In practice, Aluminium profile coatings can develop linear or lattice cracks that resemble dry riverbeds, a phenomenon known as "large cracks". Large cracks" are usually caused by insufficient solvent when the coating film is melted during the baking process. This phenomenon is aggravated by the following conditions.
1. The time taken to raise the surface temperature of the metal to 235°C is too long.
2. relatively fast flash drying of the paint film before baking.
3. too thick a profile and too thick a paint film
4. too high a pigment content, e.g. white or off-white.
The following precautions should be observed in Aluminium profile coating to minimize the occurrence of "big cracks".
1. The primer should not be sprayed too thickly, a dry film thickness of 5-10 μm is recommended.
2, the primer should not flash dry too fast before spraying the top coat, otherwise, it will produce the dry absorption of the topcoat solvent, adding slow drying solvent can extend the wetting time of the primer film.
3, the topcoat should not be too thick, it is recommended not to exceed 35um, evaporation should not be too fast, it is recommended to use slow drying solvent, ethylene glycol monobutyl ether or butyl carbitol, generally its flash drying time does not exceed 0.5 hours.
4. The process of heating the furnace to the set temperature should not be delayed, too slow or the existence of cold spots can lead to "big cracking".