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03
2020
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08
Causes and solutions for air holes in die-cast parts during mold opening.
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Zinc die casting is a casting process where molten zinc is rapidly filled into the mold cavity under high pressure. The time for filling the die-casting mold cavity with molten zinc is extremely short, generally a few tenths of a second or even milliseconds. The air holes formed during the die-casting process have smooth surfaces, typically round or oval in shape, and are often found on the surface of the casting or beneath the skin, and may also be present inside the casting. The source of these air holes mainly comes from gases entrained during the die-casting process or from gas released by the molten zinc.
1. Air entrainment during the die-casting process.
1. Die casting machines now generally adopt a three-stage injection process. In the first stage, the injection plunger advances at a relatively slow speed (usually within 0.3 m/s), which helps to expel gases from the pressure chamber; in the second stage, an appropriate flow rate is selected based on the structure and wall thickness of the casting, with an extremely fast speed at the inner gate (generally, plunger speeds range from 1 to 6 m/s, and for thin-walled parts, high gas-tightness parts, and magnesium alloy parts, speeds may exceed 8 m/s), allowing the molten zinc to fill the mold cavity. This first stage is critical for generating air holes in die castings; higher speeds are more likely to create turbulence and form air holes.
In this process, controlling air holes in die castings is mainly achieved by controlling the injection speeds of stages one and two and their switching points. The speeds of stages one and two should be kept as low as possible (but too low may affect the forming or surface quality of the casting, which should be determined based on actual conditions); the starting point for stage two injection can be chosen after areas where air holes are not allowed in the casting. Different castings may require different starting points. At the same time, as the performance of die casting machines improves continuously in terms of ejection speed, pressure building time, acceleration time, etc., air holes in castings will decrease.
2. A good die casting mold should have a well-designed pouring system and venting system. During die casting, it is important to ensure that multiple pouring channels keep consistent flow direction with the casting and avoid collisions that could create turbulence and cause air entrapment due to chaotic filling; additionally, when filling multiple pouring channels into the mold cavity, care should be taken to ensure simultaneous filling without allowing one or several streams of molten zinc to reach dead corners at the back before returning and causing turbulence. The distribution of slag pockets and venting channels on the die casting mold should be reasonable.
3. The temperature of die casting molds has a critical impact on both the quality of castings and air holes. When mold temperature is too high, release agents evaporate at high temperatures and cannot form a dense film, leading to sticking; conversely, if mold temperature is too low, moisture from un-evaporated release agents can result in poor release effects and lead to air holes in castings. Typically, preheating temperatures for molds range from 150°C to 180°C, while working temperatures are maintained between 220°C and 280°C.
4. Gases generated by coatings
a. First is the performance of the coating: a high volatility point and large gas generation directly affect air holes in castings.
b. From a spraying process perspective: excessive use of spray material or prolonged spraying time can lead to large gas evaporation volumes and may also cause mold surface temperatures to drop too low, preventing moisture on the mold surface from evaporating promptly, resulting in significant gas generation in the mold cavity after closing. During production, we should choose high-performance coatings with low volatility points that generate minimal gas.
5. Due to the characteristics of die casting involving rapid filling of mold cavities, molten zinc quickly solidifies within molds to form products; therefore, there will inevitably be air holes inside castings caused by gas entrapment in molten zinc. However, the surface layer of castings will also form a dense layer of fine grains due to rapid solidification; these fine grains possess higher mechanical properties. As long as machining allowances for castings are kept minimal, their physical properties can be ensured. Excessive machining allowances will remove this dense surface layer, exposing internal air holes and reducing physical properties.
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