Thermal conductivity of commonly used injection mold materials
The thermal conductivity of injection molds is a key performance parameter that influences the quality of plastic part molding and production efficiency. It determines the heat transfer rate and temperature uniformity of the mold during the molding process. Mold materials with high thermal conductivity can quickly transfer heat from the melt, shortening cooling time and improving production efficiency. However, uneven thermal conductivity distribution can lead to different cooling rates in different parts of the plastic part, resulting in internal stress or dimensional deviation. Therefore, a deep understanding of the thermal conductivity characteristics of commonly used injection mold materials and the factors that influence them is crucial for mold material selection and optimization.
Mold steel is the most widely used material in injection molds. Its thermal conductivity varies significantly depending on its composition and heat treatment. Carbon tool steels, such as T10A , contain a high carbon content and have a thermal conductivity of approximately 40-50 W/(m・K) . However, due to their poor hardenability, they are primarily used to manufacture simple, low-precision molds. Alloy tool steels, such as Cr12MoV , incorporate alloying elements such as chromium, molybdenum, and vanadium, reducing their thermal conductivity to 25-35 W/(m・K) . However, they offer high hardness and wear resistance, making them suitable for molding highly filled plastics or glass fiber reinforced plastics. Pre-hardened plastic mold steels, such as 718H , can achieve a hardness of 30-35 HRC after pre-hardening , with a thermal conductivity of approximately 30-40 W/(m・K) . These steels offer excellent processing properties and thermal conductivity, making them widely used in small and medium-sized precision molds. High-speed steels such as W18Cr4V , although they have excellent wear resistance, have a thermal conductivity of only 15-20W/(m · K) and are usually only used in special occasions that require extremely high wear resistance.
The use of non-ferrous metals in injection molds is increasing. Their thermal conductivity is generally higher than that of mold steel, making them suitable for applications requiring high cooling rates. Aluminum alloys, such as 6061-T6 aluminum alloy, are the most commonly used non-ferrous mold materials. Their thermal conductivity can reach 160-180 W/(m・K) , approximately 4-5 times that of ordinary mold steel . This significantly reduces cooling time for plastic parts and improves production efficiency. However, aluminum alloys have a relatively low hardness (approximately 90-110 HB ) and poor wear resistance. They are primarily used for molding small batches or parts with low corrosiveness to the plastic melt, such as PE and PP . Copper alloys, such as beryllium copper ( BeCu ), have a high thermal conductivity of 200-250 W/(m・K) and excellent wear resistance and polishing properties. They are suitable for molding parts requiring high surface quality, such as optical lens molds. However, their high cost limits their large-scale application. In addition, the thermal conductivity of zinc alloy mold materials is about 100-120W/(m・K) , which has low cost and good casting performance, but has low strength and is mostly used for simple molds or trial molds.
The thermal conductivity properties of non-metallic materials, such as mold steel composites and ceramics, offer more options for mold design. Fiber-reinforced mold steel composites, by adding thermally conductive phases such as carbon fibers or graphene to the steel matrix, can increase thermal conductivity by 30%-50% while maintaining high mechanical properties. These composites are suitable for molds requiring efficient cooling and high strength. For example, a Cr12MoV composite with 10% carbon fiber can increase thermal conductivity from 30W/(m・K) to 40-45W/(m・K) . Ceramic materials, such as alumina, have a thermal conductivity of approximately 20-30W/(m・K) , comparable to ordinary mold steel, but offer excellent corrosion and wear resistance, making them suitable for molds used in molding fluoroplastics or other corrosive melts. However, ceramics are brittle and difficult to process, and are typically used only in localized wear-resistant or corrosion-resistant areas of a mold.
The thermal conductivity of mold materials is significantly affected by temperature and microstructure. As temperature increases, the thermal conductivity of most metal materials decreases slightly. For example, the thermal conductivity of 718H mold steel is 35 W/(m · K) at 20 °C , dropping to 32 W/(m · K) at 100 °C . This is because rising temperature intensifies lattice vibrations, hindering thermal conduction by free electrons. The influence of a material’s microstructure on thermal conductivity is even more complex. For example, mold steel treated with spheroidizing annealing has refined and uniformly distributed grains, resulting in a 5%-10% increase in thermal conductivity compared to untreated material . However, materials with pores or inclusions significantly decrease thermal conductivity. For example, an aluminum alloy with 1% porosity can experience a 10%-15% decrease in thermal conductivity . Therefore, during the production and processing of mold materials, strict control of heat treatment processes and internal defects is crucial to ensure thermal conductivity stability.
The optimal selection of mold material thermal conductivity requires a comprehensive assessment of the plastic part’s characteristics and production requirements. For thin-walled parts or applications requiring rapid prototyping, high-thermal-conductivity materials such as aluminum alloy or beryllium copper are preferred to shorten cooling time and improve production efficiency. For example, the cooling time for a 1mm-thick PP part molded using an aluminum alloy mold is approximately 5 seconds, while using conventional mold steel would require 15-20 seconds. For thick-walled parts or products requiring high dimensional accuracy, materials with moderate and uniform thermal conductivity, such as 718H, should be selected to avoid internal stress concentration caused by excessive cooling. For molding reinforced plastics containing glass fiber or mineral fillers, a balance between wear resistance and thermal conductivity is important. Alloy tool steels such as Cr12MoV are typically selected, and their lower thermal conductivity can be compensated for through optimized cooling channel design. Alternatively, a mold structure combining dissimilar materials can be employed, with high-thermal-conductivity materials used on the cavity surface and conventional steel used for the mold base, to achieve a balance between cost and performance.
