Reverse Roll Coaters

A reverse roll coater is a multi-roll coater that uses a series of rolls to pre-meter and apply a coating to the web. A typical reverse roll coater features three rolls, all rotating in the same direction. In a nip fed reverse roll coater the coating is puddled in the gap formed between the metering and applicator roll, both of which are of precision steel construction. A layer of coating forms on the surface of the applicator roll as it rotates away from the coating puddle. The thickness of this layer is determined by the gap between the metering and applicator rolls, as the coating passes between them. As the coating continues to rotate about the applicator roll it is wiped onto the web, which is traveling in the opposite direction on the rubber covered backing roll. A variation of this design is a pan fed reverse roll coater. In this arrangement, the coating resides in a pan in which the applicator roll is partially submersed. 

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As the applicator roll rotates, it picks the coating up out of the pan, passes it through the gap formed with the metering roll and then delivers it to the web at the backing roll. This arrangement is particularly useful with coatings near the lower end of the viscosity range where holding them in the nip between the rolls would be difficult. The amount of coating that is delivered to the web is controlled by the gap between the metering and applicator roll as well as the relative speed of the applicator roll and web speed. The greater the gap, the thicker the layer of coating. The greater the speed of the applicator roll, the more coating that is delivered to the web. The reverse roll coater has wide coating thickness and viscosity ranges and is good for changing coating laydown rates.

Reverse roll coating is a roll-to-roll coating method for wet coatings. It is distinguished from other roll coating methods by having two reverse-running nips. The metering roll and the applicator roll contra-rotate, with an accurate gap between them. The surface of the applicator roll is loaded with an excess of coating prior to the metering nip, so its surface emerges from the metering nip with a precise thickness of coating equal to the gap. At the application nip, the applicator roll transfers all of this coating to the substrate, by running in the opposite direction to the movement of the substrate, wiping the coating onto the substrate.

Reverse roll coating machines demand high specifications in their construction, e.g. for the machining and bearings of the rollers and for highly uniform speed control. This makes them relatively expensive compared to other coating technologies. Unlike many other coating methods, they can however handle coatings with a very wide range of viscosities, from 1 to more than mPas, and are capable of producing extremely polished finishes on the coatings they apply. They have been produced in a variety of 3-roll and 4-roll configurations.[1]

Products that have been manufactured on reverse roll coating machines include magnetic tapes; coated papers; and pressure sensitive tapes.[2] The rise of slot-die coating has tended to eclipse reverse roll coaters as in most if not all cases, the same products can be made on cheaper machinery.

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References

1. ^

   Booth, George L. (). Coating Equipment and Processes. Lockwood Publishing. pp. 132&#;148.

2. ^Dennis J. Coyle; Christopher W. Macosko; L. E. Scriven (). "A simple model of reverse roll coating". Ind. Eng. Chem. Res. 29 (7): &#;. doi:10./iea046.