Download Melt-pressure transducers Part I

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