Web Coating

Selecting an appropriate coating method is a challenge because there are currently many applicators in routine use and each of these has unique capabilities in terms of the coating solution variables that they can apply and the resulting coated substrate properties. Interestingly all of the methods that have been developed since the first roll coater in 1850 are still in routine use The attached table summarizes the ~ 40 major coating method categories by the principle sued to control the wet coating weight, self metered coating, doctored coating and pre-metered coating.  Each of these methods has several possible configurations so that there is large number of method to choose from.

The typical approach to selecting the coating method for a new product or an upgraded old product is to establish the product requirement, coverage level and uniformity, number of layers, physical quality, volume, substrate, drying conditions, and solutions properties. The requirements are the compared with method capabilities and best method selected.

However, there are some additional criteria that are often ignored and should be considered when selecting a method. The coatability window is an operating diagram which indicated the stable process regions in which the product can be coated with no defects. It combines the effects of line speed, coating weight, applicator variables to determine this region.  When determined this will indicate if there is a viable operating region. The basic values in available tables do not show this.

The cost and technical effort to install and develop the coating applicator should be considered. The older established method, gravure reverse roll, Mayer Rod are inexpensive and can easily be installed with the availability of cartridge. There is also a large technical base for these methods and they can be relatively easy to operate. Slot die coater is expensive and is technically sophisticated. Considerably more technical effort is needed to  run this process. Often the simpler method is the best choice.

High line appears attractive but they may not be suitable for all methods. With low volume products the start-up time and initial losses until quality is established can adversely affect costs. This loss is minimal for a high volume long coating campaigns. However, it can be significant for a small volume product. Therefore,  for low volume product a lower line speed which is easy to start and maintain may be preferred.