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Melt-pressure transducers Part I

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					                        Melt-pressure transducers Part I
                           By John Pacini and Richard McQuiggan
                    Originally Published in Plastics Machinery & Equipment
                                    Updated by Douglas Joy

    Extrusion processors are employing melt-     control device help maintain stable output
pressure transducers more and more               and melt quality.
frequently to help them improve output and
melt quality, enhance production safety, and     The Screen Pack
safeguard machinery. To select the right
transducer to meet their needs, processors           Dirty or clogged screens cause pressure
must familiarize themselves with the             increases within the extruder barrel, which
performance characteristics of various           restricts flow from the screw to the die. If
transducers. Once a specific transducer is       the pressure upstream of the screen pack
selected, proper application and                 gets too high, it can result in excessive wear
maintenance are key factors in ensuring that     to the screw’s thrust bearing. A pressure
these instruments provide optimum                gauge mounted downstream of the screen
performance.                                     pack will alert operators to a low-pressure
    Melt-pressure instrumentation can range      condition, while a transducer upstream of
from a single transducer used for indicating     the screen pack will warn of a high-pressure
a single pressure reading to sophisticated       condition.
systems that employ a series of transducers
and accessories to record data, sound alarms,    Melt pump
take corrective action, and relay information
to a process-control system. Regardless of a        A melt pump helps eliminate fluctuations
given system’s configuration, the key points     and deliver precise amounts of melt to the
to be measured in any application are at the     die at a constant flow rate. Processors using
die, screen pack, melt pump, and, in some        melt pumps should measure both the inlet
instances, along the barrel.                     and outlet pressure at the pump to ensure
                                                 that the melt pressure at the die remains
The Die                                          constant and to prevent the possibility of
                                                 melt-pump equipment damage caused by
    While most extruder drives and takeoff       lack of polymer that lubricates the pump.
devices provide almost drift-free operation,
variations in raw material and process           Along the Barrel
conditions occur that affect the flow rate
through the die, which results in inconsistent      The extruder screw is the single most
extruder output. Be sure to at least monitor     important piece of equipment in terms of
pressure at the die and adjust the extruder      affecting the quality and quantity of melt
screw speed manually to maintain a constant      delivered to the die. Transducers are used in
pressure. Transducers placed at the entrance     the research and development of screw
to the die in conjunction with a pressure-       designs, and to evaluate the best plastics and
                                                 screws to use in specific processes. The
transducers typically used in R&D                    Capillary units exhibit greater
applications measure pressure variations         temperature stability than pushrod
within a few pounds per square inch.             transducers. Temperature gradients along
                                                 the pushrod transducer stem, particularly
PUSHROD VS. CAPILLARY-FILL UNITS                 during startup, cause different relative
                                                 expansions between the pushrod and the
    Pushrod and capillary-fill units are the     transient output shifts of up to 25 percent of
two most common types of transducers used        full-scale pressure. The temperature effect
in extrusion processes. Each incorporate a       on a capillary-type unit is a predictable 15 to
strain gauge wired in the form of a bridge       30 psi/100ºF of temperature change, which
that is mounted on a stress member. The          is a small fraction of the effect typically seen
strain-gauge bridge is mounted on a remote       with pushrod transducers.
upper diaphragm, while a lower sensing
diaphragm comes into contact with the            The key measurement points are at
material.
    A minute deflection of the stress member      the die, screen pack, melt pump,
causes a change in resistance in the strain                 and the barrel.
gauge and an imbalance in the bridge. The
amount of imbalance is proportional to the           Capillary-type transducers also exhibit a
pressure applied to the sensing diaphragm.       better-combined error specification (the sum
When voltage is applied to the bridge, a         of errors due to nonlinearity, hysteresis, and
millivolt output signal is produced that is      nonrepeatability), usually between 0.5 and
proportional to the applied pressure. The        1% of full scale. This is a particularly
electrical output generated can be used for      important consideration for transducers used
data collection, process monitoring and          in a closed-loop pressure-control system.
control, and electronic transmission of a            The most accurate way to calculate
pressure reading to a remote display.            combined error is to total all the data
    Pushrod transducers use a force rod to       deviations from a straight line that passed
isolate the strain gauge from the high-          through zero and the full-scale output point.
temperature sensing diaphragm. Due to its        This is known as the terminal method of
rigid stem design, space and temperature         determining the combined error
constraints often limit the places where         specification.
pushrod units can be installed.                      Mounting-torque sensitivity is a potential
    Most capillary-type transducers are filled   problem with pushrod units due to their rigid
with mercury or sodium potassium to isolate      design. Overtorquing a pushrod transducer
the strain gauge from the high-temperature       during installation causes minute
sensing diaphragm. These units are               dimensional changes in the stem, which
available in both rigid and flexible-stem        causes shifts in output. Capillary-filled units
models. Flexible-stem models allow the           are more flexible by design and don’t
strain gauge housing to be mounted away          exhibit any significant mounting-torque
from the high-temperature environment and
                                                 sensitivity.
can be used in installations where space
limitations are a concern.

				
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posted:7/26/2011
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