Knowledge point one:
Mold temperature: The mold should be preheated to a certain temperature before production, otherwise it will be chilled when the high-temperature metal liquid is filling the mold, causing the temperature gradient between the inner and outer layers of the mold to increase, causing thermal stress, causing the surface of the mold to crack or even crack. During the production process, the mold temperature continues to rise. When the mold temperature is overheated, mold sticking is prone to occur, and moving parts malfunction, resulting in damage to the mold surface. A cooling temperature control system should be set up to keep the mold working temperature within a certain range.
Knowledge point two:
Alloy filling: The metal liquid is filled with high pressure and high speed, which will inevitably cause severe impact and erosion on the mold, thus causing mechanical stress and thermal stress. During the impact process, impurities and gases in the molten metal will also produce complex chemical effects on the surface of the mold, and accelerate the occurrence of corrosion and cracks. When the molten metal is wrapped with gas, it will expand first in the low-pressure area of the mold cavity. When the gas pressure increases, inward explosion occurs, pulling out the metal particles on the surface of the mold cavity, causing damage, and cracks due to cavitation.
Knowledge point three:
Mold opening: During the process of core pulling and mold opening, when some components are deformed, mechanical stress will also occur.
Knowledge point four:
Production process:
In the production process of each aluminum alloy die-casting part, due to the heat exchange between the mold and the molten metal, periodic temperature changes occur on the surface of the mold, causing periodic thermal expansion and contraction, resulting in periodic thermal stress.
For example, during pouring, the surface of the mold is subjected to compressive stress due to heating, and after the mold is opened and the casting is ejected, the surface of the mold is subjected to tensile stress due to cooling. When this alternating stress cycle is repeated, the stress inside the mold becomes larger and larger. , when the stress exceeds the collapse limit of the material, cracks will occur on the surface of the mold.
Knowledge point five:
Blank casting: Some molds only produce a few hundred pieces before cracks appear, and the cracks develop quickly. Or it may be that only the outer dimensions are ensured during forging, while the dendrites in the steel are doped with carbides, shrinkage cavities, bubbles and other loose defects that are stretched along the processing method to form streamlines. This streamline is critical to the final quenching in the future. Deformation, cracking, brittleness during use, and failure tendencies have a great impact.
Knowledge point six:
The cutting stress generated during turning, milling, planing and other processing can be eliminated through center annealing.
Knowledge point seven:
Grinding stress is generated during grinding of quenched steel, friction heat is generated during grinding, and a softening layer and decarburization layer are generated, which reduces the thermal shrinkage strength and easily leads to hot cracking. For early cracks, after fine grinding, HB steel can be heated to 510-570°C and held for one hour for every 25mm of thickness for stress relief annealing.
Knowledge point eight:
EDM machining produces stress, and a self-brightening layer rich in electrode elements and dielectric elements is formed on the surface of the mold. It is hard and brittle. This layer itself will have cracks. When EDM machining with stress, a high frequency should be used to make the self-brightening layer The bright layer is reduced to a minimum and must be removed by polishing and tempered. The tempering is performed at the third-level tempering temperature.
Knowledge point nine:
Precautions during mold processing: Improper heat treatment will lead to mold cracking and premature scrapping. Especially if only quenching and tempering is used without quenching, and then the surface nitriding process is performed, surface cracks will appear after several thousand die castings. and cracking. The stress generated just after quenching is the result of the superposition of thermal stress during the cooling process and the structural strain during phase change. The quenching stress is the cause of deformation and cracking, and tempering must be performed to eliminate stress annealing.
Knowledge point ten:
Mold is one of the three essential factors in die-casting production. The quality of mold use directly affects the life of the mold, production efficiency and product quality, and is related to the cost of die-casting. For the die-casting workshop, good maintenance and upkeep of the mold is A strong guarantee for the smooth progress of normal production is conducive to the stability of product quality, reduces invisible production costs to a large extent, and thereby improves production efficiency.
Post time: Jun-28-2024
