Affected by the presence of foreign matter on the surface of the object to be coated , the coating cannot be evenly attached to produce shrinkage and the object to be coated is exposed , and the defect that the exposed surface is in a pattern with a diameter of 0.1~2.0mm is called shrinkage cavity. In powder coating , the cause of shrinkage cavity ( or similar volcanic crater ) in the coating film may be caused by the coating itself on the one hand , and may be caused by improper control of coating conditions during the coating process on the other hand .
①In the process of manufacturing powder coatings, the amount of leveling agent, defoamer and other additives is insufficient or unevenly dispersed, resulting in the fact that some powder particles sprayed on the surface of the workpiece do not contain leveling agent or defoamer.
②Due to unclean equipment such as pulverizers, uneven pulverization or contamination by impurities during the powder coating preparation process.
③ The oil and water in the compressed air used are not effective in removing, and the water and oil are brought into the powder coating for spraying, which will also cause shrinkage cavities in the coating film. In order to solve these problems, it is necessary to strictly implement the process requirements to avoid the above situation.
(1) The surface treatment of the coated object is not clean. When the degreasing process of the pre-painting treatment is not clean, oil stains are left on the coated object with rough surface or pinholes. After phosphating, the oil stain still remains on the coated object. On the object, the formed coating is prone to shrinkage, so the pre-treatment must be effective.
(2) The surface of the object to be coated is uneven, especially the surface of the object to be coated such as cast iron, aluminum and hot-rolled steel plate is uneven, and there are pores. Swelling out of the curing film and forming craters if the film does not level. The solution is:
① Preheat the object to be coated before painting to avoid the phenomenon that the air expands and breaks the coating film;
② Add defoamer to the powder coating, which can reduce the surface tension of the powder coating during the melting and leveling process, so that the air in the pores of the coated object can escape in time and the coating film can heal quickly.
(3) Residual moisture on the surface of the coated object after surface treatment. If the coated object with residual moisture is baked after coating, the moisture will evaporate and cause shrinkage cavity in the coating film, so the surface treatment should be adjusted after washing. Drying temperature and time asked to make the workpiece completely dry.
(4) When changing the type of coating, the cleaning of the equipment is not effective, causing the old and new powder coatings to mix and interfere with each other, and it is easy to cause shrinkage cavities in the coating film. Therefore, when changing the type of powder coating, the Spray Gun, powder supply tank, and powder supply pipeline , powder spraying room and other equipment clean.
In addition, the oil and moisture contained in the compressed air used for powder coating; Therefore, it is very important to ensure the cleanliness of the spraying environment.
高海伟 - 《涂料工业》