Zinc die casting is casting in which zinc liquid is filled into the mold cavity quickly and under high pressure. The time for the zinc liquid to fill the cavity of the die-casting model is extremely short, generally a few hundredths of a second or a few thousandths of a second. The pores formed during the die-casting process have a smooth surface, and the shape is mostly round or oval, which mostly exists on the surface or subcutaneous pores of the casting, and may also be inside the casting. The source of the pores is mainly gas or zinc gas evolved during the die casting process.
Gas during the die casting process.
1. The die-casting of die-casting machines basically adopts 3 levels of injection. During the 1 level of injection, the injection punch advances at a slower speed (usually within 0.3m/s), which is conducive to the gas in the pressure chamber Extrusion; 2nd level injection is to press the structure and wall thickness of the casting to select an appropriate flow rate, the gate speed is extremely fast (generally the punch speed is 1~6m/s, thin wall parts, high air tightness parts, Magnesium alloy parts may reach a speed of more than 8m/s), and the cavity is basically filled with zinc liquid. This level 1 is the key to creating blowholes in die castings. The higher the speed, the easier it is to generate vortex and form blowholes. In this process, the control of die casting pores is mainly achieved by controlling the injection speed of the first and second stages and the switching point of the first and second stages. The first and second speeds should be as low as possible (but too low will affect the molding or surface quality of the casting, depending on the actual situation); the starting point of the second injection can be selected after the part that does not allow casting air holes, different castings we You can choose a different starting point. At the same time, with the continuous improvement and improvement of the working performance of the injection speed of the die-casting machine, the pressure build-up time, the speed-up time, etc., the castings will have fewer and fewer holes.
2. A good die-casting mold should have a good pouring system and overflow system. During the die-casting process, multi-strand runners should be kept as far as possible, and the zinc flow should be consistent with the direction of the casting. Try not to collide with each other to generate eddy currents and cause gas entrapment due to filling chaos; in addition, the multi-strand runners should be filled with cavity at the same time. Filling, can't let one or several strands of zinc liquid go back to the back corner to produce vortex. The distribution of the slag bag and the exhaust channel on the die casting mold should be reasonable.
3. The temperature of the die-casting mold also has a critical effect on the quality and porosity of the casting. When the mold temperature is too high, the release agent volatilizes at a high temperature and cannot form a dense film, which is easy to cause mucous membranes; while the mold temperature is too low, the film formed by the release agent has unvolatile water, which makes the release effect poor, resulting in Porosity of castings. Usually the mold preheating temperature is 150℃~180℃, and the working holding temperature is 220℃~280℃.
4. The gas produced by the paint
a. First of all, the performance of the coating: the volatilization point is too high, and the amount of gas generated has a direct impact on the pores of the casting.
b. From the perspective of spraying process: too much spraying, too long spraying time, it is easy to cause a large amount of gas volatilization, and the surface temperature of the mold is too low. gas. In the production process, we must choose a coating with good performance, low volatile point, and small amount of generated gas.
5. Due to the characteristic of die-casting that the cavity is filled at a rapid rate, the zinc liquid solidifies quickly in the mold to form a product, so there must be pores in the casting due to the zinc liquid entrainment. However, the surface layer of the casting will also form a dense layer of fine grains due to rapid solidification. These fine grains have high mechanical properties. As long as the machining allowance of the casting is as small as possible, the physical properties of the casting can also be guaranteed. Excessive processing allowance will remove the surface dense layer, which will cause the internal pores to be exposed, and the physical properties of the casting will be reduced.
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