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.