Pressure

Melt pressure sensors were first introduced in the fifties. Until recently, they were protected by patent laws which influenced their availability and price. Today, such sensors are delivered by numerous manufacturers. The design of the sensors is based on tensiometric technology. Pressurization is carried out through the built-in membrane in the flow channel, by means of a closed capillary system filled with mercury, NaK or oil. A second membrane with a built-in tensiometer, which with changing pressure causes linear electrical resistance variation, is installed at the end of the capillary tube. The proportional resistance variation is used for measurements. Sensors with built-in amplifiers receive normalized output signals (0-10 VDC, 0-20 mA or 4-20 mA). Pressure sensors protect the extruder against excessive pressure, which can cause damage to the device. By using limit signals, the drive system of the extruder can be disabled after a predefined pressure has been exceeded.
For safety reasons, each production line should be equipped with at least one pressure sensor.

Extrusion cylinder - in order to allow further development and modernization of screws, it is necessary to install sensors along the cylinder. The data received can be used to specify material properties in each heating zone of the cylinder. A sensor, installed at the end of the extruder, will indicate the level of compression of the plastic.

Screen changer – measurement of the differential pressure behind and in front of the changer indicates when filter screens should be changed. A sensor, installed at the end of the extruder, enables scheduled replacement of screens (e.g. after completing extrusion of a desired length of material).

Melt pump – if it is used, measurement of differential pressure behind and in front of the pump must always be carried out. Proper pressure indication protects the pump against damage and helps to increase the output of the extruder.

Nozzle – a good place to install a sensor. A sensor installed in the nozzle will show the pressure required to achieve the desired shape. Pressure and temperature are the main parameters for the flow and quality of the plastic. Both parameters control the quality of and changes in the plastic, wear of the screw and cylinder and thermal changes in the technology. Pressure variation affects significantly the size of the final product.