Curtain coaters are used in coating curtains (or substrates) with a film as they pass on a conveyor belt below a holding tank. The tank releases fluid through an opening at its base onto the substrate. The coating’s thickness is determined by the speed of the conveyor belt and the rate at which fluid is released from the holding tank. The majority of these machines come equipped with catch pans that capture any excess fluid for reuse.

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The procedure uses air-free coatings of lower viscosity. The method offers several benefits compared to other coating apparatus such as roll or spray coating. These include:

Faster coating velocity is the ability to coat substrates/curtains at a continuous uniform speed allows these machines to work in a high speed/production environment.

Application of thinner coats is determined by restricting the rate at which coating fluid flows out from its holding tank. This allows for thinner coats to be applied to a substrate.

Uniform coat layers is accomplished by moving the substrate at a constant speed so the apparatus can apply a uniform layer of coating on the substrate. The procedure does not generate streaks or lines when coating is applied.

Reduced dye cost is accomplished by precise dye application via controlled pre-metering of the coating liquid employed allows for the efficient dye usage.

Less coating waste occurs by recycling excess fluid caught from side pans reduces the amount of wastage generated during the process.

Types of Curtain Coaters

Four primary types of curtain coaters are available:

Slot curtain coaters have a slot located at the base of the holding tank, which is pressurized by gravity, or pump pressurized slot die on top or external to the holding tank. As the substrate passes below the slot on a conveyor belt, the coating fluid is dropped from above at a measured rate. Any excess coating is circulated back into the holding tank. To achieve high-quality coating, the slot features a precision design capable of supporting exacting standards of execution.

Slide curtain/cascade coaters operate a series of slot dies, allowing for the application of various coating layers simultaneously where each layer of coating slides or cascades over another layer of coating. A primer, basecoat and topcoat could be applied simiultaneously to a substrate using a slide curtain or cascade coater. The use of multiple layers of coatings at once helps in the production of items such as solar cell stacks. These instruments make it possible to complete such complex operations while saving time and energy in comparison to other coating activities.

Trickle coaters have a process involving pouring a narrow stream of coating on parts such as porous castings or coils. Either during or following the coating procedure, vacuum is applied in order to meld the liquid. Sometimes resins or varnishes are applied to motor windings or similar parts to increase insulation. The approach is used to add resin to metal parts to enhance their resistance against corrosion.

Enrobers are used to coat candies, cakes, cookies and other food items with chocolate or other substances. The items are either entirely encapsulated by the substance or coated on one side. An enrober operates by first dipping the bottom part of a confection in a bath of liquid (chocolate is the primary coating used with such equipment). The item then passes through a curtain of liquid to complete the task.

Operation

Curtain coating involves pre-metered coating of an object or substrate via liquid falling through an aperture. The technology is most effective when dealing with flat substrates, as they are subject to air entrainment. It is seldom used on substrates with very rough or angled surfaces.

The process requires a minimum flow rate for the coating liquid and disqualifies any situation calling for extremely slow rates of outflow. Edges are placed on each side of the opening of the holding tank to moderate the flow of the coating fluid. This prevents surface tension from tapering the coating in. If edging alone does not suffice to prevent this, a surfactant is necessary to lower surface tension.

Each specific fluid or dye utilized for coating has its own minimum flow rate. This rate is the least amount of fluid needed to maintain the continuous outflow. These flow rates are proportional to the surface tension of the liquid; while the viscosity is inversely proportional to the metric.

As the technology operates via pre-metering, the precise coating amount needed for any substrate is calculated before beginning the process. This takes into account the ratio of the flow rate volume and substrate width to the speed at which it moves on the conveyor belt. The ability to determine the required quantity of coating fluid with accuracy for each project makes it possible to keep film thickness variation close to the target thickness.

Applications

Curtain coating is featured in a broad scope of applications, including:

• Paper, print and label media coating
• Packaging materials
• Solar cell manufacturing
• Battery manufacturing
• Pharmaceutical/drug delivery
• Display and optical film production
• Plywood/laminate manufacturing
• Medical devices
• Metal sheet and foil coating
• Fuel cell manufacturing
• Food/confection coating

Selection

Curtain coaters come in an assortment of types and specialties. Check manufacturer’s specifications to ensure that the equipment supports planned applications. When comparing a device to another type of coating apparatus, consider both the advantages and disadvantages of the technique. 

Standards

TAPPI TIP 0304-26 - Common Curtain problems: causes, detection, and correction

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