The goal of the coating process is to maintain a constant drying rate, i.e. the amount of solvent removed per hour by the dryer during all coatings. A uniform reproducible drying rate is required to insure the maximum line speed, maximum productivity, low cost and a reproducible product. However, variations in the drying rate do occur and can cause a variety of drying induced defects. The drying rate variability can be short term, during a coating campaign or long term between coating campaigns that are separated by time.
This variation will lead to several types of defects. If the drying rate decreases then the coating speed must be reduced so that coating dries with in the dryer. A low drying rate can lead to a wet product exiting the dryer; causing the wound roll to be fused together, making unwinding for further processing and packaging impossible This results in a loss of productivity, higher costs an, the inability to produce sufficient product volume. It can also affect coating quality if the process is on the borderline of the coatability g window. A low drying rate can lead to a wet product exiting the dryer; causing the wound roll to be fused together, making unwinding for further processing and packaging impossible.
If the drying rate increases then the coating can be over dried which can adversely affect product performance properties and the high temperatures can physical defects in the product. Also, this will cause productivity losses by not utilizing the entire dryer.
There are four steps that can be undertaken to insure uniform reproducible drying rates. Insure that there is constant in heat input from the drying air impinging on the wet coating solution. To do this precisely control dryer variables which effect drying rate, temperature of the impinging air, velocity of the air impinging on the film, solvent concentration of the impinging air, and accurate settings of temperatures and air velocities. These variables should be frequently calibrated.
Control coating solution properties and coating weight uniformity to insure constant solvent level being introduced into the dryer. Variables effect the amount of solvent to be removed the wet coating weight, percent solids content of the wet coating, transverse direction coating weight profile, and defects with heavy edges, such as streaks or spots
Utilize a dryer control strategy in which the dry point and film temperatures are controlled by varying impingement air dry bulb temperature, impingement air velocity and air solvent content in the varying zones This will correct for any variations that cannot be corrected by the above steps. This approach is more effective then the typical strategy of controlling air conditions within a tight range.
The inherent precision of the measurements system and equipment capabilities should be reviewed to insure that they are adequate for the reproducibility needed. If they are not adequate, upgrade the needed systems; this is usually very cost effective.