Hot pressing principle
When hard board is produced by wet method, adhesive is generally not added, and the bonding between fibers is mainly realized by the bonding effect of water, intercellular substances of wood cells and fillers between fine fibers that make up the cell wall. The filler between intercellular substance and fine fiber is a multi-component polymer compound formed by lignin and hemicellulose. During the hot pressing process, the fibers are in close contact, and the mutual diffusion movement between lignin and hemicellulose macromolecules and macromolecules themselves is gradually strengthened, until the intermolecular attraction and hydrogen bonding make the interface disappear and the bonding process is completed. During hot pressing, water plays a very important role in plasticization. Moisture not only makes the fiber softer and the slab easier to compact, but more importantly, it can reduce the glass transition temperature of polymer materials formed by lignin and hemicellulose, making the mutual diffusion movement between macromolecular segments and macromolecules easier. Then, with the evaporation of water and the gradual cooling of the hot-pressed plate, these macromolecules recombine again, thus completing the intermolecular recombination process. In dry production, because the moisture content of fiber is very low, the polymer formed by lignin and hemicellulose can't give full play to the adhesive effect, so a certain amount of adhesive must be applied to press the board to meet the quality requirements. If the moisture content of fiber is properly increased in dry production, the mechanical strength of the board can be improved or the amount of adhesive can be reduced.
Hot pressing process
Wet method and dry method are different. The hot pressing process of wet production is divided into three stages: extrusion, drying and plasticization. The curve based on the time and pressure of these three stages is called hot pressing process curve. ① Extrusion stage: use the highest pressure to make the fibers contact closely and remove the free water in the slab as much as possible; (2) Drying stage: continue to remove the water in the slab by heating and evaporation, and the pressure should be appropriately reduced to facilitate the escape of water vapor; (3) Plasticizing stage: the bonding between fibers is completed under the highest pressure. The time allocation of the three stages depends on the water content of the slab at the end of each stage. This can be judged by observing the appearance and mechanical strength of the hot-pressed plate, and whether it needs to be adjusted. Usually, the hot pressing temperature is not actively adjusted. Once the temperature fluctuates for some reason, it is also adjusted by adjusting the hot pressing process curve. The common hot pressing process parameters for producing hard plates are: hot pressing temperature is about 200℃; The pressure of extrusion section and plasticizing section is 5 ~ 7 MPa, and the pressure of drying section is about 65438±0 MPa. The hot pressing time is related to the thickness of the plate. When the thickness of the plate is 3 ~ 4 mm, it takes about 6 ~ 8 minutes.
The characteristics of dry hot pressing process are as follows: ① Due to the low moisture content of fiber and the high elasticity of dry board, the maximum pressure used to produce fiberboard with the same density is higher than that of wet method; ② The bonding between fibers mainly depends on the action of thermosetting adhesive; ③ To produce fiberboard with the same density and thickness, the cycle of dry hot pressing is shorter than that of wet hot pressing; (4) During dry hot pressing, the water evaporation of the slab is very small, so it is not necessary to add metal mesh, and double-sided fiberboard can be produced. According to the moisture content and quality requirements of slab, the dry-hot pressing process curve is usually one-time pressing or two-time pressing. One-stage pressurization, that is, the highest pressure is always maintained throughout the hot pressing cycle; Two-stage pressurization is to maintain the highest pressure for a certain time at the beginning, and then reduce the pressure appropriately until the hot pressing is over. The medium density fiberboard produced by urea-formaldehyde resin adopts the following hot pressing technology: the hot pressing temperature is160 ~170℃; The hot pressing pressure is 3.5 MPa in the first stage and 2.0 MPa in the second stage, which is related to the thickness of the plate. It takes about 8 minutes to produce a plate with a thickness of 9 mm
Hot pressing equipment
Both dry and wet methods use multi-layer hot presses and require loaders, unloaders and dunnage return devices. However, compared with wet method, dry hot pressing equipment has some different requirements because of its technological characteristics. For example, dunnage-free loading and unloading can be adopted, and there is no need for a dunnage return device. The thickness of hot pressing plates and the spacing between hot pressing plates need to be large, and the hot pressing plates need to be closed at the same time. The heat source of hot press mostly uses saturated steam, superheated steam or hot water.