Application of modern measurement techniques for analysis of injection moulding shrinkage

Abstract

The process of polymer injection moulding is complex and complicated, therefore it is characterised by a number of technological and material parametersthat influence the product quality. Setting the best injection conditions requires knowledge of the processing properties of the material, mould shape and technological capacity of the injection moulding machine. Injection shrinkage, as undesirable phenomenon, causes changes of the shape and dimensions of the injection moulding; therefore, it is one of the most important problems that have to be solved, both at the phase of mould designing and at selecting the technological parameters of injection. The paper presents classification of injection moulding shrinkage as well as factors influencing the value of the shrinkage. The conducted experimental tests were related to selected topics connected with the injection process and took into account the change of the processed material characteristics (type and amount of filler) and change of certain, relatively the simplest to regulate, injecting parameters (injection time and cooling of the piece in a mould). Geometrical dimensions of the injection mouldings produced from polypropylene filled with glass fibre were measured using two methods: tactile (electronic measuring instrument) and contactless (3D scanning). The analysis of injection shrinkage illustrates that an effective influence on its value is possible by both choosing appropriate conditions of the injection within the technical range of the machine, as well as, if needed, the processing properties of material can be modified in order to obtain a moulded piece of set geometrical and strength properties. After the analysis of the results, the connection between the injection moulding shrinkage and the content of the filler and the chosen parameters of injection was established.