MDF powder coating: A practical update

Michael Chapman Vulcan Catalytic Systems- Featured article in Powder Coating Magazine

Why aren’t more finishing lines in North America dedicated to powder coating medium-density fiberboard (MDF)? This article explains why. It discusses how Europe has gained success in the application and how the US can adopt methods to advance the market here. Small batch and high-volume two-coat systems may be the answer. The article describes these systems and provides tips to understanding the nature of MDF and powder application.

Much has been talked about, written about, and put into practice since the idea emerged of applying powder to medium-density fiberboard (MDF). Today, if companies are seriously looking for a process to successfully powder coat MDF, they will be faced with many views and ideas. However, the answer is surprisingly simple: They may just have to look and learn from a new approach at has taken root in Europe and has now spread to the US with two companies using the new approach.
Optimism for the potential pounds of powder that could be consumed has always been running high among powder coating producers for the potential applications of powder on MDF. A growing acceptance of this robust coating technique that offers new design opportunities, together with a very cost-competitive alternative to the established processes of liquid coating and vinyl (PVC) wrapping, is now attracting attention among point-of-purchas display manufacturers and kitchen cabinet door producers. Besides cost, the well-proven environmental and physical benefits brought by powder are further driving the new interests.
Historically, there have been two basic powder coating processes to choose from: ultraviolet (UV) cure and thermal cure. At the onset, the thermal process took hold as the process of choice, with convection providing the heat for preheating of the MDF and cure of the powder. UV systems, while showing great promise with the lower heat requirements, shorter curing times and more re compact lines compared with the thermal process, did not gain ground and have been unable to spawn new investors in this process.

Today, in North America, most of the original dozen or so convection users have now been reduced to a handful, and there are two or three UV users that are currently in production. So, the question arises as to why the processes with so much potential and promise are apparently stalled. The answer lies in a mixture of high capital cost for a system together with the high cost of UV powders and evidence of unreliable results for the finished parts brought on by a combination of process and material variables.

Environmental legislation prompts a new direction

If the process is stalled in North America, the reverse has been the case in Europe.
This left thermal powders as the way forward. The debate then centered on which type of heat to use for both the preheat of the MDF and the cure of the powder. Historically, the convection process required a high cost in ovens and long dwell times of more than 15minutes in the preheat oven and a further 8-plus minutes for the cure of the powder.MDF is inherently a bad conductor, hence heat transfer via conduction is poor, requiring long conveyor tracks for the heating dwell times and, consequently, long cooling times. Infrared (IR) is known as a faster method of heating the surface of flat panels compared with convection, especially if the panel is a poor conductor.
Understanding these differences in the heating dynamics of IR and the effects this has on the MDF and the powder coating would be key to designing a new process that would reduce the overall exposure of heat that is characteristic of convection. The basic difference in the two heat sources is that convection heats objects via the conduction of heat from hot, high-velocity air to the MDF substrate, and IR heats the surface by radiation. The convection process promoted in the US required that the MDF be at such a high temperature that the powder fused to the hot board. This high temperature is inherently bad for the board because it causes stress and damage to the glues that hold the board together while driving the majority of the moisture from the board, especially at the extremities of the part.
MDF moisture content. MDF has a natural water content of typically 5 percent to 7 percent. The new system developed uses this as a means to make the board conductive. By quickly raising the surface temperature of the MDF to 200°F during a total dwell time of up to 2 minutes in a catalytic preheat oven, the board becomes conductive and remains so for up to 5minutes after leaving the oven. Board temperature at application is 120°F to 150°F, well below the temperature at which the powder would fuse to the MDF. This method of applying the powder to the MDF is contrary to the convection process. In that process, the preheat lasts for some 15 minutes, virtually drying the board and raising the temperature to some 275°F at which point the powder will fuse to the board. By forcing the board to become conductive, however, the powder is easily attracted to the MDF at much lower temperatures, ensuring a high degree of transfer efficiency. The attraction causes powder to wrap around the board perimeter in a similar fashion to metal.

Powder cure.
Once the powder is evenly deposited on the board surface and around the edges, the board enters into a catalytic cure oven (Figure 2). Curing is complete within 5minutes. The catalytic heaters within the oven are arranged in such a pattern as to drive the temperature of the powder on the edges through to cure as quickly as possible. This action has a number of benefits, the primary one being to seal off the edge to any out gassing that tends to happen along the edges of the board. By concentrating the IR toward these edges, the face of the board easily absorbs the required IR to flow and cure these flat areas. MDF has a natural tendency to expand when the relative humidity increases, especially at low ambient temperatures. Some MDF expands more than others. That’s why the powder on the edges has to be well-cured to reach the full physical properties of the coating. For a line running at 10 feet per minute, the typical oven length is 45 feet to 50 feet. Throughout the oven, some 50 heaters operate on 40 zone controls. Understanding the heating profile through temperature logging is critical to achieving good cures. A sample board has small lightweight thermocouples glued to the four edges and the flat top and bottom surfaces. This board is periodically sent down the oven to compare oven settings for peak temperature and duration. The two-coat process. The process previously described is the basic method of applying and curing powder for a one-coat system. From market pressures, it became essential to achieve a homogenous- looking substrate for both the edges and the flat surfaces. MDF has an end grain, and despite well-routed profiles and sanded edges, the differences in MDF density across the end grain is difficult to hide. During the curing process, out gassing takes place. In addition, as the powder flows (and depending on the viscosity), it will “edge dive” in the less dense region in the center of the board, leaving a distinct area that telegraphs the coarser, less dense area through the finished coating (Figure 3). By simply applying a second coat, however, this area blends in perfectly with the rest of the surfaces. The challenge was to incorporate the two powder application booths and three ovens in a line process that was economical and not too distressing for the MDF with heat buildup.
MDF is a porous medium. Consequently, its moisture equilibrium changes according to the climate. As the moisture percent increases within the board, slight increases in board thickness occur; conversely, the board will shrink as moisture is given up. To accommodate these small dimensional changes, the first coat takes on the properties of a formal powder prime coat, followed by the normal low-bake powder topcoat. In the two-coat system, as the board cools down between exiting the prime cure oven and entering the topcoat booth, by simply wiping the primed edge with 220 sandpaper, any powder encapsulated fiber “nibs” are easily removed, ensuring a high-quality finish from the cured topcoat. The amount of inter-coat edge preparation depends on the degree of edge sanding done before coating and the finishing standard required.The two-coat process starts out as described with the prime coat applied to the preheated MDF panels, followed by a 3-minute cure. The board exits the oven at 330°F.A5- minute to 8-minute cool down takes place via a power-and-free conveyor system, or the board simply keeps traveling down the line to the topcoat powder booth. (See Figure 4.)


Within minutes, the board enters the final cure oven where both coats undergo a final co-cure. Before reaching the unload area, the board cools down for 30 minutes at ambient temperature or for shorter times by chilled forced air cooling. (See Figure 5.)
Powder coating economics beat other MDF coating costs

The number of parts produced on a line that operates at 14 feet per minute is around 10 kitchen cabinet doors a minute at an average size of 30 inches by 14 inches (2.9 square feet). At this rate of production with an average combined primer and topcoat thickness of 4.0 to 5.0mils, the powder usage is 120 pounds an hour. This will produce a material cost in the range of $0.12 to $0.15 a square foot. Most components will require a finish on both sides, yielding a total cost of $0.24 to $0.30 a square foot of finished MDF.
Comparing the cost to vinyl or foil (PVC) wrap MDF, powder is very competitive. PVC has a yield after wastage due to trimming of $0.60 to $0.80 a square foot, plus $0.10 for the glue line and $0.18 for the melamine backed MDF, which totals almost $1.10 a square foot. The labor content of the powder-coated door is substantially less than that for a wrapped door, and powder offers an uninterrupted flow-through process, completing the coating in a load-to-unload time of 45minutes. An additional benefit of powder coating is the absence of a parting line between the melamine and PVC. The powder completely encapsulates the MDF. Being able to produce large quantities of finished parts without human interference is another big plus for powder. (See Figure 6 for examples of MDF finished in a two-coat system.)

Powder coating on MDF today can be compared with powder coating on metal some 30 years ago. Many companies started by hand spraying powder coatings and curing the parts with a small batch oven. The two-coat MDF powder coating system allows finishers to return to this simple concept. A point-of-purchase-display or kitchen-cabinet producer is now able to make 200 to 400 parts a day with one oven, one spray booth with four handguns, and a conveyor long enough to hold a batch of production parts (Figure 8). Atypical small system has a conveyor operating at 7 feet to 10 feet a minute with a 30-foot-long oven. Half of the oven provides the preheat to make the board conductive on the first loop. Once the production run is primed, the second half of the oven is turned on to cure the primer. As the primer is cured, the part continues around to the spray booth, where the topcoat powder is applied with the board at 140°F. The line continues until all the topcoat powder parts have passed through the cure oven. The four handguns are set up with two for primer and two for topcoat. Color changes on the two topcoat guns are quick and simple, allowing for multicolor changes within any given batch with suitable spacing of parts that are traversing down the line.
For large-scale production lines that operate at 15 feet to 20 feet a minute and produce 500 to 800 parts an hour, automatic guns are used. Oven lengths are longer than those used in a small system. This is to accommodate the same dwell times required for good powder transfer to the board and to complete cure cycles for the primer and the topcoat.

As powder development continues to match the market demands for the “look and feel” of the finished MDF, an interim concept is to use powder primers followed by a liquid topcoat. This approach imparts many of the benefits of powder followed by the look of catalyzed lacquer for MDF.
The priming concept has taken another step forward in Australia through the use of catalytic technology. There, epoxy primed T-111 type plywood products are used by the construction industry. The building is pre-primed, protecting it from the elements, while the owner decides on a final color. The pre-primed plywood allows for easier topcoat application without the high absorption rate of the typical raw wood finish. This reduces labor cost and overall paint consumption.

Final thoughts
The use of powder in the engineered wood industry is in its infancy. The progress has been slow, mainly because tried-and-tested metal application and curing techniques have been applied to these heat-sensitive materials. Wood products are very much alive and adjust dimensionally to climatic changes. The coating application and chemistry, together with the curing systems, must be tailored to meet these challenges. This process is well under way with the application and market acceptance of powder coating starting to gain ground driven by environmental concerns and competitive pressures. For the strength of our industry, this is one market that needs special attention from vendors and consumers alike. PC End note

1. The Kyoto Protocol to the United Nations Framework Convention on Climate Change is an amendment to the international treaty on climate change, assigning mandatory emission limitations for the reduction of greenhouse gas emissions to the signatory nations. The objective is the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” As of December 2006, a total of 169 countries and other governmental entities have ratified the agreement (representing over 61.6 percent of emissions from Annex I countries). Notable exceptions include the US and Australia. Other countries, such as India and China, which have ratified the protocol, are not required to reduce carbon emissions under the present agreement. There is still some debate about the usefulness of the protocol, and there have been some cost-benefit studies performed. From [www.wikipedia.org].

Editor’s note
For further reading, see Powder Coating magazine’s Web site at [www.pcoating.com].Click on article Index and search by subject category. To submit a question, click on Problem solving. Michael Chapman is president of Vulcan Catalytic Systems and MDF Powder Coat Systems, 300HighpointDrive, Portsmouth, RI 02872; telephone 401/683-2070; Web site [www.vulcanmdf. com]. Vulcan Catalytic Systems has specialized in developing applications and controls for catalytic heaters and ovens on a global basis, while MDF Powder Coat Systems has developed and integrated the oven designs into successful powder coating lines for MDF.





Process tips
Here are some tips to achieving quality powder coating of MDF. It’s important to understand the nature of MDF and how powder particles behave during application to achieve an even coating.

• Powder coating MDF with stationary batch style ovens is difficult. It’s important for the board to traverse through a catalytic oven so that the board moves through heating zones to drive the powder through the heating phases required to cure the powder on the edges.Minimum line speed to produce quality parts is 6 feet a minute.

• Placing the ovens relative to the powder applicators is very important within certain limitations.Oven lengths and distance between ovens and booths are dictated by line speed.

Process tips and MDF properties required to be successful

• Controlling the powder wrap to maintain a maximum of 4.0 mils on the edges is the key to preventing blisters. With automatic guns, the wrap is controlled with grounded “robber” bars placed closely behind the MDF (Figure 1). These bars attract the powder away from the backside of the MDF. In two European applications, applying a positive charge to the bars provides a much finer control of powder application on the edges.

• Exceeding the cure temperature of the final topcoat is necessary when curing the primer in the two-coat process.
MDF properties The properties of MDF can vary based on cost and region of the country where it is produced. It’s important to select a grade of MDF produced by a given mill that matches the functionality and coatability required by the end product for edge finish, rout quality, face-sanding degree, and screw holding power. Factors that affect these properties are listed below.

• The internal bond strength of the board must be greater that 130 pounds per square inch (psi). Boards that approach 150 psi perform very well. Boards that machine well have internal bond strengths that work well for powder coating. With the IR process, the internal bond strength properties are not degraded.

• The moisture content of the board should be within 5 percent to 7 percent for optimum coating. Lower than 5 percent requires more preheat temperature, and sharp corners may become difficult to coat.

• The average density of MDF should be 45 to 48 pounds per cubic feet (la/cu ft). The density profile needs to be as flat as possible and not drop below 40 la/cu ft at the core of the MDF. Sample A, with sharp changes in density profile, will have a greater tendency for the MDF to crack during the heating cycles than sample B that has a more constant density distribution across the board thickness. (See Figure 2.) Lower quality MDF have a large differential between the core density and the face density, causing edge cracking during the heating process.

• The fiber type (soft or hard wood) of the MDF has little effect on the two coat process. The length of the fiber affects the sandability of the MDF. Shorter fibers generally provide better quality edges and are typical of MDF with higher bond strengths.