The pinhole is different from the bubble. The bubble usually refers to the “bubble” of > 1mm, the pinhole (pinhole) refers to the micropore of < 1mm, and the pinhole appearing in the coating layer is usually 0.5-1mm, which affects the compactness of the coating and the surface roughness of the casting.
Reasons for pinholes:
① Micro gas produced by chemical reaction between related components in paint
② The powder is not fully wetted by water, and there is gas adsorbed in the concave groove or inner micropore on the surface which is sealed by the coating slurry colloid. When the slurry is standing for a certain time, it will gather into micro “bubbles” and leave micropores when drying.
③ The speed of white mold in dip coating is too fast, the gas adsorbed on the rough surface of white mold can not be discharged in time and dispersed under the slurry coating, forming micropores in the drying process
④ Improper operation of coating mixing process causes air to be drawn into slurry
Pinhole elimination method:
① It is a common operation mistake that dip coating is too fast and the friction between coating slurry and white mold surface is not in place. It can be called “cutting corners”
② High speed mixing – greedy fast, high baking temperature – greedy fast, greedy speed is not achieved, the effect is opposite
③ Three kinds of aggregates that are easy to produce micro pores should be used with caution: bauxite (with micro pores inside), forsterite powder (with micro pores and a small amount of Cao), kaolin and bauxite after calcination. If these aggregates are selected, the first is to use a proper proportion, the second is to soak them with water for a period of time before mixing. Before the slurry colloid is formed, the gas in the micropore should be discharged, so that Cao can react fully with water first.