The raw materials for making particleboard include wood or wood fiber materials, adhesives and additives. The former accounts for more than 90% of the dry weight of the board. The wood raw materials are mostly from felled wood in the forest area, small diameter wood (usually less than 8 cm in diameter), cutting residues and wood processing residues. Wood chips, shavings, wood filaments, sawdust, etc. processed into flakes, strips, needles, and granules are called chips. In addition, non wood materials such as plant stalks and seed husks can also be made into boards, which are often named after the materials used, such as hemp straw (particle board), bagasse (particle board), etc. The adhesives are mostly urea formaldehyde resin adhesive and phenolic resin adhesive. The former has light grease color and low curing temperature, and has good bonding effect on various plant raw materials such as wheat straw, rice husk, etc. The hot pressing temperature is 195-210 ℃. It is widely used in production, but has the disadvantage of releasing free formaldehyde to pollute the environment. The particle shape is the decisive factor for the quality of particle board, so it is necessary to produce qualified particles first. The shavings from the residues in wood processing can be used as the core layer of particle board after reprocessing. The surface shavings are mainly produced by special processing of high-level residues (wood stumps, board edges, etc.) from logging or processing. The length, thickness and width of the shavings vary depending on the production method and the use for making the core layer or surface layer. The processing equipment for preparing shavings includes chipper, re crusher, grinder and fiber separator, and the cutting methods include cutting, cutting and crushing. In order to obtain high-quality shavings, the process of primary crushing, grinding, re crushing and screening is required. The initial moisture content of the processed shavings is about 40-60%, the moisture content of the core layer meeting the process requirements is 2-4%, and that of the surface layer is 5-9%. Therefore, it is necessary to use a dryer to dry the shavings with different initial moisture content to achieve a uniform final moisture content. The dried shavings are then mixed with liquid glue and additives. Generally, 8-12g of glue is applied on the surface area of each square meter of shavings. The glue is ejected from the nozzle into particles with a diameter of 8~35 microns, forming an extremely thin and uniform continuous glue layer on the surface of the shavings. Then the glued shavings are paved into slabs, whose thickness is generally 10-20 times of the finished product thickness. Pre pressing and hot pressing can be carried out. The preloading pressure is 0.2~2 MPa, which is carried out with a flat press or a roller press.
Hot pressing is a key process in the production of particleboard, which plays a role in curing the glue in the slab and consolidating the loose slab into a plate of specified thickness after being pressurized. The process requirements are: ① Appropriate moisture content. When the surface moisture content is 18~20%, it is beneficial to improve the bending strength, tensile strength and surface finish, and reduce the possibility of bubbling and delamination of slabs during pressure relief. The moisture content of core layer shall be lower than that of surface layer to maintain proper plane tensile strength. ② Appropriate hot pressing pressure. Pressure can affect the contact area between the shavings, the thickness deviation of the board and the rubber transfer degree between the shavings. According to different density requirements of products, the hot pressing pressure is generally 1.2~1.4 MPa. ③ Appropriate temperature. Too high temperature will not only cause the decomposition of urea formaldehyde resin, but also lead to partial premature solidification of slab during heating, resulting in waste products. ④ Appropriate pressurization time. If the time is too short, the middle resin cannot be fully cured, the elastic recovery of the finished product in the thickness direction increases, and the plane tensile strength decreases significantly. The hot pressed particleboard shall undergo a period of moisture conditioning treatment to make its moisture content reach a balanced state, and then sawing, sanding and packaging inspection. However, hot stacking is not allowed after pressure relief, otherwise, plate brittleness will increase. Molding technology refers to the technology of forming products in one operation. There are three mature processes. The hot mold method can use less or no glue. It uses lignin to activate the flow in the closed hot mold to play the role of gluing, but needs cooling and demoulding. It has large heat consumption and low productivity, and has been gradually eliminated. The box forming method is to use a special press to pressurize and press into products at one time, which is used to manufacture packing boxes. The hot pressing method is mainly used to manufacture furniture fittings, interior decoration fittings, pallets and other products. The adhesive is mainly urea formaldehyde resin, and the surface of the products is overlaid with veneer or resin impregnated paper for one-time molding. In addition, there is also the plane molding method of pressing with a template on the surface of the made particleboard or the formed slab without hot pressing to make relief patterns.