With the application of plastic products in industry and daily life more and more widely, the demand of plastic mold industry for mold steel is also growing. It is self-evident that the quality of mold plays an important role in guaranteeing the quality of products in plastic molding. Plastic mold has been developed to the direction of precision, large-scale and multi-cavity. The performance of plastic mold steel is required to be higher and higher. The performance of plastic mold steel should be determined according to the type of plastics, the use of products, production batch, dimensional accuracy and surface quality requirements.

1. Molded parts refer to the mold parts which are directly in contact with plastic, such as cavity, core, slider, insert, inclined top, side-pulling, etc.

2. The material quality of the molding parts is directly related to the quality and life of the mold. It determines the appearance and internal quality of the plastic products. It must be carefully selected according to the requirements and characteristics of the products and molds on the basis of the contract stipulations and customer requirements.

3. The selection principle of materials for forming parts and components is based on the types of plastics, the shapes of products, dimensional accuracy, the appearance quality and use requirements of products, the size of production batches and so on, taking into account the cutting, polishing, welding, etching, deformation, wear resistance and other properties of materials, while taking into account the economy and mold manufacturing. Conditions and processing methods to select different types of steel.

4. For moulds of transparent plastic products, high-grade imported steels with high mirror polishing performance should be selected for cavity and core, such as 718 mould steel (P20+Ni mould steel), NAK80 mould steel (P21 mould steel), S136 mould steel (420 mould steel), H13 mould steel, among which 718 mould steel and NAK80 mould steel are used. The hardness of S136 die steel and H13 die steel is 160-200HB, vacuum quenching and tempering after rough machining. The hardness of S136 die steel is 40-50HRC, and that of H13 die steel is 45-55HRC.

5. For the moulds which require high appearance quality, long life and mass production, the choice of moulding parts and materials is as follows:

A) High-grade imported steel with high mirror polishing performance, such as 718 die steel (P20 + Ni die steel), NAK80 die steel (P21 die steel), etc., should be selected for the cavity, which is in Pre-hardening state and need not be heat treated again.

B) Core can be made of imported P20 die steel or P20 + Ni die steel, such as 618 die steel, 738 die steel, 1.2738 die steel, 638 die steel, 318 die steel, etc., all in the Pre-hardening state; for molds with small batch production, domestic plastic die steel or S50C die steel, S55C die steel and other imported high-quality carbon steel can also be selected.

6. For the mold whose appearance quality is generally required, the material selection of its forming parts is as follows:

A) The small and precision die cavity and core are made of medium-grade imported P20 die steel or P20+Ni die steel.

B) Large and medium-sized molds, the molding plastics have no special requirements for steel, the cavity can be used low-grade imported P20 or P20 + Ni steel; core can be used low-grade imported P20 mold steel or imported high-quality carbon steel S50C, S55C, etc., can also be used domestic plastic mold steel.

C) For the cavities with dermatoglyphic etching, the 1.2738 (738 die steel) grade of P20 + Ni die steel should be avoided when the pear groove is etched.

7. For the internal structural parts without external quality requirements, the molding materials have no special requirements for the steel, and the molding parts are selected as follows:

A) For large and medium-sized molds, low-grade imported P20 or P20+Ni type steel can be selected for the cavity, as well as imported high-quality carbon steel S55C, S50C or domestic P20 or P20+Ni type plastic mold steel; imported or domestic high-quality carbon steel can be selected for the core.

B) For small moulds, if the output is high and the structure is complex, low-grade imported P20 or P20+Ni type steel can be selected for the cavity, and domestic P20 or P20+Ni type plastic mould steel can also be selected; domestic plastic mould steel can be used for the core.

C) For small moulds with simple structure and low output, domestic plastic mould steel or high quality carbon steel can be used for cavity core.

8. For molds containing fluorine, chlorine and other corrosive plastics and all kinds of plastics with flame retardants, imported corrosion-resistant steel can be selected if the product requirements are high, and domestic corrosion-resistant steel can be generally selected.

9. For moulds with strong friction and impact on steel, such as moulds for injection of nylon and glass fiber, imported or homemade H13 steels with high wear resistance, high thermal tensile strength and high toughness should be selected.

10, the forming insert is usually selected with the same material as the inserted parts. Beryllium bronze or aluminium alloy should be used as insert material for the parts which are difficult to be cooled or which require high cooling effect.

11. The selection principles for active parts in mold are as follows:

A) transparent parts should be selected with high quality imported steel, such as 718 and NAK80.

B) Non-transparent parts, generally should choose high hardness and strength of medium-grade imported steel, such as 618, 738, 2738, 638, 318, surface nitriding treatment, nitriding layer depth of 0.15-0.2 mm, hardness of 700-900HV.

C) If the mold requirements are low, low-grade imported steel or domestic steel can also be selected. Nitriding hardness is generally 600-800HV.

1. The mould base material refers to the standard of the mould base. The imported S50C or domestic SM45 are generally used as the mould base material, and the hardness is uniform, the internal stress is small and the deformation is not easy. The guide pillar material is GCr15 or SUJ2.