Acommon step in the manufacture of all wood based panels (WBP), except cement bonded particleboards, is hot pressing. Panels bonded with amide adhesives, eg UF or MUF, must be cooled after they have been taken out of the press.
If the boards were stacked together immediately, the residual heat would cause thermal degradation of the glue. On exiting the press the boards are placed in a star cooler, which holds the panels on their edges, thus exposing both faces to air. The star cooler, or coolers in some factories, are usually large enough to accommodate a sufficient number of boards so that when the boards leave the cooler their surface temperatures will have fallen to about 40oC.
The graph shows the temperature variation of a laboratory-made, 20mm thick particleboard with 450mm sides. It is clear that the internal temperatures continue to rise after the panel leaves the press and that when the surface temperature is around 40oC, the internal temperature is much higher. Commercial panels are much larger than laboratory scale panels and so the temperature differences are expected to be larger in commercial panels even though it is the thickness which is the most important variable here. Consequently, once the panels are stacked, the surface temperatures rise once again. Depending on the stack size and season, the temperature in the centre of a stack can be more than 55oC, even after 3 days of storage.
Phenolic bonded WBP, on the other hand, can be hot stacked. In fact, hot stacking is recommended for PF bonded boards as their properties are usually seen to improve by this process. It is said that the improvement is due to increased cure of the adhesive, which seems very plausible. However, some of the improvement may also be due to continued internal stress relaxation permitted by the high temperatures in the stack.
Panel cooling is also necessary from the point of view of subsequent processing steps, especially sanding, but also in order to minimise further reductions in panel moisture content, as this must be recovered before the panel is shipped.
