The function of the dryer in a web coating line is to uniformly remove the solvent from the wet coating and at the same to do no harm to the coating so that a uniform defect free dry coating exits the dryer. However, there are a wide variety of defects that can be generated in the dryer and, obviously, they need to be eliminated to give good quality, minimize defective product and to have low costs.
These defects can be generated by a variety of drying process mechanisms, variation in the wet coating and substrate properties. The basic drying properties defect mechanisms are:
• Drying rate variability,
• Impingement air velocity,
• Surface tension effects,
• Web transport substrate interactions, detrimental
• Drying air temperature effects
• Drying stresses
• Contamination in impinging air and drying enclosure.
Each of these causes can result in several different physical defects Table 1 summarizes the various defects that can be generated by the above-cited mechanisms.
An understanding of the causes of the will help in eliminating the defects by indicating process areas to consider in eliminating the defect. Another consideration in reducing dryer defects is that the dryer cannot improve any defect that is produced by the coating process. As a result, a defect free coating is needed for the best possible product exiting the dryer
The next blogs will discuss defects category in each category and how to eliminate them.
Table 1
Drying Defect Mechanisms and Types
Drying Rate Variability
Reduced Line Speed Coating Exiting Wet
Over Drying Drying Point Uniformity
Impingement Air Velocity
Mottle Drier Bands Air-knife Defects
Surface Tension
Craters Bénard–Marangoni Cells
Orange Peel Picture framing
Web Transport System
Air Bar Rubs Dryer Scratches
Detrimental Temperature Effects
Blisters Remelt
Stress Induced
Curl Mud cracking
Reticulation Starry Night
Curl & Delamination Cockle
Haze
Contamination in dryer
Condensation Spots Spots
Miscellaneous
Blooming Static Marks