Investigation of the Plastisol Knife-over-Roll Coating Process

Coating processes are used in many industrial sectors to apply thin layers of liquid to various types of substrates. Dip coating is used specifically for coating individual parts. There are numerous continuous surface coating processes: slot-die coating and curtain coating allow a liquid layer to be applied, but do not apply pressure between the deposited layer and the substrate; calender coating between two rigid rollers, and so-called “reverse roll” coating between two rollers—one rigid metal roller and the other a flexible roller with a metal core and a rubber crown—allow for the pressure-applied deposition of paint, lacquer, anti-corrosion protection, and more. This article presents the doctor blade application process designed for higher-viscosity formulations, such as those used in the production of floor coatings or wallpaper.

The doctor blade coating process is described, along with the various parameters that can be adjusted to achieve the desired coating thickness and determine the geometry of the doctor blade. Plastisol formulations, commonly used for floor coatings, are presented. They consist of PVC particles of various sizes and calcium carbonate fillers suspended in a plasticizer, as well as various additives, including swelling agents. The rheology of these formulations is complex: high viscosity at low shear rates, followed by a rheofluidizing behavior at intermediate shear rates, frequently followed by a dilatancy peak (i.e., a rheo-thickening behavior) at high shear rates, which can cause the process to stall. This rheological behavior depends on the volume fraction, size, and size distribution of the PVC particles and the filler. This complex rheology, which does not correspond to any classic case of behavior for thermoplastic polymers, makes process modeling challenging. Hydrodynamic lubrication approximations are introduced to overcome this difficulty. They are entirely justified due to the aspect ratio of the die (small variation in the gap between the scraper and the cylinder in the flow direction). The non-monotonic nature of the viscosity curve as a function of shear rate precludes the use of classical solution methods derived from continuum mechanics. Taking advantage of the fact that the shear stress, on the other hand, increases monotonically with the shear rate, hydrodynamic lubrication approximations are expressed in terms of stress and pressure variables, which complicates the numerical solution but allows for the determination of the deposited thickness as well as the forces exerted on the doctor blade for different doctor blade geometries, different coating parameters, and different plastisol formulations. Process modeling first allows for the evaluation of the impact of the squeegee’s operating parameters and the plastisol’s rheology on the coated thickness....