Knife and Doctor Blade Coating

38 and 39 show these equipments. The adhesive forms a thick fillet behind the adjustable knife. There are three systems (refer to Figs 38 1) 

Usually, adjustable springs press the knife or doctor blade against the web. There are several possible shapes for the knife (Fig. 41) in order to adjust to the viscosity and rheology and to Ae amount of adhesive required. The adhesive may 

The web speed may be quite fast if the viscosity is low enough it may reach 50 m/min with a viscosity of 3000 mPas, provided the web is strong enough to prevent tearing. 

The coating add-on is limited by the tension of the web against the knife. The lower the viscosity, the lower the thickness of adhesive. Add-on may vary from 20 to 200 g/m. If it is less than 20 g/m, the accuracy is poor, and there may be a difference of coating thickness from one side of the web to the other, because these simple systems do not allow a perfect parallel alignment of the knife and the rolls. 

They are convenient when the user wants to apply a thick coating, for instance on a textile web, to impregnate the web. The thick fillet of adhesive will provide a good impregnation, and the coating thickness may reach 200 g/m or even more. They tend to be replaced by the more sophisticated and more accurate roll coaters. 

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