Medium density fibreboard is a product that nobody fully understands and the defibration process is an example of the many mysteries that surround MDF.   The panel can be made from a wide range of lingo-cellulosic fibres, most commonly wood, which must be broken down into fibres. This is normally achieved using a thermo-mechanical pulping (TMP) process which combines heat and mechanical energy to break the wood cell bonds.   These cells are joined by the middle lamella, rich in lignin and an amorphous polymer that can adsorb small quantities of water, and so its softening temperature is moisture content-dependent.   The high temperatures and humidities in the TMP process cause significant reductions in the strength of the lignin in the middle lamella region, increasing the likelihood of fracture there when mechanical energy is applied.   Heat is applied at three stages: Pre-heating, an optional stage where the particles are heated to 40-60^oC at atmospheric pressure; in the digester where they are heated to 175-180oC, corresponding to a steam pressure of around 8.5 bar for three to five minutes; and finally during the refining step by the defibrator.   Most refiners have two discs, one stationary and another which is rotated at about 1500rpm by a powerful motor. The heated chips are fed from the digester into the refiner through a hole in the middle of the stationary disc.   Centrifugal force of the second disc forces the particles out to the periphery. As the particles move to the edge they are broken down to 'matchstick' pieces, fibre bundles and finally fibres. The refining is by the raised bars on the disc surfaces; at the centre the breaker bar pattern is coarse and at the periphery the bars are much finer. These raised bars cause the particles to be compression-stress cycled, generating more heat in them in much the same way as a piece of wire that is bent back and forth, further helping the particles to separate at the middle lamella, releasing individual fibres.   As the wood is driven across the radius and broken down, the gap between the discs must be reduced to a very small size, so the discs must be accurately   positioned to avoid them meeting. This can happen if the feeding of particles is interrupted thus reducing the pressure between the discs.   As our understanding of the intricacies of MDF improve so does the potential to develop this product further so that it can be used in a wider range of end-uses and products.