Research progress of mold materials and mold heat treatment

3.1 Pre-hardened plastic mold steel

This type of steel is fully forged in the steel mill and then made into modules. After pre-heat treatment to the required hardness (generally pre-hardened to 30″35RHC), it is used for unit-making. P20 (ie 3Cr2Mo) is the most widely used prefabricated in foreign countries. Hard plastic mold steel has been listed in the Chinese alloy tool steel standard and has been widely used in some factories in China since the 1980s. 718 is a modified P20 steel produced in Sweden, which has higher hardenability than P20 and can be quenched and tempered. It maintains uniform hardness in large cross-section dimensions and is also widely used in China.

3.2 Free-cutting pre-hardened steel[13]

In order to improve the cutting performance of pre-hardened plastic mold steel, free cutting elements can be added. The United States, Japan, and Germany have all developed some free-cutting pre-hardened steels. Foreign free-cutting pre-hardened steels are mainly S series, but also S-Se series and Ca series. But Se is more expensive. The anisotropy of S-based free-cutting steel is large, and the segregation of sulfides is more serious when the cross-section increases.

my country has developed some sulfur-containing free-cutting pre-hardened plastic mold steels, such as 8Cr2MnWMoVS (8Cr2S) and S-Ca composite free-cutting plastic mold steel 5CrNiMnMoVSCa (5NiSCa). 5NiSCa steel adopts S-Ca composite free-cutting system and spray metallurgy technology to improve the shape and distribution of sulfide and the anisotropy of steel, and the distribution of sulfide in the large section is still relatively uniform. 5NiSCa steel has high hardenability and mirror polishability. When the mold hardness is 35″45HRC, it can be processed smoothly.

3.3 Non-quenched and tempered plastic mold steel

This kind of steel can reach the pre-hardness hardness after forging and rolling without dispatching, which is conducive to saving energy, reducing costs and shortening the production cycle. This type of steel developed in China includes: medium carbon manganese boron air-cooled bainite steel, which can be used to make plastic molds and rubber molds; non-quenched and tempered plastic mold steel 2Mn2CrVTiSCaRe (FT), which is easy to add S, Ca, Re to the steel. Cutting element, has better cutting performance than S-Ca composite free-cutting steel [13]; low-carbon MnMoVB series non-quenched and tempered bainite large-section plastic mold steel (B30), adding S and Ca as free-cutting steel Element, industrial trial production shows that 400mm thick slab is air-cooled after hot rolling, and the hardness is more evenly distributed along the cross section [14].

3.4 Age hardened steel

my country has developed several low-nickel age-hardening steels. These steels are subjected to mechanical processing after quenching and tempering, and then aging, the hardness is improved by the precipitation of intermetallic compounds, and the deformation after heat treatment is small. Age-hardened steel is suitable for making high-precision plastic molds, transparent plastic molds, etc.

Such steels include 25CrNi3MoAl[3], 10Ni3Mn2AlCu(PMS)[15]] and 06Ni6CrMoVTiAl[11]. After quenching and tempering, these steels have a hardness of 20″30HRC, which can be machined and then aged to a hardness of 38″42HRC.

3.5 Corrosion resistant plastic mold steel

When plastic products use chemically corrosive plastics as raw materials, the molds must have anti-corrosion properties. Generally, corrosion-resistant steel is used to make molds. At this time, good wear resistance is also required. Commonly used steel grades are 4Cr13(420), 9Cr18, 17-4PH. PCR (0Cr16Ni4Cu3Nb) is a kind of corrosion-resistant plastic mold steel developed in my country, which has good comprehensive mechanical properties and good corrosion resistance [3].