A membrane press is a single opening press with one of the platens constructed as a heavy duty tank which is filled with water (bottom platen) or air (top platen) and covered with a 2 to 3 mm thickness flexible membrane of silicone or other synthetic rubber. Using this press, decorative surfacing materials, for example wood veneer or PVC foil, can be pressed onto the contours of premachined base panels. MDF is often used as a substrate for this application as its compacted fibre structure ensures good machining of the edge profiles and any feature grooves cut into the surface of the base panel and hence good Support for the adhesive bonded decorative surfacing material.
Fielded edge panels manufactured from wood veneered MDF are often used as alternatives to solid wood panels fitted inside the frames of kitchen doors. The good dimensional stability of MDF compared with solid wood eliminates the risk of frame joints opening or the exposure of unfinished edges when frame and centre panel doors are exposed to extreme damp or dry conditions respectively.
The MDF core panels are sprayed with polyvinyl acetate (PVAC) adhesive paying particular attention to good coverage on the profiled edges. Water is sprayed onto
the wood veneer lay-ons to make them more flexible and these are then placed on the core panel, wetted face uppermost.
Each panel assembly including a backing veneer to maintain balance is placed in the press with the veneer covered by a low friction sheet in contact with the membrane covering the top platen in an air press or the bottom platen in a water press. The panels are pressed at approximately 85°C for up to two minutes.
PVC foil laminating
Face moulded PVC panels with two or sometimes four edges covered with the surfacing foil are being used increasingly for doors for kitchen and bedroom units. The increased flexibility of PVC foil when heated in a membrane press allows it to be formed over small radius edges and pressed into contoured grooves in the surface of the panels. Again, the good edge and surface machining characteristics of MDF make it an ideal substrate for this application with complete freedom from showthrough.
The premachined MDF panels are covered by two component polyurethane adhesive, generally applied by spray, taking care to apply a generous spread on the profiled edges and inside the machined grooves to allow for the higher absorption of the MDF edges and to ensure strong bonds in the most stressed areas of the foil. An oversize PVC sheet of 0.5 turn thickness or thereabouts with a specially treated back surface to promote good adhesion is laid on the adhesive covered core panel which is supported on a jig in the press. At a pressing temperature of 85°C, the PVC sheet flows into the contours of the base panel and is held in position by the heat cured adhesive when the panel is removed from the press. As an alternative to applying two component polyurethane to the MDF, the PVC foil can be back coated with a thermosetting adhesive.
Flatness of the laminated panels is assured by using single sided melamine surfaced MDF for the core panels or by adhesive bonding PVC or paper foil to the reverse.
The decorative effect can be increased by using textured PVC or two Colour layered foil which can be routered to create a two colour finish.