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Formation of sand, rat tail and depression

Source:adminRelease:2017-11-07【large In Small

The casting surface is locally Alice tongue scar metal block. A sheet shaped shell sandwiched between a metal scar and a casting is called a sand defect. The surface of the casting is striped trench, the edge of which is the slippery tail of the rat. The surface of the castings shows irregular sags, which are called sags. Sand inclusion, rat tail and sag are common surface defects in the mold casting, which often occur in the large plane and overheating of the castings.

The causes of these three kinds of defects are basically the same, all of which belong to the expansion defects. Investment casting shell shell is a multilayer structure, good quality, each layer should let each soil block, forming a whole close structure, such as the shell binding is not strong, not to form a whole, stratification, in shell dewaxing and roasting and casting, shell due to poor thermal conductivity, the surface layer and reinforcing layer temperature. The thermal expansion is different, and in the case of the inner layer of the shell, it is easy to rise up the cavity along the layer, thus forming a sag or a tail defect on the casting. The sanding defect is formed by the rupture of the warped surface layer. For the thin wall parts may also form a penetrating hole, from the reason that in the sand and had sandinclusion castings.

The partition of the shell is the internal cause of this kind of defect, which should be prevented. There are several cases in the partition of the shell. A hierarchical is appeared between the sand grains of sand, sand particle size too small is less than 50/100, or sand dust, water is too high. Dipping paint is not blown off the upper and lower floating sand. The second layer stratification is found in the coating layer. When the coating viscosity is large, local accumulation and poor hardening, the coating is hardened by the residual hardening agent and the hardening agent on both sides, but both sides are hard to penetrate, which makes the coating itself form an unhardened interlayer. The third delamination occurs between the sand and the coating. The coating layer of the surface layer is stratified with the sand, or the upper layer of sand is too fine.