Search

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.

Missing Journal / Fehlende Zeitschrift 183, 108149 - (2019). doi:10.1016/j.matdes.2019.108149, Published by Elsevier Science, Amsterdam [u.a.]

High precision, warpage free injection molded parts are highly in demand despite posing a great challenge. According to the pvT-behavior of the polymer, local variations of temperatures and pressures lead to inhomogeneous shrinkage which results in warpage. Since the pressure is dependent on the flow path, a homogenization of the specific volume is aimed by controlling the local mold temperatures. Therefore, a mold with 18 individually controllable tempering zones was developed, each zone equipped with ceramic heating elements and liquid CO2 expansion chambers. Due to the high mass of the mold, thermal processes underlie significant delays. The precise actuation of the tempering zones therefore requires a suitable control strategy. Two control strategies for the tempering zones were investigated on a basis of experimental trials. The tempering zones were initially controlled by a PID controller since it is easy to use and has low hardware requirements. To yield superior control precision, a model predictive control approach (MPC) has been recently developed and implemented, based on a discretized one-dimensional Fourier heat equation. The control accuracy of both control strategies was tested with an asymmetrical mold tempering experiment, which showed that the MPC is far more precise than the PID control approach.

The process dependence of pressure-specific volume-temperature (pvT) measurement for an amorphous polymer, acrylonitrile-butadiene-styrene (ABS), was investigated. The influences of different measurement processes (heating, cooling, compression, and decompression with different rates) were considered in the pvT measurements. The pvT measurements of isobaric cooling and heating with different cooling and heating rates (2, 5, and 10 °C/min) and isothermal compression and decompression with different compression and decompression rates (up to 920 bar/s) were conducted. The testing temperature ranged between 40 and 230 °C and the pressure ranged between 20 and 2200 bar. The obtained results demonstrated that the pvT diagram will be significantly different depending on the direction in which the pressure or temperature is changing and also on the rate of the change. Isobaric pvT diagrams are different between cooling and heating. Fast cooling accelerates phase transitions, while fast heating reverses. Specific volume at the same pressure and temperature in decompression process is lower than that in compression. Compression and decompression leads to different pvT curves. Compression and decompression rates have different effects on specific volume in different states.

The pressure-specific volume-temperature (pvT) relationships of polymers are basic data for polymer engineering and polymer physics. The specific volume (vsp) is not only defined by the current value of pressure (p) and temperature (T) but is also dependent on the process history of p and T. The influence of measurement processes on the pvT relationships of a semi-crystalline polymer, polypropylene (PP), has been studied using a piston-die pvT testing device. Based on the isobaric cooling/heating processes and the isothermal compression/decompression processes, the specific volume was examined as a function of cooling, heating, compression, decompression, cooling/heating rates and compression/decompression rates. The specific volume increases with increasing cooling rates in the crystallization and solid state, and decreases with increasing heating rates in the crystallization and molten state. The decompression process causes lower specific volume than the compression process. The specific volume decreases with increasing compression rates but decreases then increases with increasing decompression rates.