UV Coating Application in the Wood Finishing
The UV coating is a coating that dries when the coating layer is exposed to UV light with sufficient intensity. UV coating is a fast-dry, efficient, environmentally friendly finishing with a strong and hard film layer that is highly resistant to heat, solvents, and chemicals. It is lot of used in the modern finishing industry, especially for mass production. More explanation about UV coating can be viewed in our previous article: UV coating for wood finishing.
UV coating application.
The UV coating needs a special process, and he needs special equipment and facilities in the application. Fortunately, the equipment coating technology has developing facilities to accommodate the material application. Meanwhile, the coating technology also keeps improving to provide the materials to fulfill the finishing requirement in the modern wood finishing industry. Both things have encouraged making the UV coating more flexible in application to meet the woodworking industry.
The UV coating finishing schedule consists of 3 main steps. First is layering the coating on the surface, then the UV lamp curing process. The sanding is needed when the coating film will be layered with another coating. There are some equipments that can be used to layer the UV coating on the surface: spray equipment, curtain coater, and roller coater.
Application with a roller coater
The application of UV coating with a roller is the most widely used in the wood finishing industry. Most roller coating machines are invented and designed to apply UV coating. On the other hand, most of the UV coating is designed for the roller machine. The roller coater application allows the coating to meet its advantages optimally. Application with roller can give a fast process and maximum efficiency of material usage.
The complete roller coater consists of a series of equipment to do all the finishing steps for the UV coating finishing system. The product enters the feeding and exits at the other end (output) with a dry coating layer and is ready to be packed. Basically, the finishing schedule of the UV coating finishing process is preparation sanding, coating application, drying, and sanding the coating.
- Sanding preparation
In the roller coater application, the product needs to be flat and have the same thickness. Uneven thickness will make the coating layer vary from part to part. For this reason, before entering the roller coating machine, the products should be calibrated (sanded) with a good machine so that they can have the same thickness. Sanding with a precise wide belt sander machine is the first process to ensure the products entering the machine have uniform thickness
- Coating application
The next process is the application of coating or other finishing materials. There are several types of UV finishing materials that are all applied to the surface: stain, filler, sealer, top coat, and putty that each may have different characters. To accommodate these finishing materials, there are several types of rollers for the UV coating machine: steel roller, rubber roller, and sponge roller. The steel roller is the hardest roller and is most used for filer or putty applications that require pressure. Rubber rollers are available in some hardness and are the most used to apply the top coat, sealer, or base coat. Sponge rollers are the most flexible rollers and are used for stain applications or on slightly uneven items such as distressed panels or textured panels.
In addition, there is a laser roller that is specially designed for UV top coat applications to minimize the roller marks on the resulting film layer.
- Drying the coating
After the UV coating is applied to the surface, the next process is the drying. Objects that have been coated are passed through a UV lamp oven to receive UV light in a certain intensity to let them cure and dry. In a complete UV machine series, the drying process of this UV coating is adjusted according to the finishing stages. There are 3 types of dryness in UV coating: full cure, semi cure, and sanding cure.
Full cure is the condition when the UV coating layer reacts perfectly and the film is fully dry. The full cure is usually needed in the last finishing process. The full cure dry will make the coating film perfectly dry and reach its maximum strength. Products that have passed this stage are ready to be packed or sent to the users. A full-cure coating layer is also needed when this UV coating will be overlaid with other types of coating, such as nc, pu or waterbase.
Sanding cure is a drying process where the coating layer is ready to be sanded before continuing with the next application process. A full-cure coating layer is generally very hard and difficult to be sanded, while the sanding is needed to level the film surface. Sanding cure is a condition where the coating layer is dry to be sanded before layered with another coating.
Semi-cured is half dry when the coating layer is not dry, but it won't resolve when it is layered with another UV coating. This semi-cured condition is unable to apply the multiples of thin layer coating without sanding. Coating with a high thickness tends to leave roll marks on the finishing surface, while the thinner coating layer will leave fewer roll marks. The semi-cure drying is used to apply many layers of coating thinly to minimize roller marks on the surface.
- Sanding the coating
Sanding is an important process to produce an even and smooth surface and good bonding at the finishing. There are two types of sanding machines: a wide belt sander and a brush sander. A wide belt sander is used for sanding the flat objects. A brush sander is used to sand uneven objects such as textures and distressed panels.
Application with a curtain coater
The curtain coater may not be as popular as the roller coater, but actually it gives better film coating and good efficiency. The UV coating is diluted with some solvent and additive to make it enter through the curtain coater machine. Application with a curtain coater will produce a smoother surface compared to a roller, but it still cannot be as good as a spray application. Application with a curtain coater will produce better efficiency compared to a spray application, but because there is solvent added to the coating mixture, the efficiency level is still lower than the application using a roller coater.
Application with spray
Spray application is another technique that can be used for UV coating applications. Spray application will produce a smoother and more even surface than the roller coater or the curtain coater, but with less efficiency. The automatic spray equipment that is needed to apply the UV coating. Manual spray is not recommended because UV coating is considered harmful. The monomer content in UV coating has the potential to harm human health.
In the spray application, the UV coating viscosity needs to be adjusted according to the spray equipment specification. Some solvent and additive need to be added in the mixture; as a result, it won't have 100% solid content as a roller application. The spray application will also produce dust spray, which means material waste. Thus, the efficiency will not be as good as roller application or curtain coater application.
Portable UV application machine
The portable UV application machine is one of the new technologies to apply the UV coating. Basically, it is a roller coater machine, but in a small unit. It consists of the roller coater and UV oven lamp that is put on a carrier. It can be brought to the location to do the UV coating application. It can be used to coat the floor that is installed in the house or building. The coating application will be fast and efficient as the UV roller coater machine. It could be a good option to finish the wood product in small-volume production. But this equipment is also limited to the flat products.
The UV Oven lamps
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