Intermediate coating technology
After the first coating machines were built, it was obvious that quality and speed would have to be improved. One big problem was that rolling film or paper up onto rolls required loose small rolls, otherwise pressure and cinching of the roll would cause fog to form. Festoon drying caused defects due to the holding bars.
Therefore, coating machines were devised that allowed higher speeds and the application of a second, protective overcoat and at the same time allowed for better drying. All of this led to higher quality.
The invention that allowed this was the extrusion hopper. This is a heated chamber into which emulsion is pumped and it is extruded onto the moving support in a thin, easily maintained manner. The exit gap and distance from the moving support can be adjusted easily for each product and more than one of these can be put onto the machine for easy overcoating of a product with a protective layer.
In addition, the entire device can be enclosed in a cabinet or multiple cabinets with a chill section and a hot drying section. This allows the web to move smoothly with no festoons to introduce defects.
Kodak adopted a method whereby the coating may be 41" wide on a 42" sheet of moving film or paper. This allows for a 'selvedge' edge (I have seen various spellings of selvedge and various usage). This edge is about 1/2" on each side and also contains the edge of the coating bead. The science of coating includes the fact that a ridge always forms at the outer edge of any coating.
The 1/2" edge at each side allows for the use of undercut rollers that permit automatic turns of the web so it can go up, down, or even reverse direction during drying so that the machine can take less space. The closed cabinets prevent dust and dirt from sticking to the product.
At the end, you slit off the 1" outer edges and you have 40" of good paper or film that can be further slit and chopped to the correct format.
For color products, this required one coating station for each layer, or it required one pass through the machine for every 2 layers in the product.
The coating speed of these machines could be from about 50 - 200 feet / minute. Kodak has machines of this sort in several widths. In research we had nearly a dozen of these each with different capabilities and widths.