Page 30 - Plastics News April 2020

increased free volume associated with the higher OD/ID narrow disk kneading-block elements with less intensive
ratio. At elevated screw rpm (greater than 800), the shear-stress input into the polymer, which results in
percentage increase was not as pronounced, as the more gradual melting of the polymer. The goal of the
higher screw-tip velocity seemingly had a “propeller” extended melt zone is to reduce the melt temperature
effect that somewhat inhibited feeding. and shear-stress exposure for the materials being
processed. After melting, a single kneading-block
The combination of both higher torque, lower average section was integrated into the screw design to minimize
shear, and larger OD/ID ratio has proven beneficial for temperature rise inherent in mixing.
many processes.
The corresponding melt temperatures were lower for
the 1.66/1 OD/ID ratio (even at the higher throughput
rates) due to a lower specific-energy input (kWh) into
each kg being processed and the gentler mixing effect
associated with deep-flighted 1.66/1 OD/ID screw
geometry.

The temperature profile was optimized and various
screw rpms were tested. The data in the accompanying
melt-temperature graphs was obtained with a handheld
immersion probe.
In each instance, the melt temperature with the
aggressive design was much higher (10° to 30° C) than
with the extended melting-zone design
A series of additional experiments were performed on
the ZSE-27 MAXX (1.66 OD/ID) to compare the resulting (Fig. 5). It is worth noting that the immersion probe
measured significantly higher temperatures (sometimes
melt temperature for different melting-zone screw
configurations (Fig. 4) with a 2 MFI PP pellet resin. An 20° to more than 40° C) than the flush melt probe. It is
“aggressive” melting zone with melting completed by evident that when a melt probe is not fully immersed
barrel position 3 (12 L/D) was compared with an into the polymer melt, the melt-temperature reading is
“extended” melting zone, where melting was influenced by the metal adapter setpoint—lower than
actual, and not accurate.
completed by barrel position 4 (16 L/D). A single
kneading-block set was used after melting in an attempt Higher temperatures inherent with the aggressive screw
to isolate and compare the different melting-zone design resulted in significant degradation, as indicated
configurations and the resulting melt temperature. A by smoke and discoloration at elevated screw rpm.
low-pressure discharge die was used to minimize the
effects of pressure on melt temperature. Both flush and The attainable rates were also maximized for both
immersion melt-temperature probes were utilized in the designs by targeting 85% operating torque and increasing
experiments. Tests were performed with various rates the rate until that threshold occurred. The extended
and screw rpm. melting-zone design resulted in both higher throughput
rates than the aggressive screw design and lower melt
The aggressive melting-zone design utilizes temperatures. Comparing the two melting zones
neutral/wide disk kneading-block elements and a (aggressive and extended) showed that the aggressive
reverse element to achieve full melting of the polymer melting zone caused a significant temperature rise and
by barrel zone 3. The goal of the aggressive melt zone lower attainable throughput rates than with the
might be to specify a shorter L/D, or to free up space in extended melting zone. Higher temperatures inherent
latter parts of the process for additional unit operations, with the aggressive screw design also resulted in
i.e. injection, mixing or devolatiliation. significant degradation, as indicated by smoke and
discoloration at elevated screw rpm.
In comparison, the extended screw design utilizes

APRIL 2020 27 Plastics News