Pneumatic Pressing Technology

Automatic Google-Translation from the German Original:

The advantages of gluing conventional beam structures on both sides with wood-based panels to form so-called box elements or hollow boxes are diverse and diversified. Statically, such products have an improved vibration stiffness and enable larger spans of the ceilings. In addition, the cross-section of the floor slab can be reduced by around a third in height. Functionally, the cavities between the beams offer space for insulation, soundproofing, electrical installations and more. For this purpose, the lower three-layer board is often glued in visual quality, which is also aesthetic. Above all because the new production technology works without screws.

Glued elements for the Lenzerheide mountain railways

However, this meant that nothing could be reworked, which is why the elements were completely prefabricated. They were completely insulated and provided with electrical installations, whereby heavy insulation was used in the ceilings, which was supposed to have a soundproofing.

The six-storey building (1 + 5) with its 55 rooms falls under the accommodation category in terms of fire protection and required a special risk analysis and special permit according to old Swiss law, which was in force until the end of 2014. With a holistic safety concept, which included a sprinkler system that was also integrated into the prefabricated box elements, it was possible to convince the responsible authorities that wood construction is in no way inferior to solid construction in terms of fire protection. The advantages of the box girder system were also noticeable statically. The adhesive connection increases the vibration rigidity, whereby free spans of up to 6 m were possible even without a suspended ceiling. In addition, thanks to the improved static properties, the cross-section of the floor slabs was reduced. Compared to a design with conventional beam structures as floor slabs, the total height of the building could be reduced by around 50 cm with an ideal interior height of 2.50 m.

Innovative production technology for innovative products

Up until now, box elements were mostly produced by screw pressing. The pressure on the glue joint that is required during the hardening of the glue is achieved by special screws attached at precisely prescribed intervals. Due to the type of screws and the tight spacing, this takes a lot of time and is very expensive in terms of material. In addition, this method is physically questionable. The pressing pressure cannot be measured and possible restoring forces from the wood can even cancel it out, since screws do not re-press. In addition, the element must be turned to the second side for screwing, which can easily damage the adhesive joint.

A new pressing technique should now compensate for these disadvantages. Encouraged by customers, the Swiss machine manufacturer Woodtec Fankhauser GmbH worked out a system that is finely tuned to the needs of timber construction companies. “The company has had almost 15 years of experience with box elements that are produced by our customers in our vacuum press,” explains chief engineer and managing director Thomas Fankhauser. However, these presses are designed for panel and cross laminated timber production and are not flexible enough to be operated economically by many commercial enterprises.

The new pressing device is integrated on the existing element construction table. This means that the press system does not take up any additional space in the hall and is only used when effective pressing is required. Otherwise, conventional wooden frame elements are produced on the element construction table. Based on customer experience, Fankhauser estimates that around 30 percent of the time is spent on box elements and the rest on normal walls and non-glued ceilings.

Guaranteed bonding for load-bearing components

The pressing method is based on the principle of pneumatic hose pressing. The box element is glued and assembled on the element construction table. Press brackets are then placed at intervals of 50 cm and anchored on both sides of the box in the perforated grid of the modular element table. On the underside of the press bracket there are hoses that pneumatically press onto the box element.

This has the major advantage that the pressure that is to be applied to the element can be precisely defined and automatically re-pressed. This means that the pressing pressure is kept constant over the entire duration of the pressing. In addition to other important production parameters such as wood moisture, hall temperature or planing quality, this is the decisive point to guarantee correct bonding to the engineer and the client.

3500 m2 elements in six weeks

For the staff building on Lenzerheide, 480 wall and ceiling elements were produced within six weeks. More than half of these were made in visual quality, whereby the advantages of the pneumatic press device could be fully exploited. Since screws are no longer used in the new method of pressing, the surfaces of the box elements are of excellent quality and no additional panels have to be glued on for interior decoration.

A PUR adhesive HBS 159 from Purbond with an open time of 15 minutes and a pressing time of 38 minutes was used in production. At Künzli Holz AG, the department head responsible for wood construction, Franz Hauzenberger, explains: “Instead of gluing the two wood-based panels of the box element in one operation, as is normally the case, we did this in two stages for this project due to the complex inner workings of the elements “First the first board was glued and pressed to the beams, then insulation, electrical installation and the sprinkler system were installed and in a second step the element was glued to a second board to form the finished product. Exterior walls were only glued on one side. A gypsum fiber board was used on the outside for fire protection reasons.

On average, twelve men worked alternately on different workstations. While one element was being pressed, others were in preparation or post-processing.

This intensive prefabrication and the dry construction, which can be realized thanks to the box elements, also enabled very rapid construction progress. The existing building was demolished in April and the new building was occupied in November.

Box elements also for smaller buildings

The advantages of box elements as false ceilings can be fully exploited, especially in multi-storey buildings. The entire construction from the foundation can be prefabricated on the element construction table including the pressing device, so that the added value remains in operation. Since Künzli Holzbau has been equipped with the pneumatic press device, no alternative system (CLT, stacked board ceilings, wood-concrete composite etc.) has been used for ceilings. “As soon as we offer box elements as an alternative when entering a project, architects and engineers get involved immediately,” explains Hauzenberger.

Box elements are not only used for multi-storey buildings, but also for smaller buildings such as single-family houses. Künzli Holz AG builds around 40 residential units per year, including only four to five larger multi-storey buildings. In the case of box elements, you usually work with two teams on a 34 m long element construction table, which is divided into different long work stations depending on the length of the elements. Each team has two workstations available, which work alternately. While one element is being pressed, a new one is already being prepared. That increases the efficiency enormously. In addition, according to Hauzenberger, the ambitious teams are often in healthy competition with each other as to who can best exploit the alternation process and produce the most elements. Hauzenberger estimates that the employees can produce 12 to 14 elements in this way on a normal working day, up to 1000 m2 per week.

Production in Europe

Box elements can be manufactured in the EU according to Eurocode 5 and are subject to the panel construction standard there. This means that these elements are one length ahead of cross-laminated timber, for which there is still no uniform standard.

In Germany, the Zimmererei Sieveke GmbH (Lohne) became the first manufacturer to produce box elements with the Woodtec press system since the end of 2014. Melamine adhesives are still being used at the moment, but according to the manufacturers, PUR adhesives will soon also receive approval for load-bearing components. According to Thomas Fankhauser, the fact that the system has spread so quickly in Switzerland (meanwhile 12 producers) can be attributed to the fact that box elements can be manufactured in Switzerland without monitoring or a standard concept. “A precisely regulated standard can be a hurdle, especially psychologically,” says Fankhauser. The actual stages of the glue construction permit, external and in-house monitoring are associated with very modest effort in practice, and Woodtec Fankhauser GmbH is happy to provide customers and interested parties with advice and action.