Roll coaters and adhesive spreaders (also called glue spreaders) are very similar, but there are some unique differences. Typically, adhesive spreaders will be used to bond two materials together and the applied adhesive will not be visible in the final product. With a roll coater the coating typically will be visible in the final product and will have a certain pattern or look, for example – a gloss finish.
Glue spreaders or adhesive spreaders have been around for almost 100 years as a work horse in many panel woodworking shops. The coating roll on a glue spreader is usually made out of rubber to compensate for uneven substrates, usually wood, which allows the glue to be spread into the low areas. The coating roll could be made of steel with chrome plating when applying adhesive to soft substrates such as foam. The rubber covered coating roll(s) on a glue spreader usually have a grooved pattern that have been designed to apply a specific amount of adhesive with a 5:1 ratio of coating roll speed to doctor roll speed. The coating rolls can be smooth if the adhesive is difficult to clean out of the grooves. Most times the doctor rolls on glue spreaders are made of 100 durometer Shore-A ebonite, a very hard rubber which allows consistent application and easy clean-up.
Silicone Coated Roll
The glue spreader is designed to apply adhesive to one or both sides of many different substrates. Most of the adhesives today are water-based and can be cleaned up with water, which works well with the rubber rolls made from SBR (styrene-butadiene rubber) or neoprene. When other types of adhesives are applied, such as urethanes, the smooth rubber covering on the coating roll could be a different rubber compound, like EPDM (Ethylene Propylene Diene co-Monomer) that works well with clean-up materials such as dibasic ester.
Roll coaters usually have similar machine appearance to glue spreaders or adhesive spreaders, but are usually applying a smooth coating to a substrate which could be in sheet form or web form. The smooth surface could be used to apply a very small amount of coating or a larger amount of coating that is primarily made up of something very low in viscosity like water.
The coating rolls can be made of rubber (when coating hard substrates) or steel (plated with chrome or something similar). The type of rubber is determined by the clean-up material that will be used for the coating. When applying a small amount of coating the doctor roll will be made of the opposite type (softer vs. harder) of material the coating roll is made from so the rolls can be run into compression to apply the small amount of coating. For example, most roll coaters with rubber covered coating rolls will have chrome-plated doctor rolls.
Chrome Plated Roll
When using water-based materials the chrome covered doctor rolls will have a nickel base plated under the chrome to prevent oxidation of the steel core from appearing through the chrome. To produce a smooth finish on products many times the chain and gear drives are modified on a roll coater to prevent any chatter that will leave marks in the appearance of the final product. The 5:1 coating roll speed to doctor roll speed may be modified by gears or independent driven doctor roll(s). By slowing down the speed of the doctor roll even more, it allows a smaller amount of coating to be applied and many times a smoother coating to be applied. In the case of very low viscosity coating that can be comprised of 80-90%+ of water the ratio may be changed to closer to 1:1 to allow the roll to carry more of the lower viscosity coating.
Sometimes the coatings used in a roll coater need to be cleaned up with solvent-based cleaners and the roll coater may require explosion proof electrics to prevent any risk of fire with the solvents being close to electrical cabinets with full voltage. On roll coaters many times the parts that will come into contact with solvent based cleaners are plated to protect them from corrosion. Scraper blades may also be used on the coating, doctor and/or feed rolls on a roll coater to make sure the surface remains clean for a small precise amount of smooth coating.