(1) Meet the requirements of working conditions
1. Abrasion resistance
When the blank is plastically deformed in the mold cavity, it flows and slides along the surface of the cavity, causing violent friction between the surface of the cavity and the blank, which causes the mold to fail due to wear. Therefore, the wear resistance of the material is one of the most basic and important properties of the mold. Hardness is the main factor affecting wear resistance. In general, the higher the hardness of the mold parts, the smaller the amount of wear and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material.
1. Toughness
The working conditions of molds are mostly very bad, and some of them are often subjected to large impact loads, which leads to brittle fracture. In order to prevent sudden brittle fracture of mold parts during work, the mold must have high strength and toughness.
The toughness of the mold mainly depends on the carbon content, grain size and organization state of the material.
2. Fatigue fracture performance
During the working process of the mold, under the long-term action of cyclic stress, it often leads to fatigue fracture. Its forms include low-energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture, and bending fatigue fracture.
The fatigue fracture performance of the mold mainly depends on its strength, toughness, hardness, and the content of inclusions in the material.
3. High temperature performance
When the working temperature of the mold is higher, the hardness and strength will decrease, leading to early wear of the mold or plastic deformation and failure. Therefore, the mold material should have a higher anti-tempering stability to ensure that the mold has a higher hardness and strength at the working temperature.
4. Heat and cold fatigue resistance Some molds are in a state of repeated heating and cooling during the working process, which causes the surface of the cavity to be pulled and pressure to change the stress, causing surface cracks and peeling, increasing friction, hindering plastic deformation, and reducing dimensional accuracy. Mold failure. Heat and cold fatigue is one of the main forms of failure of hot work molds, and this type of mold should have high resistance to cold and heat fatigue.
6. Corrosion resistance
When some molds such as plastic molds are working, due to the presence of chlorine, fluorine and other elements in the plastics, strong corrosive gases such as HCI and HF will be resolved after being heated, which will erode the surface of the mold cavity, increase its surface roughness, and aggravate wear failure.
(2) Meet the process performance requirements
Mold manufacturing generally involves several processes such as forging, cutting, and heat treatment. In order to ensure the quality of mold manufacturing and reduce production costs, the material should have good forgeability, machinability, hardenability, hardenability and grindability; it should also have small oxidation, decarburization sensitivity and quenching Deformation and cracking tendency.
1. Malleability
has low hot forging deformation resistance, good plasticity, wide forging temperature range, low forging cold cracking and low tendency to precipitate network carbides.
2. Annealing processability
The spheroidizing annealing temperature range is wide, the annealing hardness is low and the fluctuation range is small, and the spheroidizing rate is high.
3. Machinability
Large cutting amount, low tool loss, low surface roughness.
4. Sensitivity to oxidation and decarburization
Good oxidation resistance when heated at high temperature, slow decarburization speed, insensitive to heating medium, and little tendency to produce pitting.
5. Hardenability
has uniform and high surface hardness after quenching.
6. Hardenability
After quenching, a deeper hardened layer can be obtained, and it can be hardened by using a mild quenching medium.
7. Quenching deformation cracking tendency
The volume change of conventional quenching is small, the shape is warped, the distortion is slight, and the tendency of abnormal deformation is low. Conventional quenching has low sensitivity to cracking, and is insensitive to quenching temperature and workpiece shape.
8. Grindability
The grinding wheel is relatively low wear, and the non-burn limit grinding consumption is large. It is not sensitive to the quality of the grinding wheel and the cooling conditions, and it is not easy to cause abrasion and grinding cracks.
(3) Meet the economic requirements
When selecting materials for the mold, the principle of economy must be considered, and the manufacturing cost must be reduced as much as possible. Therefore, under the premise of satisfying the performance, the first choice is the lower price, carbon steel can be used without alloy steel, and domestic materials can be used without imported materials.
In addition, when selecting materials, the production and supply of the market should also be considered. The selected steel grades should be as few and concentrated as possible and easy to buy.
Selection of plastic mold materials
At present, plastic products are increasingly widely used in daily life, of which injection molding technology accounts for about 80%. Injection molding is widely used in many fields such as automobiles, construction, household appliances, food, medicine, etc. because of its one-time molding, accurate size, possible inserts, high productivity, easy modernization, and low post-processing. The selection of plastic molds is very critical to whether the plastics industry can receive good economic benefits. Therefore, it is necessary for mold designers to understand the basic requirements of mold materials and select appropriate materials.
The working conditions of plastic molds are different from those of cold stamping dies. Generally, they must work at 150°C-200°C. In addition to being subjected to a certain pressure, they also have to withstand temperature. According to the different use conditions and processing methods of plastic molding molds, the basic performance requirements of steel for plastic molds are roughly summarized as follows:
1. Sufficient surface hardness and wear resistance
The hardness of the plastic mold is usually below 50-60HRC, and the heat-treated mold should have sufficient surface hardness to ensure that the mold has sufficient rigidity. When the mold is working, due to the filling and flow of the plastic, it has to withstand greater compressive stress and friction. It is required that the mold maintain the accuracy of the shape and the stability of the dimensional accuracy to ensure that the mold has a sufficient service life. The wear resistance of the mold depends on the chemical composition and heat treatment hardness of the steel, so increasing the hardness of the mold is beneficial to improve its wear resistance.
2. Excellent machinability
Most plastic molding molds require certain cutting processing and fitter repairs in addition to EMD processing. In order to extend the service life of cutting tools, improve cutting performance, and reduce surface roughness, the hardness of the steel used for plastic molds must be appropriate.
3. Good polishing performance
For high-quality plastic products, the surface roughness of the cavity is required to be small. For example, the surface roughness value of the injection mold cavity is required to be less than Ra0.1～0.25, and the optical surface requires Ra<0.01nm, and the cavity must be polished to reduce the surface roughness value. For this reason, the selected steel requires less material impurities, fine and uniform structure, no fiber directionality, and no pitting or orange peel defects during polishing.
4. Good thermal stability
The shape of the parts of the plastic injection mold is often complicated and difficult to process after quenching. Therefore, it should be selected as much as possible with good thermal stability. When the mold is formed by heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, and the dimensional change caused by the temperature difference The rate is small, the metallographic structure and the mold size are stable, and the processing can be reduced or no longer needed to ensure the mold size accuracy and surface roughness requirements. 45 and 50 grades of carbon steel have certain strength and wear resistance, and are mostly used as mold base materials after quenching and tempering. High-carbon tool steel and low-alloy tool steel have higher strength and wear resistance after heat treatment, and are mostly used for forming parts. However, high-carbon tool steel is only suitable for manufacturing small-sized and simple shaped parts due to its large heat treatment deformation.
With the development of the plastics industry, the complexity and precision of plastic products are becoming more and more demanding, and higher requirements are placed on mold materials. For the manufacture of complex, precise and corrosion-resistant plastic molds, pre-hardened steel (such as PMS), corrosion-resistant steel (such as PCR) and low-carbon maraging steel (such as 18Ni-250) can be used, all of which have good Cutting, heat treatment and polishing performance and high strength.
In addition, when selecting materials, you must also consider preventing scratches and bonding. If there is relative movement between the two surfaces, try to avoid choosing materials with the same structure. Under special conditions, one side can be plated or nitrided to make the two sides have different The surface structure.
