Influence of increased injection pressure load on the captured acoustic emission signals and dimensional accuracies of polypropylene test specimens

This paper presents experimental results regarding the acoustic emission signals measured during the injection moulding of those standard test specimens commonly used for examining the shrinkage behaviour of various thermoplastic materials. The acoustic emission was measured on an injection mould with a visible crack on the cavity's surface using two contact PZT sensors under normal and increased injection pressure loads. A low-cost polypropylene material with a high shrinkage value was used for the experimental purposes. Accurate measurements of the test specimens' shrinkages were obtained using an optical 3D scanner. After completing the first part of the experiments under normal injection pressure load, we were able to record, for almost one minute, those AE bursts that originated from the cavity surface crack as a result of injection mould cooling. During the second experiment, under increased injection pressure load, we discovered through evaluation of the AE signals that, as expected, their amplitude and energy values were higher during both the filling and packing stages of the injection moulding cycle compared to those obtained at a normal injection pressure load. The final results obtained showed that the captured AE signals largely depended both on the size and duration of the injection pressure load, both of which are essential in order to achieve and produce test specimens within acceptable dimensional tolerances.