Page 58 - Plastics News Issue September2025

TECHNOLOGY NEWS
of screw flighted length for the feed section, the
total cored depth for the cooling hole would be
43 inches.
Once the screw is installed into the extruder, the
plumbing components required for water flow
can be assembled. Figure 2 shows the assem-
bly of the screw-cooling system, and a list of the
components needed for the system is provided
in Table 2. These components were developed
for use with plant tower cooling water. A pho-
tograph of an installed screw-cooling system is
shown by Figure 3.
Hex-Head Bushing Prevents Wobble
A key part of the screw-cooling system is a
hex-head bushing welded to the outer pipe that
connects the rotary union to the shank of the
screw. This bushing will keep the outer pipe cen-
58 PLASTICS NEWS
tered in the drive quill bore. A schematic of the
outer pipe with the hex-head bushing is shown
by Figure 4. The outside diameter of the bush-
ing should have a slip fit to the bore of the drive
quill. The length of the pipe should be as short
as possible. The bushing will maintain the pipe
centered in the quill such that the cooling device
doesn’t wobble with screw rotation.
Typically, the return water temperature out of
the rotary union needs to be between 38-50 C
0
for optimum temperature control on the root
of the screw in the feed section. The ultimate
method of controlling the screw-cooling system
is to use a mold temperature control unit (TCU).
When using a TCU, the temperature of the cool-
ant medium that’s used within the screw-cooling
system can be precisely controlled just like bar-
rel zones.
The first three turns of the screw in the feed
section are only augering the resin, it’s not con-
veying the resin. This portion of the screw is still
in the feed-throat housing, which is oversized
to the flight outside diameter. The feed-throat
housing is also cooled between 38-50 C. The ac-
0
tual solids conveying of the resin doesn’t begin
until about the fourth turn of the screw where
the first heated zone begins.
Screw cooling is very advantageous when the
feedstock that is being processed has a high
percentage of regrind, and the material has poor
September 2025