When a mold designer designs a mold, he or she needs to choose correct mold steel for the mold, so the mold designer need to know some basic requirements for the performance of plastic mold steel:
High rigidity and sound abrasion resistance, surface hardness of cavity should be 30—60 HRC and hardenability>55HRC, sufficient hardening depth is required and sufficient obdurability is demanded at the heart therein to avoid brittle fracture and deformation.

Injection molds are an indispensable tool in the production process of plastic products. With the advancement of technology, plastic products such as tableware and car parts have become ubiquitous and have penetrated into every aspect of our lives. These products are mainly manufactured through the injection molding process, which involves heating the plastic until it melts, injecting it into the mold, and then cooling it to form the various shapes and sizes we need.

The quality of the mold is directly related to the quality of the plastic product, so it is crucial to choose high-grade mold steel. High-quality mold steel must be able to withstand:

Long-term high-pressure conditions,

High-speed impact of molten plastic at high temperatures.

In addition, molds are reused repeatedly, often millions of times, which requires mold steels with excellent wear resistance, corrosion resistance and sufficient toughness.

Causes of Injection Mold Damage
Before delving into the characteristics required of mold steel, it is important to understand how molds are usually damaged:

Mold Deformation Molds that work under repeated high-temperature and high-pressure conditions will deform over time. Specifically, the mold cavity may be dented, affecting the dimensional accuracy of the molded product and may cause the formation of flash (excess material).

Wear The clean (sharp) edges of the mold gradually become blunt over time, resulting in rounded corners and surface depressions. The most significant result of this wear is the production of flash.

Fracture After a period of use, the mold will develop cracks due to material fatigue. These cracks tend to expand over time and with continued use, eventually leading to the breakage of some parts of the mold.

Corrosion When using acidic plastic materials or materials that release corrosive gases during processing (such as PVC and POM), the mold surface may corrode over time. This corrosion causes surface unevenness, the so-called pitting, which has an adverse effect on the appearance of the final product.

Basic requirements for mold steel materials
Hardness, wear resistance and toughness The hardness and wear resistance of steel are crucial in mold manufacturing. This is because the mold is subjected to continuous wear and tear from the molten plastic during the injection process, especially when the plastic contains hard fillers such as glass fiber, which can cause greater damage. In addition, the mold is subjected to impact during operation, so a certain toughness is required to prevent cracking. When processing high-temperature plastics, the need for wear-resistant steel is exacerbated, and high-performance mold steel is required.

Polishing performance High-quality mold steel should have high purity and a fine and uniform structure to ensure that the mold surface is perfectly polished. Porosity, impurities or other defects on the mold surface can adversely affect the appearance of the final product. For example, S136 steel is known for its excellent polishing properties.

Weldability Molds are inevitably subject to wear and accidental damage during use, and sometimes need to be adjusted due to design changes. Therefore, the weldability of mold steel is crucial, which determines the difficulty and feasibility of mold repair and modification.

Corrosion resistance For molds that process corrosive plastics, the use of corrosion-resistant steel is essential. Common corrosion-resistant steels include S136, 2316, 420, 2083, M333, etc. In addition, for molds that are frequently changed in production batches, good corrosion resistance can reduce the need for rust prevention maintenance.

Small deformation during heat treatment Mold steel will harden during heat treatment, increasing the difficulty of processing and thus increasing costs. In order to improve production efficiency and reduce costs, it is necessary to use steel with uniform internal structure and small deformation during heat treatment. This material will not produce excessive internal stress or deformation, ensuring the accuracy and life of the mold.

In summary, choosing the right mold steel material is not only crucial to ensuring the quality of plastic products, but also crucial to improving the durability of the mold, reducing production and maintenance costs, and improving the efficiency and economy of the entire production chain.

Pre-hardened vs. through-hardened tool steels
Why choose pre-hardened tool steels?

Typically, molds need to be hardened through or case hardened to reach hardness levels between HRC50-60. This hardness range increases processing difficulty and cost.

To achieve such hardness requires heat treatment, especially in the case of through quenching, there is a risk of deformation and extended manufacturing cycle. Therefore, pre-hardened tool steels were developed, usually with a hardness between HRC30-40 – a range that is easier to manage in terms of processing, but hard enough for many plastic injection molds (such as PP, PE, ABS, etc.).

Pre-hardened tool steels
Pre-hardened tool steels are steels that have been hardened before leaving the factory. Its hardness usually meets the requirements for use, which means that it can be directly used in mold processing and production without the need for additional heat treatment by the user.

The main advantage of this steel is its ease of use, as it saves production time and cost by reducing the risk of dimensional deformation during heat treatment. Pre-hardened tool steels are usually used for shorter production cycles and smaller batches, or when the mold size makes heat treatment impractical.

Common grades of pre-hardened tool steel include: P20, 718/718H/718HH, 738H, NAK80, S136H and 2083H.

Through-hardened tool steel
In contrast, through-hardened tool steel is supplied in a softened state and requires heat treatment during or after the mold manufacturing process to achieve the required hardness. The hardness of this steel can be precisely controlled through heat treatment, resulting in higher wear resistance and toughness.

Hardened tool steel is suitable for molds that need to run for a long time and withstand high wear, especially when the same plastic products are produced in batches. However, the need for post-production heat treatment increases production costs and requires careful handling to prevent deformation and stress problems caused by uneven heating or cooling.

Common through-hardened tool steel grades are: 2083, S136, H13, SKD61, 8407 and SKD11

In summary, pre-hardened and through-hardened tool steels each have advantages and disadvantages. The choice between them depends on specific production needs, mold design and economic efficiency. Pre-hardened tool steels offer convenience and speed and are ideal for small batches or molds of a specific size; whereas hardened tool steels are more suitable for mass production molds that require very high wear resistance and long-term use.