The choice of mould steel is of great importance if the blow mold is to function effectively.When plastic is molded, due to molecular stretching and shearing, residual stress is left in the product, which causes the product to warp and even crack.Residual stress is also generated during the machining of the mold steel. This residual stress also causes the mold to deform or even crack.The prime function of any mould steel must be that of meeting the service requirements imposed upon it.Moulding requirements vary.

The mould has high requirements for steel. Whether the steel is properly selected has a great influence on the mold’s service life, processability, and precision. from simple prototype work undertaken on soft aluminum construction moulds to fully hardened alloy steel volume production moulds moulding to close dimensional tolerances.The plastic mould application must be thoroughly investigated and fully understood before a selection decision can be made to avoid costly long-term mistakes.Once decided upon，the selected material should be written into the tooling specification for future procurement purposes.Using a tooling specification laying out the mould requirements before tool quotation or construction greatly reduces the risk of fundamental mistakes at the toolmaking stage.

The requirements vary depending on the structure of the mould, the service life of the mould, the plastic used for moulding, and the performance, appearance, and dimensional accuracy of the moulded product.As a result of the service conditions encountered during injection moulding, i.e., relatively high compressive cyclic loadings, the material core strength is relevant to the mould designer.Wear resistance may be imparted to a mould tool steel by various means，usually by hardening the material or adding property modifying alloying elements.

The choice of which method depends on the material in question.Glass fibers are often added to increase strength and wear resistance. In addition, metal fiber can also be added to the plastic to get conductive plastic to protect against radio wave interference.The biggest problem of these additives in molding is that the friction of various fibers in the composite material in the cavity is intensified, causing the wear of the mold cavity and affecting the dimensional accuracy of the products.The intended material should sustain a good long-term surface finish without the additional requirement of polishing between production runs.