Feed Screws, Designs

Post-Consumer Resin Screw Design

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Technology Brief
POST-CONSUMER RESIN SCREW DESIGN
Manufacturers utilizing both post-consumer resins (PCRs)
and virgin resins often have difficulty producing a
homogenous materials mix. The ability to obtain a
homogeneous PCR/virgin mix is affected by such PCR
materials characteristics as varying melt points, viscosity
levels, densities, and form (flake or pellet). An often
overlooked remedy to a poor materials mix is to change the
screw design of the machine being used in product
manufacture.
Optimizing the screw design for use with PCR materials
enables manufacturers to obtain consistent processing
characteristics and contributes to maximizing material
properties. This may result in improved output, increased
recycled content, or better performance properties.
This technology brief provides background information on
plastics processing screws and highlights the important
points to consider when evaluating screw design for use
with PCR materials.
The Plastic Screw
The plastic screw is an essential part of all extrusion,
injection and blow molding machines. The screw is a
helically flighted shaft that rotates within a cylindrical
housing called a barrel. The screw’s purpose is to
mechanically mix, melt and advance the material being
processed into a mold or die (2).
The typical screw is composed of three sections, each
designed for a specific purpose. The feed section
contains deep channels between the flights to begin the
process of melting and mixing, and to make sure the
material constantly moves forward into the next section.
The transition section gradually decreases the flight depth
to further compact, melt, and mix the material. Finally, the
metering section further reduces the flight depth to create
compression and finalize the melting process. The melted
material is then pumped forward, often through a screen,
into the mold or die (5).
Preventing Material Degradation
During resin processing, low shear and controlling melt
temperature are important factors in preventing
degradation of PCR/virgin materials mixes. Because
PCRs have already undergone multiple heat histories,
preserving melt quality by
preventing excessive heating is
essential. Poor melt quality can
lead to insufficient materials
properties and poor surface quality
of the final product.
Materials degradation also occurs
when maximum shear rates are
exceeded within the screw and
barrel. Shear rate is defined as “the surface velocity at
Key Words
Materials: Plastics or Post-Consumer Resins.
Technologies: Extrusion; injection molding; blow
molding.
Applications: Plastics products.
Market Goals: Optimize plastics manufacturing with
recycled resins.
Abstract: Design considerations in selecting a
screw for plastics processing.
Typical Screw. Schematic courtesy of Xaloy, Inc.
# PL-95-1
NIST MEP
Environmental Program
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the barrel wall divided by flight depth.” All plastics have
maximum allowable shear rates. Many PCR materials are
heat sensitive and have lower permissible shear rates than
do virgin materials. Since flight depth is inversely
proportional to shear rate, PCR materials may require
deeper screw channels than virgin materials to lower shear
rates and prevent degradation. Deeper screw channels
also have the added benefit of allowing a larger volume of
material to be processed and a greater output (2).
However, deeper flight depths increase the probability of
poor circulation of material within the screw. If PCR
materials are to be used in combination with virgin
materials, processors should select a PCR material with
shear rates closely resembling virgin resins. Processors
should also select a flight depth to optimize mixing ability
and output.
Better Mixing
There are many mixing devices that can be used to ensure
material homogeneity. Generally employed at the
discharge end of the screw, these devices seek to achieve
the desired quality of the materials mix while minimizing
the rise in melt temperature. Common mixing device
designs include parallel interrupted mixing flights, pins
protruding from the root, helical grooved channels, and
fluted constructions (1, 2).
Some plastics reclaimers use extruders with two screw
diameters to reduce the need for pre-processing. A larger
screw with deep channels serves to densify the material.
The smaller screw melts, mixes and conveys the material.
The screws can be combined into one long shaft to
function as a single unit (3).
Ogando cites a number of approaches to obtain low
temperature, low shear mixing in blown film applications.
The first involves the use of extruders in the 30:1
length/diameter (L/D) range. Average L/D for blown film
extruders is 25:1. The longer-than-average extruders
blend materials better than shorter extruders, and have the
flexibility to process more types of material. The second
method emphasizes the use of shorter extruders, 20:1 L/D,
in order to decrease the residence time (and potential
degradation) of the material (4).
Barrier screws provide another method to obtain optimal
mixing. Barrier screws are designed to accommodate
blends of multiple materials. In the proper application,
barrier screws serve to minimize melt temperature,
improve output rate, maximize output stability and maintain
acceptable melt quality levels.
Contaminant Control
Screw design also plays an important role in removing
unwanted vapors during material processing. PCR
materials may possess high moisture levels or contain
contaminated materials that release volatiles. Excessive
moisture levels can cause splay leading to poor surface
finish. Volatiles may complicate processing and create
voids that lower product performance. To remove
moisture and volatile elements from the melt, processors
may employ two-stage screws in vented-barrel extrusion
or molding equipment. These screws have a barrel with
an opening to the atmosphere or a vacuum source that can
remove moisture, air, and/or other volatiles (1,2).
PCR materials may contain contaminants such as metals
or pebbles that can wear and even break screws in
extreme conditions. Screws can be coated with harder
wear-resistant alloys, such as Colmonoy 83, in order to
provide additional protection (3).
Conclusions
In situations where potentially dissimilar materials such as
PCR and virgin resins are blended, selecting the correct
screw design is an important step in optimizing materials
processing and maximizing materials properties. Any
company purchasing new equipment, or established
processors, that intend to use PCR materials should benefit
from a screw design evaluation.
References
1. Berins, Michael. Plastics Engineering Handbook of the
Society of Plastics Industry, Van Nostrand Rheinhold, 1991.
2. Hoyt, Gunther H. "Screws and barrels - overview", Modern
Plastics Encyclopedia, 1992.
3. Klein, Ron. "Screw and barrels - design trends," Modern
Plastics Encyclopedia, 1992.
4. Ogando, Joseph. "Blowing Film With PCR Is Not That
Tough", Plastics Technology, December 1994.
5. Rauwendall, Chris. Polymer Extrusion, Hanser Pub., 1986.
Fact Sheet Issue Date: November 1995
For More Information
For a copy of the report, Post-Consumer Resin Screw Design (PL-95-1),use the CWC Publication Order Form. For more
information call CWC at (206) 443-7746, email info@cwc.org, or visit the CWC Internet Website at www.cwc.org.
This technology brief was prepared by CWC, Managing Partner of the Recycling Technology Assistance Partnership (ReTAP).
ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's
National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the
American Plastics Council.
CWC is a division of the Pacific NorthWest Economic Region, 2200 Alaskan Way, Suite 460, Seattle, Washington, 98121.