Thermal optimization of thermoplastic injection moulds

Thermoplastic injection molding is a cyclic process used to shape parts made of amorphous or semi-crystalline polymers, whether reinforced or not. The process involves injecting molten polymer into a temperature-controlled mold cavity, where the material solidifies and takes on its final shape. Minimizing cooling time is a key factor in the process' profitability. Nevertheless, the thermal history during cooling has a direct impact on the quality of the part produced, and must therefore be kept under control. From the thermicist's point of view, the mold or tool can be considered as a heat exchanger in a periodic regime, which must be dimensioned to obtain the most uniform cooling possible in the shortest time. The aim is therefore to determine, for a minimum cycle time, the tooling thermal design that minimizes surface thermal gradients, while achieving a temperature level in the part that enables it to be ejected from the mold. This article will help the tool designer to address this issue.

The approach consists firstly in a global study of heat exchange in the average regime, in order to define the power to be supplied to the tooling. Secondly, one-dimensional analytical models are used to calculate the cooling time of amorphous and semi-crystalline polymer parts. These models do not require the use of advanced calculation codes, and call on a limited number of parameters. They take into account the imperfect thermal contact between the polymer and the mold cavity surface. The results obtained from these models are validated experimentally, based on tests carried out on a "school" part injected into a tool fitted with pressure and heat flow sensors.

The cooling time obtained for the 1D case is transposed to the case of a real 3D part, in order to design a control system that achieves good thermal homogeneity on the mold cavity surface, while keeping time to a minimum. Different optimization strategies are proposed, with no preconceived ideas as to the shape, number, position and temperature of the mold control channels. Experimental validation is proposed on an industrial case study.

Finally, the choice of control fluid is proposed, as well as the thermal flow metrology to be implemented in a thermoplastic injection tool.

The aim of this article is to help the engineer determine the cooling time of a thermoplastic injection-molded part, quickly and without the need for a calculation code, and thus associate a cost with the production of this part. The designer should also be able to position the control channels optimally to achieve this time on a complex part.