The MDF powder coating process developed by Heraeus Vulcan consists of a preheat oven, powder application equipment and the curing oven. The key to success is to carefully understand the required temperature profile for the parts as they pass through the pre heat oven and then the main cure oven, after the application of powder. This is achieved using ovens that have been designed to ensure an even distribution of heat across all exposed surfaces of the part, including the difficult to reach cut edges. Because we use a series of individual gas catalytic infrared heaters, appropriately positioned, this can be achieved. However, it is then critical to control the heat output from each group of heaters. This is achieved using PLC controls, which enable individual programs of heat profiles to be created and saved for future recall. Further details are given below which explain further how our systems operate.
MDF Moisture Content
MDF has a natural water content of typically 5–7 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 deg.F for a total dwell time of up to 2 minutes in a catalytic IR Preheat Oven, the board becomes conductive and remains so for up to 5 minutes after leaving this oven. Board temperature at application is 120 deg.F to 150 deg.F which is well below the temperature at which the powder would fuse or melt to the MDF. Once the board is conductive, the powder is easily attracted to the MDF, ensuring a high degree of transfer efficiency. The attraction causes powder to wrap around the board perimeter in a similar fashion to metal.
Once the powder is evenly deposited on the board surface and around the edges, the board enters into a catalytic oven for the 5 minute curing process. The gas 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 towards these edges, the face of the board easily absorbs the required IR to flow and cure these flat areas.
MDF Powder Coating Process - Two Coat Technique
This is the basic method of applying and curing powder for a simple 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), the liquid powder will edge dive in the less dense region of the edge before the powder reaches the gelled state. This leaves a distinct area that telegraphs the coarser, less dense area through the finished coating. 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 is economical and not too distressing for the MDF with heat buildup. The answer is a continuous operating line showing two powder applications.
This two-coat process starts out as described with the prime coat applied to the preheated MDF panels, followed by a 3-minute cure in the primer gel oven. The board exits this oven at 300 deg.F. A 5–8 minute cool down takes place as the board travels down the line to the topcoat powder booth. Within minutes, the board enters the final cure oven where both coats undergo a final co-cure. The board cools down for 30 minutes at ambient temperature before reaching the unload / load area.
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.
For the Two Coat system, line speeds may range from 8 to 20 ft. per min. with outputs ranging from 100 400 parts per hour. The process is unique, in that the majority of the moisture content is preserved within the MDF board. Additionally, the infra red energy only heats the outside of the board raising the surface temperature sufficiently to cure the powder, without compromising the integrity and physical properties of the MDF board.
MDF Powder Coating History and why there should be more finishing lines in North America dedicated to powder coating MDF
This article tries to explain 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 process that 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-purchase display manufacturers and kitchen cabinet door producers. Besides cost, the well-proven environmental and physical benefits brought by powder are further driving this new interest.
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 pre-treatment of the MDF and cure of the powder. UV systems, while showing great promise with the lower heat requirements, shorter curing times, and more 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 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 the New Developments in MDF Powder Coating
If the process is stalled in North America, the reverse has been the case in Europe. The wood industry in Europe has fallen under the tough new environmental legislation of the Kyoto Protocol 1. This in turn has forced solvent-based wood and MDF finishers to look for alternatives to bring them into compliance. Seven such companies throughout Europe have adopted powder coating processes to produce MDF components ranging from kitchen cabinet doors, point-of-purchase displays, and office furniture.
Planning for this potential use of powder coatings started some 3 years ago when a leading powder coatings producer collaborated with Heraeus Vulcan, a catalytic oven manufacturer. A new approach and understanding of the variables that had apparently held back the anticipated growth potential was required to kick-start the use of powder coatings for MDF in North America and Europe.
Early decisions had to be made as to which process would produce a forgiving and economical system. UV powders, while having some distinct benefits, were eliminated on the grounds of high powder cost, issues with long-term reliability and operating costs for the UV generators, and issues with color matching.
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 15 minutes 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.
Some Tips From The World Leaders In MDF Powder Coating Technology
Here are some tips from Heraeus Vulcan to improve your MDF powder coating process.
Understand your MDF
It’s important to understand the nature of MDF and how powder particles behave during application to achieve an even coating.
The internal bond strength of the board must be greater than 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. Lower quality MDF have a large differential between the core density and the face density, causing edge cracking during the heating process. (click image to enlarge)
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 sand ability of the MDF. Shorter fibers generally provide better quality edges and are typical of MDF with higher bond strengths.
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.
No Batch Ovens
Powder coating MDF with stationary batch style ovens is difficult. It’s important for the board to traverse through a gas catalytic IR oven so that the board moves through the heating zones in such a way that the heating phases required to cure the powder on the edges act effectively.
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.
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. 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.
Good Quality MDF is the Key
MDF properties can vary based on the region of the country where it is produced. For best results of powder coating 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. The following factors affect these properties.
Internal bond strength
The internal bond strength of the board must be greater than 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. Within 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 (lb/cu ft). The density profile needs to be as flat as possible and not drop below 40 lb/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. Lower quality MDF has a large differential between the core density and the face density, causing edge cracking during the heating process. Therefore MDF properties should be carefully considered if a good quality finish is to be achieved.
Compare Cost Per Sq Ft. of Powder Coated v. Foil Wrapped
Consider a production line which is running at 14 feet/min (4.3 m/min) and the number of kitchen cabinet doors produced each minute (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 5.0 mils, the powder usage is 120 pounds an hour. This will produce a material cost in the range of $0.12 to $0.15 per 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.
Foil Wrapped MDF more than four times as expensive to produce
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 a powder coated door is substantially less than that for a wrapped door, while powder coating offers an uninterrupted flow through process completing the coating in a load to unload time of approximately 45 minutes.
Powder Coated MDF for Toughness
An MDF part that has been powder coated is significantly tougher in use than either a foil wrapped part or a liquid finished part, thus offering a better quality product at less cost. The latest powder technology that is coming to market, enables either single or two coat processes that can now be used in high humidity areas such as bathrooms, and can be cured in less than 5 minutes at 130°C.
Gas Catalytic Infrared v. Electric Infrared v. Convection Oven
The use of powder coating infrared ovens as opposed to convection ovens for the pre heat and cure cycles is the most cost effective method to cause the MDF to become conductive and to cure the powder without removing the water content from the MDF board. Catalytic infrared is more effective over electric infrared in as much as the IR leaving the catalyst surface is multi directional and emerges from a cloud of water vapor and carbon dioxide which are bi products of the catalytic chemical reaction. These hot gases provide a low intensity long wave infrared that is more easily absorbed by the powder on the surface of the MDF, raising the surface temperature quickly and uniformly through out the thickness of the powder.
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.
Economics of Powder Coating MDF Proven
It is clear that the argument has been proven. A powder coated MDF part, cured in a gas catalytic infrared oven is cheaper to produce than any other finish. and tougher in use.
Heraeus Vulcan has been at the forefront of the latest developments in oven cure technology and PLC controls to ensure the optimum cure.
Vulcan Catalytic® is a worldwide leader in the development of pre-heating and curing systems for successful application of powder on MDF. We have been at the forefront of New Developments in MDF Coatings. Using unique catalytic oven designs with our high efficiency heaters, together with the latest in low bake powder technology, the process has become more predicable and consistent.
Powder coating has now evolved to offer many advantages over liquid paint application. Advances in technology continue to increase the efficiency and cost effectiveness of the process. As part of our ongoing research, Heraeus Vulcan has found significant opportunities in powder coating, but the most exciting of these existed in the wood industry, and in particular, M.D.F.
MDF (Medium Density Fiberboard) is well known for its versatility, particularly in the furniture and construction industries, with new applications being developed continually. Although the material itself is relatively inexpensive, much of the production cost of mass produced products is associated with the finishing of the product’s surface, either by conventional wet paint spraying, vacuum coating or membrane pressing. The method adopted is dependent on the type of board, production requirements, the finish and of course the cost.
While long established and proven systems, they do have intrinsic drawbacks. Our research indicates a significant potential for an effective finishing process giving the benefits of wet paint without the distinct disadvantages. Heraeus Vulcan has spent considerable time developing and refining the process of applying powder to MDF. resulting in an extremely reliable and efficient system for a two coating process that enhances the sealing and protective properties of the MDF. To understand the significance of powder for MDF, we must look at the established methods of finishing MDF.
Wet Paint applied using a spray gun there has an ongoing cost of over sprayed paint, maintenance of the paint booths and the disposal of the residual material. Of greater concern is that virtually all‑usable wet systems have the major problem of V.O.C. (Volatile Organic Compounds) emissions. Ever increasing environmental legislation has prompted the search for an 'environmentally friendly' alternative to wet paint.
The common alternative to wet painting is Membrane Pressing. This process involves the application of glue to the substrate, and the subsequent pressing of a vinyl onto the surface. While application of vinyls gives a very good finish, the major problems include the time taken to apply the glue and the timing between the application of the glue and the pressing of the vinyl. Also, differing conditions from day to day can effect the glues and their subsequent performance. A further drawback can be found in the pressing process. Sharp corners, acute angles, and thick boards can result in stretching, bubbles and the bucking up of the vinyl, producing rejects, a large proportion of which can not be re worked and have to be disposed of.
With the application of powder coating, liquid paint problems can easily be overcome. As powder is a dry film when applied solvents are not required, thus alleviating the major hurdles imposed by legislation on VOC emissions. Also, because powder is a dry film and subsequently heated to melt and cure the film, the over-sprayed powder can be potentially collected and re‑used, thus minimizing waste. The design and shape of the MDF. board can utilize unusual shapes, which would be impossible for membrane pressing. Finally, powder coating is easily automated, not only the application but also the recovery and introduction of fresh virgin powders into the system.
A simple online pre‑conditioning of the board, again using I.R., has overcome the problem of coating consistency from one batch of MDF. to another or due to the shaping of the surface. Preconditioning is in fact an essential part of the process whether using I.R. and U.V., or simply in the final cure. This draws moisture and resins to the surface of the MDF. facilitating uniform attraction of the charged powder particles to the substrate. Preconditioning may be achieved using I.R. or hot air circulating ovens or a combination of the two. The ultra low bake system is a proven and highly utilized system which has now been put to use in an application which previously would have been inconceivable! Capital investment is lower, therefore pay back times for the investment are shorter.
The requirements for the final cure of ultra low bake powders will vary depending upon a number of factors, the MDF. board and of course the conveyor speed, whether IR or hot air re-circulating ovens are to be used, or a combination of the two. These factors may be determined during trials at Eurotec headquarters.
A great deal of excitement and interest is now being generated by the fact that this is now a proven production process using standard powder coating application equipment and virtually standard format coating powders. This means of course that manufacturers can now mass‑produce a variety of products finished with all the advantages of powder coating.
Vulcan Catalytic® will, of course, be at the forefront of new technology development in this area.
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