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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 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.