Molds are known as father of industry. Due to increasing automation of modern industry, scope of use of molds is wider. Among many small and medium-sized mold manufacturers in china, mold life is still very low, only 1/3 to 1/5 of same industry in foreign countries. Low life of mold not only reduces quality of product, but also has serious consequences such as wasting mold materials and increasing processing time, which makes cost of product high and seriously affects production efficiency. Increasing life of mold has great economic benefits. Generally, mold tooling cost accounts for about 25% of production cost in trial production stage, and only 10% in production. Mold life is related to moulding die design, manufacturing, use and maintenance. Through analysis of a large number of failed molds, 45% of various factors are related to improper heat treatment. Unreasonable mold structure and material selection account for about 25%, untimely lubrication and equipment maintenance factors account for about 20%. Improper handling accounted for about 10%.

 

A reasonable moulding die design is basis for ensuring its longevity, and many factors need to be considered here. Batch size, shape and precision of finished product determine structural scheme of mold, and pay attention to economical use of mold; good structural scheme not only has a compact structure, flexible and convenient operation, but also enables each component to have sufficient rigidity; corners of each surface of mold parts should be rounded as far as possible to eliminate stress concentration; combination and inlay can be used to eliminate stress concentration of concave mold, cavity, part of punch and core. Appropriate measures are taken to protect elongated punch or core; wear of sliding fittings and frequent impacting parts in long-term use also have a great influence on life of mold. Reduction of wear should be considered in key consideration; when designing cold stamping die, a mechanism should be provided to avoid blockage of parts or waste. Reliable guiding mechanism of mold is helpful to avoid mutual bruise between punch and die. Multi-station mold should not be guided by only 2 guide columns, 4 guide columns should be guided as much as possible, so that guiding performance is good. Gap between convex and concave molds is ensured to ensure that punch and concave mold do not collide.

Principle of selecting mold material is based on processing batch, mold manufacturing process, and processing object in moulding die design. For ordinary molds that are easy to deform and break, high-strength and high-toughness carbon tool steel should be selected. For punching molds with easy edge wear, material with high hardness, wear resistance, good processing performance, small deformation and high hardenability during heat treatment quenching should be used. Main failure mode of stamping die is surface peeling caused by surface fatigue crack, steel with good surface toughness is used; material with particularly low friction coefficient is suitable for drawing die; hot forging die requires material with strong toughness, strength, wear resistance to cold and heat fatigue, alloy tool steel is more suitable; for die-casting molds subjected to cyclic thermal stress, hot work die steel with high heat fatigue, high temperature environment should be selected; plastic mold should use materials that are easy to cut, have a dense structure and a good surface finish; when designing punch and die, matching of molds with different hardness or different materials should be considered to prolong their respective life. In addition, selected mold material should have a weak affinity with finished part to prevent part from sticking to mold, wear parts of mold, and shorten service life of mold.

 

Mold manufacturing process refers to molding process, heat treatment specifications and surface treatment technology. In order to ensure life of mold, mold manufacturing process should also be mentioned as a very important position, and factors affecting its service life are mainly processing method and precision for mold manufacturers in china.

Mold materials are mostly high-carbon, high-alloy steels, and there are defects such as composition segregation, tissue segregation, and carbide segregation exist in varying degrees, which cannot be directly used for molding. At the same time, shape and size of raw materials used are difficult to match with module, forging is an indispensable means of obtaining required internal organization and performance, reducing amount of machining. Through forging can effectively improve carbide segregation of tool steel, and generally can reduce carbide segregation level 2 after forging, up to level 3.
Die steel generally has poor thermal conductivity, and heating speed must be slow and uniform. Large forgings generally use preheating or step heating to control heating speed. Position of steel in furnace should be appropriate, and sometimes it should be repeatedly turned over so as to make heat as uniform as possible. In order to maximize breaking and uniform carbide, it is necessary to adopt a deformation process in which pier is thick, long and repeated repeatedly, and finally, like kneading dough, it is turned up and down, front and back, left and right, so that internal deformation is sufficient and uniform. After billet is forged, it should be slowly cooled with furnace or cooled in a hot ash box. However, for Cr12 steel, if it is slowly cooled after forging, it is easy to precipitate network carbide on grain boundary, thus affecting quality of blank. Therefore, it should be quickly cooled to about 700℃, then pit cold or into furnace slow cooling. Forgings should minimize number of forging fires to control oxidation and decarburization of billet.

There are many ways to improve performance of mold. Use of new heat treatment technology is a cost-effective and effective way. Mold heat treatment process mainly includes matrix strengthening and surface strengthening treatment; strengthening and toughening of matrix is to strengthen toughness and strength of matrix, reduce occurrence of fracture and deformation. Surface strengthening is mainly to increase wear resistance, corrosion resistance and lubrication properties of surface.

Main causes of material failure are stress concentration and fatigue fracture. In order to increase toughness, reduce brittleness and fracture of ordinary cold work die steel, low temperature quenching and low temperature tempering process can be used; high temperature quenching and high temperature tempering process can significantly enhance toughness and thermal stability of hot work die steel. When mold cavity is large and wall is thin, upper limit of normal quenching temperature is required to increase amount of retained austenite so that mold does not swell. Rapid heating method has a short heating time, a reduced tendency to oxidative decarburization, a small grain size, quenching deformation of large tool of carbon tool steel is small. High-speed steel adopts low quenching and high-returning process, quenching temperature is low, tempering temperature is high, which can greatly improve toughness. Although hardness is reduced, mold can be improved in resistance to fracture and fatigue damage. In order to reduce residual stress, mold should be tempered after quenching. Effect of tempering is to release material due to internal stress generated by quenching in a short time. Tempering should be sufficient, and tempering is insufficient to cause pre-grinding cracks.

 

It has been found that wear and adhesion occur on surface of mold, common fatigue and fracture often start from surface. In order to improve life of mold better, it is necessary to enhance wear resistance of surface of part, and surface strengthening treatment of main forming parts is the most direct way. Surface strengthening process of mold mainly includes gas nitriding, ion nitriding, electric spark surface strengthening, boronizing, thermal diffusion carbide coating, chemical vapor deposition, physical vapor deposition, laser surface strengthening, ion implantation, plasma spraying, and so on. In actual production, different surface strengthening processes are adopted according to different uses of mold. For example, in order to enhance wear resistance and compressive strength of surface layer of blanking die for mold manufacturers in china, a strengthening method such as electric spark or hard alloy surfacing may be adopted; for surface of hot working die (die-casting mold, plastic mold), nitriding method may be used to enhance wear resistance, heat fatigue resistance and corrosion resistance; drawing die and bending die are mainly wear caused by friction in production, and sulfur permeability process can be used to reduce friction coefficient in order to enhance wear resistance of material. Carbonitriding is suitable for surface strengthening of various types of molds. PVD and CVD in surface coating hardening technology have made great progress in recent years. Vacuum evaporation, vacuum sputtering and ion plating are commonly used in PVD. Among them, ion plating has strong adhesion, fast deposition speed, no pollution. Etc. Ion plating process can plate tic and TIN on surface of mold, and its service life can be extended several times to several tens of times.

Machine-added process directly affects service life and product quality of mold. Practical research has found that if surface roughness of mold cavity is doubled, service life can be increased by 50%, and premise is to ensure correct processing technology. Mold manufacturing process first needs to solve problem that processing deformation and residual stress after processing are not too large. When roughing, it is best not to make surface roughness Ra greater than 3.2μm. In particular, it should be noted that smooth transition should be made at the corner of working part of mold to reduce thermal stress caused by heat treatment. Amount of cutting is small during finishing, and no knife marks are allowed. For precision grinding of precision molds, attention should be paid to influence of ambient temperature, and constant temperature grinding is required. Roughness of forging die cavity directly affects life of forging die. Since roughness value is high, metal flow resistance is increased, so that forging is not easy to demold. A mold having a low working surface roughness value has low frictional resistance and strong seizure resistance and fatigue resistance.