When a controlled amount of coating needs to be applied to a flat surface, several application methods can be considered. One of these is curtain coating, a technique used in industrial finishing processes where consistent application, production speed and material efficiency are important.
A curtain coater creates a continuous curtain of liquid coating through which the workpiece passes. The coating is applied without direct contact between the application head and the product, while excess material can be collected and returned to the system.
But how does curtain coating work, which factors influence the coating result, and when is it a suitable choice for a production line?
What is curtain coating?
Curtain coating is an industrial application method in which a continuous, free-falling curtain of liquid coating is deposited onto a moving workpiece.
The technique is particularly suitable for flat or nearly flat surfaces, including wooden panels, parquet boards and furniture components.
Unlike roller coating, curtain coating does not require an application roller to touch the product surface. Unlike spray coating, the coating material is not atomised into small droplets.
Instead, the workpiece moves through a controlled flow of liquid coating.
Depending on the coating material and process configuration, curtain coating can be used to apply different coating weights. It is particularly interesting for continuous production processes where consistent application and efficient material use are important.
How does a curtain coating machine work?
A curtain coating machine uses a coating head positioned above a conveyor system. The coating material is pumped from a reservoir to the application head, where it forms a continuous curtain.
The process generally consists of five steps:
- Coating supply: The coating material is pumped from the reservoir to the coating head.
- Curtain formation: The coating flows through the application head and forms a continuous curtain of liquid.
- Product transport: A conveyor transports the workpiece through the falling curtain.
- Coating application: The liquid coating is deposited onto the surface of the moving product.
- Material recirculation: Excess coating is collected and returned to the system, with filtration where required.
Because the coating is applied as a continuous liquid curtain rather than a spray, conventional overspray caused by atomisation is avoided.
However, maintaining a stable curtain and suitable coating conditions is essential for consistent results.
The Trivec Curtain Coater is designed for industrial finishing processes and offers different configurations depending on the coating material, workpiece and production requirements.
Depending on the machine configuration, one-sided or two-sided coating is possible.
More information about the machine’s capabilities and technical specifications can be found on the product page.
What determines the coating thickness?
The final coating result depends on several process parameters working together. These include coating flow rate, conveyor speed, coating properties and the condition of the substrate.
Coating flow rate and conveyor speed
The relationship between coating flow and conveyor speed is important in determining the amount of coating applied.
At a constant effective coating flow, increasing the conveyor speed generally reduces the amount of coating deposited per unit of surface area. Reducing the speed generally has the opposite effect.
However, these parameters cannot be adjusted independently without considering the stability of the coating process.
The liquid curtain requires sufficient flow to remain continuous. At the same time, the conveyor speed must remain within a suitable operating range to prevent coating defects.
The correct settings depend on the coating formulation, application system and required finish.
Coating viscosity and flow behaviour
Viscosity also plays an important role in curtain coating.
The coating must flow consistently through the application head and form a stable liquid curtain.
If the coating properties are unsuitable, the curtain may become unstable or break up, resulting in an inconsistent application.
Besides viscosity, other properties such as surface tension, temperature and the behaviour of the coating during circulation can influence the process.
Air bubbles in the coating system can also affect application quality.
For this reason, coating compatibility should be assessed before defining the operating parameters.
Understanding coating weight and layer thickness
In industrial wood finishing, coating weight describes the amount of coating applied to a given surface area. It is commonly expressed in grams per square metre (g/m²).
The required coating weight depends on the coating material, substrate, desired finish and application process.
Coating weight and coating thickness are related, but they are not identical. The relationship depends on the density and composition of the coating.
It is also important to distinguish between the wet coating layer and the final dry or cured layer. For coatings containing water or other volatile components, the final layer thickness is influenced by the amount of material remaining after drying.
For reliable production, both application weight and finished coating quality should be evaluated through practical measurements and coating trials.
Which products are suitable for curtain coating?
Curtain coating is particularly suitable for flat products that can move continuously through the coating curtain.
Common applications in industrial finishing include:
- Parquet and flooring boards: Applying controlled coating layers to flat wooden flooring products.
- Furniture panels: Finishing flat surfaces where a consistent coating is required.
- Interior panels: Applying lacquers or protective coatings to flat sheet materials.
- Other sheet materials: Certain plastic, metal and other substrates may also be suitable, depending on their surface properties and the selected coating.
Different coating systems can be used with curtain coating equipment, provided their properties are suitable for the application process.
The Trivec Curtain Coater supports selected water-based lacquers and paints, two-component lacquers and UV-curable coating systems.
However, the shape of the workpiece remains an important consideration.
Products with deep profiles, complex geometries or surfaces requiring coating on several sides may be more difficult to finish consistently using a conventional curtain coating arrangement.
For these applications, spray coating, vacuum coating or another application method may be more suitable.
Advantages and limitations of curtain coating
Like every industrial coating method, curtain coating has advantages and limitations.
Advantages
Controlled coating application
Coating flow and conveyor speed can be adjusted to achieve the required application weight within the suitable operating range of the process.
Reduced coating waste
Excess coating that does not land on the workpiece can be collected and recirculated when the coating formulation and process allow it. This can reduce material loss compared with spray applications where overspray cannot be recovered.
Suitable for continuous production
Curtain coating can be integrated into continuous industrial finishing lines, making it suitable for manufacturers looking for consistent application and efficient production.
Non-contact application
Because the application head does not touch the workpiece, there is no direct mechanical contact between an application roller and the coated surface.
Limitations
Curtain coating is not suitable for every product or coating material.
A stable liquid curtain is necessary for consistent application. Inappropriate flow conditions, air bubbles, contamination or unsuitable coating properties can affect the result.
Another important factor is air entrainment, where air becomes trapped between the coating and the moving substrate. This can occur under unsuitable application conditions and may cause defects in the coating layer.
The geometry of the product also matters. Deep profiles and complex shapes can make it difficult to coat all required surfaces consistently.
Finally, the required coating weight must be achievable within a stable operating range. Very low application weights may require specific coating formulations, equipment configurations or a different application method.
These considerations make coating trials an important part of evaluating the process.
Curtain coating vs roller coating vs spray coating
Curtain coating, roller coating and spray coating are different application methods used in industrial finishing.
The most suitable technique depends on the substrate, coating material, required application weight and desired finish.
Curtain coating
Curtain coating applies liquid coating through a continuous falling curtain. It is well suited to flat surfaces in continuous production.
Excess coating can be collected and returned to the system, depending on the coating formulation and equipment configuration.
Roller coating
Roller coating uses rotating rollers to transfer coating material directly onto the workpiece.
It is commonly used for flat panels and boards where the coating can be applied through direct contact with the surface.
The coating result depends on factors such as roller configuration, application settings and coating properties.
Spray coating
Spray coating uses atomised coating material to cover the product surface.
It offers flexibility for different shapes, profiles and product geometries.
However, some of the sprayed material may miss the workpiece, resulting in overspray. The amount of material lost depends on the spraying technology, product geometry and process setup.
Which method is most suitable?
For flat products requiring consistent application in a continuous process, both roller coating and curtain coating may be suitable.
When the coating material and required application weight are compatible with curtain coating, the method can offer advantages in non-contact application and material recovery.
For complex shapes or products requiring coating on multiple sides, spray coating or vacuum coating may be more appropriate, depending on the specific product geometry.
There is no single application method that performs best in every situation.
Curtain coating in a complete finishing line
A curtain coater is only one part of an industrial finishing process.
Before coating, the substrate may require sanding, brushing or cleaning to prepare the surface.
After application, the coating may need time to level before entering the drying or curing stage.
The required treatment depends on the coating system.
Water-based coatings generally require drying to remove water. UV-curable coatings require suitable UV exposure to achieve curing.
For water-based UV-curable coatings, the process typically includes water evaporation before UV curing. Other UV-curable formulations may not require the same preliminary drying stage.
For compatible UV-curable coatings, a Trivec UV Dryer can be integrated into a finishing line.
Conveyor systems, product handling and stacking also need to match the required production capacity.
A coating machine capable of operating at high speed offers little advantage if the drying, curing or handling process cannot maintain the necessary output.
For this reason, the complete finishing line should be considered when selecting an application method.
How to choose the right coating process
Choosing the right coating technology starts with understanding the production requirements.
Several questions are worth considering:
- What material and surface geometry need to be coated?
- Which coating formulation will be used?
- What coating weight is required?
- What surface quality and finish are expected?
- What production capacity is needed?
- What drying or curing process is required?
- How important are material efficiency and process consistency?
The answers help determine whether curtain coating, roller coating, spraying or another application method is most suitable.
For example, a manufacturer producing flat parquet boards may benefit from investigating curtain coating when the required coating and production conditions support the process.
A manufacturer processing heavily profiled wooden components may achieve better results with a different technique.
For profiles requiring coating on several sides, vacuum coating can also be considered. Read more in our article When a Vacuum Coater Works and When It Doesn’t.
Ultimately, the most important question is not simply:
Which machine can apply the coating?
It is:
Which application method produces the required finish in the most stable and efficient production process?
Discuss your coating process with Trivec
Not sure whether curtain coating is suitable for your product and coating material?
At Trivec, we can look at the workpiece, coating formulation, required application weight and production requirements together before selecting the most suitable application technology.
This helps determine which process fits your requirements and how it can be integrated into a complete finishing line.
Get in touch with Trivec to explore the most suitable coating solution for your products and production requirements.
