Analysis of the causes of die burst in die technology

Due to different stamping processes and working conditions, there are many reasons for the die bursting. The following is a comprehensive analysis of the reasons for the bursting of the die in terms of the design, manufacture and use of the die, and find out the corresponding improvement measures.

1. The mold material is poor and easy to be broken in subsequent processing

The service life of dies made of different materials is often different. For this reason, two basic requirements are put forward for the materials of the working parts of the stamping die: ① The service performance of the materials should be high hardness (58~64HRC) and high strength, with high wear resistance and sufficient toughness, small heat treatment deformation, and a certain degree of thermal hardness; ② The processing property is good. The processing and manufacturing process of working parts of stamping dies is generally complex, so they must have the adaptability to various processing technologies, such as forgeability, machinability, hardenability, hardenability, quenching crack sensitivity and grinding machinability. Generally, mold materials with excellent performance are selected according to the material characteristics, production batch, accuracy requirements, etc. of the stamping parts, taking into account its processing and economy.

2. Heat treatment: deformation due to improper quenching and tempering process

Practice has proved that the hot working quality of the die has a great influence on the performance and service life of the die. According to the analysis and statistics of the causes of die failure, the die failure ‘accidents’ caused by improper heat treatment account for more than 40%. The quenching deformation and cracking of the working parts of the die and the early fracture in the use process are all related to the hot working process of the die.

1) Forging process: this is an important link in the manufacturing process of die working parts. For the die of high alloy tool steel, the technical requirements for the metallographic structure such as carbide distribution of the material are usually put forward. In addition, the forging temperature range shall be strictly controlled, the correct heating specification shall be formulated, the correct forging force method shall be adopted, and the slow cooling or timely annealing after forging shall be carried out.

2) Preparation heat treatment: annealing, normalizing or quenching and tempering and other preparation heat treatment processes shall be adopted according to the different materials and requirements of die working parts to improve the structure, eliminate the structural defects of forging blanks and improve the processing technology. The high carbon alloy die steel can eliminate the network secondary cementite or chain carbide, spheroidize and refine the carbide, and promote the uniformity of carbide distribution after proper preparation heat treatment. This is conducive to ensuring the quality of quenching and tempering and improving the service life of the die.

3) Quenching and tempering: this is the key link in the heat treatment of dies. If overheating occurs during quenching and heating, it will not only cause greater brittleness of the workpiece, but also easily cause deformation and cracking during cooling, seriously affecting the life of the die. Special attention shall be paid to prevent oxidation and decarburization during quenching and heating of the die. The heat treatment process specification shall be strictly controlled. Vacuum heat treatment can be used if conditions permit. Temper in time after quenching, and adopt different tempering processes according to technical requirements.

4) Stress relief annealing: the working parts of the die shall be subject to stress relief annealing after rough machining, aiming to eliminate the internal stress caused by rough machining, so as to avoid excessive deformation and cracks caused by quenching. For molds with high accuracy requirements, stress relieving tempering treatment is also required after grinding or electric machining, which is conducive to stabilizing the mold accuracy and improving the service life.

3. The flatness of the mold grinding is not enough, resulting in deflection and deformation

The surface quality of die working parts is closely related to the wear resistance, fracture resistance and adhesion resistance of the die, which directly affects the service life of the die. In particular, the surface roughness value has a great impact on the life of the die. If the surface roughness value is too large, stress concentration will occur during operation, and cracks will easily occur between its peaks and valleys, which will affect the durability of the die, and also affect the corrosion resistance of the workpiece surface, directly affecting the service life and accuracy of the die. Therefore, the following matters should be noted:

1) During the processing of mold working parts, it is necessary to prevent grinding from burning the part surface, and strictly control the grinding process conditions and methods (such as grinding wheel hardness, particle size, coolant, feed rate and other parameters);

2) During processing, macro defects such as knife mark, interlayer, crack and impact scar shall be prevented on the surface of working parts of the die. The existence of these defects will cause stress concentration and become the root cause of fracture, resulting in early failure of the mold;

3) The use of grinding, grinding, polishing and other finishing and fine processing, to obtain a smaller surface roughness value, improve the service life of the mold.

4. Design process: the mold strength is not enough, the knife edge spacing is too close, the mold structure is unreasonable, the number of template blocks is not enough, there is no pad foot, the mold guidance is inaccurate, and the gap is unreasonable.

1) Layout and overlap. Unreasonable reciprocating feeding layout method and too small overlap value often cause sharp wear of the die or gnawing of the convex and concave dies. Therefore, while considering improving the utilization rate of material judgment, it is necessary to reasonably select the layout method and overlap value according to the processing batch, quality requirements and die fit clearance of parts, so as to improve the die life.

2) The precision of the guide mechanism of the die. Accurate and reliable guidance has a great impact on reducing the wear of working parts of the die and avoiding the gnawing of male and female dies, especially the blanking die with no clearance and small clearance, compound die and multi station progressive die. In order to improve the service life of the die, it is necessary to correctly select the guide form and determine the precision of the guide mechanism according to the requirements of the process nature and the part precision. In general, the precision of the guide mechanism should be higher than the fit taper of the convex and concave dies.

3) Geometric parameters of die (male and female die) cutting edge. The shape, fit clearance and fillet radius of male and female dies have great influence not only on the forming of stamping parts, but also on the wear and life of dies. For example, the fit clearance of the die directly affects the quality of the blanking part and the life of the die. For those with high accuracy requirements, smaller clearance values should be selected; On the contrary, the clearance can be appropriately increased to improve the service life of the die.

5. Wire cutting process: artificial wire cutting, improper wire cutting clearance treatment, no influence of angle cleaning and wire cutting metamorphic layer.

The cutting edge of stamping die is usually processed by wire cutting. Due to the thermal effect and electrolysis effect of wire cutting, a certain thickness of metamorphic layer is produced on the surface of the die, resulting in the reduction of surface hardness and the appearance of micro cracks, which makes the die for wire cutting prone to early wear, directly affecting the maintenance of die blanking clearance and the edge is easy to collapse, shortening the service life of the die. Therefore, a reasonable electrical gauge should be selected in wire cutting to minimize the depth of metamorphic layer.

6. Selection of punch equipment: punch tonnage, insufficient blanking force and too deep mold adjustment.

The precision and rigidity of stamping equipment (such as press) are very important to the life of stamping die. The stamping equipment has high precision, good rigidity and greatly improves the service life of the stamping die. For example, the die material of complex silicon steel sheet is Crl2MoV, which is used on an ordinary open press and has an average regrinding life of 1-3 million times. However, when used on a new precision press, the regrinding life of the die can reach 60 to 120 thousand times. In particular, the press with high precision and good rigidity must be selected for small clearance or no clearance die, cemented carbide die and precision die, otherwise, the life of the die will be reduced, and even the chess pieces will be damaged in serious cases.

7. Stamping process: the thickness tolerance of raw materials of stamping parts is out of tolerance, the material performance fluctuates, the surface quality is poor (such as rust) or not clean (such as oil stain), etc., which will lead to the adverse consequences such as increased wear of die working parts and easy blade collapse. To this end, it should be noted that:

1) The raw materials with good stamping process shall be used as far as possible to reduce the stamping deformation force;

2) Before stamping, the brand, thickness and surface quality of raw materials shall be strictly checked, and the raw materials shall be wiped clean. If necessary, the surface oxide and rust shall be removed;

3) Softening treatment and surface treatment can be arranged when necessary, and appropriate lubricants and lubrication processes can be selected according to the stamping process and the type of raw materials.

8. Correct use and reasonable maintenance of production operations.