Roll coaters are often used to industrially apply liquids to the surface of a part. Roll coaters can be used to apply liquid adhesives, paints, oils and coatings such as varnishes or clear topcoats. Roll coaters can take many forms, from simple paint rollers to complex coaters with multiple rollers. A roll coater works by transferring a layer of coating from the surface of the roller to the surface of the part. When this happens, a phenomenon known as "film splitting" occurs. The coating on the surface of the roll splits - part of it remains on the roll and part of it sticks to the surface of the part. The percentage of coating that adheres to the part (substrate) depends on the surface characteristics of the roller and substrate.
With most roll coaters, there are ways to control the thickness of the coating on the surface of the roll before contacting the substrate. The three most common methods of controlling coating thickness are metering blades, metering rolls, and transfer from another roll.
Metering Blade: The applicator roller picks up the coating from the container. Since the paint is close to the roller and is carried by the rotation of the roller, only a certain amount of paint can pass through the gap between the metering blade and the roller surface. The excess flows back to the tank. Metering blades are usually made with some means of adjustment, so coating thickness is changed by moving the blade to open or close the gap.
Metering rolls: Roll coaters In metering roll systems, the thickness of the coating is controlled by adjusting the gap between the two rolls. In a typical basecoat application, the coating is taken from a storage tank and conveyed by one roller to a nip point, then the excess is metered by a second roller, which also serves as an application roller. A similar concept is used when a coating must be applied to the top surface of a part. The coating is held in the valley formed between the metering roll and the applicator roll, and the gap between the rolls is adjusted to vary the coating thickness.
Reverse Roll Coating: In reverse roll coating, the surface of the part is passed through the coating roll in the opposite direction to the direction in which the surface of the coating roll travels. This allows the applicator roll to function as both an applicator roll and a wiper roll. Roll coaters create a small coating puddle at the interface of the roll and substrate, which flows onto the part as it passes through the coating roll. Reverse roll coating is used to apply heavier coats than forward roll coating. Very smooth coatings can also be applied by using the reverse roll technique. Since the coating is heavier, it tends to flow out better, and the flow from the puddle at the roller to the part interface tends to smooth the coating. The result of these two effects working together can provide a very flat and smooth coating.