Causes of Blistering of Coatings in Lost Foam Castings

1. In the composition of water-based paint, quartz powder is used as the powder of the paint for the infiltration of lost foam castings. Although the effect of its particle size and particle size distribution on coating properties has not been systematically investigated, generally speaking, the finer the powder particles, the better the suspension stability of the coating and the better the calcination performance of the coating. However, the finer the powder, the more binder is required, and the higher the moisture content in the coating. This not only reduces the high-temperature water permeability of the coating but also seriously affects the water permeability of the coating during drying. The water that needs to evaporate from the coating will reduce the breathability of the coating itself, so the water vapor generated when the bottom of the coating dries up cannot break through the tension of the coating and escape in time, which increases the possibility of blistering.

2. Among the coating components, the type and amount of suspending agent have a great influence on the mixing and foaming of the coating. The foaming property of bentonite suspension is greater than that of other clays. When the content of bentonite is too high, the air foam is usually dispersed in the paint, and it is not easy to gather and raise. When the plastic pattern is impregnated, the shear rate is lower than that of brushing and painting, and the viscosity of the slag coating of the lost foam casting is reduced. Small air bubbles dispersed in the coating are not expelled but stored in the coating. Through the drying temperature, the small air bubbles are dispersed, and the volume expands and penetrates into each other, thereby increasing the possibility of foaming.

3. When the plastic pattern is produced, sometimes silicone oil is coated on the mold to facilitate the mold being raised. If the silicone oil on the pattern is not cleaned before spraying, under severe conditions the coating will not hang and will not show white. The wettability of the coating is slightly reduced. Dip coating is used to apply low shear rate coatings. The solidified film formed by bentonite in the paint and some network structures and irregular macromolecular screw groups in the polymer solution does not disintegrate, and the viscosity is high. Coatings cover patterned surfaces that are not as clean as plastic film. There is gas where the coating and pattern are not wet, and it is easy for the coating to bubble when it is dry and heated.

4. For the same paint recipe and the same batch of paint, there will be blistering defects after double-coating. However, not all products have blisters, and few products have coating blisters during the drying process. The results show that the plastic pattern with its simple structure and short inner wall has a shorter immersion time in the coating, and the coating using lost foam casting technology generally does not produce foaming defects. Instead, there are blistering defects.