Time:2023-12-12 Browse: 30
The uneven surface defect in aluminum alloy die castings can be caused by multiple factors including mold design, die casting process, alloy materials, and equipment. Below, the editor from Aude Aluminum Alloy Die Casting Factory elaborates on the relevant causes in detail:
If the draft angle is excessively small, the friction between the aluminum alloy casting and the mold surface increases during demolding in the die casting process, which is prone to causing scratches and uneven surfaces on the casting. A proper draft angle is particularly crucial for castings with complex shapes and large depths.
Unreasonable layout of cooling channels leads to uneven cooling rates across different parts of the casting. Areas with faster cooling experience greater shrinkage, while those with slower cooling have smaller shrinkage, resulting in internal stress and subsequent unevenness on the casting surface. For instance, cooling channels that are either too close or too far from the casting surface will adversely affect the cooling effect.
If the mold surface has insufficient machining accuracy with defects such as tool marks and scratches, or uneven surface hardness, the molten aluminum will fill these defects during die casting, leading to an uneven casting surface. Furthermore, molds with poor surface quality are prone to die sticking, which further damages the casting surface.
Die casting temperature includes the pouring temperature of molten aluminum alloy and the mold temperature. Excessively high pouring temperature prolongs the cooling and solidification time of molten aluminum in the mold, increasing shrinkage volume and easily causing defects such as shrinkage cavities and porosity, which result in an uneven casting surface. Conversely, excessively low mold temperature causes the molten aluminum to cool too quickly during filling, reducing its fluidity. This may lead to incomplete filling or cold shuts on the casting surface, also contributing to surface unevenness.
An overly high injection speed causes turbulent flow of molten aluminum in the mold cavity, trapping a large amount of gas and exerting significant impact on the cavity surface. This tends to make the casting surface rough and even result in defects like pitting and depressions. An overly low injection speed, however, may lead to insufficient filling of molten aluminum, causing material shortage and uneven surfaces on the casting.
The function of pressure holding is to continuously provide pressure during the solidification and shrinkage of molten aluminum, ensuring adequate feeding for the casting. Insufficient pressure holding time means the casting cannot receive enough supplementary molten aluminum during solidification, leading to the formation of shrinkage cavities and porosity, which cause surface unevenness.
Exceeding the standard impurity content in aluminum alloy affects the fluidity and solidification characteristics of the alloy. For example, excessively high iron content forms hard and brittle intermetallic compounds, reducing the toughness and fluidity of the aluminum alloy and making the casting surface susceptible to defects. An excessively high or low silicon content also changes the shrinkage rate of the aluminum alloy, increasing the likelihood of uneven surfaces on the casting.
Inadequate stirring during the melting of aluminum alloy results in uneven distribution of alloy components. During die casting, the unevenly composed molten aluminum shrinks inconsistently during solidification, leading to an uneven surface on the casting.
Malfunctions in the pressure system of the die casting machine cause significant pressure fluctuations during injection, failing to provide stable pressure. This affects the filling and solidification process of molten aluminum in the mold cavity, resulting in inconsistent density across different parts of the casting and subsequent surface unevenness.
After long-term use, the precision of the die casting machine—such as mold clamping precision and the motion precision of the injection mechanism—deteriorates. For example, uneven gaps between the moving mold and fixed mold during clamping lead to flash and burrs on the casting at the parting line, affecting surface flatness. The reduced motion precision of the injection mechanism causes unstable injection processes, impairing the filling effect of molten aluminum.
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