Six suggestions for injection molding material selection

The choice of mold material is a more important problem in mold design and manufacture, which directly affects a series of aspects such as the mold manufacturing process, mold service life, molding quality of plastic parts, and mold processing cost.

Because of this, domestic and foreign scholars have developed new die steels with good service performance and machining performance and small heat treatment deformation, such as pre-hardened steel, new quenched and tempered steel, and maraging steel, etc.

In addition, to improve the life of the injection mold, much research and engineering practices have been done on the surface strengthening treatment of the mold-forming parts, and good results have been achieved.

So far, the above research and development work is still being carried out in-depth, and the results obtained are being vigorously promoted.

The working conditions of plastic mold generally must be carried out at 150~ 200℃. In addition to being affected by a certain pressure, there is also the influence of temperature.

Now, according to the different conditions and processing methods of plastic molding molds, Zhenye Injection Molding Hui also believes that the basic performance requirements of steel for plastic molds are summarized as follows:

Adequate surface hardness and wear resistance

The hardness of the plastic die is usually below 50~60 HRC. The heat-treated die should have enough surface hardness to ensure that the die has enough stiffness. 

Due to the filling and flow of plastic in the work of the mold to bear greater compressive stress and friction, the mold is required to maintain the stability of shape accuracy and dimensional accuracy, to ensure that the mold has enough service life. 

The wear resistance of the die depends on the chemical composition of the steel and the heat treatment hardness, so improving the hardness of the die is beneficial to improve its wear resistance.

Good mechanical processing performance

To choose easy to cut, and after machining can get high precision parts of the steel. For this purpose, medium carbon steel and medium carbon alloy steel are most commonly used, which is especially important for large molds. For the parts that need EDM, the burn-hardening layer of the steel is also required to be thin.

Excellent polishing performance

High-quality plastic products require the cavity surface roughness value is small. Therefore, the working surface of injection molding parts needs to be polished to the mirror, Ra≤0.05μm, the need for steel hardness 35~40 HRC is appropriate, the hard surface will make polishing difficult.

For this reason, the selected steel material requires fewer impurities, the microstructure should be uniform and compact, no fiber directionality, polishing should not appear with pitting or orange skin defects.

Good thermal stability

Plastic injection mold parts shapes are more complex and difficult to process after quenching, so should choose as far as possible with the good thermal stability of materials, makes the injection molding parts after heat treatment the linear expansion coefficient, heat treatment deformation is small, the size of the rate of change caused by the temperature difference is small, the microstructure and the mold size stability can reduce or no longer carry on the processing, To ensure the mold size accuracy and surface roughness requirements.

Good wear resistance and fatigue resistance

The cavity of the injection mold is not only scoured by high-pressure plastic melt but also subjected to alternating cold and hot temperature stress. General high carbon alloy steel can be heat treated to obtain high hardness, but poor toughness, easy to form surface cracks, should not be used.

The selected steel should make the injection mold can reduce the number of polishing repair die, can maintain the dimensional accuracy of the cavity for a long time, and achieve the service life of mass production.

Corrosion resistance

For some plastic varieties, such as polyvinyl chloride and flame-retardant plastics, it is necessary to consider the choice of steel with corrosion resistance.

In addition, the selection of materials must also be considered to prevent abrasion and gluing, such as the existence of relative movement of the two surfaces, trying to avoid the choice of the same structure of the material, under special conditions can be plated on one side or nitriding, so that the two sides have different surface structure.