Cavity pressure

The cavity pressure describes the pressure that prevails in the tool during the primary forming .

The measurement of cavity pressure is common in the manufacture of plastic molded parts. It is used to monitor the manufacturing process. The pressure is recorded by sensors and usually evaluated electronically. The measurement is used to reduce scrap when setting up the injection molding process. The measurement is also used for quality assurance .

Cavity pressure has been used for decades to determine the physical properties of an injection molded part during production. In numerous studies, the relationship between cavity pressure and certain properties such as B. molded part dimensions, orientations, etc. proved.

Piezoelectric measurement technology has established itself for this application over the years, as the sensor itself is ideally suited for this. Due to the sometimes very fast injection processes, requirements arise that only very compact and rigid sensor designs can meet. Other technologies such as B. the DMS technology do not meet these requirements by far.

The piezoelectric measuring method

The way this method works is based on the direct piezoelectric effect. If the crystals are deformed by the action of external mechanical forces, electrical charges occur on certain crystal surfaces, which can be measured. The charge that occurs is directly proportional to the mechanical stress. In the unloaded state, the crystal surfaces are uncharged, i. H. electrically neutral and have no charge.

If the quartz element is exposed to an external load, e.g. B. the cavity pressure is deformed, so occur due to the crystal lattice shift on the surfaces of electrical charges. If these surfaces are made electrically conductive with special materials, the size of the charge can be measured.

The quartz crystal retains this property, depending on the type of cut, up to the Curie temperature (> 450 ° C).

In order to avoid discharge processes, good insulation with a relatively high insulation resistance is necessary. This insulation can be greatly reduced by impurities in the plug connections, which leads to a premature discharge process and thus to measurement errors.

The measurement of electrical charge requires charge amplifiers that convert the charge into a proportional voltage. Measurement is thus made possible by simple electrical measuring devices.