Plastic Molds: Basic Properties of Steel for Plastic Molding

Release Date:

2021-03-19 15:39

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Based on the varying applications and manufacturing processes of plastic molds, the fundamental performance requirements for steels used in plastic mold making can be summarized as follows:

According to Plastic mold Based on the various uses and processing methods, the fundamental performance requirements for steel materials in plastic molds are summarized as follows:

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I. Sufficient surface hardness and wear resistance

1. The hardness of molds is generally below 50–60 HRC. After heat treatment, the mold should possess adequate surface hardness to ensure sufficient rigidity.

2. In the manufacturing process, due to plastic filling and flow, the mold must maintain stable shape and dimensional accuracy to ensure a sufficient service life.

3. Wear resistance depends on the steel’s chemical composition and its heat‑treatment hardness; therefore, increasing the hardness of Chinese molds can help enhance their wear resistance.

II. Excellent Machinability

It is used for steels that are easy to cut, enabling the production of high‑precision parts after machining. It is commonly employed with medium‑carbon steels and medium‑carbon alloy steels and is particularly important for large molds. For components requiring electrical discharge machining, the case‑hardened layer in these steel grades must also be thinned.

III. Excellent Polishing Performance

High‑quality plastic molds require a low surface roughness on the cavity. The working surfaces of injection‑molded parts must be polished to a mirror finish, with a Ra value of ≤0.05 µm. A steel hardness of 35–40 HRC is optimal; excessively hard surfaces can make polishing more difficult. The selected steel should contain few impurities, exhibit a uniform and dense microstructure, show no fiber‑orientation effects, and remain free of pitting or orange‑peel defects during polishing.

IV. Excellent Stability

Plastic mold The geometry of component parts is often complex and difficult to machine after quenching; therefore, it is advisable to select materials with excellent thermal stability, ensuring that Chinese mold‑forming components exhibit a low coefficient of linear thermal expansion, minimal distortion during heat treatment, and reduced dimensional variability caused by temperature differentials. This results in stable microstructures and dimensions, thereby minimizing or eliminating the need for subsequent machining operations and enabling precise dimensional accuracy and compliance with surface‑roughness requirements.

V. Corrosion Resistance

1. For certain plastic grades, such as PVC and flame-retardant plastics, corrosion-resistant steel grades should be considered.

2. In addition, when selecting materials, it is essential to consider preventing abrasion and adhesion. If there is relative motion between two surfaces, it is advisable to avoid choosing materials with identical microstructural characteristics. Under special circumstances, one surface may be coated or nitrided to impart distinct surface properties to the two sides of the plastic mold.




Keywords:

Plastic mold

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