First, the cause of damage to the die-casting production mold

        In die casting production, the most common form of mold damage is cracking and cracking. Stress is the main cause of mold damage. Thermal, mechanical, chemical, and operational shocks are sources of stress, including mechanical and thermal stresses, which are generated by:

In the mold manufacturing process

1) Quality problems of rough forging

Some molds produce cracks only when they are produced in a few hundred pieces, and the cracks develop rapidly. It is possible that only the outer dimensions are ensured during forging, and the loose defects such as dendrites, inclusions, shrinkage cavities, and bubbles in the steel are stretched and elongated along the processing method to form a streamline, and this streamline is for the future. The final quenching deformation, cracking, brittle cracking during use, and failure tendency have a great impact.

2) Cutting stress generated during final machining such as turning, milling, planing, etc., which can be eliminated by intermediate annealing.

3) Grinding stress occurs during hardening of hardened steel, friction heat is generated during grinding, softening layer and decarburization layer are generated, thermal fatigue strength is reduced, and thermal cracking and early cracking are easily caused. After the fine grinding of h13 steel, it can be heated to 510-570 ° C, and the thickness is maintained every 25 mm for one hour for stress relief annealing.

4) EDM produces stress. The surface of the mold produces a white bright layer rich in electrode elements and dielectric elements, which is hard and brittle. This layer itself has cracks and stress. EDM should use a high frequency to minimize the white layer. It must be removed by polishing and tempered. The tempering is carried out at a three-stage tempering temperature.

2. During mold processing

Improper heat treatment will lead to cracking of the mold and premature scrapping. In particular, only quenching and tempering, no quenching, and surface nitriding will occur, and surface cracking and cracking will occur after several thousand die casting.

The stress generated when the steel is quenched is the result of the superposition of the thermal stress during the cooling process and the structural stress during the phase change. The quenching stress is the cause of the deformation and cracking, and the solid must be tempered to eliminate the stress.

3. In the process of die casting production

1) Mold temperature

The mold should be preheated to a certain temperature before production. Otherwise, when the high temperature molten metal is filled, chilling occurs, which causes the temperature gradient of the inner and outer layers of the mold to increase, forming thermal stress, causing the surface of the mold to crack or even crack.

During the production process, the mold temperature is continuously increased. When the mold temperature is overheated, the mold is easily generated, and the moving parts are broken, resulting in damage to the mold surface. The cooling temperature control system should be set to keep the working temperature of the mold within a certain range.

2) Filling type

The metal liquid is filled with high pressure and high speed, which will inevitably cause severe impact and erosion on the mold, thus generating mechanical stress and thermal stress. During the impact process, molten metal, impurities, and gases also create complex chemical interactions with the mold surface and accelerate corrosion and crack generation. When the metal liquid is wrapped with gas, it will first expand in the low pressure region of the cavity. When the gas pressure rises, inward blasting occurs, and the metal dots on the surface of the cavity are pulled out to cause damage, and cracks are generated due to cavitation.

3) Open the mold

In the process of core pulling and mold opening, mechanical stress is also generated when some components are deformed.

4) Production process

In the production process of each die casting, due to the heat exchange between the mold and the molten metal, a periodic temperature change is generated on the surface of the mold, causing periodic thermal expansion and contraction, and periodic thermal stress is generated. For example, when the mold is poured, the surface of the mold is subjected to compressive stress due to the temperature rise, and after the mold is ejected from the casting, the surface of the mold is subjected to tensile stress due to the temperature drop. When this alternating stress is repeatedly circulated, the stress accumulated inside the mold becomes larger and larger, and when the stress exceeds the fatigue limit of the material, cracks are generated on the surface of the mold.

Second, measures to prevent mold damage

1. Good casting structure design

The wall thickness of the casting is as uniform as possible to avoid the occurrence of thermal joints to reduce the thermal fatigue caused by the local heat concentration of the mold. Appropriate cast fillets should be placed at the corners of the casting to avoid stresses due to sharp corners on the mold.

2. Reasonable mold structure design

1) Each component in the mold should have sufficient rigidity and strength to withstand pressure without deformation. The mold wall thickness should be sufficient to reduce deformation.

2) The gating system is designed to minimize impact and erosion on the core.

3) Correctly select the tolerance fit and surface roughness of each component.

4) Maintain mold heat balance.

3. Specification of heat treatment process

The metallurgical structure of the material can be changed by heat treatment to ensure the necessary strength, hardness, dimensional stability at high temperatures, thermal fatigue resistance and material cutting performance.

The correct heat treatment process will result in optimum mold performance, while the properties of the steel are controlled by quenching temperature and time, cooling rate and tempering temperature.

4. Die casting production process control

1) Temperature control: preheating temperature and working temperature of the mold; alloy casting temperature, with a lower pouring temperature under the premise of ensuring good molding.

2) Reasonable die casting process: specific pressure and filling speed.

3) Adjust the clamping force of the machine to make the mold evenly stressed. Pay attention to cleaning the debris on the surface of the mold, so as to avoid the uneven force on the surface of the mold when the mold is closed, causing deformation.

4) Strict control of alloy smelting to reduce gas in molten metal.

5. Mold maintenance and maintenance

1) Regular stress relief

2) Mold repair