Abstract
One of the largest challenges in production using melt-based processes is the manufacturing of precise parts. Especially due to high differences in temperature of the produced parts while processing, shrinkage cannot be avoided, since the processing material is typically heated up, molded and cooled down. In injection molding, the molten plastic is loaded by high variations in temperature and pressure while being processed. The gradients can lead to a significant change of the local specific volume, shrinkage potential and inner stresses, which can result in part warpage. In order to increase the precision of the manufactured parts, the shrinkage potential has to be homogenised to achieve an even shrinkage and therefore minimise part warpage. In this work, the approach for homogenisation of the shrinkage potential is a homogenisation of the specific volume and density respectively. Pursuing the goal of a homogeneous distribution of the specific volume leads to manipulation of the influencing factors namely temperature and pressure. The impact of temperature and pressure changes on the specific volume can further be quantitatively described by the material specific pvT-data. Based on geometric restrictions, a manipulation of the local pressure inside the cavity in most cases is impossible. However, on the other hand, a local control of the temperature is possible using highly dynamic tempering techniques. Based on this line of arguments, the paper describes the development of a highly segmented dynamic temperature control in injection molding to locally influence the mold and part temperature. Thereby, the specific volume of the part will be locally adjusted to reduce warpage as well as to compensate process variations occurring by changing material properties or varying ambient conditions. Due to the nature of thermal processes a special control strategy has to be developed to enable accurate temperature control, which is able to compensate slow thermal reaction in a highly dynamic process.
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