Curtain Coating
Encyclopedia
Curtain coating is a process that creates an uninterrupted curtain of fluid that falls onto a substrate. The substrate is transported on a conveyor belt
at a regulated speed through the curtain to ensure an even coat of the die. The curtain is created by using a slit at the base of the holding tank, allowing the liquid to fall upon the substrate. Most manufactures will also include a catch pan to retrieve and reuse the excess fluid.
Curtain coating is a premetered method, which means that the amount of liquid required is supplying from the tank to the screen in order to be deposited on the substrate.
from causing the coating to taper in; sometimes edging alone is not enough and a surfactant
must be added to lower the surface tension.
is inversely proportional.
Curtain Coating is a pre-metered coating method, which means that the exact amount of coating needed to coat the substrate can be calculated before the process is actually accomplished. This is can be done by using the ratio of the flow rate (with respect to volume) and width of the substrate to the speed at which the substrate passes under the "curtain" of coating fluid.
Because of the ability to calculate the exact (or nearly so) amount of fluid needed for a given project, film thickness variations on the finished product can be kept within +/- 0.5% of the target thickness
.
Conveyor belt
A conveyor belt consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley...
at a regulated speed through the curtain to ensure an even coat of the die. The curtain is created by using a slit at the base of the holding tank, allowing the liquid to fall upon the substrate. Most manufactures will also include a catch pan to retrieve and reuse the excess fluid.
Process
Curtain Coating is a process in which the object or substrate to be coated is guided through a curtain of fluid located in a gap between two conveyors. The mechanism is formed by a tank of fluid from which a thin screen falls down in between the two conveyors. The thickness of the coating layer that falls upon the object is mainly determined by the speed of the conveyor and the amount of material leaving the tank (Pump Speed).Curtain coating is a premetered method, which means that the amount of liquid required is supplying from the tank to the screen in order to be deposited on the substrate.
Edging
To be certain the flow of the liquid is disrupted, an edging along both sides of the slit is required. This is to prohibit the surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
from causing the coating to taper in; sometimes edging alone is not enough and a surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
must be added to lower the surface tension.
Flow Rate
It is also important to note that each fluid or dye has its own minimum flow rate. The minimum flow rate is the smallest amount of dye at a given moment to keep the curtain flowing continuously. The minimum flow rate is directly proportional to the surface tension while viscosityViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
is inversely proportional.
Curtain Coating is a pre-metered coating method, which means that the exact amount of coating needed to coat the substrate can be calculated before the process is actually accomplished. This is can be done by using the ratio of the flow rate (with respect to volume) and width of the substrate to the speed at which the substrate passes under the "curtain" of coating fluid.
Because of the ability to calculate the exact (or nearly so) amount of fluid needed for a given project, film thickness variations on the finished product can be kept within +/- 0.5% of the target thickness
.
Benefits of Curtain Coating
- Faster coating velocity
- Ability to produce a thinner coat
- Easily coat abstract surfaces
- Lower cost of dyes
- Lower waste of coating
- Coats a more uniform layer
Deficits of Curtain Coating
- Minimum flow rate
- Air entrainment
- Rough surfaces are more likely to have air entrainment
- Management of airflow around the apparatus
Geometrical Possibilities
- Curtain coating is usually only effective on relatively flat substrates. Because of the possibility of air pockets being formed beneath the coating, it is not advisable to use curtain coating on substrates with extremely rough or angular surfaces with grooves and pits etc.
Production Rates
- Production rates are only limited by the maximum speed that the fluid can be laid down on the substrate without breaking the curtain or getting air bubbles between the substrate and the coating material. This is dependent mainly upon the viscosity of the coating fluid.