Fine-tuning the Solidiwood production process

The necessary pre-profiling is carried out to match as closely as possible the optimal drying and compression requirements and the product profile after pressing. The need for compression of the wood material, i.e. for forming work, then remains very small.

The pressing forces required on opposite surfaces should ideally be as nearly equal as possible. It follows that either the opposing shapes must be very similar, or their pressing forces must be made at least approximately equal, for example through pre-profiling of the blank’s shapes, or by means of temperature or moisture.

One possibility is to replace the traditional tongue-and-groove joint with thin female grooves pressed into both side faces and a plastic tongue strip.

Isostatic pressing with Solidiwood’s elastic pressing tools makes possible products, including their surfaces, that follow the topography of the wood and are otherwise of high quality in many respects. These can be pressed with, for example, a liquid-filled bladder or an elastic plastic sheet.

In Solidiwood products, surface densification is achieved, in addition to the isostatic pressing mentioned above, primarily by means of coating agents rather than by compression. Further assistance comes from the rapid heating that takes place from the pressing tools and is transferred by liquids. Here too, product-specific optimisation of the various factors is the basis for achieving the best overall result.

Even for products that are left without actual surface treatment, the formation of the profile and of dense, intact, more homogeneous, smooth, topographic or textured surfaces is improved by using, for example, a primer, an impregnating agent or some other substance, or at least water.

In Solidiwood hot pressing, it is mainly only the surface layers of the product that are densified. The product blank heats up, and therefore compresses, rapidly at its surfaces. The cooler wood material inside and the back-pressure of the water vapour forced out during pressing slow down the compression of the interior.

At its smallest, the need for pressing covers the elimination of the very small profile and dimensional inaccuracies of sawn timber produced with modern sawing technology. A substantially greater need arises only in the manufacture of load-bearing floorboards. Even there, the reduction in the thickness of the sawn timber is limited to 15 %.

To achieve a good overall result, it is important that the blank is brought to a uniform temperature of over 150 °C right through the wood, and to a dryness of 3-4 %, at least equivalent to that of conventional heat treatment. The necessary dwell time in this state is also essential. Only in this way can good heat treatment, relief of the stresses in the wood and lasting product properties be achieved.

In the final stage of the pressing process, the blank is first pressed very close to the final dimensions of the product, until the drying or moistening of the blank to the desired moisture content, the equalisation of moisture and the warping of the blank have ended.

The Solidiwood process must end with cooling in the pressed state. During this phase, the wood’s own binding agents, which are in a melt-like state, together with the adhesives and surface treatment agents added to the blank, solidify and lock the product in its pressed state. At this point the pressing tools are closed against their stops and the product is in its finished state. The product is brought to its exact dimensions as the metal tools bear against the metal stops.

Unless a product has been manufactured in this way, there is also no overall result to test, such as the irreversibility of the compression. Individual functions can be tested by using the process only insofar as the result can be verified.

In some cases, it is justified to manufacture products intended for the most humid applications also at a temperature of < 150 ºC, drying them only to a moisture content slightly below that of their place of use. Their product properties and the permanence of those properties remain the weakest, but are nevertheless satisfactory for their purpose. For these products, compression to a higher degree is not justified.

What matters most is that Solidiwood process technology enables product-specific optimisation of all factors to achieve the best overall result.