Direct forward gravure coating on unsupported web

Abstract

This experimental study of forward gravure coating considers the effects of operating variables on air entrainment, ribbing instabilities and the thickness of the film formed. The data show that this coating method can yield very thin films of thickness of order of 15 - 20% at most of the equivalent cell depth of a gravure roller. Air free and non ribbed stable uniform films can however only be obtained in a narrow window of operating conditions at very low substrate capillary number (CaS ~ 0.02) equivalent to substrate speeds typically less than 20m/min. The paper draws a similarity with flow features observed with smooth forward roll coating and slide coating. It is shown that the onset of ribbing and the flux distribution between the gravure roller and the substrate at the exit of the nip obey approximately the same rules as in smooth forward roll coating, whereas the onset of air entrainment actually corresponds to a low-flow limit of coatability similar to that observed in slide coating.