BLADE | BlackClawson

Blade Coater

The most popular method used today for top coating of board is the blade coater. Due to its ability to coat high coat weights with high solids and high viscosity at high speeds, this coater tends to replace the Air Knife Coater for high quality coated board applications such as solid bleached sulphate. It can be used as a pre coater for all double coated applications.

Types of Blade Coater Application.

Flooded Nip Roll Applicator
Fixed Dwell Fountain application
Vari Dwell Fountain application

Roll Blade Coater

The Roll Blade Coater utilises a pan fed driven applicator roll to apply the coating to the sheet. The term “flooded nip” comes from the coater operation which maintains an excess of coating at the nip, keeping the gap flooded at all times. Consequently there is a back fall resulting in a substantial overflow and recirculation from the pan. The applicator roll and the coating pan assembly move as a unit. In the up position they are held firmly against adjustable stops on the mainframe so that the gap between the applicator and backing rolls is controlled and constant. The applicator roll is rubber covered.

Some features incorporated in the Roll Blade Coater are:

The volume of coating in the system is minimised by contouring in the pan to the applicator roll relationship also eliminates dead pockets.
A jacketed pan for chilled water circulation to promote sweating of the pan surfaces preventing build up of dry coating which extends pan life between cleanups.
Coating transfer is controlled by applicator roll speed and gap adjustment.

Fig 1. Flooded Nip Roll Blade Coater – Economy design for slower and narrower machines.

Fig 2. Flooded Nip Roll Blade Coater.

Fountain Blade Coater.

The Fountain Blade Coater utilises a fountain to apply an excess coating to the sheet replacing the applicator roll. The fountain is made from a body with lips mounted to the top. The gap between the lips supplies coating to the web. The fountain is mounted in fixed position in close proximity to the sheet, with the ability to rotate the fountain away from the backing roll for cleanup.

Vari Dwell Fountain Blade Coater.

The Vari Dwell Blade Coater is characterised by its ability to vary, over a wide range, the dwell time/distance of the coating on the base shett between the points of application and metering by the blade.

The applicator and metering blade assemblies of this coater are arranged in two units entirely independent of each other. This permits the applicator section (jet fountain) to be displaced along the backing periphery, and thus vary the distance between the points of application and metering. Adjustment range is between 80mm and 500mm.

The primary goal in developing the blade coater was to add this key variable application feature without compromising the important operating features.

Fig 3. Vari Dwell Fountain Blade Coater

Blade Configurations.

There are two major types of blade arrangements, the bevelled blade and the bent blade. The rules of operation for a bent blade are easy to understand and apply – the coat weight increases with speed, solids, viscosity, and lower blade pressures. The bevelled blade is the most widely used and unfortunately is much more complex.

The final coat weight and other qualities can be optimised by a series of factors in the bevelled blade coating process. The major factors include:

Blade force or pressure
Speed
Blade thickness
Blade angle
Application method
Solids
Viscosity and rheology (pigment shapes and polymer properties)
Raw stock
Coating formulation (levels and selection of materials)

Types of Blade Coater Metering.

Bevel Blade (Rigid)
Bent Blade
Rod

Fig 4. Blade Metering Variations

Most operations optimise these factors by a series of pilot or production trials. These trials are often guided by the engineer’s concept of how the bevelled blade coater works, as the subject is too complex for a thorough, theoretical examination of all factors combined.

Blade coaters are used today for a variety of reasons compared to other methods. The most important reasons are

Smoother and more level surface for post printing.
High speed of operation
Higher solids and viscosity

Since the Blade Coater controls weight by scraping off the excess coating, it deposits more coating in the paper’s depressions and less on the very top of the fibers. The differences in these local coat weights can be extensive, sometimes up to 20:1. This is an advantage for creating a level surface, but it is a limitation where uniform coverage is desired to prevent mottling. Blade coaters are rarely used alone to cover up dark or brown rawstocks with white coatings. This limitation can be overcome with a very smooth rawstock and heavy coat weights, where the product can afford the effort to achieve a smooth rawstock, the Blade Coater becomes the coater of choice because it allows high solids as in thermal and CF coated papers. Here uniform layers are required for cost and final image development reasons.

Strengths