Page 35 - Plastics News February 2021

FeAtures

For new equipment: However, with some resins the associated internal parting

Ensure capabilities are engineered for a range of resins. line becomes a hang-up point for potential material
buildup and degradation, resulting in potential black
• Include extended-field-range motors. specs in the material Another compromise to enable
• Consider vented and plugged extruder barrels for multiple-resin performance requires larger die bodies,
greater flexibility.• which unfortunately push the die farther from the nip,
creating other challenges.
• Choose a flexible screw design for multiple materials
as needed, or optimized screw designs to fit your Dies must be able to accommodate a sheet gauge of 8 to
requirements. 32 mils and typical widths from 32 to 58 in. For die-lip
radius, 25 to 50 microns are preferred versus the 150- to
To support melt delivery from the extruder to the die 200-micron standard.
on existing or new equipment, melt filtration is always
a consideration. You can use a single slide-plate screen Roll-Stand Configurations and Purpose
changer for virgin, or clean in-house regrind/resin blends. Choosing the right roll stand is also important. Here is a
When looking to process rPET or other recycled materials, summary of each type:
a continuous-type screen changer should be considered
for uninterrupted runtime. Various styles of continuous • Vertical downstack: Conventional arrangement;
units are available for sheet applications, with a dual-bolt horizontal die-to-nip approach; improved operator
rotary unit the most common for that application. visibility; equal web wrap; smooth sheet processing.
Lower melt-strength resins should use a smaller-
Melt Pumps diameter top roll. Recommended for PMMA.
Since most sheet processes use a blend of regrind and • J-stack: Most flexible design with an angled die-to-nip
virgin resin with variable bulk density, we recommend approach; good for low-melt-strength resins; vertical
using a melt pump. Here’s why: top and center roll for improved operator visibility;
• Melt pumps reduce the output pressure variability to increased web wrap and more cooling. Recommended
the feedblock and die. for rPET, PETG, GP-PS, PP.

• They improve machine-direction thickness control. • Horizontal stack: Vertical die-to-nip approach; best
for lower-melt-strength materials; typically used on
• They support die performance for transverse thickness thinner-gauge sheet; limited operator visibility; equal
control (stable flow-through die manifold).
web wrap; difficult to string up. Recommended for
• Melt pumps offer melt-stream bead stability in the PC.
roll-stand primary nip.
• They operate around 700 to 1000 psi inlet pressure
and 2000 to 3500 outlet pressure.
• Special continuous leaking pumps can control leakage
flow and prevent contamination of clear sheet.
• Melt pumps are essential when using a two-stage
screw design.
Die Selection
Dies built for the specific resin to be processed are
always best. General-purpose dies will operate with some
sacrifice of performance such as the need to adjust die lips In choosing a roll stand, evaluate what type of roll-gap
more frequently due to resin pressure changes within the control would be best for your operation. There are three
die. A restrictor bar may be required to allow processing a types: manually actuated worm gear; gear motor; and
variety of materials, as this allows the operator a means hands-free. Here are the differences:
of changing the die’s internal pressure distribution.

35 Februar y 2021 Plastics News