Injection Mold Material Selection

Introduction to Mold Steels

Mold steels can generally be divided into three categories: cold work mold steels, hot work mold steels, and plastic mold steels. These materials are used in forging, stamping, cutting, die casting, and various other manufacturing processes. While aluminum molds for injection molding have gained popularity for certain applications, traditional mold steels remain essential for many high-performance requirements.

Due to the diverse applications and complex working conditions of various molds, mold steels must possess specific properties tailored to their intended use. These properties typically include high hardness, strength, wear resistance, sufficient toughness, as well as high hardenability, hardenability, and other technological properties. The performance requirements vary significantly depending on the application, whether we're discussing traditional steel molds or modern aluminum molds for injection molding.

Cold Work Mold Steels

Cold work molds include cold stamping dies, wire drawing dies, drawing dies, embossing dies, thread rolling dies, wire rolling plates, cold heading dies, and cold extrusion dies. Unlike aluminum molds for injection molding, which are valued for their thermal conductivity, cold work mold steels must possess high hardness, strength, wear resistance, sufficient toughness, as well as high hardenability, hardenability, and other technological properties to withstand their specific working conditions.

Alloy tool steels used for these purposes are generally high-carbon alloy steels with a carbon content of more than 0.80%. Chromium is an important alloying element in these steels, typically present at no more than 5%. However, for some molds requiring very high wear resistance and minimal quenching deformation, the maximum chromium content can reach 13%. To form large amounts of carbides, the carbon content in these steels is also very high, up to 2.0% to 2.3%.

Cold work die steels have a relatively high carbon content, and their structure is mostly hypereutectoid or ledeburitic steel. Commonly used steel types include high-carbon low-alloy steels, high-carbon high-chromium steels, chromium-molybdenum steels, and medium-carbon chromium-tungsten steels. While aluminum molds for injection molding offer advantages in certain production scenarios, cold work mold steels remain irreplaceable for applications requiring extreme hardness and wear resistance.

Microstructure of cold work mold steel showing carbide distribution - a key factor in wear resistance compared to aluminum molds for injection molding

Hot Work Mold Steels

Hot work molds are divided into several main types: hammer forging, die forging, extrusion, and die casting molds. This category includes hot forging dies, press forging dies, stamping dies, hot extrusion dies, and metal die casting dies. Unlike aluminum molds for injection molding, which excel in rapid cooling applications, hot work molds must withstand not only enormous mechanical stress but also repeated heating and cooling, which causes significant thermal stress.

In addition to high hardness, strength, red hardness, wear resistance, and toughness, hot work die steels must also possess good high-temperature strength, thermal fatigue stability, thermal conductivity, and corrosion resistance. Furthermore, they require high hardenability to ensure consistent mechanical properties across the entire cross-section.

For die casting mold steels, additional properties are required: the surface layer must not crack after repeated heating and cooling, and it must withstand the impact and erosion of liquid metal flow. These steels are generally medium-carbon alloy steels with a carbon content of 0.30% to 0.60%, belonging to hypoeutectoid steels. Some steels become eutectoid or hypereutectoid due to the addition of more alloying elements (such as tungsten, molybdenum, vanadium, etc.).

Commonly used steel types include chromium-manganese steels, chromium-nickel steels, and chromium-tungsten steels. While aluminum molds for injection molding can handle moderate temperatures, hot work mold steels are specifically engineered to maintain their properties at extreme temperatures encountered in metal casting processes.

Plastic Mold Steels

Plastic molds include thermoplastic plastic molds and thermosetting plastic molds. Plastic mold steels require certain properties such as strength, hardness, wear resistance, thermal stability, and corrosion resistance. Additionally, they must have good processability, such as minimal heat treatment deformation, good machinability, corrosion resistance, good grinding and polishing performance, good welding performance, high roughness, good thermal conductivity, and dimensional and shape stability under working conditions.

In general, injection molding or extrusion molding dies can use hot work die steels. Thermosetting molding and molds requiring high wear resistance and strength can use cold work die steels. Increasingly, manufacturers are also considering aluminum molds for injection molding as a viable alternative for certain plastic molding applications, particularly where rapid cycle times and cost efficiency are priorities.

Mold Steel Classification

1. Cold Work Mold Steels

Comparison of various mold steel grades showing different microstructures and finishes, highlighting the diversity of options compared to aluminum molds for injection molding

Mold Steel Properties

1. Machinability

Machinability encompasses several aspects of material processing:

Cold work die steels are mostly hypereutectoid steels and ledeburitic steels, with relatively poor hot working and cold working properties. Therefore, it is necessary to strictly control the process parameters of hot working and cold working to avoid defects and waste products. This is one area where aluminum molds for injection molding often have an advantage, as aluminum generally offers superior machinability compared to high-alloy steels.

On the other hand, by improving the purity of the steel, reducing the content of harmful impurities, and improving the microstructure of the steel, the hot working and cold working properties of the steel can be improved, thereby reducing the production cost of the mold. To improve the cold working performance of die steel, research began in the 1930s to add S, Pb, Ca, Te and other free-cutting elements or elements that cause graphitization of carbon in die steel, developing various free-cutting die steels to further improve their cutting performance and grinding performance, reduce tool abrasive consumption, and lower costs. These improvements help narrow the machinability gap between traditional steels and aluminum molds for injection molding.

2. Hardenability and Hardenability

Hardenability mainly depends on the chemical composition of the steel and the original microstructure before quenching; hardenability mainly depends on the carbon content in the steel. For most cold work die steels, hardenability is often one of the main considerations. For hot work die steels and plastic die steels, mold sizes are generally larger, especially for manufacturing large molds, where their hardenability is more important.

Additionally, for various molds with complex shapes that are prone to heat treatment deformation, in order to reduce fire deformation, it is often necessary to use quenching media with weaker cooling capacity, such as air cooling, oil cooling, or salt bath cooling. To obtain the required hardness and hard layer depth, it is necessary to use die steels with better hardenability. These properties are particularly important for large or complex molds where aluminum molds for injection molding would not provide sufficient structural integrity.

While aluminum molds for injection molding offer advantages in terms of thermal conductivity and ease of machining, they cannot match the hardenability characteristics of properly heat-treated tool steels. This makes steel molds indispensable for applications requiring deep hardening and high wear resistance over extended production runs.

Mold Component Material Selection Principles

The materials for mold cavities, cores, mold bases, or other key components should follow the materials specified by the customer. For general mold structural parts, the mold manufacturer can select them according to actual needs, but must ensure reliable operation, wear resistance, durability, and service life meet the requirements of the "Technical Contract". When considering material options, it's important to evaluate both traditional steel solutions and alternatives like aluminum molds for injection molding based on specific application requirements.

Injection mold assembly showing various components made from different materials, including both specialized steels and aluminum molds for injection molding in certain applications

Mold Component Material Guidelines