Three principles selecting materials for low-pressure casting molds

Mold material selection is a very important part of the entire mold making process. The selection of materials for the mold needs to meet three principles. The mold should meet the work requirements of wear resistance and toughness. The mold should meet the process requirements. At the same time, the mold should meet the economic applicability.
(1) The mold meets the requirements of working conditions
1. When the wear-resistant blank is plastically deformed in the mold cavity, it flows and slides along the surface of the cavity, which causes intense friction between the cavity surface and the blank, resulting in the failure of the mold 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, the wear resistance is also related to the type, number, form, size and distribution of carbides in the material.
2. The working conditions of strong and tough molds are mostly very bad, and some often bear large impact loads, which leads to brittle fracture. In order to prevent the mold parts from being brittlely broken during work, the mold should have high strength and toughness. The toughness of the mold mainly depends on the carbon content, grain size and microstructure of the material.
3. Fatigue fracture performance During the working process of mold, under the long-term effect of cyclic stress, it often leads to fatigue fracture. Its forms are 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.
4. High temperature performance When the working temperature of the mold is higher, the hardness and strength will be reduced, resulting in early wear or plastic deformation of the mold and failure. Therefore, the mold material should have high stability against tempering to ensure that the mold has high hardness and strength at the working temperature.
5. Cold and thermal fatigue resistance Some molds are in a state of repeated heating and cooling during the work process, which causes the surface of the cavity to be pulled and the stress becomes variable, causing surface cracking and spalling, increasing friction, and hindering plastic deformation. Reduced dimensional accuracy, resulting in mold failure. Hot and cold fatigue is one of the main forms of hot work die failure. This type of mold should have high resistance to cold and hot 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 plastic, after heating, strong erosive gases such as hci and hf are resolved to erode the surface of the mold cavity and increase the surface roughness. , Exacerbating wear failure.
(2) The mold meets the process performance requirements
The manufacture of the mold generally goes through several processes such as forging, cutting, and heat treatment. In order to ensure the manufacturing quality of the mold and reduce the production cost, the material should have good forgeability, cutting workability, hardenability, hardenability and grindability; it should also have small oxidation, decarburization sensitivity and quenching Deformation and cracking tendency.
1. The forgeability has low hot forging deformation resistance, good plasticity, wide forging temperature range, low forging cracking and low tendency to precipitate network carbide.
2. The annealing process has a wide spheroidizing annealing temperature range, low annealing hardness and small fluctuation range, and high spheroidizing rate.
3. The cutting amount is large, the tool loss is low, and the roughness of the processed surface is low.
4. Sensitivity to oxidation and decarburization The high-temperature anti-oxidation energy is good at high temperature heating, the decarburization speed is slow, it is not sensitive to the heating medium, and the tendency to produce pits is small.
5. Hardenability has a uniform and high surface hardness after quenching.
6. After hardening, a deeper hardened layer can be obtained, which can be hardened by using a mild quenching medium.
7. Quenching deformation cracking tendency The conventional quenching volume change is small, the shape warpage and distortion are slight, and the abnormal deformation tendency is low. Conventional quenching has low cracking sensitivity and is insensitive to quenching temperature and workpiece shape.
8. The relative wear of the grindable grinding wheel is small, and the maximum amount of grinding without burns is large. It is not sensitive to the quality of the grinding wheel and the cooling conditions, and it is not easy to cause scratches and grinding cracks.
(3) Mold meets economic requirements
In the selection of materials for molds, the principle of economy must be considered to reduce manufacturing costs as much as possible. Therefore, on the premise of satisfying the use performance, first of all, choose the lower price, carbon steel can be used without alloy steel, domestic materials can be used without imported materials. In addition, the production and supply of the market should also be considered when selecting materials. The steel types selected should be as few and concentrated as possible and easy to purchase