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.
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