Roll Coating, Different Techniques

With open systems, water or volatile solvents can evaporate on the rolls. This causes thickening of the adhesive and consequently may cause a variation in the amount of adhesive applied. 

This can be aggravated if heat is generated by the shear forces resulting from mixing. When the rotation speed increases, splashing and air entrapment may also occur. 

Let us start with the simple and small systems. Fig. 43 shows the more simple roll coaler where a roll turns inside a glue trough, the parts pass on top of the roll and pick up the glue. 

Starting from this basic system, we may add several improvements step by step  

In the basic system of Fig. 45, the glue will rise on the roll if it wets and adheres to the roll. In order to have this, the glue should not be too fluid because it would then immediately run down and there would not be enough glue on the roll. It should also not be too thick because the pick up roll would pick too much glue and the machine would get dirty very quickly. Therefore, the adapted viscosities should be between 1000 and 30,000 mPas. When the adhesive is fluid, the coating system may be reduced to one roll only, and the excess will run down. Coating speed a few metres per minute. 

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Whether or not a core is suitable for coating depends on its hardness, shape, surface, size, heat sensitivity, and tendency to interact with the coating material. Moreover, since sugar and film coating processes involve very different techniques, they place different demands on the cores to be used [53]. Tablets used as cores must be biconvex in shape to prevent them from sticking together like coins in a roll. 

There are different coating techniques. The woven and nonwoven fabrics normally involves three steps as it passes from the unwind roll to the rewind roll. Powder is metered onto the fabric, heated in an oven (gas or electric) that are usually divided into several heating zones, and cooled by a chill roll. 

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