Ceramic coatings protect against the corrosive acid gases generated in PVC processing. They also have excellent compressive strength. But a thin ceramic coating can be brittle and have poor impact resistance. "When two hard-coated rolls are used in a nip position on sheet, there's no forgiveness for uneven pressures across the rolls, and denting or 'worming' of the roller surface can occur," explains Jon Glidden, market development manager of specialty products at American Roller. Dents or "worming" results in imperfections in the sheet surface. Consequently, polished chrome has been the standard for sheet rolls: "Until recently, it was the only material available that provided the smooth, hard surface needed in this process," Glidden says. But even chrome plate can be subject to denting.
'It Didn't Dent'
Since late 1997, two sheet lines using American Roller's thicker, dent-resistant ceramic coatings have been running at Trio Products, a unit of Ivex Packaging Corp. in Elyria, Ohio. Trio makes PVC sheet for thermoformers of blisters, trays, and point-of-purchase displays. Trio had two sheet lines with two thin ceramic-coated rolls and a third polishing roll of nickel-plated chrome.
Soon after the lines arrived, Trio experienced brittleness and denting problems with the thin ceramic coatings. One ceramic roll was damaged accidentally when an operator bumped it with a tool. A week later, another ceramic roll needed to be replaced. This time, worming occurred on the roll surface as a result of high nip pressure in a concentrated area of a roll, which dented the steel core through the ceramic coating.
Trio's PVC sheet process is especially challenging to roll coatings because the PVC comes from industrial regrind. The regrind is more difficult to extrude than virgin PVC because PVC gains molecular weight when it's reprocessed, says Trio operations manager Scott Cargill.
By coincidence, a roller on one of the lines had been recoated with extra-thick ceramic by the original owner to enlarge it slightly. After a year and a half, it had suffered no problems. "We later came to realize how well that roller had worked. It didn't dent," recalls Glidden.
Trio then ordered a pair of additional chill rolls with the new multi-layer ceramic coating called Arcotherm TH200. This coating is 0.060 in. thick, whereas American Roller's previous single-layer ceramics are typically 0.008-0.015 in. The surface layer is chrome oxide. A bottom layer of corrosion-resistant metal alloy bonds the ceramic to the steel roller and gives added protection to the steel core. The three-layer ceramic has a "near-mirror" finish with a "roughness average" (Ra) of 4-6 micro-inches--only slightly rougher than single-layer ceramics with an Ra of 4 micro in. (A true "mirror"-finish polished chrome typically has an Ra of less than 1 micro-in.)
Another Tough Ceramic Roll Coating
Another extra-thick roll coating was developed in the late '80s by F.R. Gross Co. in Stow, Ohio, using ceramic coatings from Praxair Surface Technologies in Indianapolis. The "SC2" coating from F.R. Gross consists of three layers: a ceramic base coat, an aluminum oxide layer, and then a chrome oxide coating. Total thickness is up to 0.10 in. The "SC2" also targets calendering and high nip load applications, including PVC sheet.