Discharge pressure measurement is important in an extruder for various reasons. Typically, melt pressure is measured just behind the breaker plate/screen pack at the screw tip for safety and process monitoring, and can also be measured beyond the screen pack for control purposes. The industry-standard measurement device is an electronic transducer with a 0.31in (8mm) diaphragm inserted in a port in the barrel wall. Although the transducers are accurate and fairly durable, they have several problems due to the location and port geometry.
The transducer’s diaphragm is flat, but the inner surface of the barrel is curved, so a pocket is created. On small diameter machines (1” and under) this pocket can be quite large, and become a point of stagnation, degradation, contamination, and corrosion. The severity of this issue increases with materials prone to degradation or corrosion.
The port position must clear the extruder’s front barrel clamp and flange, so is some distance behind the screw tip. This is usually located over the screw flight, in some cases far enough from the tip that it is reading pressure much lower than the actual discharge value. Also, because the flight passes under the diaphragm it is subject to large pressure variations, confusing the actual value.
US-Extruders has developed a new and unique method of measuring extruder discharge pressure (patent pending) utilizing the axial force of the screw against the thrust bearing as an indirect means of measurement. The screw is forced rearwards by the melt pressure at the tip, much like a hydraulic cylinder. It is also forced rearwards by the jacking force of the rotating screw. The extruder is equipped with a force sensor behind the thrust bearing, and this value is input to an algorithm along with drive torque to calculate a virtual pressure value which is scaled to conventional units. The Pressure transducer and rupture disc, and their barrel ports, are eliminated. Virtual Pressure is displayed on the HMI or via a special discrete instrument complete with alarms and shutdown limits. The design also allows for the last barrel heater to cover the area formerly occupied by the transducer and rupture disc, which created a cold spot.
Experiments were carried out to compare the calculated ‘Virtual Pressure” to that measured by a conventional transducer on a 2.5in diameter extruder running various polymers of various viscosities and properties. The graph below shows the close agreement between the two measuring methods. These polymers were all run using the same algorithm and variables in the Virtual Pressure calculation. No changes are required for different materials or temperatures. The variables only change for machine size and power and are factory set.
Virtual Pressure has been proven to be accurate, reliable, and very beneficial especially in small extruder applications in Medical, Pharma, and in Fluoropolymer where the transducer port pocket is relatively large, and degradation and contamination are critical.