1.How does the surface condition of the substrate itself affect the surface roughness of the color coated coil?
Substrate type and roughness
Hot-rolled substrate: The surface is rough (Ra is usually 3-5μm), and it is easy to have residual oxide scale or rolling defects (such as roller marks). It needs to be cold-rolled or finished, otherwise the coating is easy to show "rough shading".
Cold-rolled substrate: The surface is relatively smooth (Ra 0.8-1.5μm), but if the roller wears unevenly during the rolling process, it will cause the substrate to have "striped" roughness differences (such as the Ra deviation between the edge and the middle> 0.5μm).
Galvanized substrate: The size of the zinc flower directly affects the roughness - the large zinc flower substrate (Ra 2-3μm) is easy to retain the zinc flower texture after coating, and the small zinc flower or zinc flower-free substrate (after finishing, Ra 0.8-1.2μm) has a smoother coating.
Substrate surface defects
Pits, protrusions, scratches or oil residues on the substrate will cause uneven local coating thickness (such as thin coating and easy bottom exposure at pits, thick coating and easy accumulation at protrusions), forming "local high roughness points".
2.How does the pretreatment process affect the surface roughness of color coated coils?
Degreasing and rust removal effect
If the local oil and oxide scale of the substrate are not cleaned, the coating cannot be evenly spread in this area, and it is easy to form "pinholes" or "shrinkage holes", resulting in increased local roughness.
Uniformity of chemical conversion film
Uneven thickness of phosphating film will lead to increased surface unevenness; if the film layer is too thin or missed, the substrate will be partially exposed, and the coating will easily form "depression" here, both of which will cause roughness fluctuations.
3.How does the coating process affect the surface roughness of color coated coils?
Parameters of the coating roller system
Surface roughness of the coating roller: The Ra value of the steel roller needs to match the target coating. If the roughness of the coating roller itself is uneven, it will be directly copied to the coating surface.
Coating roller pressure and parallelism:
Excessive pressure: The coating is over-extended, the local coating becomes thinner, and the roughness decreases;
Too little pressure or parallelism deviation: The coating transfer amount is uneven, and local accumulation forms "bumps", and the roughness increases.
Speed ratio of the coating roller to the substrate: When the speed ratio deviates from the set value, the coating is prone to "drag marks" or "accumulation", forming periodic roughness fluctuations.
Coating performance
Viscosity: High-viscosity coatings have poor fluidity and are easy to retain the texture of the coating roller, resulting in a high Ra; low-viscosity coatings have good leveling and smoother coatings, but excessive leveling may have a weak ability to cover up substrate defects. If the substrate has tiny bumps, the coating roughness will still be affected.
Pigments and fillers: Uneven pigment particle size or dispersion will cause a "grainy feel" on the coating surface; excessive addition of fillers will increase the roughness of the coating.
Drying speed: If the paint dries too quickly, it will cause uneven shrinkage of the surface, forming "orange peel texture" and significantly increase the Ra value.
4.How does the curing process affect the surface roughness of the color coated coil?
Oven temperature distribution
Uneven temperature will lead to differences in coating leveling: the coating in the high temperature area is fully leveled, and the low temperature area is insufficiently leveled, forming a "yin and yang surface".
Too high temperature: the coating is over-crosslinked, and "pinholes" are easily formed on the surface; too low temperature: incomplete curing, insufficient coating hardness, and subsequent processing is prone to "scratches", which indirectly increases the roughness.
Matching curing time with line speed
Line speed is too fast: the coating is cured before it is completely leveled, retaining the microscopic bumps during coating;
Line speed is too slow: the coating is excessively leveled, which may cause the coating to become thinner, expose the subtle defects of the substrate, and increase the roughness.
Cooling method
Uneven air cooling will lead to different local shrinkage rates of the coating, forming "microcracks" or "wrinkles", and the Ra value will increase; if the roller surface of roller cooling is uneven, "imprints" will be pressed on the coating surface, directly changing the roughness.
5.How do equipment and environmental factors affect the surface roughness of color-coated coils?
Equipment accuracy
If the guide roller and tension roller of the coating unit are parallel, the substrate will "deviate" during operation, the coating thickness at the edge will be uneven, and the roughness will be significantly different from the middle.
Wear of the coating roller bearing will cause the roller surface to jump and periodic "bumps" will appear on the coating surface.
Environmental cleanliness
If there is too much dust in the coating workshop, dust falling on the wet film surface will form "impurity bumps" and the local Ra value will rise sharply; too high humidity will cause the surface of the water-based coating to "re-moisten", the leveling property will decrease, and the roughness will fluctuate.