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Duration of the Solidiwood production process

Duration of the Solidiwood production process

Solidiwood process technology hot-forms green sawn timber in 2-3D into high-grade processed products, right through to surface treatment, in a single machine / process – in as little as a few hours, even for the most massive products. The duration of the process depends on the drying.

The starting point for developing the process comes from Junckers. At Junckers, sawn timber approx. 30 mm thick with 80% moisture content dries to 1% moisture content in 2 hours with 1D press drying alone, without the additional Solidiwood functions mentioned below.

Production time from green to joinery-dry

Junckers and Solidiwood® vs. press kiln and chamber kiln

Junckers’ pressing elements are heated with hot oil, and Solidiwood’s alternatively also with electric cables. In both, heat passes from the elements into the wood through firm contact between them and the wood and under strong pressure, so the transfer of heat into the wood is in itself very efficient.

In the press kiln shown in the picture, heat is supplied to the press plates by air, so heat transfer is poor even into the pressing elements. Liquid and electric cable conduct heat 25 times more efficiently than air.

At Junckers and in Solidiwood, the pressing force and the transfer of heat and moisture all act in the same direction. The same applies to any resin and other natural substances of the wood that may be released. In other techniques, moisture has to escape against the direction of heat and moisture transfer, and in the press kiln shown in the picture also against the pressing force.

According to the second law of thermodynamics, heat always flows spontaneously from a warmer body to a colder one, and the greater the temperature difference, the greater the driving force of heat transfer.

The driving force of heat and moisture transfer is large because the temperature difference between the heating element and the outside air is large. With the exception of Junckers and Solidiwood, in other techniques the heat and moisture transferred from the kiln air into the wood have to escape from inside the wood into the same, equally warm kiln air.

The starting point for the development of Solidiwood process technology is the large-scale industrial 1D press drying that Junckers Oy has carried out in Denmark for more than 60 years. It is done with heating plates that press the faces of the sawn timber. In it, sawn timber approx. 30 mm thick with 80 % moisture content dries to 1 % moisture content and is further processed through heat treatment, the relief of internal stresses, and the straightening and densification of its faces and an increase in bending strength. All of this takes place in two hours. The pressing temperature is 160 °C, the pressing pressure 12 bar, and the reduction in thickness, i.e. compression, of the load-bearing parquet blanks 20 %.

Only in Solidiwood

Other techniques have no edge pressing at all, let alone pressing of the end faces and control of the escape of moisture / heat through them. Solidiwood technology also enables material savings in wood products, even absolute dryness, weight reduction, and all-round treatment protection against moisture and other damage.

In addition to delivering its own heat capacity, edge pressing directs the 3 factors described above (pressing force and the transfer of heat and moisture) from every point of the faces and edges to the area that is most difficult in terms of improving drying efficiency: the centre of the blank’s cross-section.

Moisture removal can be accelerated further by means of a groove or grooves made in the edge-pressing elements and/or in both edges of the blank.

In massive products in particular, such as columns and intermediate floor and wall elements, moisture escapes efficiently through longitudinal holes / grooves made possible by their multi-part construction. The same applies to other products glued together from several parts in the process, such as wide covering products.

Pieces of sawn timber with grooved faces can be dried and otherwise processed by pressing two pieces on top of each other, grooved faces together. Another option is for every other face-pressing element to be perforated and channelled for moisture removal, as in the press kiln shown in the picture.

The use of other options that further accelerate moisture removal depends on the wood species and the product. Their size, shape, placement, etc. should be adapted with the cost-effectiveness of the overall situation in mind.

Making grooves, i.e. holes, and other pre-profiling in the blanks is most economical when the timber is sawn.

The solutions above and below provide a versatile range of options for each need. There is plenty to use, change and adjust, not just temperature and pressing pressure. The more of them there are, the more certain it is that the best application can be found for each function. Edge pressing in particular brings options. Among other things, it can be used to limit the release of pressure. Grooving/perforating the wood for its part adds further possibilities.

The pressure built up by edge pressing and the steam generated, and the driving force of heat/moisture transfer – all of which can be optimised product by product – together with the grooving/perforation of the edge-pressing tools and/or the blank, form an extensive range of options that makes Solidiwood pressing and its benefits possible in all situations.

The same range of options can also be used to optimise, case by case, the counter-pressure to the escape of moisture and pressing pressure. It is needed especially when compressing the surface layers.

To remove moisture, the face and/or edge plates can of course also be opened and/or grooved / perforated face plates used.

Copied from the following pages of the current FI patent publication 1287 B (= EU, USA and Canada):

  1. 4: “Longitudinal holes inside wood-based products/product elements, which may be, for example, interior panelling, floor covering, glulam, log, beam or column elements, and/or for getting impregnating agent deep into the blank through their end faces by applying strong negative pressure relative to the outside air. Changes in moisture, pressure, temperature, adhesive and surface-treatment agent within the blank can then be controlled from inside the surfaces of the blank by controlling the amount of release out through the central part of its ends. Pressure and temperature in the blank are approx. 10 times those of the outside air, so moisture removal is truly efficient. Regardless of its cross-sectional area, the green blank is dried to joinery-dry and at the same time further processed into a surface-treated end product in as little as a few hours. Instead of applying negative pressure, the space formed by the longitudinal holes inside the wood blank, e.g. in an intermediate floor element (hollow-core slab), can also be pressurised to remove the moisture passing from the blank into the space, by reducing the difference between the vaporisation pressure and the release out through the central part of the ends of the blank. At the same time, the heat of vaporisation heats the inner surfaces, which then also act as heat-transfer surfaces. If desired, the required overpressure and additional heat can be introduced into the space from outside, e.g. by means of air or steam. The overpressure provides a counter-pressure on the inner-surface side to the pressure compressing the outer surfaces of the blank. This makes it possible to use a higher pressing pressure on the outer surfaces and to achieve the desired profiling and dimensioning of all parts of the product. By means of pressure introduced from outside the space, additional heat can also be delivered to the inner surfaces and, if necessary, they can be protected at the end of the process with a surface-treatment or impregnating agent.”
  2. 6: “According to another aspect of the present invention, it is essential for its use that the moisture, temperature and vaporisation pressure of the blank are monitored, and that the use of binders and treatment chemicals/materials is optimised from inside the blank by means of the release out through the ends of the blank and the pressing pressure on the surfaces of the blank.”
  3. 7: “During the essentially single-stage production process according to the invention, it is possible to monitor and optimise, among other things, the moisture, temperature and vaporisation pressure of the product/intermediate product being made from the blank, and the use of binders and treatment chemicals/materials, from within the blank by means of the amount of release out through the ends of the blank and the pressing pressure on the surfaces of the blank.”
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