Conventional wood based products are like pancakes: they are flat. They are used in the construction industry, for booth construction, for shop fittings and particularly for furniture production.

However, applications are restricted. Wherever shaped or curved surfaces are required wood based products have some drawbacks: the panels available are flat. With the usually-used thermosetting adhesive, it is not possible to deform a panel after it has been delivered from the manufacturer.

Three-dimensionally shaped elements are, however, commonly made. The material is moulded plywood. During production an adhesive is applied to several veneer sheets, which are stacked together and pressed in a curved moulding press to the shape desired. Since the deformation from flat takes place before curing, a thermosetting adhesive can be used. The procedure is far from ideal. It consists of numerous work steps, most of which are carried out manually.

The resulting long cycle times and the comparatively high material costs are major disadvantages of this type of mould production.

To overcome these disadvantages, a new concept has been developed. It uses fibreboard rather than plywood. The fibreboards are produced on plants of the wood based panel industry as semifinished products (pre-pregs). They are not completely cured when they leave the factory; they are thus post-mouldable by the customer, who processes the semi-finished boards by pressing them in moulds to the 3-D shape required.

Because of lower material costs (of fibres versus veneer) and the reduction of timeconsuming work steps, the new process should lead to lower costs.

In a research project the production of pre-pregs and the subsequent post forming to moulds were investigated. In addition, it was examined whether post mouldable fibreboards can be covered with veneers or films and post-formed in one production step. The objective was to substitute several inner veneer layers with the post-mouldable fibreboard. The project was a cooperation between the Institut für Holztechnologie Dresden (IHD), Swiss Krono Group, and BASF SE.


The aim of this study was to develop a suitable adhesive system for manufacturing pre-pregs, such that they were capable of being subsequently moulded. It was intended to have them produced from thermo-mechanical wood pulp, but, in contrast to the state-of-the art in other industries, without artificial fibres (such as are used in the automotive sector) or other plastics. For this reason, thermosetting curing adhesive systems have been used. However, they had to be modified in order to keep the post-formability of the fibreboards. Furthermore, the existing parameters, both of the industrial production of fibreboards on continuous lines, and of the post-forming process, should be altered as little as possible.

Two-stage curing systems

The aim of the adhesive modification was its applicability to a two-stage curing process. The first stage is to provide sufficient adhesion between the fibres to allow cutting and transport of the pre-pregs. The second stage should be activated during post-forming at the mould manufacturer. Here, the high final strength of the moulded part is intended to be obtained.

The period between manufacture of the semi-finished fibreboard and final production in the mould may be several days or weeks. The second stage of the adhesive system must be still able to be activated after this time.

The two-stage curability of the adhesive is realised by formulation of a thermoplastic adhesive in combination with a thermo-setting one. Melamine-urea formaldehyde (MUF) adhesive proved to be suitable as the thermo-setting component. In contrast, urea-formaldehyde (UF) adhesives are too reactive for this application.

Production of Post-Mouldable Fibreboards Glued In A Blender

Initially, MDF fibres were glued in a blender with different adhesives and adhesive contents. The manually-formed fibre mats were pressed in a program controlled laboratory press to pre-pregs at a press temperature of below 140°C.

The pre-pregs (570 x 470 mm, with bulk density less than 400kg/m³ ) were cut into three parts. One part was used to investigate physical and mechanical properties of the pre-pregs. The middle part was used for the production of the mould. The third part was pressed into a flat board.

The properties of the finished board were evaluated on these pressed flat boards. In order to transfer the data determined from the flat pressed boards to the moulds, exactly the same press parameters were used for both. The parameters were selected in such a way that the industrial process in the mould manufacturers was imitated as closely as possible. This means that the mouldings and the flattened plates were produced in the laboratory at comparatively low pressing temperatures (120-160°C) and longer pressing times (approx 20s/mm). With this approach, the properties of both the semi-finished product and the repressed MDF could be generated from each plate and the formability could be assessed.

To evaluate the cross linking during post-forming of the thermo-setting adhesive, the thickness swell after 24 hours water storage at 20°C (DIN EN 317) was determined (Fig 1). As expected, the pre-pregs showed no moisture resistance and decayed during storage in water. This means that the thermo-setting component has not yet, or only slightly, reacted.

However, the post-moulded boards showed lower thickness swelling values. With increasing pressing temperature of the pre-pregs, the thickness swell of the post moulding increases. These findings can be explained by an undesired partial curing of the thermo-setting adhesive (the stagetwo part of the adhesive) at high pressing temperature prematurely in stage one of the pre-preg process.

The internal bond values lead to the same conclusion: The post-formed boards showed lower values at pressing temperatures of the pre-pregs of 120 and 140°C than at 95°C. As expected, the internal bond of the pre-pregs was very low.

Production of Post-Mouldable Fibreboards Glued In A Blowline

Since blowline gluing is the dominant gluing technology in the MDF industry, an important part of the trials was carried out using this technology. Thermo-plastic and thermo-setting adhesive components were injected separately from each other over cooled nozzles into the blowline, which is part of the laboratory refiner system of the IHD.

The adhesive system TP1/MUF1, which showed promising results in the trials with blender gluing, did not work when applied via the blowline. The adhesive probably pre-cures in the dryer, or even while still in the blowline.

More promising was the use of thermoplast type TP3. A combination with MUF and UF systems was tested. Pre-pregs with properties of sufficient strength could be produced. As described above, the prepregs were divided into three parts for the further investigations.

Depending on the pressing temperature, the post-formed fibreboards revealed sufficient values for internal bond. The variants with the melamine-enhanced adhesive system showed a thickness swelling of 25%. In comparison, the UF variant revealed higher thickness swelling and a lower strength, because of precuring during the production of the prepreg.

Industrial Upscaling

After the laboratory tests at the IHD, pre-pregs fabricated there were postformed into moulded parts (the backrest of a chair) in the factory of an industrial mould manufacturer. Both coated and uncoated moulds were produced. Beech veneers or melamine films were used as coating materials. They were applied together with an adhesive film on the top and bottom of the pre-preg. Most variants were easily post-formed in one working step and exhibited sufficient strength.

The devolatilisation behaviour of the fibre moulds differed from that of plywood and required an adjustment of the pressing programme – an additional ventilation cycle. Pre-pregs with a bulk density below 350kg/m³ were particularly suitable.


The project developed a two-stage curing adhesive system for the production of post-formable fibreboards. The new adhesive system enables the production of pre-pregs which can be stored and transported and later post-formed into moulds. The production of semi-finished products on a laboratory scale is based on the industrial process for the production of MDF.

The commonly-used industrial blowline gluing process is suitable.

First trials on an industrial post-forming process demonstrated the suitability of the developed pre-pregs for uncoated and coated moulds. The result therefore has been the development of a cost-effective alternative to the production of moulded plywood.


The project MF110158 was funded by the Euronorm GmbH within the framework of the INNOKOM-Ost programme by the Federal Ministry of Economics and Labour on the basis of a decision of the German Bundestag.