As plastic products are increasingly used in industry and daily life, the demand for cast steel in plastic mold industry is also growing, it goes without saying that the quality of the mold plays a role in the injection molding process to ensure product quality, plastic molds to precision, large, multi-cavity direction, the performance of plastic mold steel requirements are increasingly high, the performance of plastic mold steel should be based on the type of plastic, product use, production batch , dimensional accuracy and surface depending on the requirements for quality.

Factors affecting the selection of ductile iron

Plastic mold parts are divided into two major categories.

One type includes structural parts, including: pouring system, guide parts, top mold, ejector mechanism, support parts, etc..

The other type is casting, including: cavities, cores, inserts, etc.

One type of parts can be designed according to the requirements of mechanical parts for strength and structure, and the materials are usually chosen from low and medium carbon structural steel, alloy steel and carbon steel. Due to the complex structure of the molded parts, the workpiece requires high dimensional accuracy, low surface roughness and good sealing of the weld seam, which puts forward professional requirements for mechanical properties, wear resistance and mold material processing technology.

1、Basic mold failure mode

Due to high working temperature and high mold pressure, some plastics are easily worn and corroded on the mold surface, and sometimes collision occurs due to demolding. There are three main failure modes as follows.
（1）Surface wear

Due to the reinforcing resin filler in the plastic, the surface of the mold cavity is scoured, worn and corroded, affecting the surface roughness value of the mold cavity and the dimensional overspill.

（2） Deformation

Plastic deformation occurs locally in the mold, causing surface damage such as depression, crease, crater, collapsed edge, etc.

（3）Fracture

Fracture phenomenon caused by local stress concentration in the mold.

2、Mold manufacturing and use requirements

Plastic mold strength and toughness requirements are lower than the hot and cold processing requirements, according to its failure mode and work requirements, its main performance requirements are as follows.

（1） Adequate wear resistance

Since surface wear is one of the main forms of mold failure, the mold must have sufficient hardness to ensure the mold wear resistance and mold life, usually need to choose the right material and suitable heat treatment methods to meet the hardness requirements, but when the hardness reaches a certain value, the impact of hardness on wear resistance improvement is not obvious.

（2） Reduce the effect of heat treatment deformation

As the shape of injection molded parts is often very complex, plastic molds are difficult to process after hardening, and sometimes even impossible to process, heat treatment is needed to increase hardness, appropriate measures should be taken to reduce the impact of heat treatment deformation, for the mold to be processed after heat treatment, the material with small heat treatment deformation should be selected.

（3）Excellent cutting performance

Mold materials need good cutting properties to increase tool life and ensure surface finish, pre-hardened materials also need good cutting properties after quenching.

（4）Good polishability and pickling

In order to obtain high quality plastic products, the surface of the cavity in the mold must be polished to reduce the surface roughness value, and must be of high material purity, fine organization, uniform, dense, and without fiber orientation.

（5） Good corrosion resistance

When injecting PVC or plastic products with additives such as flame retardant, it will decompose corrosive gas and produce certain chemical corrosion effect on the surface of the mold, and the steel with certain corrosion resistance should be selected when making such molds.

Types and characteristics of commonly used mold steel

1、Carburized steel

Carburized steel due to its low hardness, good ductility after annealing, can be obtained by cold extrusion, improve the efficiency and reduce costs, extrusion of complex cavities, after the hardness