High frequency

  • Fast processes
  • Very cost-efficient

Music to our ears, the comeback of the HF/RF technology

In all processes which require heat penetration of thick layers of wood, the HF/RF technology has clear advantages over conventional contact heating methods. Different electrode arrangements allow the manu-facture of many products, for instance solid wood panels etc. Especially in cases where the full heat exposure of the outer layers would create undesirable tensions in the workpiece or drying cracks in the edges, HF bonding is the method of choice. The a. c. voltage applied stirs up water molecules, creating heat by friction, which in turn directly warms up the glueline and speeds up evaporation. It is not necessary to additionally expose the substrates to heat.

In general, all water-based glues can be used for HF bonding. Specially formulated 1-component PUR prepolymer adhesives, however, can also cure by the HF method after the part to be bonded was moistened. Due to the shorter pressing times, more efficient work processes and lower thermal stress on the wood, this method also offers advantages when the timber to be bonded has a lower thickness. Jowat supplies special dispersions and 1-component PUR prepolymers which were developed for the specific requirements of the HF technology. Compared to conventional products, these are dis-tinguished by clearly shorter pressing times, a lower thermoplasticity, higher initial strength data and lower tendency to discolour in the bondline, when bonded in a HF field with a.c. voltage. In general, HF bonding should be carried out with the lowest possible moisture content of the wood, ideally at approx. 6-10 %.

Higher levels of wood moisture require longer pressing times in comparison to dryer timber, since a large part of the available energy is heating up the part itself, and is lost for the glueline. Wood moisture content above 12 % should in any case be avoided since at this level, there might be a risk of excessive voltage. If the adhesive has a chance to penetrate into the wood before entering the HF field, this will clearly improve the bonding result. For this end, the adhesive should be allowed to remain on the part under pressure for a time before heating starts.

Ideally, the bondline temperature is set by generator power and HF time to about 50-65 °C. In panels with one layer bonded with dispersion adhesives, the best HF times have proven in field applications to run from 10 to 20 s at ampere levels of 4-6 A; in bonding of windowframes, however, the HF settings would preferably be higher due to the larger dimensions, going up to 60 s. When using 1-component PUR prepolymers, the HF exposure is approx. 2-6 minutes, depending on the type of adhesive. Due to the thermoplastic behaviour of dispersions, it is necessary for the press to remain shut after HF exposure, until the bondline temperature levels off to about 40 °C.