There are several potential contamination sources in the dryer that can result in a variety of spot scratch drying defects being introduced into the wet coating.
Condensation Spots
Condensation spots are a spot defect with a clear center with high edges, figure 1. They are caused by solvent condensing on the interior surface of the top of dryer and then dripping on to the wet coating from the top of the dryer and distorting the wet coating. They occur in areas in the dryer where the exhausting air solvent laden from is stagnant and becomes saturated with solvent in the top of the dryer. If the dryer enclosure is not well insulated, then the interior temperature of the top of the dryer can fall below the dew point of the solvent and it will condense and cause drops to fall on the wet coating.
To eliminate condensation posts, insure all of the impinging air from the nozzles is drawn into the return air system and there are no stagnant areas in the top of the dryer. Insulate the dryer enclosure so there are no cool spots inside drying chamber.
Particulate Spots.
Any particles of dirt, hair, grease, oil, bacteria, fibers, filter medium and particles from solvent recovery units etc that get into the impinging air from the nozzles can cause a discrete spot defect, in the drying coating. Therefore, the drying air must be contamination free. This requires that the drying air be purified by filters to remove contaminates. The filter pressure will increase as particles build up in the filter and cease to be effective. When filter pressure increase to a set value, it must be changed. All of the ducts and dampers should be sealed so that no outside air is drawn into impinging air. Also if there ay access ports to ducts they should be checked to insure they are closed. They often can be accidentally left open, particularly after maintenance shutdowns. Seals on the drying enclosure should not have any leaks where outside air can be drawn into dryer. Occasional leaning of dryer interior and ducts will help reduce contamination.
Dryer Scratches
An unstable substrate in the dryer can result in scratches or rubs on the drying coating, by contacting the drying nozzles. This contact can cause scratches by two methods. If the coating is wet when the contact is made; some of the wet coating will transfer to the nozzle and dry. The dried coating will then build-up on the nozzle and form a “stalactite”. The stalactite can lengthen so that it can contact web even if it is stable. The close clearances in floater dryers are particularly susceptible to this defect. Also, if there any sharp areas in the nozzle exit it can result in a scratch even if the coating is dried.