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UV roll coaters have been used since the first UV MDF and particle-board fillers were invented in the 1970s. They were used for sealing and filling engineered wood for use in the ready-to-assemble furniture market. This market is now dominated by coated laminated papers, but UV coatings and inks proved to be a viable emerging technology for the wood products industry. Roll coating required a relatively viscous (thick liquid) coating so it was a good fit for UV because the technology of the era allowed for only viscous UV finishes.
Jim Arvin, president of DuBois Equipment, a manufacturer of roll coaters in Jasper, Ind., explains, "Most roll coaters applying UV finishes in the field are differential roll coaters. This type of roll coater allows for the chrome metering roll to be adjusted for speed and direction. This adjustment gives the coater precise control of the coating film thickness and appearance."
During the 1970s and '80s the prefinished flooring and prefinished paneling markets soon accepted UV clear finishes as the standard for their industry. These industries are some of the largest users of UV clear wood finishes in the world. The roll coater was now the applicator of choice for UV sealers, fillers and topcoats.
The only problem with the roll coater is that it could only coat two-dimensional surfaces. Any 3-D part could not use UV because UV was too thick to spray or vacuum coat. The roll coater worked well on finely calibrated parts with little variation in dimension. "The most common uses for UV roll coating is in the prefinished hardwood flooring industry, paneling industry, kitchen cabinet industry, RTA furniture, some case goods, flat solid core veneered doors and various flat parts," Arvin says.
Low viscosity UV finishes: A new era in UV coatings
Some industries were so desperate to use UV finishes that they diluted thick UV finishes with ketone solvents and sprayed them through conventional spray or air-assisted airless spray equipment. This usually produced a tough finished film but did not help the finisher's VOC problems. Some of these sprayed solvent reduced UV finishes are as high as 3.0 to 5.0 VOCs. This might be a slight improvement over conventional finishes, but it wasn't the "green" solution that some manufacturers were looking for. As the 20th century drew to a close, UV raw material manufacturers developed lower viscosity materials for use in 100% solids UV wood finishes. These new raw materials proved to be critical in making UV coatings that could be "vacuum coated" and "spray applied" without the addition of solvent. These new products spawned a new UV coating application technique called "vacuum coating."
During the past decade a water-based UV has been developed. This product has a low viscosity, along with the toughness usually attributed to UV-cured finishes. It can also be applied at low film thickness since it is usually about 40 percent solids. Both the 100% solids UV finishes and the water-based UV finishes had flash points over 200 F. This means no more flammable storage and less chance of a solvent-related fire.
Vacuum coating with UV
Vacuum coaters work well on parts that have a consistently shaped profile such as picture frame mouldings and door trim parts, which are a large market for wood parts that are vacuum coated. The vacuum coating technique has been around for awhile. Sid Ely, area manager for Delle Vedove, a manufacturer of UV vacuum coaters remembers that "the original vacuum coaters were used as a technique for applying W/B stain to wood mouldings."
They became a popular technique for applying water-based stain and coating and several manufacturers started to pop up to fill this market. Paint-O-Matic, Shiele, AM&D, Universal Finishing and others served this new market. In the early '90s UV finishes finally became low enough in viscosity that they could also be applied by the vacuum coating technique. Then more manufacturers became interested in building vacuum coating equipment to service this market. The equipment became more accessible to the medium-sized manufacturer of mouldings and lineals.
Parts are generally coated with a 100% solids UV wood sealer and then a 100% solids UV topcoat with some type of automated sanding between coats. An intermediate UV coating can also be used in the vacuum coater. It can be used as a wood sealer and a topcoat. This type of coating compromises the sanding properties of the sealer and the scratch resistance of the topcoat, but is adequate for many wood applications. The automated sanding may include abrasive brushes, flap wheel brushes or semi-solid wheels that have been cut to the profile of the moulding. Some lower-end applications may require only one coat of UV vacuum coat finish.
Ely suggests that the vacuum coater may be over 97 percent efficient in coating transfer efficiency since it is a closed loop system. He explains that the UV vacuum coater can be operated at speeds up to 200 fpm while taking up very little in plant floor space, sometimes as little as 12 ft. for the coating and curing portion.
The major disadvantage of a vacuum coater is the templates must match the moulding and have to be changed for each profile. This template change would require several minutes per profile change. Vacuum coaters are an excellent choice for the manufacturer that has a lot of parts to coat and they are only a few different profiles per day.
Spraying with UV
Spraying UV coatings is a relatively new process. This application technique started with spraying 35 percent solids UV solvent diluted products in the 1980s and 1990s. This had very little if any environmental benefit and was not well accepted by most of the wood coatings industry. But during the last decade of the century, UV coatings underwent a tremendous reduction in viscosity. It was now possible to spray a UV coating that contains little or no solvent, but resistance from equipment suppliers and some coatings manufacturers has made for a slow transition.
A new generation of water-based UV spray finish is now making its way into the market. Recent advances in UV water-based spray technology has introduced a product that is easy to spray and easy to get a lacquer look due to its lower solids. Typical water-based UV formulations run about 40 percent solids. This allows the applicator to spray a little too much coating without getting a "plastic look" that old-generation UV coatings were giving with the thick UV spray finishes that were typical 10 or more years ago.
Reciprocating spray machines
Most sprayed UV products are sprayed on automated or semiautomated spray lines. The most common type of automated spray line is the automated reciprocating spray machine. These machines are usually triggered by a motion sensor to spray parts that are lying on a conveyor belt. This application technique works best for raised-panel doors for cabinets, drawer fronts and other beveled panel surfaces.
David King, operations manager of Superfici America, a manufacturer of automated reciprocating spray machines, sees a shift in UV spray toward water-based UV. Previously much of the UV reciprocating spray was solvent diluted with acetone. "Typically, the reciprocating machine uses air-assisted airless spray guns. They usually are equipped with 4 to 8 guns per machine. Most machines have two circuits and can switch from clear to tinted with the flip of a switch. Automated reciprocating spray machines can spray wood parts on the line at 3 to 6 m/min., (9.75 to 19.5 fpm) on a 4- to 5- ft.-wide conveyor belt," King explains.
Special solvent/water drying ovens have been developed for speeding the dry time of the water-based UV. These ovens combine jet drying, IR and laminar flow to achieve a fast dry without solvent or water popping. Immediately after the water or solvent is dried out of the coating, the conveyor runs under one to three high-intensity UV curing lamps to crosslink the finish. The parts are then cooled and stacked and ready for assembly or shipment.
These automated reciprocating spray machines can be designed to capture the over-spray from the belt and recycle this material, thus increasing the spray transfer efficiency of the unit to 85 percent or more.
Misting spray machines
Arvin says the mist coater is capable of applying very thin films of 100% solids UV spray. "We have sprayed UV finish as low a 0.3 mil with a smooth film," he says.
The mist coater employs conventional air spray guns or HVLP guns that allow for better atomization of the 100% solids UV than reciprocating spray technology is capable of achieving. The mist coater is also designed to recycle excess coating that has been sprayed onto the conveyor belt. It is equipped with a baffled exhaust system and is capable of recycling atomized UV finish that is sucked into the exhaust system. DuBois Equipment has tested this misting spray equipment and has achieved over 98 percent material usage from its mist coater with the recycling system, Arvin says.
He goes on to explain that, "the mist coater's advantage over a reciprocating spray unit is it can spray at thinner films than the recip machine, and it can achieve a higher material usage efficiency."
The thinner films allow more flexibility to the coater in regard to the "open grain look" that some wood products manufacturers require.
This type of equipment can be used for coating raised panel doors, drawer fronts, drawer sides as well as wider trim parts. Usually two coats of 100% solids UV are required for mist coaters. The first coat acts as a sealer and the second coat is a topcoat.
Older moulding spray machines can be adapted to spray UV finishes of 100% solids UV or solvent diluted UV. These machines are used to spray everything from prefinished hardwood flooring to architectural mouldings like chair rail and crown mouldings. This application method is limited to narrow parts and can only coat three sides of the wood. It is becoming more popular with facilities that need fast turnaround in the coating operation but cannot afford to invest in all new equipment. The coating and curing process is similar to the process for misting spray and automated reciprocating spray machines except that the guns must be adjusted for every new size of moulding or lineal that is sprayed.
Improved UV curing unit technology
The "UV curing unit" or "UV oven" has been improved to run much cooler. The lower running temperature improves the look of all the coatings cured in such units. The UV curing units of the past were "ovens" in that they provided far too much thermal energy to the wood. This excess heat led to unnecessary air popping and heating of the sensitive wood substrate. Old UV curing systems delivered an inordinate amount of UV energy to the top of the substrate and just enough energy to the sides of the door. Newer units are designed to distribute the UV energy to the recessed areas and sides of the parts.
Summary
Advancements in both UV equipment design and coating chemistry brought UV finishes closer to all facets of the wood industry. Water-based UV spray and 100% solids UV spray are now in the forefront of the cabinet industry on cabinet doors and frames and not just side panels and drawer boxes. UV finishes that were once only dominant in the paneling and hardwood flooring industries are becoming more commonplace in the cabinet, furniture and moulding industries.
Ed. Note: Donald Hart is currently the technical service manager for Mid-America Protective Coatings, Inc. Please feel free to e-mail Donald Hart at donphart@aol.com.
author: By Donald Hart