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Measurement and Control of Melt Pressure

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When it comes to the placement of melt pressure sensors the extrusion engineer faces considerably fewer problems compared to the placing of temperature sensors. One of the basic principles of hydraulics pump is that the pressure in a fluid is equalin all directions.

This may not be strictly true in the case of a very viscous, moving fluid, but it is close enough for our purposes. One of the few things that the extrusion industry was able to standardize is the shape and size of the mounting hole in the extruder for a melt pressure sensor. This mounting was diagramed.

A typical pressure sensor (see Fig. 6.9) will have a thin diaphragm at the end,in contact with the plastic. This diaphragm deforms in the presence of a hydraulic force, and it is the deformation of the diaphragm which is a measure of the pressure.The pressure is ultimately measured by a pressure sensitive transducer of some sort.

Unfortunately, none of the practical pressure transducers can stand the elevated temperature of the plastic stream, so it is necessary to locate them in the remote end of the sensor body. The problem of coupling the diaphragm to the transducer body has led to the diversity in the different pressure sensors available. The earliest ones simply used a rod to couple the two. The problem here was the unpredictable thermal expansion of the rod. One end was very hot, being in contact with the diaphragm, which in turn is in contact with the hot plastic. The other end is in contact with the pressure transducer,located in the other end of the sensor housing. The expansion of the rod in between, due to temperature, was very erratic.

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