross-sectional volume in the screw. The deep channel conveys the relatively low
bulk density feedstock pellets into the machine. The feedstock is conveyed forward
into the transition section or melting section of the screw. The transition section
increases in root diameter in the downstream direction, and thus the channel
depth decreases. Here, the feedstock is subjected to higher pressures and temperatures,
causing the feedstock to compact and melt. As the material compacts, its
bulk density can increase by a factor of nearly two or more. As the feedstock compacts,
the entrained air between the pellets is forced back and out through the
hopper. For example, a pellet feedstock such as ABS resin can have a bulk density
at ambient conditions of 0.65 g/cm3
while the melt density at 250 °C is 0.93 g/cm3
.
Thus for every unit volume of resin that enters the extruder, about 0.3 unit volumes
of air must be expelled out through the voids in the solid bed and then out
through the hopper. The transition section is where most of the polymer is converted
from a solid to a fluid. The fluid is then conveyed to the metering section
where the resin is pumped to the discharge opening of the extruder. In general, the
metering section of a conventional screw has a constant root diameter, and it has a
8 1 Single-Screw Extrusion: Introduction and Troubleshooting
much smaller channel depth than the feed section. The ratio of the channel depth
in the feed section to the channel depth in the metering section is often referred to
as the compression ratio of the screw.