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 pretreatment 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 Protocol1. 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.