Requirements for the selection of materials for injection molded parts
The selection of mold materials is a crucial issue in mold design and manufacturing, directly affecting a range of aspects such as mold manufacturing process, mold lifespan, molding quality of plastic parts, and mold processing costs.
For this reason, scholars both domestically and internationally have developed new types of mold steel with excellent performance and machinability, and minimal deformation after heat treatment, such as pre-hardened steel, new quenched and tempered steel, and martensitic aging steel.
Furthermore, to improve mold lifespan, much research and engineering practice has been conducted on surface strengthening treatment of mold forming parts, achieving excellent results.
To date, the aforementioned research and development work continues to deepen, and the achievements are being vigorously promoted.
Plastic molds generally operate at temperatures between 150 and 200°C, subject to both pressure and temperature effects.
Based on the different usage conditions and processing methods of plastic molding dies, the basic performance requirements for steel used in plastic molds are summarized as follows:
Sufficient surface hardness and wear resistance
The hardness of plastic molds is typically below 50-60 HRC. Heat-treated molds should have sufficient surface hardness to ensure adequate rigidity. During operation, molds withstand significant compressive stress and friction due to the filling and flow of plastic, requiring them to maintain shape and dimensional accuracy to ensure a sufficient service life. The wear resistance of a mold depends on the chemical composition and heat treatment hardness of the steel; therefore, increasing the mold’s hardness improves its wear resistance.
Good machinability
Steel grades that are easy to cut and produce high-precision parts after machining should be selected. Medium carbon steel and medium carbon alloy steel are most commonly used for this purpose, which is especially important for large molds.
For parts requiring electrical discharge machining (EDM), the steel grade also needs to have a thin burn-hardened layer.
Excellent polishing performance
High-quality plastic products require low surface roughness of the mold cavity. Therefore, the working surfaces of injection molded parts often need to be polished to a mirror finish, with Ra≤0.05μm.
A steel hardness of 35~40 HRC is ideal, as excessively hard surfaces will make polishing difficult. Therefore, the selected steel should have few impurities, a uniform and dense microstructure, no fiber orientation, and should not exhibit pitting or orange peel-like defects during polishing.
Good thermal stability
The shapes of plastic injection mold parts are often complex and difficult to process after quenching.
Therefore, materials with good thermal stability should be selected as much as possible so that the molded parts have a small coefficient of linear expansion after heat treatment, small deformation after heat treatment, small dimensional change rate caused by temperature difference,
stable metallographic structure and mold size, and can reduce or eliminate processing, thereby ensuring the mold dimensional accuracy and surface roughness requirements.
Good wear resistance and fatigue resistance
The injection mold cavity is subjected not only to the scouring of high-pressure molten plastic but also to the thermal stress of alternating hot and cold temperatures.
While high-carbon alloy steel can achieve high hardness through heat treatment, it suffers from poor toughness and is prone to surface cracking, making it unsuitable.
The selected steel grade should minimize the frequency of polishing and mold repair, maintain the dimensional accuracy of the cavity over a long period, and achieve the service life required for mass production.
Corrosion resistance
For some types of plastics, such as polyvinyl chloride and flame-retardant plastics, it is necessary to consider using steel with corrosion resistance.
In addition, when selecting materials, it is necessary to consider preventing scratches and adhesion.
If there is relative movement between the two surfaces, try to avoid selecting materials with the same structure. In special cases, one side can be plated or nitrided to make the two sides have different surface structures.