Influence of Insert Component Position and Geometry on Shrinkage in Thermoplastic Insert Molding

摘要

In modem thermoplastic injection molding, the overmolding of inserts allows a high level of functional integration. An important quality criterion for the manufacturing of those technical thermoplastic parts is the detailed and comprehensive understanding of the factors that influence shrinkage. Often, local shrinkage differences result in loads on the inserts or cause dimensional tolerance inaccuracies. This study investigates the effect of different insert part geometries and positions along the flow path on the shrinkage of the overmolded part. A mold was designed to overmold square- and cylindrically shaped metal inserts at six different mold positions. Overmold materials were polypropylene (PP), polybutyleneterephthalate (PBT), and polyamide (PA). The results of simulation and experimental measurements indicated significantly lower shrinkage of the overmold with inserts parallel and normal to the flow direction. Square-shaped inserts in most cases induced greater impact on shrinkage behavior compared to the cylindrical inserts. The insert position caused reduced shrinkage with inserts placed at the end of the flow path compared to positions near the feeding point. Additionally, normal to flow shrinkage decreased between central and lateral insert positions. Asymmetrical insert positions especially caused a partial shrinkage onto or rather off of the insert resulting in mechanical load, critical for e.g. sensitive insert parts like RFID transponders. Overall, these findings contribute to a more precise estimation of the achievable dimensional part accuracy to improve overmolding processes in injection molding.