Basis for selecting materials for injection mold parts
The selection of materials for injection mold components is a critical step in mold design and manufacturing, directly impacting mold life, molding accuracy, production costs, and part quality. Different mold components are subject to varying loads, temperatures, friction, and other conditions during operation, necessitating the selection of appropriate materials based on their function and operating environment. The selection of mold materials requires comprehensive consideration of factors such as the component’s stress state, wear resistance, corrosion resistance, processing performance, and cost, ensuring a balance between cost-effectiveness and reliability while still meeting the mold’s intended use.
The operating conditions of mold components are the primary consideration for material selection, as the environments faced by different parts during the injection molding process vary significantly. The cavity and core are in direct contact with the high-temperature melt, subjecting them to high injection pressure (typically 30-150 MPa), temperatures (150-300°C, depending on the type of plastic), and friction. Therefore, materials with excellent wear resistance, high heat resistance, and high strength are essential. For example, pre-hardened steel (such as 718H) can be used for the cavity and core of common plastics like ABS and PP. Its hardness of 30-35 HRC offers excellent processability and wear resistance. However, when molding harder plastics like PC and POM, or those containing glass fiber, hardened and tempered steel (such as SKD61) is preferred. Its hardness can reach 45-50 HRC, providing enhanced wear and corrosion resistance. Ejector parts such as ejectors and inclined ejector rods move frequently during operation and are subjected to repeated impact loads and friction. Therefore, materials with good toughness and high wear resistance, such as SKD61 or ASP60, need to be selected. At the same time, surface nitriding treatment is required to increase surface hardness.
The material properties and quality requirements of plastic parts significantly influence the selection of mold materials. When polyvinyl chloride (PVC) is used as a molding material, hydrogen chloride gas is released during the molding process, which can corrode the mold. Therefore, components such as the cavity and core should be made of corrosion-resistant materials, such as stainless steel (e.g., S136), and mirror-polished to prevent corrosion from affecting the surface quality of the part. For applications requiring a scratch-free, high-gloss surface (such as appliance housings and automotive interiors), the cavity and core should be made of materials with good polishing properties, such as pre-hardened 718H steel or S136 stainless steel. These materials can achieve a surface roughness of Ra0.02μm after fine polishing, ensuring surface quality. If the plastic part contains reinforcing materials such as glass fiber, which have poor flowability and cause severe wear on the mold, mold components should be made of highly wear-resistant materials, such as powdered high-speed steel ASP60, to extend the mold’s life.
Mold production batch size and production cost are important economic factors in material selection. For molds used in small-volume production (such as trial molds or annual production of less than 10,000 pieces), lower-cost materials such as carbon structural steel (e.g., 45 steel) can be used for the cavity and core. Surface hardening can be used to increase surface hardness and reduce mold manufacturing costs. For molds used in medium-volume production (10,000-100,000 pieces per year), pre-hardened steel (e.g., 718H) is often used. This steel can be directly machined without subsequent heat treatment, reducing processing steps and costs while also offering excellent wear resistance and strength. For molds used in large-volume production (over 100,000 pieces per year), high-performance alloy tool steels (e.g., SKD61 and H13) are essential. These materials, after quenching and tempering, possess exceptional hardness and wear resistance, capable of withstanding long-term production wear. While the material cost is higher, this can extend mold life and reduce mold costs per unit. In addition, the processing performance of the material will also affect the production cost. Pre-hardened steel has good cutting performance and high processing efficiency, and is suitable for the processing of parts with complex shapes; while high-speed steel is difficult to process, requires special processing equipment, and has high processing costs, so it is only used in occasions with special requirements.
The machinability of mold components is a crucial factor in material selection, directly impacting mold manufacturing precision and production efficiency. A material’s cutting properties, heat treatment properties, and polishing characteristics all influence the machining process. Pre-hardened steel (such as 718H) already has a certain hardness (30-35 HRC) upon shipment, allowing for direct milling, grinding, and EDM machining without the need for subsequent heat treatment. This avoids part deformation associated with heat treatment and ensures machining accuracy, making it suitable for machining complex cavities and cores. Quenched and tempered steel (such as SKD61) requires quenching and tempering after forming to improve its hardness and wear resistance. However, this heat treatment process can cause part deformation, necessitating a machining allowance, increasing the number of steps and cost. This makes it suitable for relatively simple parts. Stainless steel (such as S136) offers excellent polishing and corrosion resistance, but has poor cutting properties and is prone to tool sticking during machining. It requires specialized cutting tools and processing parameters, resulting in higher processing costs. Therefore, when selecting materials, it is necessary to consider the shape complexity of the parts and the processing equipment conditions and select materials with good processing properties.
The heat treatment performance and dimensional stability of mold components are also important considerations in material selection. For mold components requiring high precision (such as guide pins, guide bushings, and locating pins), the material must exhibit excellent dimensional stability to resist deformation during heat treatment and use. Alloy tool steel (such as Cr12MoV), after proper heat treatment, achieves a uniform microstructure and high hardness, offering excellent dimensional stability and making it suitable for high-precision guide components. Carbon structural steel (such as 45 steel), on the other hand, has poor hardenability and is prone to deformation and cracking after heat treatment, making it suitable only for parts requiring less precision. Furthermore, the material’s coefficient of thermal expansion can affect the mold’s dimensional stability. For large mold components, materials with a low coefficient of thermal expansion, such as cast iron (e.g., HT300), should be selected to minimize dimensional deviations caused by temperature fluctuations. For mold components operating in high-temperature environments (such as nozzles in hot runner systems), materials with excellent heat and oxidation resistance, such as heat-resistant steel (e.g., H13), should be selected to prevent oxidation and softening under prolonged high temperatures.
