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Plastic mold life and injection mold structure design

Die life and mold structure design

Die life is a comprehensive reflection of mold design, mold material performance, mold manufacturing level, mold heat treatment level, and use and maintenance levels for a certain period of time. The level of mold life reflects the level of metallurgical industry and machinery manufacturing industry in a country and a region to a certain extent.

Die life refers to the number of times the mold can be formed in the premise of ensuring the quality of the part. For example, if the mold life of a cavity is 100,000 times, it means that 100,000 qualified parts can be produced. When the life of a mold with 4 cavities is 100,000 times, it means that 400,000 qualified parts can be produced. It includes repeated sharpening and replacement of consumables until the main part of the mold is replaced by the total number of qualified parts formed. In the actual operation of the factory, when the cost of re-maintenance is often about 1/3 to 1/2 of the cost of re-production, it is determined that the mold has failed, and then maintenance is often not worth the loss.

The failure of the mold is divided into abnormal failure and normal failure. Abnormal failure (early failure) means that the mold cannot be serviced if it does not meet the industry-recognized life. Normal failure means that the mold cannot be used for continued service due to slow plastic deformation or relatively uniform wear or fatigue fracture after mass production. Despite the wide variety of molds, the working conditions are very different, and the damaged parts are different, but according to the failure form, it can be divided into three types: wear failure, fracture failure, and plastic deformation failure. 1) Wear is due to the relative motion of the surface, the contact surface gradually loses material; 2) The fracture can be divided into plastic fracture and brittle fracture, and the brittle fracture can be divided into one-time fracture and fatigue fracture; 3) plastic deformation is when the mold When the stress of a certain part exceeds the yield limit of the mold material at that temperature, plastic deformation occurs due to lattice slip, twinning, grain boundary slip, etc., changing the geometry or size, and cannot be repaired. In service, it is characterized by upsetting, bending, swelling of the cavity, and collapse. The plastic deformation of the mold is the yielding process of the mold metal material.

The service life of the mold is related to the mold design level, the mold structure, the heat treatment of the mold material, the material selection, the machining process, and the smoothness of the mold. According to the analysis and statistics of a large number of failed molds by relevant personnel, among the various factors causing mold failure, about 25% of the mold failures are caused by the unreasonable mold structure. Therefore, designing a reasonable mold structure improves the quality of the mold and The service life will play a multiplier role. Reasonable mold structure design should make the mold work evenly, not easy to be biased, and the stress concentration is small.

Regardless of the type of mold, the formwork should have good rigidity, and the formwork should not be too thin. When the working machine space is sufficient, the thickness should be increased as much as possible, not only to meet the strength requirements of the formwork, but also to consider the rigidity of the formwork. Judging from the large-scale imported molds, the domestic molds generally have a thin template phenomenon, which is mainly due to insufficient understanding of the rigidity of the mold base.

For multi-station molds, it is often difficult to ensure the accuracy of the guide by two guide pillar guides. Four guide pillar guides should be used, and six guide pillar guides should be considered for large molds. When using multiple guide pillars, the positional accuracy of the relevant parts should be given special attention.

For the forming part of the mold, the radius of the corner of the working part not only affects the forming process and the quality of the part, but also affects the failure form and life of the mold. Under the premise of meeting the requirements of the product, try to increase the size. The working part transitions the fillet radius, which will have an unexpected effect on increasing the life of the mold. For the case where the corner radius cannot be increased and the complicated structure of the die, the insert structure can be considered, which also reduces the stress concentration.

For the stamping die, under the premise of ensuring the dimensional accuracy of the workpiece, the gap between the convex and concave molds is reasonably increased to improve the stress state of the working part of the punch, so that the punching force, the unloading force and the pushing force are reduced, and convex The die edge wear is reduced, thereby increasing the life of the mold.

Multi-station high-speed progressive stamping die should pay attention to the problem of scrapback of the scrap, and increase the corresponding measures in the structure to avoid the phenomenon of micro-slack rebound caused by the instantaneous vacuum effect during high-speed stamping, so that the scrap enters the working area and the convex die blade The mouth is cracked.

For high-temperature molds such as hot forging dies, heat dissipation and cooling are not negligible in structural design. Local temperature should be avoided and plastic deformation of the mold material should be avoided.

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