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Most plastics generally need to be coated with primer before vacuum coating, mainly because of the following points:
The first, after molding, the surface of plastic must have a certain roughness, such as 0.5um roughness. The vacuum coating layer is very thin, so it is difficult to cover the unevenness of the substrate surface. If the primer coating technology is adopted, the thickness of the light-cured coating is about 10~20um, and the roughness of the coating itself is below 0.1um, so the brightness of the coating can be greatly improved.
Second, plastic contains moisture, residual solvent, monomer, low polymer, plasticizer, etc. Volatile small molecules will escape from the surface in vacuum or heating environment, which will seriously affect the adhesion of vacuum coating to substrate. However, primer coating technology can hinder the escape of these small molecules and improve the adhesion of vacuum coating to substrate.
Third, The thermal expansion coefficients of plastic substrate and vacuum coating are very different from each other, and the film is easy to crack during the process of heating and cooling in vacuum coating. The thicker the film is, the greater the possibility of cracking is. Therefore, choosing a suitable coating as the transition layer can reduce the accumulation of internal stress and the occurrence of cracking
