TI111 - Technical Process Recommendations for Powder Coating MDF
Goals of MDF pre-treatment
Among others, the major objectives of preparing medium density fi bre panel MDF) substrates include:
- homogeneously prepared MDF substrate by e.g. “grinding”
- avoiding defects in / on the surface
- avoiding grinding marks
Process steps for MDF preparation
The more finely the MDF fibers were ground during manufacturing, the better the surface quality, intermediate layer, and homogeneity of the panel.
Currently (August 2014), EGGER MBP-L is one of the MDF panels we recommend for powder coating.
The cut blanks for the parts to be milled are sawn from large, full-size boards. If the surface sanding supplied by the manufacturer does not meet the required quality, the boards must be sanded with a grain size of at least 220 before further processing. The edges must at least be chamfered (radius > 1 mm). After sanding and cleaning with compressed air, the surface is evenly finely sanded and free of any contaminants, small scratches, dust, grease, etc.
Next, the material is cut to the required shape on a CNC machine or a double-end tenoner, the narrow edges are sanded, and various other processing steps are carried out (drill holes, cutouts, etc.). After processing, the cutting and sanding dust on the material surfaces and narrow edges must be thoroughly cleaned off with compressed air. The cleaner and more dust-free the material is, the cleaner the area where the workpieces are suspended and coated remains. This is an important quality factor for the final result.
To ensure a wider production window, we recommend conditioning the MDF individually depending on the type, quality, and ambient climatic conditions.
Suspension
Most workpieces have 1 or 2 drill holes for suspension. The goods carriers can have a thread on the suspension point to prevent workpieces from slipping off. Suspension points without threads must have at least the same diameter as the drill hole to prevent the workpieces from slipping off.
In order to guarantee perfect discharging, the contact surface must be bare and clean.
The goods carrier is hung onto a grounded transport system. This ensures the necessary discharging for every workpiece. Before coating, every workpiece is checked for faults in its suspended state and with side-lighting.
Before the material goes through the preheater it must be free of all defects and dust. Every flaw, scratch, or contamination will impair the quality of the coating result.
Preheating
In the preheater, IR radiation heats up the workpiece. During preheating of the raw MDF, the maximum temperature on the surface should not exceed 100°C. Wood fibers on the surface dry/shrink at different rates and this shows up under the coating.
Depending on the geometry and MDF type, the optimal MDF surface temperature at the pistols is between 40 – 60°C. Heating significantly increases the conductivity of the panel.
In the 2-coat process, the primed workpiece can be heated to a higher temperature before application of the top coat because the primer coat prevents any further drying of the MDF surface.
Application
During application, a homogeneous coating must be guaranteed. The surface and narrow edges must have the same coating thickness.
The powder output of the pistols should not exceed 250g/min. If the powder output is higher, this causes lower electrical charging of the powder and therefore poorer initial application effectiveness.
The application settings can be chosen within a wide window. They depend on whether raw or already coated MDF is to be coated and what coating thicknesses are required. The voltage (kV) can be set from 20–90 kV, and the current between 5 and 50 μA. The coated parts must be visually inspected to determine the optimal application settings. The workpiece should be homogeneously coated with the required coating thickness, without any backspray on the corners or narrow edges.
An adjustable return electrode can be used to adjust the coating thickness on the narrow edges.
The guns’ feed and coating parameters must be adjusted individually after visual inspection. It is important to ensure that the optimal powder coating quantity is applied to the front and back narrow edges.
See the respective technical data sheets to find the minimum coating thicknesses.
Dry-through process
Due to the restricted thermal conductivity of the substrate, we recommend using infrared (electric/gas-catalytic) or combined circulating air-infrared furnaces.
The surface temperature is determined on a powder coated MDF panel using attached thermal sensors (preferably type-K thermal elements). We recommend fixing the thermal sensors with temperature-stable glass textile tape (type 69 from 3M). See the corresponding technical data sheet for the product-specific hardening conditions. In order to determine the ideal curing conditions, we recommend always performing practical trials that are adapted to the object in question and the curing oven.
Removal
When the suspended workpieces are removed and stacked after coating, the surface temperature should not be above 40°C.
Test methods
The following tests can be performed to determine the quality of the coating:
- Acetone test (IGP AA 341.58)
- Concealed hinge test (IGP AA 341.54)
If you have any questions, please contact us.
