A numerous range of extruder screws are
these days available for processing different materials like polymers,
metals or rubber. The screw is actually a long shaft that has a thread
helically wrapped around it. The thread is known as a flight. A channel in a
an extruder screw refers to the area in between adjacent sections of the
flight. The following image shows a basic design of an extruder screw.
Components of Extruder Screw
There are many types of screw designs, but irrespective of the design screws
mostly have three primary sections and two secondary sections. They are
feed, transition (or compression), metering, devolatization zone and finally
mixing zone.
An extruder screw's detailed design is extremely complex. Channel depth is
an important variable to screw. Compression is typically largest and
uniform in the feed section while smallest in the metering section. Channel
depth decreases gradually along the transition section. The flight depth
will also decrease in the metering area to provide a precise material feed
rate in the die area. A screw compression ratio is the ratio of depth of
feed section flights to the metering section flights. Ideally in plastics,
common compression ratios in the extruders can range from 2 - 4:1.
Functions of an Extruder Screw
The extruder screw serves the following basic functions:
- Bringing the feedstock into the extruder and then moving the material
along the screw. This is performed side by side while compressing and
removing volatiles.
- Softening of the melt by heating (from both internally generated
shear forces as well as externally applied heat).
- Pump the fluid in a steady manner.
- Mixing and melting to eventually produce a homogeneous melt that is
without any impurity.
- Applying constant pressure (free of any pulsation) very much needed
for forcing the material through the die.
Advantages of Extruder Screw
An extruder screw enjoys the following advantages:
- An Increase in the melting efficiency due to total elimination of
solids bed breakup.
- A considerably higher melting rate due to prolonged exposure of the
solids bed to the barrel surface where melting takes places.
- Better stability, due to complete elimination of channel plugging
that is a source of trouble at the entrance to the barrier section.
- Improved mixing in the melt channel as melt material is subjected to
moderate shear when it passes over the barrier flight.
Extruder Screw Materials
Extruder screws are mostly made up of alloy steel and then undergo further
secondary treatment operations depending upon the nature of steel. Some of
these treatments include chrome plating, nitriding, flight surface hardening
etc. The following table highlights some of the screw materials and gives
the treatment options to make it fit for use as extruder screw.
| Screw Material |
Treatment |
| Alloy steel 4140 |
Flame hardened |
| Nitralloy 135-M |
Nitrided |
| Tool steel CPM 9V |
Heat treatment |
| Special alloy Nickel 718 |
Age hardened |
What is a Barrier screw?
Barrier screw is a specialized form of extruder screw. Barrier screws
have an altered transition section, this makes it possible to have a
quicker melting for improved output rate. This is made possible by
addition of flights in the metering section that produces melt channels
between the two flights. As a result the solid channel is separated from
the melt channel. This permits the solid material to experience a higher
shear. Thus allowing a quicker melting at a considerably lower
temperature.
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