Thermal diffusivity of plastics and corresponding mold temperature
The thermal diffusivity of plastics is an important parameter for measuring their heat transfer capacity. It indicates how quickly a plastic transfers heat under a temperature gradient; a larger value indicates better heat transfer performance. The thermal diffusivity is closely related to factors such as the plastic’s composition, structure, and density. Different types of plastics exhibit significant differences in their thermal diffusivity. For example, polyethylene has a relatively large thermal diffusivity, indicating rapid heat transfer; whereas polycarbonate has a smaller thermal diffusivity, resulting in a relatively slow heat transfer rate. Determining the thermal diffusivity is crucial for developing plastic molding processes. It helps us understand the temperature variations of plastics during the molding process and provides a basis for optimally setting mold temperatures.
Mold temperature is a key process parameter in the plastic molding process and is closely related to the plastic’s thermal diffusivity. For plastics with high thermal diffusivity, due to their rapid heat transfer, heat can be quickly transferred from the plastic to the mold during the molding process. Therefore, the mold temperature can be set appropriately lower to accelerate the plastic’s cooling rate and improve production efficiency. Conversely, for plastics with low thermal diffusivity, due to their slow heat transfer, heat is slowly transferred within the plastic. Setting the mold temperature too low can lead to uneven cooling of the plastic , generating internal stresses and, in turn, defects such as deformation and cracking of the plastic part. Therefore, it is necessary to appropriately adjust the mold temperature based on the plastic’s thermal diffusivity to ensure part quality.
In actual production, mold temperature settings also need to consider the crystallization properties of the plastic. Crystalline plastics, such as polyethylene and polypropylene, have relatively large thermal diffusivity. A certain mold temperature is required during the molding process to promote crystal growth and perfection, thereby improving the mechanical properties and dimensional stability of the plastic part. For example, when molding polypropylene parts, the mold temperature is typically set between 40°C and 60°C. This promotes the orderly arrangement of polypropylene molecules and the formation of a stable crystal structure. In contrast, for amorphous plastics, such as polyvinyl chloride and polystyrene, the thermal diffusivity is relatively small. Mold temperature settings are primarily focused on controlling the cooling rate, typically set at a lower temperature, such as between 20°C and 40°C, to accelerate cooling and finalization.
Mold temperature uniformity is also affected by the plastic’s thermal diffusivity. For plastics with low thermal diffusivity, due to slow heat transfer, even slight differences in mold temperature can lead to inconsistent cooling rates across the part, resulting in internal stress and warping. Therefore, when designing molds for these plastics, measures must be taken to ensure mold temperature uniformity, such as optimizing the mold’s cooling water channel layout to ensure uniform distribution of cooling water flow rate and flow rate. For plastics with high thermal diffusivity, mold temperature uniformity is relatively easy to maintain due to their rapid heat transfer. However, a properly designed cooling system is still required to prevent localized overheating or underheating, which can affect part quality.
Furthermore, the thermal diffusivity of plastics also affects the molding cycle length. Plastics with high thermal diffusivity cool quickly, shortening the molding cycle and improving production efficiency. Plastics with low thermal diffusivity, on the other hand, cool slowly and have a relatively long molding cycle. In actual production, the molding cycle can be optimized by adjusting the mold temperature to appropriately alter the plastic’s cooling rate. For example, for polycarbonate (with a low thermal diffusivity), increasing the mold temperature appropriately can slow the cooling rate, facilitating adequate filling and degassing of the plastic, but at the expense of a longer molding cycle. Lowering the mold temperature, on the other hand, can accelerate cooling and shorten the molding cycle, but care must be taken to avoid defects. Therefore, while ensuring part quality, it is necessary to rationally set the mold temperature based on the plastic’s thermal diffusivity and production efficiency requirements to achieve a balance between quality and efficiency.
