What are the common causes of mold cracking

What are the common causes of mold cracking

The material of mold

If the mold material is not good, it is easy to crack in the subsequent processing, so the mold material should meet the following two requirements:

1.The performance of the material should have high hardness (58~64 HRC) and high strength, high wear resistance and sufficient toughness, small heat treatment deformation, and certain hot hardness;

2. Good process performance. Usually, according to the material characteristics of stamping parts, production batch, precision requirements, etc., the mold material with excellent performance is selected, while taking into account its processability and economy.

Heat treatment

The cooling medium during quenching is not properly selected, and the cooling rate required during severe quenching in the cooling process is to ensure that the part does not cause the decomposition of austenite during the cooling process, and the martensite structure is obtained. Austenite has different stability at different temperatures, requiring different cooling media, different temperatures, and different cooling rates. If the cooling medium and quenching process are not properly selected, the mold will crack due to excessive thermal stress.

Insufficient tempering or tempering in the tempered brittle zone The mold is tempered in the tempered brittle zone, or insufficient tempering at the normal tempering temperature, it is difficult to eliminate the residual stress inside the material, thereby reducing the service life of the mold and even causing cracking.

If the interval between quenching and tempering is too long, the quenched martensite structure is generally obtained after the mold is quenched. At this time, there is a huge internal stress inside the mold, which needs to be tempered in time to eliminate the internal stress. If the interval between quenching and tempering is too long, under the long-term action of quenching stress, micro-cracks or even cracking will occur inside the mold

Machining

(1) A large amount of electrical discharge machining produces more stress, and stress relief tempering should be adopted

(2) EDM produces white bright layer, decarburization, etc., which should be clear to avoid induced cracking

(3) The cutting/grinding is overheated, and the surface temperature causes surface quenching, resulting in cracks

(4) There is a decarburized layer on the surface of the steel, and the decarburized layer has micro-cracks, which induces the risk of cracking

Design of mold
(1) The accuracy of the guide mechanism of the mold

Accurate and reliable guidance has a great impact on reducing the wear of the mold workpiece and avoiding the pressure injury of the convex and concave molds, especially the small gap blanking die, compound die and multi-station progressive die are more effective. In order to improve the life of the mold, it is necessary to correctly select the guiding form and determine the accuracy of the guiding mechanism according to the requirements of the process nature and the accuracy of the parts. In general, the accuracy of the guiding mechanism should be higher than the matching accuracy of the convex and concave molds.

(2) The geometric parameters of the cutting edge of the die (convex and concave die)

The shape, fit clearance and fillet radius not only have a great influence on the forming of stamping parts, but also have a great influence on the wear and life of the die. For example, the matching clearance of the mold directly affects the quality of the blanking parts and the life of the mold. For higher precision requirements, a smaller gap value should be selected; otherwise, the gap can be appropriately increased to improve the life of the mold.

Craft of pressing
1. Raw materials for stamping parts.

In actual production, due to the excessive thickness tolerance of raw materials of external pressure parts, large fluctuations in material properties, poor surface quality (such as rust) or uncleanness (such as dirt), etc., it will cause adverse consequences such as increased mold wear and easy chipping. . To this end, it should be noted that:

(1)Use raw materials with good stamping technology as much as possible to reduce stamping deformation force;

(2)Before stamping, the grade, thickness and surface quality of the raw materials should be strictly checked, and the raw materials should be wiped clean, and surface oxides and rust should be removed if necessary;

(3)According to the stamping process and the type of raw materials, softening treatment and surface treatment can be arranged if necessary, and appropriate lubricants and lubricating processes can be selected.

2. Layout and edge.

Unreasonable reciprocating feeding and nesting method and too small overlap value often cause sharp wear of the mold or pressure damage of convex and concave molds. Therefore, while considering improving the utilization rate of materials, it is necessary to reasonably select the nesting method and edge value according to the processing batch, quality requirements and mold matching clearance of the parts, so as to improve the life of the mold.