What Factors Affect the Mold Life in Aluminum Die Casting?

1. Structural design

The die design manual introduces in detail the problems that should be paid attention to in the design of aluminum die casting die. It should be emphasized that the structural design of the die should try to avoid sharply rounded corners and excessive section changes. The stress concentration caused by a sharp fillet can be up to 10 times the average stress. Thus, it is easy to cause early failure of the die. In addition, attention shall be paid to the subsequent heat treatment deformation and cracking caused by unreasonable structural design. In order to prevent heat treatment deformation and cracking, the section size shall be uniform and the shape shall be symmetrical and simple. The blind hole shall be opened into a through hole as far as possible, and the process hole can be opened if necessary. Geometric notch shall be avoided in structural design. Including knife marks, included angles, scouring grooves, holes and abrupt changes in section, etc., to reduce the occurrence of part structure and heat treatment defects.

2. Materials

The influence of die material on die life is reflected in three aspects: whether the choice of die material is correct, whether the material is good and whether the use is reasonable. Statistical data show that due to improper material selection and heat treatment, about 70% of the early failure of the die is caused by improper material selection and heat treatment. Aluminum die-casting mold belongs to hot working mold, and its service conditions are very bad. The melting point of aluminum is 580-740 ℃. During die casting, the temperature of molten aluminum is controlled at 650-720 ℃. In the case of die casting without preheating the die, the temperature of the cavity surface will rise from the room temperature to the liquid temperature, and the cavity surface will bear great tensile stress. When the top part of the mold is opened, the surface of the cavity bears great compressive stress. After thousands of times of die casting, the surface of the die will have cracks and other defects. Therefore, the requirements for aluminum die materials are high.

3. Machining

Incorrect machining is easy to cause stress concentration, and insufficient finish and machining do not completely and evenly remove the decarburized layer formed by rolling and forging. May lead to early failure of materials. In addition, in the process of processing the mold. The thickness of thicker formwork cannot be ensured by superposition. Because the thickness of the steel plate is 1 time, the bending deformation is reduced by 85%, and the lamination can only play the role of superposition. The bending deformation of two plates with the same thickness as the single plate is 4 times that of the single plate. During the processing of the cooling water channel, special attention shall be paid to ensuring the concentricity during the processing of both sides. If the head corners are not concentric with each other, the connecting corners will crack during use. The surface of the cooling system shall be smooth and preferably free from machining marks. With the progress of technology, we should pay attention to the tracking and use of advanced machining technology, which is conducive to the improvement of the machining accuracy of mold parts, so as to prolong the service life of the mold.

4. Grinding and EDM

Grinding time may lead to local overheating of metal surface, high surface residual stress, and structural changes, which may lead to grinding cracks. In addition, grinding cracks can be caused by improper pretreatment of the original structure, carbide segregation, coarse grain size, and insufficient tempering. Therefore, under the condition of ensuring the material, pay attention to selecting the appropriate coolant to control the grinding cooling. Control the grinding speed to reduce cracks.

EDM can form a white bright hardening layer of quenched martensite on the die surface after quenching and tempering. The thickness of the hardening layer is determined by the current intensity and frequency during machining. It is deep during rough machining and shallow during finish machining. The hardened layer will cause great stress on the die surface. If the hardened layer is not removed or the stress is eliminated, the die surface is prone to electric crack, pitting corrosion, and cracking during use. The quenching layer or stress relief can be achieved by removing the quenching layer with oilstone or grinding. Without reducing the hardness, the stress can be reduced below the tempering temperature, which can greatly reduce the surface stress of the die cavity.

5. Heat treatment

Improper heat treatment is an important factor leading to the early failure of dies. The deformation of heat treatment is mainly caused by thermal stress and structural stress. When the stress exceeds the yield strength, the material will produce plastic deformation. When the stress exceeds the strength limit, it will cause a quenching crack of the parts. 

6. Production operation

When determining the injection speed of aluminum die-casting mold, the speed should not be too high. Too high speed will cause mold corrosion and increase the deposits on the cavity and core. However, too low speed will easily cause defects in the casting. Therefore, for aluminum die casting mold, the minimum injection speed is 18m /s, the maximum injection speed should not exceed 53m/s, and the average injection speed is 43m/s. The casting process shall be strictly controlled during the use of the die. Within the scope of the process permit. Try to reduce the casting temperature and injection speed of molten aluminum, and increase the preheating temperature of the mold. The preheating temperature of aluminum die casting mold is increased from 100-130 ℃ to 180-200 ℃, and the service life of the mold can be greatly improved. At the same time, avoid improper adjustment and operation of the machine tool, resulting in early failure of the die.

7. Mold maintenance

– Timely remove the mold cavity deposits. After the mold is used for a period of time, there will be deposits on the mold cavity and core. These deposits are formed by the combination of demoulding agent, coolant impurities, and a small amount of die-casting metal under high temperature and high pressure. Some deposits are very hard and adhere firmly to the core and cavity surface, which is difficult to remove. When removing sediment, it can be removed by grinding or mechanical method. Pay attention not to damage other surfaces and cause dimensional changes.

– Regular maintenance to keep the mold in good condition. After the new mold is tested, the stress relief tempering shall be carried out when the mold is not cooled to room temperature. When the new mold is used for 1/6-1/8 of the design life, that is, the aluminum die casting mold is used 10000 times, the mold cavity and mold base shall be tempered at 450-480 ℃, and the cavity shall be polished and chlorinated to eliminate internal stress and slight cracks on the surface of the cavity. After that, the same maintenance shall be carried out every 12000-15000 mold times. When the mold is used for 50000 mold times, it can be maintained every 25000-30000 mold times. Through maintenance, the speed and time of cracking caused by thermal stress can be significantly reduced. Effectively improve die life and casting quality.

– Timely repair mold defects. In case of serious erosion and cracking, the mold surface can be nitrided to improve the hardness and wear resistance of the mold surface. During nitriding, pay attention to the bonding firmness of the substrate and nitriding layer, and the thickness of the chlorination layer shall not exceed 0.15mm. A too thick nitriding layer is easy to fall off at the parting surface and sharp corners. The mold surface defects can also be repaired by welding. During welding, pay attention to the composition matching of welding rod and weldment, cleaning of welding surface and drying of welding rod, trimming and finishing of mold cavity after welding, and stress relief tempering after welding.