A Review of Gravure Coating Systems

Gravure roll coating is a technique used to coat fluids of a wide range of viscosities (up to 1500mPa s)onto substrates at speeds of up to 900 m/min (Booth,1970,1990). Coat thicknesses in the range of less than 1 micron up to 50 microns can be achieved, making this a versatile process which is finding application across a growing number of market sectors. Gravure roll coating differs from many of the conventional roll coating techniques in that one of the rolls is patterned with a surface engraving (the ‘gravure’ or ‘anilox’ pattern). Both the shape and the size of the gravure pattern can be varied which affects the final properties of the coating. The term ‘gravure roll coating’ covers a number of distinct gravure coating arrangements. Two common variants of these are direct gravure coating and offset gravure coating, see Figure 1. Direct gravure coating is where the fluid transfer takes place directly from the gravure roll to the web, whilst in offset gravure coating, fluid is transferred first from the gravure roll to a smooth deformable roll (often termed the applicator roll), and then from the deformable roll to the web. Whilst both are described as gravure coating processes, the fluid mechanics of these two processes are very different and will be described separately. Gravure ‘coating’ is distinct from gravure ‘printing’ (or roto-gravure), in that gravure coating is designed to give uniform coverage on the substrate whilst gravure printing is designed to print specifi c patterns. The quality of gravure coating can be defi ned in terms of the thickness and variation of the coating, whilst the quality of gravure printing will include quantifi cation of print characteristics such as resolution and edge definition. This article will focus on gravure coating, but many of the emerging markets, for example in the manufacture of electronic products such as solar cells, require both large areas of uniform coating but with good edge definition. This article will first describe the range of gravure cells available and typical manufacturing techniques, together with the important parameters that specify the gravure roller, before describing in more detail the two distinct gravure roll coating processes.