Advantages
Modern coil coating lines have integrated devices for treating waste
water and exhaust fumes and generally exceed legal requirements
concerning emission levels. Coil coated metal is "clean" and
lends itself well to further processing. The number of processing steps
for the fabricator are reduced as well as the need for:
- Post-painting
with its expensive pre-treating and rinsing steps
- Storage
of paint and chemicals
- Laboratory
- Equipment
maintenance
- Expensive
waste water and air treatment
- Disposal
of waste paints
- Insurance
costs and space needed
- A flat surface is
easier to coat than an irregular shape
This
means improved productivity and lower costs.
Quality, capacity, environmental aspects and costs have already convinced many metal sheet fabricators to switch from post-painting to using coil coated metal.
What coils?
Coils are large coils of metal sheet stock. Coil coating is often confused with coils for electric motors and transformers outputted from flat rolled products. Coils represent a variety of metals including, but not limited to, cold rolled steel, hot dip galvanized, electro galvanized, Galvalume¨, Galvanneal¨, zinc-nickel, terne, aluminum, stainless steel, copper and brass.
What exactly is coil coating?
Coil coating is a continuous and highly automated process for coating metal before fabrication. In one continuous process, a coil of metal is unwound and both the top and bottom sides are cleaned, chemically treated, primed, oven cured, topcoated, oven cured again and rewound for shipment.
Even more specifically, bare coils of metal are placed on an unwinder or decoiler where the metal is observed for defects. The metal is then cleaned and chemically treated in preparation for painting. Brushes can be used to physically remove contaminants from the sheet, or the metal may be abraded by flap sanders to further enhance the surface. Pretreatments may be used to provide the bond between the metal and the coating, in addition to adding any corrosion resistance. The type of chemical treatment varies with the type of metal being used.
After drying, the strip enters a coating room for a coat of primer usually on both sides of the sheet. The pickup roll transfers the coating liquid from the pan to the applicator roll. The liquid is then pumped into the pan, and then overflows back to the supply reservoir, where it is remixed and filtered.
The direction of the rotation of the applicator roll plays a part in determining the type of coating. Reverse roller coating, when the applicator roll turns in the opposite direction of the strip, is used to apply thick coatings. Direct roller coating, turning in the same direction as the strip, is used for thinner coatings, 0.5 mils or less.
The paint sheet then enters an oven in which the coating is baked at high temperatures for 20 to 30 seconds. The strip exits the oven and is cooled with air and water. A majority of specifications call for two coats (primer and top coat) on each side of the sheet, requiring a pass through a second coater room and oven. The fully painted sheet then exits the second oven and is cooled before inspection and rewind.
Since the process allows metals to be finished in one continuous pass, there is flexibility in how the metal is coated and
the types of coatings. Organics and inorganics, such as polyesters, epoxies, vinyls, plastisols, acrylics, water-born emulsions, fluorocarbons, dry lubricants, treatment and primer combinations, can be applied. Prepainted coils can also be printed, striped and embossed to create special visual effects.
The key word in this entire process is control. By its very nature provides for controls that are virtually impossible to attain with most other painting processes. Dealing with a flat sheet allows for mechanical cleaning in addition to the spray cleaning. The flat sheet also enables excellent control of coating weights of both the pretreatment and the paint to within a tenth of a mil or less, depending upon the equipment and the paint system being applied. Such advantages, along with the economic and environmental benefits, make coil coating the choice for success.
The flexibility of finished coils also extends beyond the coating process to fabrication. Coils can be rewound or drawn, formed, bent or blanked. Prepainted metal can be fastened with bolts, screws, rivets, Tog-L Loks¨ and similar devices. It also can be lockseamed, adhesive bonded and sometimes welded. No matter what the end product requirements, there is a coated coil that meets the need. |
