The causes of burrs in precision castings

The causes of burrs in precision castings

The small protrusions on the surface of castings in precision casting are similar to cucumber thorns on casting defects. The characteristic of cucumber thorns is that there are many isolated and scattered short metal thorns on the surface of the casting. The direct reason for this is that the surface layer of the mold shell is not dense, and there are small holes with different shapes, which are caused by the metal liquid drilling into it during pouring. Today, precision casting manufacturer Tianyao Metal will discuss in detail the reasons for burrs in precision castings.

Generally speaking, in investment casting, the molten metal only replicates the original condition of the shell surface, and the final condition of the casting surface is only a reflection of the true condition of the shell surface. Therefore, the surface of the shell before pouring should have voids. So, what situation would cause so many pores or non compactness on the surface of the mold shell?

1、 The viscosity of the surface coating is insufficient, and the powder to liquid ratio is too small. When the powder to liquid ratio of the surface layer is small, the surface coating is relatively thin, and due to the small powder to liquid ratio, the shrinkage of the surface layer is relatively large when it is dry, and the spacing between silica particles is relatively large. When the solvent evaporates, the gaps between particles are relatively large, causing the surface layer to be less dense and have too many pores. In addition, due to the relatively thin surface layer, when the sand particle size of the surface layer is large or when the sand in the transition layer directly penetrates the surface layer film, it will cause pores on the surface layer. Once the metal liquid enters these pores, small protrusions will form.

2、 The wettability between the coating and wax mold is poor, and selective adsorption occurs after sanding. Some people may not understand what selective adsorption is? The so-called selective adsorption is when the wettability between the coating and the wax mold is worse than that between the coating and the sand particles. After sanding, certain parts of the coating slurry adhering to the wax mold will be "sucked" by the sand particles and leave the surface of the wax mold, forming irregularly sized and shaped pores.

3、 The mixing time of the surface coating is short, not evenly mixed, and the bonding agent and powder are not sufficiently wetted, resulting in an inappropriate thickness of the surface coating, varying coating densities and densities in various areas.

4、 Another possibility is that the sanding method is improper, using a floating sand bucket to hang sand on the surface layer, and the air pressure is too high, causing sand particles to penetrate the surface layer coating, resulting in pores on the surface layer.

In fact, there is often more than one reason for a defect in a casting, which may be the result of a combination of multiple factors. For example, if the viscosity of the surface layer is low, the powder to liquid ratio is low, the difference in sand particle size between the transition layer and the surface layer is large, or the particle shape is not spherical but angular, these combined forces may cause cucumber thorns to appear on the surface of the casting. Sometimes we cannot simply determine the cause of this defect, we can only determine which factor may have a greater impact.

Precision machining