Die-casting is the use of the die-casting machine will be aluminium, zinc, magnesium, tin and other alloy melt into the mold after the molten state of pressure forming and forced cooling in a short period to produce a large number of high dimensional accuracy; the internal structure of good denseness of the casting, to reduce the machining allowance and ensure the inherent quality of the casting. Therefore, the reasonable use of mold cooling circulating water in the complex die-casting mold is particularly important.
The influence of circulating cooling water on the die-casting mold
Die-casting mold cooling can be divided into external cooling and internal cooling; external cooling is in the casting after demolding with a nozzle on the surface of the mold cavity spray cooling, and spray release agent; internal cooling is in the internal mold pass circulating cooling water, is the main cooling mode of the mold. In the first domestic car aluminium cylinder die-casting project of Dongfeng Honda Engine Co., Ltd, its complex cylinder die-casting mold has cooling water passed inside the movable core and most of the hole-making pins to control the temperature of various parts of the mold.
The high-temperature control of the mold, although it is conducive to the filling of the aluminium liquid in the mold and the forming of the casting when the temperature of the mold is close to 300 ℃, it will produce the obvious sticky aluminium phenomenon, making the surface of the casting rough or missing pieces, requiring frequent grinding of the mold. At the same time, the casting is not fully cooled in the scheduled time to open the mold, the casting is not fully formed due to insufficient cooling and the release gap between the mold, coupled with the phenomenon of sticky aluminium, the release will lead to the module and the hole-making pin is too much force and cracking damage (the ringing sound when opening the mold will also be significantly increased). Low mold temperature control will affect the liquidity of the aluminium and cause cold separation of the casting. We hope that the temperature of each part of the mold does not differ too much. Otherwise it will cause the module to crack due to uneven thermal expansion and contraction of the mold, and the tiny crack will be continuously expanded due to the stress concentration phenomenon and the infiltration of aluminium liquid so that the module will be partially cracked, which directly affects the service life of the mold. In the production practice, we have encountered that the poor water quality of cooling water produces scale and causes part of the cooling pipe blockage. As a result, the moving die side of the active core temperature is too high (the surface temperature is close to 300 ℃), resulting in the surface of the mold cavity being heavy, sticky aluminium; almost every shift to polish the mold, and in the module corners cracking phenomenon increased significantly. The situation has improved significantly by taking measures to clear the pipes and improve the water quality.
The influence of mold cooling water on casting quality
The temperature of each part of the mold varies depending on the need, the workpiece wall thickness of the larger parts of the aluminium liquid solidification is slow, and the mold temperature can be slightly lower; tendons tabs part or thin wall to prevent hard partition, to improve the liquidity of aluminium, the mold temperature should be relatively high. Control the temperature of each part of the mold, can also change the distribution of shrinkage in the casting state (see Figure 2). Our production of aluminium cylinder blanks, water jackets and bolt holes between the shrinkage is easy to produce, resulting in water and bolt-hole leakage; if you strengthen the cooling of the hole-making pin, you can make the bolt hole around the aluminium liquid to speed up the solidification, while other parts of the cooling are slow, shrinkage is always in the aluminium liquid last solidified position formed. The shrinkage between the water jacket and the bolt hole is transferred, and the leakage rate of the casting is controlled.
Poor cooling of the mold so that the mold temperature is high, in addition to causing sticky aluminium to affect the surface finish of the workpiece, but also because the workpiece is not sufficiently cooled to form and produce tensile stress when the mold is removed, resulting in an internal dark crack. In the cylinder block die-casting production, we encountered poor cooling of the fixed die in the test pressing of the blank, resulting in obvious leakage.
To reduce the air holes in the casting, the mold should have a certain riser volume and a perfect exhaust channel. If the exhaust gap between the static and dynamic mold is too small, the air in the cavity of the aluminium liquid in the filling process can not be fully discharged, which will increase the pores in the casting; if the gap is too large, the aluminium liquid will rush out from the gap, forming a too large flying edge. To solve the contradiction of smooth exhaust and not make the aluminium liquid leakage, it can be added to the cold iron at the venting part of the outside of the riser with water-cooled pipes when the aluminium liquid to the mold cavity gas through the riser from the larger gap of the air channel smooth discharge, aluminium liquid encountered with internal cooling of the cold iron rapid condensation and will not cause excessive burr. To enhance the cooling effect of the cold iron, in addition to the use of cooling water internal cooling, some also use high thermal conductivity alloy materials.
Cooling water system
We adopt a closed cooling tower circulation system to ensure the pristine water quality of the existing mold cooling water; to avoid scale blocking the pipeline, the circuit is equipped with a multi-functional high-frequency electronic descaling device and dosing anti-corrosion device. To prevent rusting, all metal components in the reservoir are made of stainless steel, and the water suction port is at a certain distance from the bottom of the pool to avoid inhaling sediment. According to the needs of the cylinder mold, the cooling capacity of the cooling water system is 100RT (RT-Japanese cold ton, 1RT=3320kCal/h=3.861kW). The maximum cooling water circulation volume is 13090L/min, the water supply pressure is 0.3-0.4MPa, the water supply temperature is 20-35℃, and the temperature difference between import and export water is 5℃.
During the training period of Honda Motor Co., Ltd. in Japan, we carefully observed and analyzed the cooling water system of the cylinder block die-casting mold. In addition to the closed-circulation system, they also adopted a computer control system, which can set the temperature of each point on the mold. Their mold is equipped with sensors to detect the temperature of each part and convert the temperature of each place into an electric signal to give feedback to the computer control system to compare with the set temperature to adjust the cooling water flow in each place.
The internal cooling water of the complex mold directly affects the life and quality of the mold. Like die-casting cylinder body such a complex mold, its cooling water in and out of the branch pipe has hundreds of cooling water in each part of the mold for the corresponding adjustment; if the valve is controlled, not only is the layout difficult, and can not make the quantitative adjustment. Only by using a computer control system is it easy to adjust and stabilize the temperature of each point of the mold.
The structure of the mold, the wall thickness and complexity of the workpiece, the change of temperature, etc., have different requirements on the cooling water flow of the mold, which needs to be tested and figured out through a certain period to obtain the best data.