3.Machining and processing HPL
Hot melt adhesives are processed on machines using the hot-cold process. This creates a bond through the physical process of cooling. When refilling adhesive, it has to be ensured that the required temperature in the melting tank is maintained. Adhesive residue and contaminations have to be removed from the walls and ceiling of the melting tank at regular intervals. The residue would cause the adhesive temperature to be lowered. The processing temperature of the adhesive in the adhesive tank and on the application roller has to be reduced to the temperature recommended by the adhesive manufacturer when the edge banding machine is at a standstill (e.g. during work breaks), otherwise the adhesive will degrade. The applied adhesive quantity depends on the texture of the substrate edge. Porous workpiece edges with many voids require a higher adhesive quantity to ensure reliable bonding of the edging material. If the substrate or the edging material is too cold, the hot melt adhesive layer will cool down suddenly, resulting in incomplete adhesion. A draft in the room after adhesive application can also cool down the boundary layer (skin formation). Preheating the edging material with an infrared radiator, heating bar or hot air fan directly before passing it through the machine can be beneficial in this case.
The following factors have a crucial influence on the quality of the adhesion:
Laser/plasma processes have been developed as an alternative to the conven- tional bonding methods. Functional layers adapted to the process are applied to the rear side of the edging by the edging manufacturer. Afterwards, the processing company activates the functional layers using special machines. This creates an almost seamless transition between board and edge.
For the thermal activation process (cold bonding activation process), modified PVAc adhesive is applied to the workpiece edges in the edge banding machine using adhesive rollers or spray units. The adhesive is then dried and activated with hot air or infrared radiation. The pre-coated edging material with activated adhesive layer is then introduced. The two activated adhesive films are then bonded together in the pressure zone (rollers or belt). Immediate further processing is possible. Cleanliness must be ensured during processing. We recommend applying a suitable release agent to the workpiece surface to prevent adhesive residue from adhering to the surface.
For stationary methods, the edging material is pressed on either with edge presses, bonding wheels or stands or using c-clamps with a rigid support layer. The adhesive is applied to the workpiece edge cold with a brush or roller. The applied quantity has to be selected so that the evenly applied adhesive sufficiently fills the porous workpiece edges and wets the full surface of the edging material. The applied edging material is pressed on with the recommended tools. Excessive contact pressure can result in uneven surfaces. Excessive temperature can result in surface damage on the edging material. If PVAc or condensation resin adhesives are used, the curing times can be reduced with heating bars. The temperature of the heating bar depends on the adhesive type used. The actual temperature of the heating bar can easily be checked with melting salts or heat sensitive pens. Processing solvent adhesives requires special care. The adhesive is applied to the substrate material and edging material using a brush or spray gun. A primer coat is recommended for strongly absorbent workpiece edges. Thorough curing (finger test) before joining the workpieces is important; the contact pressure should not be below 0.5 N/mm2.
