Plastic materials are
formed to shape by a process defined as extrusion. This
is accomplished by first softening the material with
heat. Through the use of a drive screw, which is rotated
by a motor, the material is forced toward and then
through an opening, called a die, used to shape the
plastic melt.
Various compounds, colorants and additives, can be mixed
with the plastic materials as they move along the screw
path. The heated materials are shaped by the die and/or
other post extrusion equipment and then cooled to retain
their shape. |
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Where
and Why Transducers are Used |
Melt pressure transducers can be effectively used along many points of the
extrusion process for a variety of reasons:
1. From a quality control viewpoint, a transducer should be located in the die.
The measurement of the melt pressure at this point is used as an indication of
flow rate.
2. To indicate when a screen is in need of changing and also to insure the
safety of personnel and equipment alike, a transducer will be located somewhere
ahead of the screen changer. These are most likely located either in the adapter
or along the screw path within the barrel. An even more accurate determination
of screen plugging can be made by reading the differential pressure between
transducers located on either side of the screen, one being in the adapter, the
other located in the barrel ahead of the screw tip.
3. For research and development purposes, Tempco transducers should be located
at various points along the barrel in order to accurately monitor the pressure
and mixing characteristics of the melt.
4. Transducers are also used for pressure sensing on post-extrusion equipment
such as blow molding heads, extrusion pumps, and spinnerettes.
5. Locating transducers anywhere along the apparatus also serves to improve the
safety of the extruder. |
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End Products of Extrusion
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The end results of the extrusion process can be found in various products.
Some examples include:
1. The feedstock for other plastic packaging systems used for compounding and
mixing.
2. Plastic film used to create bags and packaging materials.
3. Plastic tubing, hose, and pipe to contain water, gases, or chemicals.
4. Insulated cable and wire housing.
5. Filaments used to create textiles, brushes, rope, and twine. |
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Tempco Tip Material Options
The standard Tempco transducer diaphragm is machined out of a single
piece of type 15-5 PH stainless steel (.0045") and then heat
treated and finally Armoloy coated. This material gives Tempco
transducers the transverse strength and toughness needed for most
standard applications.
There are, however, certain extrusion processes that require different
types of diaphragm materials and/or coatings. Tempco is able to supply
customers with diaphragms and coatings specifically suited to their
needs and applications. |
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Electroless
Nickel Coated Diaphragm
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The Electroless Nickel
diaphragm option gives the transducer an advantage in
abusive environments. The Electroless Nickel offers
abrasion resistance and corrosion resistance. This is
due to a phenomenon called reduce skin friction. This
material will also cut down on diaphragm failures due to
adhesion of melt to diaphragm during the process.
There are two different versions of this diaphragm
option available. The first is a standard thickness
(0.0045") diaphragm made of 15-5 PH stainless steel
and then coated with a 0.0002" electroless nickel
coating. This version is applicable for use in any
pressure range plastic extruder. The second version is a
0.0080" thick diaphragm made of 15-5 PH stainless
steel coated with a 0.0002" electroless nickel
coating. This version is applicable for use in plastic
extruders with pressure ranges of 7,500 PSI and up. |
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Titanium
Nitride Diaphragm |
| The titanium nitride
diaphragm is offered for its excellent abrasion
resistance. Its abrasion resistance is superior to the
electroless nickel coated diaphragm and like the latter
diaphragm the titanium nitride diaphragm comes in two
different versions. The first is a standard thickness
(0.0045") diaphragm made of 15-5 PH stainless steel
and then coated with a 0.0002" titanium nitride
coating. This version is applicable for use in any
pressure range plastic extruder. The second version is a
0.0080" thick diaphragm made of 15-5 PH stainless
steel coated with a 0.0002" titanium nitride
coating. This version is applicable for use in plastic
extruders with pressure ranges of 7,500 PSI and up. |
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Internal
Resistance Calibration |
Tempco strain gauge
sensors rely on the small change in resistance of each
strain gauge to generate an analog signal that is
proportional to the applied physical input. This
resistance change is generated by straining a structural
element to which the gauges are attached. The same
output can be accomplished by electrically offsetting
the resistance of one of the strain gauges through a
simple shunt resistor network. This offsetting
resistance network is built into each Tempco transducer.
During manufacturing, each Tempco transducer is pressure
calibrated using highly accurate pressure sources and
instrumentation. The signal output versus pressure input
characteristic is thereby precisely known. The internal
resistance network is adjusted so that the output
generated by the shunt resistor simulation method
matches precisely the calibrated output of the
transducer at a selected point on its calibration curve.
The standard simulation value is 80% of the full range
rating of each transducer but other values may be
chosen. |
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Special 0.006" thick
Inconel® diaphragm with a proprietary coating
This special diaphragm is designed to be used in
extremely abrasive environments. Superior to all other
diaphragm materials for corrosion and abrasion
resistance, examples of applications requiring this
diaphragm option are ceramics or glass filled nylon.
Recommended use: Applications that are extremely
abrasive. |
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Internal
Resistance Calibration Tracking |
| An internal compensation
circuit insures that the shunt calibration output will
track any changes in pressure sensitivity (output) due
to changes in temperature of the strain gauge housing.
The simulated output, therefore, is 80%, ±0.25% of the
full scale pressure output over the entire operating
temperature range. |
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Applications of Melt
Pressure Transducers
Pressure monitoring is a fundamental quality control
technique used in modern extrusion processing.
Typical applications include:
Film Adaptable
for either blown process or slit casting, pres-sure monitoring
can help produce thinner, more uniform film
at faster process speeds. The pressure transducer also pro-vides
primary process information helpful for maximizing produc-tivity
and minimizing start-up scrap.
Synthetic Fibers
Accurate, reliable pressure monitoring helps deliver greater
consistency with less waste by reducing high speed variations,
even with high per-formance fibers.
Wire Coating
Pressure monitoring right in the crosshead die where the wire is
coated with plastic insulation improves throughput, quality, and
profits. This process parameter has become even more important
as wire take-up systems go to higher and higher speeds.
Pipe, Tubing, and Profile
A basic process parameter, pressure monitoring allows tighter
toler-ances, improves pro-duct quality and significantly
improves cost effectiveness even for complex and multi-hollow
extrusion. |
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HCL