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Curtain coating, also known as conformal or contour coating, is an industrial process where a smooth curtain of fluid falls like a waterfall on the surface to be coated. The object to be coated is transported on a conveyor belt and moves at a predetermined, controlled speed. It passes through the falling curtain of fluid where it gains an even coating, whose thickness is determined upon the amount and viscosity of the fluid that leaves the tank and the speed of the conveyor belt. Any excess fluid is collected in a trough and is pumped back into the pouring tank. The liquid falls in a thin screen onto the object through a narrow adjustable slit in the base of the pouring tank, and the upper surface of the object that passes through the screen is coated evenly with the fluid.
It is one of the best methods to cover large areas of an object with a coating of fluid or adhesive and has been extensively used in the photographic industry for decades. Curtain coating is beginning to be used in the paper industry for special applications and applications that involve coating cardboard packing materials or printing papers. In the furniture industry, curtain coating is widely used to coat plain surfaces, such as cupboard and door panels; it is also utilized to coat sheet metal. This coating method is considered to be a premetered coating method, which means that only the necessary amount of fluid needed to coat the object's surface is sent from the tank because it is accurately precalculated.
This method offers a great many advantages over other coating methods, such as air knives, blades, and spin coating. There's very little excess fluid, and the surface to be coated gets a very even coating of uniform thickness. Not only can this process coat even slightly uneven surfaces, such as cardboard and paper, but also, the coating is almost totally free from streaks and sharp lines that characterize other methods due to the absence of blades and rolls. As there is little fluid pressure at the point of contact, the coating is applied in a much more gentle manner that is ideal for delicate surfaces like paper. The high speed of application, lower material cost, reduced wastage, and the ability to produce a controlled thin coat make curtain coating a much favored process in industrial coating applications.
Some of the disadvantages of curtain coating include the inability to use it on grooved or pitted surfaces; air pockets could be formed under the coat of fluid. It's also not possible to use every type of fluid in this method because the liquid has to be of a certain viscosity that allows it to form a thin, smooth waterfall. Additionally, if the liquid flow rate is low, the curtain may split into different streams — the stability of the falling fluid curtain is a key issue. In case the object is moving too fast, the fluid doesn't make contact with the object evenly, leading to defects such as pinholes and bubbles. If the object moves too fast, then the curtain bulges out from the point of contact, leading to other defects.