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How to prevent die casting production mold damage and measures

Posted by: Alan Xia 2021-10-21 Comments Off on How to prevent die casting production mold damage and measures

Reasons for the damage of the die-casting production mold

In die casting production, the most common forms of mold damage are cracks and cracks. Stress is the main cause of mold damage. Thermal, mechanical, chemical, and operational shocks are all sources of stress, including mechanical stress and thermal stress. Stress occurs in:

(1). In the process of mold processing and manufacturing

  • 1. Quality problems of rough forging;Some molds have cracks after only a few hundred pieces are produced, and the cracks develop very quickly. It is possible that only the external dimensions are guaranteed during forging, and the loose defects such as dendrites, inclusion carbides, shrinkage cavities, and bubbles in the steel are extended and elongated along the processing method to form a streamline. This streamline affects the future The final quenching deformation, cracking, embrittlement during use, and failure tendency have a great influence.
  • 2. Cutting stress generated during final processing such as turning, milling, planing, etc., this stress can be eliminated by intermediate annealing.
  • 3. Grinding stress is generated during grinding 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 precision grinding, h13 steel can be heated to 510-570°C, with a thickness of 25mm for one hour for stress relief annealing.
  • 4. EDM produces stress. A bright white layer enriched with electrode elements and dielectric elements is formed on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. High frequency should be used in EDM to minimize the white layer, polishing method must be used to remove, and tempering treatment, tempering is carried out at the third tempering temperature.

(2). During mold processing

Improper heat treatment can lead to cracking of the mold and premature scrapping, especially if only quenching and tempering is used, and the surface nitriding process is carried out without quenching, surface cracks and cracks will appear after several thousand die castings.

The stress produced during quenching of steel is the result of the superposition of the thermal stress during the cooling process and the structural stress during the phase transformation. The quenching stress is the cause of 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, chilling will occur when the high-temperature molten metal is filled, which will increase the temperature gradient of the inner and outer layers of the mold, form thermal stress, and cause cracks or even cracks on the mold surface.

In the production process, the mold temperature continues to rise. When the mold temperature is overheated, it is easy to produce mold sticking, and the failure of moving parts causes damage to the mold surface.

A cooling temperature control system should be installed to keep the mold working temperature within a certain range.

2. Filling

The molten metal is filled with high pressure and high speed, which will inevitably produce fierce impact and erosion on the mold, thus generating mechanical and thermal stress. During the impact process, molten metal, impurities, and gas will also produce complex chemical interactions with the surface of the mold, and accelerate the generation of corrosion and cracks. When molten metal is encased in gas, it will expand first in the low-pressure area in the cavity. When the gas pressure increases, inward blasting occurs, pulling out the metal particles on the surface of the cavity and causing damage, and cracks due to cavitation.

3. Open the mold

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

4. Production process

In the production process of each die casting, due to the heat exchange between the mold and the molten metal, the surface of the mold produces periodic temperature changes, causing periodic thermal expansion and contraction, and generating periodic thermal stress. For example, the surface of the mold is subjected to compressive stress due to the temperature rise during pouring, and after the mold is opened to eject the casting, the surface of the mold is subjected to tensile stress due to the temperature drop. When this alternating stress is repeatedly cycled, the stress accumulated inside the mold becomes larger and larger. When the stress exceeds the fatigue limit of the material, cracks occur on the surface of the mold.

Measures to prevent mold damage

1. Good casting structure design

The wall thickness of the casting should be as uniform as possible to avoid hot spots and reduce the thermal fatigue caused by the local heat concentration of the mold. The corners of the castings should have appropriate casting fillets to avoid stress caused by 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 wall thickness of the mold must be sufficient to reduce deformation.
  • 2) The gating system is designed to minimize the impact and erosion on the core.
  • 3) Correctly select the tolerance and surface roughness of each component.
  • 4) Maintain the thermal balance of the mold.

3. Standardize the heat treatment process

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

The correct heat treatment process can get the best mold performance, and the performance of the steel is controlled by the quenching temperature and time, cooling rate and tempering temperature.

4. Die casting production process control

  • 1) Temperature control: mold preheating temperature and working temperature; alloy pouring temperature, under the premise of ensuring good molding, use a lower pouring temperature.
  • 2) Reasonable die-casting process: specific pressure, filling speed.
  • 3) Adjust the clamping force of the machine to make the mold evenly stressed. Pay attention to cleaning the residual debris on the surface of the mold, so as not to cause the uneven force on the surface of the mold and cause deformation when the mold is closed.
  • 4) Strictly control alloy smelting to reduce gas in molten metal.

5. Mold maintenance and maintenance

  • 1) Regular stress relief
  • 2) Mold repair

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