Explain in detail the pressure reduction process of vacuum hot press
At the end of the hot pressing process of the vacuum hot press, there is still a lot of steam and superheated water in the high temperature area in the middle of the slab. In order to improve the production capacity of the hot press and ensure that no quality accident occurs during the reduction process, the reduction process should be divided into three stages according to the dynamic state of water evaporation.
The first stage of pressure reduction is from working pressure to "balance pressure". At this stage, the pressure of the hot press on the slab is higher than the steam pressure inside the slab, so the moisture inside the slab is basically not evaporated, so rapid depressurization can be adopted. The "balance pressure" is the pressure of the hot pressed board that is basically in balance with the internal steam pressure of the slab. It is 3-4 kg/cm2 in the hot pressed plywood.
The second stage of pressure reduction is from "balance pressure" to zero. At this stage, a large amount of superheated water inside the slab vaporizes, which has a large expansion force. If the pressure drops too fast, there will be a sharp pressure difference between the inside and outside of the slab, making a large amount of water vapor escape from the interior of the slab to the surrounding at a high speed (washing out the adhesive layer), causing local degumming (bubbling). Therefore, the pressure should be reduced slowly at this stage, and the speed of pressure reduction is required to basically adapt to the speed of steam escaping from the plywood. When mastering the details, factors such as the number of plywood layers, hot pressing temperature, moisture content of veneer, and glue amount should be considered. In general, when the number of plywood layers is large, the temperature of pressing plate is high, the moisture content of veneer is high, and the glue content is large, the reduction speed should be slow; On the contrary, it should be faster. Reduce the required time, generally 30-50 seconds for urea formaldehyde resin adhesive plywood and 50-80 seconds for multilayer board. There is more water in the protein bonded slab, so the reduction time should be extended accordingly.
The third stage of reduction is from the pressure on the slab is zero to the full opening of the hot pressing plate, which relieves the final constraint of the hot pressing plate on the plywood. This stage can be carried out at a faster speed. However, for multi-layer hot press, due to the dead weight of the platen, the lower layers of slabs bear greater pressure, so the opening speed of these layers should be appropriately slowed down. In hot pressing, most of the water in plywood is discharged at this stage, which can generally reduce the water content by 6-8%.