In the last issue I discussed panel cooling, which raised a few questions about mat temperature during hot pressing and so in this issue I intend to discuss what happens inside the mat during hot pressing.
I have already touched on this subject in this column before. Those of you who have carefully kept previous issues can read my views on how steam affects the maximum panel width and density profile of hot-pressed products in WBPI 24(1):60 and 24(2):56.
The core layer of a panel is mainly heated by the penetration of steam generated in the surface layers of the mat. Consequently, the maximum temperature of the core is limited to the steam pressure inside the mat up until the point that the mat is completely dry, then conduction and convection mechanisms can raise the core temperature to that of the platens. Of course, this never happens in a commercial press because the press cycles are too short.
Panel core temperature is limited by the same mechanisms as a pan of boiling water on a stove. If the stove is turned up then the water boils more vigorously, but its temperature remains at 100oC until all the water has evaporated, at which point the pan becomes extremely hot. When there is water in the pan, the evaporation of the water removes energy from the pan, so if more energy is put in, the water evaporates faster, thus maintaining 100oC.
A pressure cooker cooks food at a higher temperature because it is pressurised. The same occurs in a mat. Its internal temperature is governed by the internal steam pressure. A higher steam pressure creates higher temperatures, but increases the risk of ‘blowing’ the boards when the press opens.
Which shows the data collected from a particleboard panel as it was pressed in a laboratory-scale hot press, illustrates the phenomena described above.
Four thermocouples were inserted into the mat as it was formed so that the thermocouple tips were in the centre of the plane of the panel and distributed through the thickness in the top half of the mat.
Understandably, the thermocouple on the surface of the mat attained the temperature of the press platens very quickly; the delay of almost one minute was caused by the slow closure of the press. The thermocouple just below the surface (2mm) heats up more slowly, demonstrating the insulating properties of wood. The fact that the temperature 2mm into the mat is quickly 50 to 60oC higher than those at 6 and 10mm indicates that the top layer dries out within the first two minutes.
The inner thermocouples show the characteristics of steam heating: no increase in temperature during the initial stages of pressing, followed by a rapid increase to a plateau of 100-110oC. The plateau is caused by the escape of water vapour from the sides of the mat. After about six minutes, even the centre is dry and so the temperature begins to rise steadily.
The speed at which the core temperature rises is normally the rate-determining step for panel manufacture, because once the core is hot the adhesive can cure rapidly. Computer models are becoming more accurate and providing insights into how best to optimize core heating. One way to achieve it is by direct steam injection, but that is another story.