Cutting Processing
Both raw material processing and final product processing require the application of cutting techniques, such as rotary cutting and planing of veneers, cutting of wood chips and shavings, grinding and separation of fibers, as well as sawing and sanding during final processing. Cutting wood into units of different shapes and recombining them into various boards in a certain way can improve certain properties of wood, such as anisotropy, heterogeneity, swelling and shrinkage. Plates composed of large units have higher mechanical strength, while those composed of small units have better homogeneity. Accurately controlling the thickness error of rotary cutting veneer can increase the yield by 2-3%. The shape of the cut shavings affects the overall physical and mechanical properties of the particleboard; The fiber morphology is also closely related to the strength of fiberboard. The final sawing, grinding, and other processes of the board also affect the specifications and quality of the product.
dry
Including veneer drying, particle drying, fiber drying in dry fiberboard process, and heat treatment of wet fiberboard. The drying process and process control are different from that of finished wood drying. The process control of wood drying is based on the relative humidity of the drying medium, and attention must be paid to preventing the generation of drying stress; The drying of sheet and granular materials used in artificial boards is carried out under relatively high temperature, high speed, and continuous conditions, and immediately enters the deceleration drying stage at the end of the heating stage. Materials such as veneers and shavings are thin, have a large surface area, and the influence of drying stress is minimal or non-existent. In addition, during the cutting process, the wood tissue undergoes varying degrees of relaxation, resulting in low resistance to water diffusion. As a result, the internal water diffusion law of the wood loses its significance for veneers, shavings, and other materials.
Dry heat sources mostly use steam or combustion gases. Infrared drying consumes too much energy, requiring 5500-18000 kilojoules per kilogram of water evaporated; And steam drying only requires 4200 to 5000 kilojoules. The advantage of high-frequency drying is that when the moisture content of the dried material is high, the drying speed is fast and the final moisture content is uniform, but the drying cost is too high. If combined with steam to achieve compound heating, it is advantageous. Vacuum drying is not only expensive, but also has low production efficiency. When using steam as the heat source, for every 1 kilogram of water evaporated, 1.75-2 kilograms of steam are required for drying veneers, about 1.8 kilograms of steam are required for drying shavings, and 1.6-1.8 kilograms of steam are required for drying soft fiber slabs.
Glue application
Including veneer coating, shavings, and fiber bonding. Single board coating still uses traditional drum coating in Europe, while many plywood factories in the United States have switched to spray coating since the 1970s. Chinese plywood factories also use roller coating for adhesive. The glue pouring method is suitable for the process of whole sheet and automated billet assembly. Particle and fiber glue are mainly applied by spray coating method.
In the late 1970s, some European and American countries studied non adhesive bonding technology, and the most advanced one was to activate lignin molecules and use lignin bonding under certain conditions; Alternatively, the hemicellulose in wood or other materials can be processed to convert it into a binder for bonding. In the early 1980s, Canada successfully used steam residue to produce adhesive free particleboard. Chinese research institutes and universities are also conducting research on adhesive free bonding technology and have achieved preliminary results.
Forming and pressurization
The assembly of plywood, the forming and pressing of particleboard fiberboard slabs all belong to the forming process of artificial board manufacturing. The study of wood structure in woodworking reveals that the arrangement of wood fibers in natural wood is hierarchical and directional, thus capable of withstanding certain external forces exerted by nature on wood. The evolution of artificial board manufacturing technology is undoubtedly influenced by this understanding: the layers of particleboard and fiberboard slabs have changed from single-layer to 3-layer and multi-layer structures; The arrangement of shavings and fibers in the slab has also shifted from random to directional; The perpendicular arrangement of fibers in adjacent layers of plywood improves the anisotropy of wood under natural growth conditions and enhances dimensional stability.
Pressure is divided into pre pressure and hot pressure. When using a non pad system, it is necessary to pre press the slab. It prevents the slab from being damaged during the pushing of the hot press. The hot pressing process is a key process that determines the production capacity and output of enterprises. The commonly used hot pressing equipment in the artificial board industry is mainly multi-layer hot press machines. In addition, single-layer large format hot press machines and continuous hot press machines are gradually being adopted. Particle board factories often use single-layer hot press machines. In the manufacturing of medium density fiberboard, single-layer press machines can achieve dual heating using high-frequency and steam, which is beneficial for shortening the compression cycle and improving the uniformity of product cross-sectional density.
Final processing
After being unloaded from the hot press, the board undergoes a cooling and moisture balance stage, which includes sawing and sanding. The hard fiberboard needs to undergo heat treatment and humidity control treatment. In the past, the cutting of board edges was done using both cold and hot sawing methods, but the hot sanding method must not be used as it can damage the surface quality of the finished product. According to usage requirements, some boards require special treatments such as impregnation, painting, coating, and edge sealing.