Plastic mold steel material and mold heat treatment process

Plastic mold steel material and mold heat treatment process
The output value of molds for plastic molding has taken the first place in the total output value of the mold industry. There was no special steel for plastic molds in China in the past. In recent years, while introducing foreign steel for plastic molds, some new special steels for plastic molds have been independently researched and developed.

1. Pre-hardened plastic mold steel

This type of steel is fully forged in the steel plant and made into modules, and is pre-heated to the required hardness (usually pre-hardened to 30~35RHC), and then used for unit molding. P20 (ie 3Cr2Mo) is the most widely used pre-hardened plastic mold steel abroad, and has been included in my country's alloy tool steel standard. It has been widely used in some factories in my country since the 1980s. 718 is a modified P20 steel produced in Sweden. It has higher hardenability than P20. After quenching and tempering, it can maintain uniform hardness in large cross-sectional dimensions. It is also widely used in my country.

2. Easy-cutting pre-hardened steel

In order to improve the cutting performance of the 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. S-based free-cutting steel has a large anisotropy, and when the cross-section increases, the segregation of sulfides is more serious.

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, and when the mold hardness is 35~45HRC, various processing can be carried out smoothly.

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 my country 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), the steel is added with S, Ca, Re for easy Cutting elements have better cutting performance than S-Ca composite free-cutting steel; low-carbon MnMoVB series non-quenched and tempered bainite large-section plastic mold steel (B30), adding S and Ca as free-cutting elements to the steel, industrial The trial production shows that the 400mm thick slab is air-cooled after hot rolling, and the hardness is more evenly distributed along the cross section.

4. Age hardening steel

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

Such steels include 25CrNi3MoAl, 10Ni3Mn2AlCu (PMS) and 06Ni6CrMoVTiAl. After quenching and tempering, the hardness of these steels is 20~30HRC, which can be machined and then aged, and the hardness can reach 38~42HRC.

5. Corrosion resistant plastic mold steel

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

5. Cemented carbide and steel bonded cemented carbide

Cemented carbide is a type of composite material manufactured by powder metallurgy. Cemented carbide has high hardness, good wear resistance, high elastic modulus and high working temperature. Used to make some molds, the service life of the mold can be increased several times or more than tens of times. However, cemented carbide is brittle, has poor bending strength and toughness, and cannot be machined. Cemented carbide is used as a mold material and is mainly used for wire drawing dies, cold extrusion and cold punching dies that are not subject to much impact. At present, my country has been able to produce various grades of cemented carbide, which can basically meet the needs of the domestic market.

In order to meet the needs of manufacturing micro-drills for drilling holes in integrated circuit boards, dot matrix printing needles for computers, precision tools and molds, etc., in recent years, various countries have developed some microcrystals (WC grains less than 1 micron) and ultra-fine crystals. Granular cemented carbide (WC grains less than 0.6 microns), in traditional cemented carbide, the WC grain size is 1.3 to 1.5 microns. Ultra-fine grained cemented carbide makes up for many shortcomings of conventional cemented carbide, expands its application range, and has achieved good results in the manufacture of wear-resistant and impact-resistant molds. Some research institutes and cemented carbide factories in my country have developed a variety of grades of microcrystalline cemented carbide and ultra-fine grained cemented carbide. The development of high-performance ultra-fine grained cemented carbide is still a hot spot in cemented carbide research.

Steel-bonded cemented carbide is a composite material formed by carbide as the hard phase and steel as the bonding phase. Steel-bonded cemented carbide has good wear resistance, its strength and toughness are generally higher than that of cemented carbide, and has some process properties such as heat treatment, machinability, forgeability and weldability. Die is the main application field of steel-bonded cemented carbide. my country began to develop this material in the 1960s, and has developed a variety of grades of steel-bonded cemented carbide, which is used as a steel-bonded cemented carbide for molds. The hard phase mainly uses TiC and WC, and the steel matrix mainly uses low-alloy chromium. Molybdenum steel, medium and high alloy tool steel or high-speed steel, such as TiC series GT35, R5, D1, T1 and WC series TLMW50, GW50, GJW50. Steel-bonded cemented carbide has been used to make cold heading dies, extrusion dies, drawing dies, punching dies, wire drawing dies, hot heading dies, etc.

The development of powder metallurgy technology and the application of hot isostatic pressing led to the production and use of segregation-free powder high speed steel in the 1970s. Its main characteristics are toughness, grindability, isotropy, and heat treatment processability. High-speed steel, and has a relatively high service life. In the future, this technology will be used to produce high-carbon, high-vanadium, and high-wear-resistant cold die steels that cannot be produced by conventional processes. This type of steel has better machinability and grinding performance, and has better toughness. Some cemented carbides are similar. Many grades of powder metallurgy high wear-resistant cold die steel have been produced abroad, and there is still little research in China.