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The selection of mold steel directly affects the lifespan of the mold, product quality, and manufacturing cost. Choosing the right one yields twice the result with half the effort, while choosing the wrong one often leads to frequent repairs or even premature scrapping. So, from the perspective of just procurement, our goal is to complete the project with the least cost. So, how should we choose the mold steel?

I. Three Basic Principles for Selecting Materials

Selecting materials is not based on intuition or guesswork; it requires a comprehensive consideration of three aspects.

Firstly, it is necessary to meet the performance requirements for use. During the use of the mold, it will be subjected to various loads, including impact, wear, high temperature, and corrosion, etc. The selected material must have the ability to resist these failure forms. This is the primary principle for material selection.

Secondly, it is necessary to consider the process performance. The material must be able to be processed into the required shape of the mold, including cutting processing performance, heat treatment processability, forging performance, and welding performance, etc. Even the best steel is of no use if it cannot be processed.

Finally, it is necessary to focus on economy. Under the premise of meeting the usage requirements, choose materials with lower costs. Not all expensive steel is better. Adequate and applicable materials are the best choice.

II. Three Categories of Mold Steel

Based on different working conditions, mold steel is mainly divided into three categories: cold working mold steel, hot working mold steel, and plastic mold steel.

Cold working mold steel is mainly used for metal forming processing at room temperature. Typical applications include punching dies, cold extrusion dies, wire drawing dies, and cold heading dies. This type of steel needs to have high hardness and high wear resistance. Common steel grades include Cr12, Cr12MoV, CrWMn, and D2, etc.

Hot working mold steel works under high-temperature conditions and withstands thermal fatigue and thermal wear, mainly used for die casting dies, hot forging dies, and hot extrusion dies. This type of steel needs to have good thermal stability and resistance to thermal fatigue. H13 is currently the most widely used steel grade, and there are also 3Cr2W8V and DIEVAR, etc.

Plastic mold steel is used for various injection molds, blow molds, and extrusion molds. Depending on specific requirements, it may need to have corrosion resistance, high polishing performance, or pre-hardening characteristics. Common steel grades include P20, 718, NAK80, and S136, etc.

III. How to Choose Cold-Working Die Steels

The main factor to consider is the production batch size. For quantities less than 100,000 pieces, T10A is sufficient as it is inexpensive and reliable. For quantities between 100,000 and 1,000,000 pieces, CrWMn is recommended due to its good hardenability and small thermal treatment deformation. For quantities over 1,000,000 pieces, Cr12MoV or D2 should be used, which is more expensive but more durable. If the production quantity starts at several million pieces, high-speed steel or hard alloy would be more cost-effective.

In addition, the material to be processed also needs to be considered. For soft materials like aluminum and copper, low alloy steel is sufficient. For high-strength steel or stainless steel, Cr12MoV or high-speed steel should be used for better wear resistance.

IV. How to Choose Hot-Working Die Steels

Hot-working die steels work at high temperatures. The key factors are heat resistance and fatigue resistance. The most commonly used is H13, which has good toughness and heat resistance, and can be used for aluminum die casting and hot forging dies. The working temperature is approximately 590 degrees Celsius. At higher temperatures, such as copper die casting, 3Cr2W8V should be used, which can handle up to 650 degrees Celsius, but with slightly poorer toughness. For large aluminum die casting dies, DIEVAR is recommended. It has improved the formula based on H13, with better toughness and thermal stability.

By the way, in recent years, the mainstream improvement direction for H13 is low silicon and high molybdenum, which can enhance hardenability and anti-recovery ability, resulting in better comprehensive performance.

V. How to Select Plastic Mold Steel

When choosing plastic mold steel, factors to consider include production volume, whether the plastic is corrosive, and whether mirror finish is required.

For production volumes below 100,000 times, 45 steel or P20 can be used, which has the lowest cost. For production volumes ranging from 100,000 to 1 million times, P20, 718 or NAK80 can be used. These materials are pre-hardened during production, eliminating the need for heat treatment during processing, ensuring stable dimensions. For production volumes above 1 million times, S136 or H13 can be used. After processing, they are quenched and tempered, resulting in high hardness and better wear resistance.

If the plastic is corrosive (such as PVC or POM), stainless steel must be used. S136 or 2316 are the preferred options. If the product requires a mirror finish (such as optical lenses or high-gloss components), NAK80 has good polishing properties, while S136 offers both corrosion resistance and mirror finish. The choice depends on your preference.

VI. Selection of Pre-hardened Steel and Quenched Steel

In the field of plastic molds, the selection of pre-hardened steel and quenched steel is a common issue.

Pre-hardened steels such as P20 and 718 have been pre-heated to a hardness of 30 to 40 Rockwell degrees at the factory. Customers can directly perform mechanical processing, and after the processing is completed, there is no need for further heat treatment, and it can be directly assembled and used. The advantage of this material lies in its good dimensional stability, as it will not deform due to heat treatment. It is particularly suitable for medium and large-sized molds.

Quenched steel such as S136 and H13 is in an annealed state when it leaves the factory, with a lower hardness that is easier to process. After the mold processing is completed, it needs to be sent to the heat treatment workshop for quenching and tempering, with a hardness of 48 to 52 Rockwell degrees. Its wear resistance and lifespan are significantly better than pre-hardened steel. However, the disadvantage is that there is a risk of deformation during the heat treatment process, and sufficient machining allowance needs to be reserved and fine machining should be carried out after heat treatment.

In simple terms, for convenience and dimensional stability, pre-hardened steel is chosen; for extreme lifespan and wear resistance, quenched steel is chosen.

VII. Material Selection Checklist

When selecting materials in practice, it is recommended to confirm the following issues item by item: Is the mold cold working, hot working, or a plastic mold? What is the production batch size? What is the hardness and corrosion resistance of the processed material? Is the mold size large enough that high-quenching-strength materials are required? Is mirror-polishing of the product surface needed? Is the working temperature normal or high? Is the cost budget allowing for the selection of imported or high-end domestic steel?

After answering these questions, the range of suitable steel materials has been basically determined.