The basic reasons for shrinkage

Shrinkage cavities are caused by external temperature. When the external temperature drops, the liquid alloy will cool down and shrink. After shrinkage, the volume will decrease, and gaps will appear inside the accessories. Therefore, in order to prevent the occurrence of shrinkage cavities, we should choose good materials and solidify them according to the method of directional solidification. Let's understand the causes and formation process of shrinkage cavities.

Cause

The shrinkage holes have irregular shapes and rough pore walls. The condition to prevent shrinkage cavities is that the alloy crystallizes at a constant temperature or within a very small temperature range. The casting wall solidifies layer by layer. The process of shrinkage cavity formation is shown in the figure, after the liquid alloy fills the casting mold. Because the casting mold absorbs heat, the metal near the surface of the cavity quickly drops to the solidification temperature and solidifies into a shell. The temperature drops, the alloy solidifies layer by layer, the solidified layer thickens, and the remaining liquid inside is

Shrinkage formation process

The liquid shrinks and the solidification shrinkage of the supplementary solidified layer causes the volume to decrease, the liquid level to drop, and voids to appear inside the casting until the interior is completely solidified, forming shrinkage holes on the upper part of the casting. When the casting with shrinkage cavity continues to cool to room temperature, the outer dimensions of the casting will slightly change due to solid state shrinkage.

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Measures to prevent shrinkage and porosity:

① Reasonable selection of casting alloy;

② Solidify according to the principle of directional solidification;

③ Reasonably determine the location of the ingates and the pouring process;

④ Reasonably apply process measures such as risers, chilled iron and subsidies.

Causes of cavitation in electrophoresis

1. The car body is not properly degreased before cleaning or is contaminated by oil and dust after cleaning.

2. The electrophoresis tank liquid or the post-electrophoresis cleaning tank liquid is contaminated with oil, and there is oil floating on the liquid surface or the oil is emulsified in the tank liquid.

3. The color-to-base ratio of the bath liquid is unbalanced, and low pigment content is prone to shrinkage holes.

4. The resin in the replenishing paint is not well dissolved or neutralized.

5. The coating environment is poor (including the oven), and the air contains oil mist, paint mist or silicone substances that pollute the coated object or wet coating film. The first four reasons can basically be verified through experiments, such as confirming the status before and after pre-treatment (especially the oily condition), manual oil removal, hanging plate test, laboratory electrophoresis test and other methods. If it is the last situation, it is difficult to solve. Once the cause is found, the solution can be easily determined.

Solve the problem of shrinkage holes in low pressure castings that require polishing of oxidized aluminum castings

1. The uneven wall thickness of the casting is very prone to defects such as shrinkage holes. The available solution is to add shrinkage compensation channels to the casting at the thickness change point, that is, to increase the casting fillet, because in casting, try to avoid shapes with vertical angles.

2. Increasing the temperature gradient during the solidification of the casting, that is, ensuring the solidification order of the casting from bottom to top, can effectively reduce the generation of shrinkage holes. Another method is to reduce the amount of gas generated in the casting cavity. Too much gas generation in the cavity will also cause many voids in the casting. Solution to the problem of a large number of shrinkage holes after spraying and powder coating. A large number of shrinkage holes after spraying and powder coating are caused by excessive static electricity in the spray gun, which is left by static electricity breakdown. The position broken down by static electricity repels powder with the same charge, and the powder cannot be adsorbed. The spray gun voltage can be reduced by 20-40KV, the powder output can be increased, and the distance between the spray gun and the workpiece can be controlled.