Case Study Of A Quality Complaint On Aluminum Die Casting Products - The Importance Of Process Quality Capability Improvement To Ensure Stable Product Quality (Ⅲ) (Continued from the last issue) ▌The impact of melting process on the black spot of castings Melting is the first process of die casting. Melting is usually carried out by mixing the raw material (aluminum ingot) and the return material according to a certain proportion, and the material of aluminum ingot is set according to the customer requirements. In this process, we need to pay attention to: whether the material of the recycled material is the same as that of the aluminum ingot; whether the appearance is polluted; whether any non-aluminum alloys are mixed in; whether there are cutting oil or cutting fluid on the return materials; the proportion of recycled material added; whether the recycled material has zone management where the primary and secondary recycled materials are identified, and whether the primary and secondary recycled material are added at the same proportion. Those unclean aluminum alloy chippings must be cleaned or directly scrapped so as to avoid unclean substances mixed in the return material, causing pollution of molten alloy during smelting and producing black spots on the casting surface.
▌The impact of die casting process on black spots of castings The die casting process is the key process of thermostat housing production. The compliance of process requirement in this process has a great impact on the internal quality and appearance of the product, with hidden potential risks of product failure. Therefore, in this process, we need to highly focus on the following factors: gating system and mold structure; mold opening time; mold cavity temperature; mold surface temperature at mold opening; mold release agent ratio; spray amount of mold release agent; whether soft water is used for mold release agent; whether the water content of mold release agent in mold cavity is blown clean or completely volatilized.
▌The impact of melting system and mold structure on black spots of castings We need to pay attention to whether the gating system is far away from the gate, the eddy current, the large plane of the casting, and the mold cavity and the final filling parts, which may lead to the surface blackening of the die casting parts. The main reason is that there is more flue gas gathered in these parts, the contact time between the alloy liquid and the mold surface and the flue gas is long, and the amount of vortex entrapped gas in the alloy liquid is large, which aggravate the oxidation of the alloy liquid on the surface of the die casting and make the degree of adhesion of oil fume as well as carbon black on the mold surface worse; Additionally, the flow speed of the alloy liquid is slow in the final filling area, the degree of erosion on the mold surface is light, and the oil fume accumulated on the mold surface cannot be washed away by the alloy liquid. For large plane castings, most of the alloy liquid is spray filling, which exceeds the velocity of flue gas flow and removal. The more the alloy liquid contacts and mixes with the flue gas, the more the amount of oil fume gas in the alloy liquid, the easier the alloy liquid is oxidized and polluted, and the phenomenon of blackening appearing on the surface can be even more serious.
Therefore, improving the gating system of the mold, improving the position, size and direction of the inner gate and overflow slot, changing the flow direction and flow rate of the alloy liquid, changing the generation, flow direction and discharge status of the oil fume are all effective measures to reduce and avoid the blackening of the casting.
The blackening conditions in die casting process can be summarized as: blackening on the surface, showing oil smoke; blackening on the surface, showing flake shape; blackening on the surface, showing black spot.
When the casting surface seems to have black smoke with irregular shape and inconsistent depth, this is because in the process of alloy liquid’s filling the mold cavity, when the release agent meets the high-temperature alloy liquid, it burns insufficiently, causing a large amount of smoke gathering in the mold cavity, and the smoke are not completely and quickly discharged from the mold cavity. The accumulated oil fume pollutes the surface area of the alloy liquid and the mold cavity, resulting in the blackening of the oil fume on the surface of the die casting.
When the surface of the die casting is blackened, like a cloud piece, irregular shape, but the color of black is uniform, this is because the surface of the mold cavity is contaminated by the carbon black and oil fume of the mold release agent, and the carbon black on the mold surface is printed on the surface of the casting, resulting in a shape close to the blackening pattern on each surface of the die casting, with lighter black color but darker than lampblack.
When the black spots on the surface of the die casting parts are different in size, approximately round in shape and dark in color, this is because the black particles formed by the release agent are on the surface of the mold or casting, with each black particle causing black on the casting surface. These black particles may be mold release agent deposits or aggregates, which are relatively viscous and are not easy to be completely decomposed at high temperature. (To be continued) |