The steel used for plastic molds covers a wide range, from ordinary steel to special steel. This was more obvious in the early days of plastic molds and is now becoming more advanced.

Special steel has its own system. The chemical composition of mold steel is basically the same as that of alloy tool steel, but the metallurgical quality is higher, the processability is better, and it is conducive to heat treatment.

Plastic mold steel has strict performance requirements and is difficult to heat treat.

In order to avoid deformation of mold parts or other heat treatment problems during toughening, it is more common for mold steel to be supplied to the market in a pre-hardened state.

Plastic products made from petrochemical products have varying degrees of corrosiveness, so corrosion-resistant steel is also more commonly used for such injection molding.

With the application of pre-hardened steel and non-quenched and tempered steel, convenient cutting is also a feature of plastic mold steel.

Selection and heat treatment of mold steel
In the manufacturing cost of plastic molds, processing and polishing account for 70% to 80%. Therefore, when selecting mold steel, it is necessary to ensure the performance requirements of the mold and improve the processing performance of the mold.

The selection method can be carried out as follows:

According to the type and quality requirements of plastic products.

According to the production batch of plastic parts. When other factors are determined, the smaller the production batch, the lower the requirements for mold wear resistance and service life, so steel with lower performance can be selected. For small batches, quenched and tempered steel or carbon steel can be selected. For medium batches (300,000 to 1 million injections), P20, 5NiSCa, SM2, etc. can be selected; for large batches, 5NiSCa, PMS, etc. can be selected.

Choose according to the size and precision requirements of the plastic parts. For large and high-precision injection molds, such as plastic bottle cap molds, when the production volume of plastic parts is large, pre-hardened steel is often used to prevent heat treatment deformation.
Choose according to the complexity of the plastic parts. Mold manufacturers should fully consider the processing technology of the mold and try to choose materials that are easy to cut, have small heat treatment deformation, and have good wear resistance.

Certain heat resistance and erosion resistance.
Due to the generally complicated structure of plastic molds, it requires low surface roughness and high precision for the cavity and excellent technical performance should be ensured. The specific requirement is as follows:

a) Slight deformation during heat treatment and sound hardenability.
b) Sound performance for cutting processing and superior polishing property and abrasion resistance.
c) For plastic molds employing cold extrusion forming process, it requires sound cold extrusion molding ability, and to make it convenient for molding, the hardness after anneal should be low, the plasticity good, and deformation resistance small. The deformation resistance after quenching is high.
d) Other processing properties, such as forging and jointing properties, should be fine.

Below I would like to share some basic principles for mold steel selection:

1. Wear resistance

When the blank undergoes plastic deformation in the mold cavity, it flows and slides along the surface of the cavity, causing severe friction between the cavity surface and the blank, causing the mold to fail due to wear. Therefore, the wear resistance of the material is one of the most basic and important properties of the mold.

Hardness is the main factor affecting wear resistance. Generally speaking, the higher the hardness of the mold parts, the smaller the wear amount and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material.

2. Toughness

The working conditions of the mold are mostly very bad, and some are often subjected to large impact loads, resulting in brittle fracture. In order to prevent the mold parts from breaking suddenly during operation, the mold must have high strength and toughness.

The toughness of the mold mainly depends on the carbon content, grain size and organizational structure of the material.

3. Fatigue fracture performance

During the working process, the mold often suffers from fatigue fracture under the long-term action of cyclic stress. The forms include small energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture and bending fatigue fracture.

The fatigue fracture performance of the mold mainly depends on its strength, toughness, hardness and the amount of inclusions in the material.

4. High temperature performance

When the working temperature of the mold is high, the hardness and strength are reduced, resulting in early wear or plastic deformation of the mold and failure. Because the mold material should have a high tempering resistance stability to ensure that the mold has a high hardness and strength at the working temperature.

5. Resistance to cold and hot fatigue

Some molds are in a state of repeated heating and cooling during the working process, which makes the surface of the cavity subject to tensile stress and compressive stress, causing surface cracking and peeling, increasing friction, hindering plastic deformation, and reducing dimensional accuracy, thus causing mold failure. Cold and hot fatigue is one of the main forms of failure of hot working molds, and should have high resistance to cold and hot fatigue.

6. Corrosion resistance

Some molds, such as plastic molding, when working, because the plastic contains elements such as chlorine and fluorine, after heating, they decompose to produce highly corrosive gases hci and hf, which erode the surface of the mold cavity, increase its surface roughness, and aggravate wear.

The mold meets the process performance requirements
The mold manufacturing process generally includes several processes such as forging, cutting, and heat treatment. In order to ensure the quality of the mold and reduce production costs, the material should have good forging, cutting, hardenability, hardenability and grindability, and low oxidation, decarburization sensitivity, quenching, and deformation cracking tendency.

1. Forgeability

Hot forging deformation resistance is small, plasticity is good, forging temperature range is wide, forging cracking tendency is small, and network carbide precipitation is small.

2. Annealing processability

Spheroidizing annealing temperature range is wide, annealing hardness is low, fluctuation range is small, and spheroidization rate is high.

3. Cutting processability

The cutting amount is large, the tool loss is small, and the surface roughness is low.

4. Oxidation and decarburization sensitivity

When heated at high temperature, the oxidation resistance is good, the decarburization speed is slow, it is insensitive to the heating medium, and the tendency to produce pitting is small.

5. Hardenability

The surface hardness is uniform and high after quenching.

6. Hardenability

A deep hardened layer can be obtained after quenching, and it can be hardened with a mild quenching medium.

7. Quenching deformation cracking tendency

Conventional quenching has small volume change, warping, slight distortion, and low abnormal deformation tendency. Conventional quenching has low cracking sensitivity and is insensitive to quenching temperature and workpiece shape.

8. Grindability

The grinding wheel has small relative loss, large maximum grinding volume without burning, is insensitive to grinding wheel quality and cooling conditions, and is not easy to produce wear and grinding cracks.

The mold meets the economic requirements

In the selection of mold materials, the principle of economy must be considered to reduce the cost of mold manufacturing process as much as possible. Therefore, under the premise of meeting the performance, the price must be low first, carbon steel can be used instead of alloy steel, and domestic materials can be used instead of imported materials. In addition, the production and supply situation of the market should also be considered when selecting materials, and the selected steel types should be as few and concentrated as possible for easy procurement.

Selection of automotive stamping mold materials

Due to the different functions of each component in the mold, the requirements for materials are also different.
The selection and selection principles of mold materials are also different. Therefore, the reasonable selection of mold materials is also a very important task in mold design. It is not only related to the cost of parts, but also seriously affects production.

With the development of the automobile industry, people’s demand for automobiles is increasing, and the biggest factor affecting production is how to use mold materials.

Although the mold working parts should use better materials than other parts, generally speaking, there are four main types according to the use conditions and requirements of the mold:

1) For mold working parts with simple shapes and small stamping parts, carbon tool steels such as T8A, T10A, etc. are often used.

2) For mold parts with complex shapes and large stamping parts, alloy tool steels or high-speed tool steels such as Cr12, CrWMn, Cr12MoV, W18Cr4V, W6MoSCr4V2, etc. are often used.

3) For mold working parts with high requirements for stamping part precision or mold life, cemented carbide or steel-bonded cemented carbide such as YG15, YG20, GW50, etc. are often used.

4) For the working parts of large molds (such as automobile cover molds), ordinary cast iron or cast carbon steel is used. Some also use welding reinforcement at the cutting edge, such as gray cast iron HT250, cast steel 2G270-500, etc.

In the selection of mold parts materials, in addition to the selection of working parts, the pillars, guide sleeves and other parts are more prominent. Carburizing and quenching generally use low-carbon steel or bearing steel in high-quality carbon steel.

It must be pointed out that the selection of mold parts materials is the most important and the most stringent, and the requirements for its heat treatment industry must be reasonable.

In addition to the above parts, medium carbon steel or high-quality carbon steel can also be used in carbon structural steel.

Large-scale drawing parts, automobile cover deep drawing mold materials

Alloy cast iron or high-strength ductile iron can be selected. Ductile iron can be immersed in lubricating oil, and the graphite in the organization has a self-lubricating effect, which can effectively reduce friction during stretching, and has low cost and easy processing.

High-strength ductile iron can be subjected to dual-medium delayed cooling martensitic isothermal quenching to obtain higher strength and toughness, with a hardness of 55~58HRC. First, the mold is slowly preheated, then heated to 880~900℃, air-cooled after insulation, and then quenched with brine to about 550℃, and then switched to oil cooling. When the mold temperature drops to about 250℃, 180~200℃ hot oil is added to keep it warm at medium temperature for 2~3 hours, then the oil temperature drops to about 170℃, kept warm for 5~7 hours, and finally switched to air cooling.