Page 57 - Plastics News Issue September2025

TECHNOLOGY NEWS
Building a Screw-Cooling System
crews have been cored for cooling since
the early 1960s. In many processes, how-
Sever, screw cooling is not utilized. Here’s
an optimal design for a screw-cooling system.
Poor solids conveying in smooth-bore, single-
screw extruders can limit rates, cause flow surg-
ing, and reduce the profitability of a line. Exam-
ples of poor solids conveying were presented
for a polystyrene (PS) resin and in the August
2025 issue for a polycarbonate (PC) resin. In
both cases, the screws became too hot in the
feed section, limiting the forwarding forces for
solids conveying. These processes were made
stable, and rates were increased with the use of
screw cooling.
Screw cooling is one of the most misunderstood
operations in the extrusion process. Screws have
been cored for cooling since the early 1960s. In
many processes, however, screw cooling is not
utilized. This article describes the optimal design
for screw-cooling systems.
Cooling the feed section of a screw is performed
by pumping water through a duo-flow rotary un-
ion into a passageway on the shank end of the
screw, as shown by Figure 1. The passageway is
September 2025
a gundrilled hole into the shank that extends to
the end of the feed channel. The inflowing wa-
ter enters a pipe at the rotary union. The pipe
extends to about 1 inch from the end of the gun-
drilled cooling hole. The water then flows back
toward the shank through the annular passage
between the pipe and the gundrilled hole. The
water then flows back out of the screw, through
the outer pipe. This outer pipe is attached by a
pipe thread to the screw shank and the rotary
union. Water flows out through the rotary union.
The typical bore sizes of cooling holes and the
tap sizes for screws of different diameters are
shown in Table 1. Screws with diameters less
than 2 inches are very seldom bored for cool-
ing because it greatly weakens the screw’s tor-
sional strength unless its base material is made
of 17-4 PH stainless steel or other high modulus
steel. Screw cooling becomes very important for
screws that are 8 inches in diameter and greater.
When screw cooling is used to improve solids
conveying, the screw is bored to a depth equal
to about the end of the feed section. In most
cases, the depth will be the length of the drive
shank plus five to six diameters of the screw. For
example, on a 4.5-inch diameter screw with a
drive shank length of 16 inches and six diameters
PLASTICS NEWS 57