During gluing, a precisely dosed amount of adhesive is added to the flakes, fibres or strands and evenly distributed. To determine the required amount of adhesive, the weight of the wood material is first measured with a belt weigher. To achieve specific board properties, various substances can be added to the adhesive, e.g. to improve fire protection or moisture resistance.
The flakes must be glued separately according to middle layer and top layer. According to the given recipe the glue is sprayed into a mixer. An even wetting of the particle surface takes place by friction in the compulsory mixer (ploughshare mixer).
Gluing takes place immediately after the fibers have left the refiner system. Due to the previous boiling, the fibres have a high moisture content and must therefore be dried. In combination with gluing there are therefore two fundamentally different possibilities:
- The BlowLine
In the so-called BlowLine the glue is added to the moist fibres. The fibres and adhesive are then dried. Wood fibres for MDF production require a drying process with short holding times in the dryer tube. Fibre dryers therefore work according to the principle of flow tube drying. The fibres glued in the blow line are injected centrally into the flow tube and after a few seconds already discharged again in the cyclone.
- Gluing in the mixer
The fibres are first dried in the flow tube dryer and then glued in the mixer.
After sorting and grading, the strands are temporarily stored in the dry particle silos. They are then conveyed to the gluing machine via the belt weighers for middle layer and top layer. These ensure that the right amount of strands is available for the required production output. At the same time they ensure that the gluing ratio (i.e. the ratio of strands to the quantity of glue and chemicals used) is maintained. The delivery rate of the dosing pumps automatically adjusts to the weighed strand quantity. The strands are circulated in large-volume drums with internal lamellas, so-called blender drums. Atomizers (also: rotary sprayers, atomizers) spray glue, paraffin, hardener and water finely distributed. The quantity of glue and chemicals used varies depending on the board quality, board thickness and type of gluing.