Bolt surface coating peeling off research
The test method selects the 2mm thick cold-rolled steel plate with the same material as the panel, cuts it into 130mm-250mm test plate, and drills 6 holes of the same size as the panel on each test plate, and makes different coating systems according to the following numbers. The model is then simulated assembly and the results observed. The acrylic polyurethane topcoat is the original topcoat. When the temperature is 5e, the hardness of the coating film is 56H and the impact toughness is less than 30kg/cm2. The acrylic polyurethane topcoat is the improved topcoat. When the temperature is 5e, the coating is tested. The film hardness was 23H, and the impact toughness was more than 50 kg/cm2. The phosphating film layer and each coating layer are still made by the original units according to the original process to find out the cause of the coating falling off.
Influence of deformation factors When assembling, the bottom layer of the panel has a thickness of 215mm asbestos pad. Because the hardness is relatively low, the screw hole is too large. When the bolt is tightened (the bolt size is M8, the torque is 12Nm), the asbestos pad is affected by Compressed and compressed, a deformation similar to the shape of a circular disk (B) is produced on the periphery of the panel bolt. It is found that the greater the tightening force of the bolt, the larger and deeper the deformation of the shape of the circular disc, and the more serious the peeling phenomenon. This phenomenon can be explained by the fact that when the panel is bent and deformed, the outer side is stretched, the coating is subjected to tensile stress, the inner side is pressed, and the coating generates compressive stress; especially when two adjacent positive and negative bends occur simultaneously. When the two stress values ​​are superimposed on each other, it is highly likely that the adhesion value is greater than the adhesion value, and the more severe the deformation, the greater the stress value generated, and the greater the possibility of separation.
Phosphating film layer The phosphating film layer has two main functions, one is to have certain rust resistance, the other is to increase the adhesion of the bottom layer and the coating, but also has different brittleness due to its kind and thickness. Since the zinc-based phosphating film is composed of Zn2Fe(PO4)24H2O and Zn3(PO4)24H2O, it belongs to the crystalline film and has a certain brittleness. When the workpiece is deformed, the inside of the thick or coarse-grained phosphate layer is easily produced. Crystallized to stratify and fall off. Therefore, in practical applications, the underlying phosphating film used as the surface coating of the easily deformable workpiece should be prevented from being too thick. Through the detection and observation of the simulated test plate, it was found that the phosphating film was rough and the thickness was severely uneven. The thickest part was 13Lm (corresponding to 1339g/m2) and the thinnest part was 3Lm (corresponding to 36g/m2). After 4 days of storage, dark rust spots appeared on the surface, and the distribution density was very similar to the white point on the surface of the coating on the production site. Therefore, in addition to the thickness is not applicable, the phosphate film layer on the panel has some problems in its quality. Therefore, the phosphating process of the phosphating cooperative unit was investigated and found: the original electricity was scarce due to the shortage of electricity after entering the winter. In order to meet the normal production requirements, the original hot water washing in the phosphating process was changed to normal temperature washing; the phosphating film drying was changed to Naturally dry; to ensure that the workpiece does not return rust, the phosphating time is extended from the original 10min to 25min.
Therefore, it is known that the reason why the phosphating film layer affects the peeling of the coating layer is that since all the cleaning processes are changed to the normal temperature and the first immersion washing, the cleaning effect is highly susceptible to the environmental temperature change and the cleaning water replacement cycle. It is speculated that the white tiny dots seen in the peeling off of the coating should be corrosives caused by the cleaning, which promotes the peeling of the coating. After the original zinc-based phosphating film drying process is changed to natural drying, the rust-preventing property and the coating adhesion force are affected by the inclusion of amorphous water. It is reported that the optimum drying temperature of the zinc-based phosphating film should be 100-120e. Therefore, the generation of white tiny dots should also have a certain relationship with the natural drying process of the phosphate film. If the phosphating time is too long, the thickness of the phosphating film is too thick, and the brittleness increases when deformed, causing the peeling of the coating when the coating is pressed and deformed. Therefore, for such a phosphating film whose main purpose is to improve adhesion, it is preferable to control the phosphating time within 10 minutes. Improper maintenance of the phosphating solution resulted in rough crystallization of the phosphating film and severe uneven thickness.
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