Coil coating is the process of applying organic coating materials to rolled metal strip substrates in a continuous process. The process includes: unwinding and cleaning, chemical pre-treatment of the metal surface (single or double sides), single or multiple applications of (liquid) paint or coating powder, subsequent curing or lamination with plastic film, cooling and rewinding of the same roll for transport to sheeting machines, slitting machines or manufacturing plants.
One of the fundamental principles of coil coated materials is that the material passes through the various process stages in a continuous manner! Therefore, the coated material has very consistent properties. In addition to the obvious labor efficiency, environmental benefits and energy savings benefits; coil coated materials are inherently more consistent than those products that are produced on a part basis.
Coating surfaces can have a variety of colors, gloss levels, and surface textures. The choice of coating depends on the end use application, if the finished coating has non-stick properties, great flexibility or high chemical resistance.
Substrates in coil coating lines：
A typical organic coil coating line consists of unwinder, entry strip accumulator, washing, chemical pretreatment, primer application, curing, final paint application, curing, exit accumulator and recoiler. The size of the coating line varies greatly, with rated outputs between: 3000 tons and 200,000 tons of coated metal per year, maximum widths from 150 mm to over 2500 mm and speeds from 20 meters per minute to 200 meters per minute.
Continuously Operated Coil Coating Line - Accumulators
The basic principle of this continuous process is achieved by using inlet and outlet accumulators.
Inlet Accumulator - It allows the operator to stop the input end and splice in another coil while the coil being processed continues to run through the line, unaffected by the activity on the input end. When the input end is stopped, this accumulator continues to feed material to the coil line as the upper and lower accumulator rolls converge. When the splice is complete, the input end restarts, the accumulator rolls diverge, and the entire line is once again in a steady state. This ability to continuously splice coils together without stopping the entire line creates an uninterrupted web of virtually unlimited length. This process is repeated until the last coil of metal is processed.
Outlet Accumulator - It performs the opposite function of the inlet accumulator. When the input end is stopped, the inlet accumulator feeds material to the line while the outlet accumulator takes material from the line. This allows the exit end of the line to be stopped without affecting the process. In this case, the upper and lower coils deviate as more material is put into the accumulator, and the exit-end operator removes the completed coil and prepares the exit-end to rewind another coated metal coil.
Inlet, cleaning and pre-treatment.
Paint Application：
1. Reverse Roll Coating Process The paint circulates in the paint pan, the rotating take-up roller is partially immersed in the coating and rotates counterclockwise, then transfers the coating to the coating roller rotating in a clockwise direction. This special configuration is used for most products and is able to achieve a coating thickness of 25 microns in most coatings.
2. Direct coating process Thinner coating and printing use a similar configuration, but the direction of the rollers is opposite, so the coating roller has the same surface direction as the web, and the pick-up roller has the same surface direction as the coating roller. Direct coating requires additional equipment because there is a dangerous pinch point between the two rollers.
3. Three-roll coating process A third configuration is also possible if the coater is equipped with additional rollers. Three-roll coating is often used for coatings that require additional thickness, such as PVC coatings and coatings that are applied as stripes with bare edges. This is quite common when producing materials for garage doors and lighting fixtures. Three-roll coating uses a third roller located behind the take-up roller, and the rotation direction of all three rollers is clockwise. In almost all configurations, the wet film is controlled by three factors: the speed of the take-up roller, the speed of the coating roller, and the gap between the take-up roller and the coating roller. The gap between the coating roller and the web has little effect on the wet film thickness and is adjusted more for aesthetic reasons to accommodate different surface textures of the metal being coated. Some coil coating lines can include the application of laminating films using rolls of thin polymer films. Glue is applied to the metal surface immediately before lamination, or a paint layer (such as primer) is used to achieve the same purpose. The laminates are directly bonded and may require limited exposure to heat in an oven to fully cure the bonding layer. This is usually rolled onto the strip under pressure after a top coat oven which is used to cure the adhesive applied by a top coat roller coater.
The content of coil coating is too long, we will describe more in the next article