In the process of production, it is often encountered that the gate of zinc alloy die casting mold has serious sticking or crack phenomenon. Zinc alloy die casting adheres to the mold wall around the gate, which is not easy to remove. This situation is more in the new zinc alloy die casting die-casting mold. There are many factors leading to mold sticking of zinc alloy die casting, such as unqualified chemical composition of zinc alloy, because zinc alloy and iron have strong affinity, it is easy to react with H13 die under certain conditions, leading to sticky mold. The effect of release agent is poor, the process parameters of zinc alloy die casting are unreasonable, the inner gate design of zinc alloy die casting mold is unreasonable, the rigidity or surface roughness of zinc alloy die casting mold is not enough, which needs to be solved in the specific production process.
In this article, we discuss what causes sticky mold at the gate of zinc alloy die castings and reveal the solutions to mold sticking of zinc die casting.
1. Recognition of chemical composition of zinc alloy die casting parts
Zinc alloy and iron have a strong affinity. When the iron content in zinc alloy is less than 0.7%, zinc alloy is easy to react with H13 mold to produce compound, which adheres to the surface of zinc alloy die casting mold and causes mold sticking. However, iron can reduce the tendency of mold sticking of zinc alloy and facilitate die casting. With the increase of iron content, the mechanical properties decrease, especially the impact toughness and plasticity decrease, the hot cracking tendency increases, and the hard spots appear in the zinc alloy, and the machinability becomes worse. Therefore, the iron content of zinc alloy die casting should be controlled within 0.7% ~ 1.3%.
Therefore, we should first analyze the chemical composition of zinc alloy. According to spectral analysis, the iron content of ADC12 zinc alloy used in this product is about 0.9%, and the alloy element composition is no problem.
2. Adjustment of process parameters of zinc alloy die casting parts
The setting of the process parameters of zinc alloy die casting also has a great influence on the mold sticking, such as the size of pressure and speed, the pouring temperature of alloy, the temperature of the mold, etc., so the reasonable die-casting process parameters should be calculated according to the structure and application requirements of the zinc alloy die casting.
(1) Adjustment of pressure and speed. High metal pressure flow will increase the adhesion between the alloy and the mold. The friction heat between different metals under high pressure makes the welding between metals, which is also a kind of die sticking phenomenon. The pressure of metal flow needs to calculate and set the minimum pressure needed to produce high quality zinc alloy die casting products. According to the requirements of the product, the required boosting pressure is 24 ~ 26mpa, which can avoid metal flow bonding under too high pressure, reduce the tightening force of the mold, and avoid the occurrence of cracks. The results show that the high speed metal flow washes the mold wall, the acceleration die-casting mold is worn, the high speed metal flow enters the mold wall in the form of fog, adheres to the die-casting mold surface, and cannot fuse with the subsequent metal flow to form surface defects. Pressure and speed are two factors that complement each other and restrict each other. In order to meet the technological requirements of zinc alloy die casting, the pressure and filling speed of die casting should be adjusted steplessly.
(2) Mold and alloy temperature control. Mold temperature is very important for mold sticking. The higher the mold temperature is, the easier mold sticking is. The water inlet side of the mold is the high temperature area of the temperature. In this area, we should increase the number of sprayed copper tubes, or use the spray gun to cool the mold locally, and check the cooling water effect of the mold. In the actual production process, we can use the infrared thermometer to detect the mold sticking part, and control the mold temperature between 150 ℃ and 220 ℃, so that the mold can achieve heat balance. The pouring temperature is set lower according to the requirements of zinc alloy die casting, between 610 ℃ and 680 ℃, so as to reduce the formation of mold sticking.
(3) Through the above process debugging. The mold sticking at the gate was alleviated to some extent, but it was still unstable and scrapped. So start to improve the mold runner. The high ingate speed increases the temperature of the local mold of the metal flow impact wall and accelerates the formation of mold sticking. Therefore, it is necessary to consider reducing the ingate speed. The ingate speed = injection speed * punch area / gate area. It can be seen from the formula that to reduce the ingate speed, the cross-sectional area of the ingate can be increased, the injection speed can be reduced and the pressure chamber can be replaced. We increase the cross-sectional area of the ingate and adjust the injection speed to reduce the speed of the ingate and reduce the formation of mold sticking.
When the runner with internal gate is used, the direct impact of metal flow on the mold surface is easy to destroy the dense oxide metal film on the mold surface, which makes the mold surface uneven and causes mold sticking. By modifying the runner to make the metal flow contact the cavity surface at a small angle, the arc angle can also be applied to the runner.
3. How to select the mold release agent accurately
The release agent of zinc alloy die casting helps to reduce the sticking of mold, so it is necessary to use good release agent. The release agent can form a protective film between the zinc alloy die casting and the mold, so as to avoid the direct contact of the molten soup with the mold, so as to prevent the occurrence of the sticking mold of the die casting. Even on the polished surface of zinc alloy die casting mold, there are many uneven areas in micrometers. The release agent can fill the uneven surface and produce lubrication effect, but the release agent will be washed away under the vortex flow of molten metal, forming adhesive mold. Good release agent must have enough strength to bear the separation or impact of liquid metal. The cooling water temperature of the mold should be lower than the surface temperature of the zinc alloy die casting mold, so that the release agent can be fully attached to the surface of the zinc alloy die casting mold, and play an ideal protective role. If the temperature of zinc alloy die casting mold is too high, the release agent is difficult to adhere to the surface of zinc alloy die casting mold. This is because the release agent will be heated to a very high temperature, so that it will soon be evaporated and unable to adhere to the mold surface, so that the tendency of mold sticking phenomenon increases. The mold release agent can form a protective film on the mold surface, and the ideal temperature is 200 ℃ ~ 250 ℃. When the surface of zinc alloy die casting mold is roughened by the pressure injection metal flow, the trend of forming the mold will increase. In the places where the mold is more serious, some special pressure grease and mold release paste can be applied to the mold. These zinc alloy die casting parts have good effect of moistening and stripping mold and reducing the formation of the sticking mold.
4. Treatment of mold surface of zinc alloy casting parts
Some surface treatment methods can prevent the occurrence of mold sticking. Special materials with high melting point should be used to treat the mold surface. This zinc alloy can be mixed with iron and adhere to the position where mold sticking occurs on the mold surface to avoid the occurrence of mold sticking; It can also use various materials to prevent mold sticking to treat the surface of zinc alloy die casting die, improve the hardness of the material on the surface of zinc alloy die casting mold at high temperature, and reduce the surface activity of zinc alloy die casting to avoid mold sticking.
1. Analysis and inspection of chemical composition in zinc alloy die castings.
2. Reduce the mold temperature to lower where the mold is prone to sticking.
3. The release agent and special mold release products sprayed on the surface of zinc alloy die casting mold can effectively reduce the occurrence of sticking mold.
4. In order to ensure the quality of zinc alloy die casting, the temperature of molten metal should be reduced as far as possible.
5. The direction of zinc alloy solution entering the mold must contact the cavity surface at a small angle.
6. Under the condition of meeting the technological requirements of zinc alloy die casting, the pressure and filling speed of die casting should be adjusted steplessly.
7. Surface treatment can prevent the occurrence of sticking mold, improve the hardness of the material on the surface of zinc alloy die casting mold at high temperature, and reduce the activity of the surface of the die casting mold to avoid sticking mold.