Thermal diffusivity, heat transfer coefficient, specific heat capacity and density of common plastics
Plastics are widely used in modern industry. Their thermal properties, such as thermal diffusivity, heat transfer coefficient, specific heat capacity, and density, have a crucial impact on the molding process and performance of plastic products. Understanding these parameters can help engineers optimize the injection molding process and improve the quality of plastic products. It also helps to rationally select plastic materials based on the actual use requirements of the product.
Take polyethylene ( PE ), a common thermoplastic, as an example. Low-density polyethylene ( LDPE ) typically has a thermal diffusivity between 0.12 and 0.15 × 10⁻⁶ m²/s , a heat transfer coefficient of approximately 0.35 to 0.45 W/(m・K) , a specific heat capacity of 2.3 to 2.6 J/(g・K) , and a density of 0.91 to 0.94 g/cm³ . LDPE offers excellent flexibility and processability, but its low thermal diffusivity means that heat transfer within the material is relatively slow, potentially leading to longer cooling times during injection molding. However, its moderate heat transfer coefficient and specific heat capacity ensure relatively stable temperature changes during heating and cooling, facilitating control of the molding process. High-density polyethylene ( HDPE ) has a thermal diffusivity of approximately 0.14-0.17×10⁻⁶m²/s , a heat transfer coefficient of 0.4-0.5W/(m・K) , a specific heat capacity of 2.2-2.4J/(g・K) , and a density of 0.94-0.97g/cm³ . Compared to LDPE , HDPE ‘s higher density imparts greater rigidity and strength. In terms of thermal performance, HDPE has slightly higher thermal diffusivity and heat transfer coefficients, meaning it transfers heat faster. This allows for slightly faster cooling during injection molding, but also requires greater attention to temperature uniformity to avoid defects such as deformation in the product due to uneven cooling.
Polypropylene ( PP ) is also a commonly used plastic. Its thermal diffusivity ranges from 0.11 to 0.14×10⁻⁶ m²/s , its heat transfer coefficient from 0.12 to 0.29 W/(m・K) , its specific heat capacity from 1.9 to 2.2 J/(g・K) , and its density from 0.9 to 0.91 g/cm³ . PP exhibits high heat resistance and good chemical stability. Its low heat transfer coefficient indicates relatively good thermal insulation properties, making it advantageous in certain plastic product applications requiring thermal insulation. During the injection molding process, due to its thermal diffusivity and specific heat capacity, the PP melt cools and solidifies at a relatively moderate rate in the mold, making it suitable for molding products with complex structures. However, it is important to note that PP is prone to significant shrinkage during the molding process. This is related to its thermal performance parameters and crystallization characteristics, which need to be considered and compensated for in mold design and process control.
polyvinyl chloride ( PVC ) vary depending on the formulation and processing technology. Generally speaking, the thermal diffusivity of rigid PVC is approximately 0.13-0.16×10⁻⁶m²/s , the heat transfer coefficient is 0.16-0.29W/(m・K) , the specific heat capacity is 0.9-1.4J/(g・K) , and the density is 1.38-1.58g/cm³ . PVC has excellent electrical insulation and corrosion resistance. Its high density makes its products relatively heavy. However, PVC is widely used in applications where weight is not a concern but weather and chemical resistance are a priority, such as building doors and windows, and piping . During injection molding, PVC has a relatively narrow processing temperature window due to its poor thermal stability and susceptibility to decomposition at high temperatures. Its thermal performance parameters dictate the need for precise temperature control during heating and cooling to ensure product quality. Due to the addition of plasticizers and other additives, the thermal performance parameters of soft PVC will change. The thermal diffusivity and heat transfer coefficient may decrease slightly, the specific heat capacity will increase, and the density will fluctuate depending on the type and amount of plasticizer.
Polystyrene ( PS ) has a thermal diffusivity of 0.08-0.11×10⁻⁶m²/s , a heat transfer coefficient of 0.08-0.13W/(m・K) , a specific heat capacity of 1.3-1.5J/(g・K) , and a density of 1.04-1.09g/cm³ . PS has excellent transparency and gloss, making it commonly used in optical products, packaging containers, and other applications. Its low thermal diffusivity and heat transfer coefficients result in relatively slow heat transfer, which in turn affects the cooling rate during injection molding, requiring extended cooling times. However, its excellent thermal insulation properties also make it advantageous in some packaging applications requiring heat preservation. PS ‘s relatively moderate specific heat capacity allows for relatively stable temperature changes during heating and cooling. However, PS is relatively brittle, requiring careful control of processing conditions during the molding process to avoid cracking due to excessive internal stress.
Common plastics vary in thermal performance parameters, such as thermal diffusivity, heat transfer coefficient, specific heat capacity, and density. These parameters directly impact the processing properties of plastics and the performance of finished products. In the design and production of actual plastic products, engineers must fully consider the thermal characteristics of plastics based on specific application requirements, rationally select plastic materials, and optimize injection molding process parameters to achieve high-quality plastic products. With the continuous advancement of materials science, new plastic materials are constantly emerging, and their thermal performance parameters are constantly being optimized and improved, providing greater room for innovation and development in plastic products.
