Pressure Measurement
Melt pressure sensors were already developed in the 50's. Until a few years ago they were protected by patents. That affected price and availability. Today melt pressure sensors of different manufacturers are available. Due to the influence of the VDMA perfectly compatible products resulted. Most of the transducers work according to the proven principle of the strain gauge technology. The pressure at the measuring diaphragm standing concisely at the melt channel is transmitted through a closed capillary system to a measuring diaphragm. The capillary system is alternatively filled with mercury, NAK or oil. The strain gauges, are applied onto the measuring diaphragm, they show a linear change of their resistance by a pressure load. This change of resistance is used to create an electrical signal. With transmitters the signal is already converted in the sensor head to standardized signals (0-10Vdc, 2.10Vdc, 4-20mA or CAN bus).
Mounting Locations
In the Nozzle
- the optimal location for a melt pressure sensor is in the nozzle. There the sensor measures the process pressure. Pressure and temperature are both substantial parameters which represent the viscosity of the melt. Thus changes of the material specs, wear of screw/cylinder, thermal deviations can be determined. Changes of the process pressure also effects the dimensions of an extruded product. Die swell of the material after the nozzle depends essentially on the melt pressure. In addition the melt pressure transducer fulfills a safety function, dangerous high pressure can be detected and via alarms switch off the extruder. EACH extruder should be equipped with at least one melt pressure sensor.
At the melt filter
- with a differential pressure measurement, before and behind the filter, the melt pressure sensor measures the degree of the contamination of the filter. Filter change can easily be scheduled.
At the melt pump
- when using melt pumps a differential pressure measurement (before and behind the pump) must be always made. The correct differential pressure can protect your (expensive) melt pump against damage and improve your output.