Curtain coating is an industrial process where a smooth curtain of fluid falls onto a surface to be coated. It offers advantages over other coating methods, but has limitations such as the inability to use it on grooved or pitted surfaces.
Curtain coating, also known as conformal or contour coating, is an industrial process in which a smooth curtain of fluid falls as a waterfall onto the surface to be coated. The object to be coated is transported on a conveyor belt and moves at a predetermined and controlled speed. It passes through the falling fluid curtain where it obtains a uniform coating, the thickness of which is determined by the amount and viscosity of the fluid leaving the tank and the speed of the conveyor belt. Any excess liquid is collected in a trough and pumped back into the casting tank. The liquid falls in a thin screen onto the object through a narrow adjustable slot in the base of the sprue tank and the top surface of the object passing through the screen is evenly coated with the fluid.
It is one of the best methods for covering large areas of an object with a fluid or adhesive coating and has been used extensively in the photographic industry for decades. Foil coating is starting to be used in the paper industry for specialty applications and applications involving the coating of cardboard packaging materials or printing papers. In the furniture industry, curtain lining is widely used to cover smooth surfaces, such as cabinet panels and doors; it is also used to coat sheet metal. This coating method is considered a pre-measured coating method, which means that only the amount of fluid required to coat the surface of the object is sent from the tank because it is accurately pre-calculated.
This method offers many advantages over other coating methods, such as airknives, blades and rotary coating. There is very little excess fluid and the surface to be coated gets a very uniform coating of uniform thickness. Not only can this process coat even slightly uneven surfaces, such as cardboard and paper, but the coating is also almost totally free of the streaks and sharp lines that characterize other methods due to the absence of blades and rollers. As there is little fluid pressure at the point of contact, the coating is applied much more gently which is ideal for delicate surfaces such as paper. The high application rate, lower material cost, reduced scrap, and ability to produce a controlled thin layer make powder coating a popular process in industrial coating applications.
Some of the disadvantages of powder coating include the inability to use it on grooved or pitted surfaces; air pockets could form under the fluid layer. Also, not every type of fluid can be used in this method because the liquid must have a certain viscosity that allows it to form a thin, smooth waterfall. Furthermore, if the liquid flow rate is low, the curtain may split into several streams: the stability of the falling fluid curtain is a key issue. In case the object moves too fast, the fluid does not contact the object evenly, resulting in defects such as pinholes and bubbles. If the object moves too fast, the curtain inflates from the point of contact, causing other defects.
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