Drying air impinging on the wet coating entering the dryer, can disturb the coating and result in the liquid coating flowing and causing a variety of defects. The specific defects that can be formed depend on the viscosity of the coating, the impinging air velocity, the distance from the nozzle to the coating and the airflow distribution in the dryer. The major defects that are encountered are, mottle, dryer bands and air knife defects.
Mottle
Dryer mottle is an irregular pattern on the surface of a coating that can vary in severity from small to several centimeters. It is caused by non-uniform airflow, creating a slight flow in coating surface and changing the uniformity in the surface appearance of the coating. This surface difference is what appears as mottle. It can be formed in the space between coating applicator and in the initial drying zone. The actions that should be taken to eliminate mottle Are:
• Make the coating more resistant to motion by concentrating the coating so that viscosity increases and it is more resistant to flow
• Use thickener additives to increase viscosity
• Reduce air velocity in the initial a drying zone
• Improve air uniformity in first zone
• Enclose the transition area from coater to dryer so no room air impinges on the coating.
• Insert Infrared heater before first impingement zone to increases viscosity with no airflow on coated web.
Dryer Bands
Dryer bands are subtle machine direction non-uniformities that are similar to streaks but are wider and jagged in appearance. The surface of the defect area has a different appearance then the normal coating. They are often uniformly spaced across the web and this spacing correlates with a physical characteristic of the dryer such as a non-uniform transverse direction velocity profile in slot type dryers.
They are caused by non‑uniformities in the air impinging on the web and the reinforcement of that non‑uniformity over a wide range of nozzles. There can be flow in the coating to cause the bands. Also,
solvent concentration differences between adjacent areas there cause migration of coating ingredients that will affect coating performance and appear as bands in the final product. The cumulative effect of having a velocity variation in the same place for all nozzles in the dryer amplifies the surface disturbance and drying rate variation and increases their severity,
The actions to minimize dryer bands are:
• Insure uniform slot exiting air velocity with no discontinuities.
• Increase viscosity as discussed in mottle.
• Modify dryer system to insure uniform air return.
• If nozzles have a spike in transverse direction profiles. Adjust nozzles so that spikes are not in same location throughout dryer.
Air-knife Defects
Air-knife defects are a significant reduction in the coating weight caused by the high velocity air exiting the impingement nozzles and moving the coating. The initial slots dryer slots can function as doctor blade and remove or disturb the coating. The defect appears as gross cross web non-uniformity very similar to coating chatter. The difference between air-knife and mottle is that the air knife defect is gross and can easily be detected. This will occur in the initial drying zone where the viscosity of the applied coating is low and very sensitive to disturbances.
The same actions as used for mottle are effective for this defect.