The Microporous Metal in the Polymer Processing - Carbon Dioxide Assisted Mould Temperature Control

In this thesis a carbon dioxide assisted cooling technique for primarily injection moulds, a part of the so called ToolVac™ moulding technology, has been studied. The cooling technique is based on the injection of liquid carbon dioxide into microporous mould steel. The study is an introductory examination focused on experimental work, of the cooling technique and the possibilities it offers. A test apparatus for the thermal simulation of the thermoplastic injection moulding process has been developed. The function of the test apparatus may be summarised with that a heated polymer plate is pressed against a microporous insert equipped with accessories for the carbon dioxide assisted cooling. The temperature measurement during the cooling is primarily intended to be done on the interfacial surface between the polymer and the mould. It has been found that the evolution of the mould surface temperature during the carbon dioxide assisted cooling may be divided into three stages. In the initial stage when a heated polymer is put into contact with the microporous mould the temperature forms a plateau at a predictable level. In the second stage the temperature starts to decrease proportionately fast and in the third and final stage the temperature levels out again. It has also been found that the rate of the temperature reduction in the second stage, of the temperature evolution during cooling, is highly dependent on the dosage frequency of the carbon dioxide, with a fixed total opening time of the cooling media supply valve. The connection is that the higher the frequency is the faster is the reduction of the mould temperature. The explanation to that the cooling rate increases with an increasing carbon dioxide dosage frequency, when a fixed valve opening ratio is assumed, has been found to be that the actual amount of supplied cooling media increases with increasing dosage frequency. To achieve a high cooling rate it is better to have a proportionately high frequent dosage with short valve openings than the opposite. The results presented in this thesis, which are more generally applicable, are primarily intended as guidance for the designer and the user of the studied carbon dioxide assisted cooling technique. The presented results may also be seen as a knowledge basis for further research work of the carbon dioxide assisted cooling technique. Further research should be focused on more specific models for the creation of some sort of a database of the cooling technology