Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 19

Rapid heat cycle molding (RHCM) is an advanced injection molding technology for producing spraying-free plastic products with excellent appearance (20). Rapid mold heating and cooling is the key technique of RHCM.

Despite being widely used in practice, the regular rapid mold heating and cooling methods still have some obvious defects. Thus, a new rapid mold heating and cooling method has been developed, characterized by electric heating and annular cooling.

Here, the temperature and pressure responses in the heating and cooling periods were experimentally investigated (20). The results of the study showed that the tool surface temperature increases almost linearly with the heating time after a short response time. The larger the heating power or the smaller the distance from heater to tool surface, the faster the heating rate.

The introduction of air bubbles into the working fluid can remarkably reduce the pressure growth of working fluid without affecting the heating rate. In the investigated range of flow rate, the cooling rate first increases significantly with the flow rate, and then reaches a plateau, while the running pressure of working fluid increases linearly with the flow rate in the whole range.

The optimum flow rate is around 6.0 l min^–1, corresponding to a Reynolds number of 6700 (20). The Reynolds number helps to predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent (21).

The heat transfer coefficient in the cooling period increases sharply at the initial stage, and then reduces gradually, and finally reaches a plateau. The larger the Reynolds number, the higher the heat transfer coefficient. In particular, the heat transfer coefficient and the Reynolds number show a linear relationship on the double logarithmic scale. Finally, a mathematical model was developed for predicting and controlling the temperature fluctuation range of tool surface (20).