Steel for plastic injection molds: A deeper understanding of selection and application

Steel used for plastic injection molds is crucial in the plastic manufacturing industry. The choice of the right type of steel affects the mold’s durability and lifespan and directly impacts production costs and product quality. This article looks at commonly used steel types, their characteristics, selection processes, and applications in plastic injection mold production.

1. Commonly used steel types

a. Stainless steel

Stainless steel is widely used for molds requiring high corrosion resistance. It is particularly useful in the production of plastic products for food or medical applications.

• Composition: Contains at least 10.5% chromium, which helps create a protective layer against corrosion.
• Applications: Producing plastic products such as water bottles and medical devices.

b. High-strength steel

High-strength steel is typically used for molds that must withstand high pressure during the injection process. They exhibit good strength and heat resistance.

• Characteristics: Can achieve tensile strength of up to 2000 MPa.
• Applications: Molds for manufacturing automotive parts and electronic equipment.

c. Alloy steel

Alloy steel consists of various steel types with different alloying elements such as nickel, chromium, and molybdenum. Two commonly used types are P20 and S7.

• P20: Commonly used for general-purpose injection molds due to its hardness and good machinability.
• S7: This steel has high hardness and impact resistance, often used for molds made from difficult-to-machine materials.

d. Cast steel

Cast steel is often used for large and complex molds. The casting process allows for the creation of intricate shapes without extensive machining.

• Process: Steel is cast into the desired shape and then undergoes heat treatment to improve mechanical properties.
• Applications: Molds for large products such as plastic containers and industrial equipment.

2. Key characteristics of mold steel

a. Hardness

The hardness of steel is an important factor that affects the wear resistance of the mold. Mold steel typically has a hardness of 45-60 HRC.

• Hardness testing: Use a Rockwell hardness tester to determine the hardness of the steel.

b. Heat resistance

Plastic injection molds often need to withstand high temperatures during production. Therefore, steel needs good heat resistance to avoid deformation.

• Heat resistance index: Some steel types can withstand temperatures up to 300°C without deforming.

c. Ductility

The ductility of steel is its ability to undergo deformation without fracture. This is crucial to ensure the mold can operate for a long time without issues.

• Tensile testing: Assess ductility through tensile testing to determine the steel’s ability to deform.

d. Machinability

Mold steel should be easy to machine to create complex shapes. The machinability of steel affects the time and cost of mold production.

• CNC machining: Use CNC machining technology to ensure high precision in mold shaping.

3. Choosing the right steel

a. Identify needs

Before selecting steel, clearly define production needs, the type of plastic products to be manufactured, and special requirements such as heat resistance and corrosion resistance.

• Product analysis: Consider the mechanical and chemical property requirements of the final product.

b. Expert consultation

If unsure about which steel type is appropriate, consult industry experts or steel suppliers.

• Choose reputable suppliers: Seek suppliers with experience and a good reputation in supplying steel for the plastic industry.

c. Cost comparison

Compare costs between different steel types, not just the purchase price but also maintenance costs and the lifespan of the mold.

• Total cost of ownership: Calculate the total cost of ownership (TCO) to evaluate the long-term costs of each steel type.

4. Mold production process

a. Mold design

• Use CAD software: Design 3D molds to simulate and optimize shapes before production.

b. Machining

• Cutting and machining: Use CNC cutting machines and milling machines to create precise mold components.

c. Heat treatment

• Heat treatment: Perform heat treatment processes to enhance the hardness and durability of the steel.

d. Assembly and testing

• Assemble the mold: Assemble the mold components and verify accuracy.
• Quality testing: Assess the quality of the mold before production.

Learn more about the Plastic Injection Molding process here

5. Mold maintenance

a. Regular maintenance

• Schedule maintenance: Establish a regular maintenance schedule to inspect and repair molds.

b. Mold cleaning

• Regular cleaning: Ensure molds are kept clean to avoid clogs and other production issues.

Learn more about plastic injection mold maintenance items here!

Conclusion

Choosing the right steel for plastic injection molds is an important decision that significantly impacts production efficiency and product quality. By understanding the various types of steel, their characteristics, and the mold production process, you can make the best decision for your project. Always stay updated on new technologies and maintenance processes to ensure your molds operate efficiently and have a long lifespan.