Fiberboard is a board made by crushing, soaking and grinding waste such as bark, shavings and branches into wood pulp, then pressing and drying. Due to the different temperature and pressure during molding, it can be divided into three types: hard, semi-hard and soft.
3. Particleboard is a kind of board made of wood shavings or wood fibers, with or without glue. Particleboard has low density and uniform material, but it is easy to absorb moisture and has low strength.
4. The joinery board is an artificial board which is made from the corner waste produced in the wood processing process, after shaping, shaving and sizing, splicing and veneering. Generally, the core of the board is made of fully dried short wood strips, and the board surface is made of single-layer veneer or plywood. Blockboard is not only an effective measure for comprehensive utilization of wood, but also the obtained board has uniform structure, stable size, large width and thickness. Besides surface decoration, it can also be used as a structural material.
5. Other interpretation versions and emerging technologies about wood-based panel manufacturing technology:
Except plywood, most of the raw materials used in wood-based panels come from logging and processing residues, as well as small-diameter timber (below 8 cm in diameter). Sheet, strip, needle and granular materials made by crushing or chipping and then crushing can be used to make particleboard. Wood chips are used for fiberboard manufacturing after fiber separation. This can improve the utilization rate of wood by 20 ~ 25% compared with the traditional utilization method. In the 1970s, people began to pay attention to the use of bark and sawdust as raw materials for wood-based panels, but bark can only be used in the middle layer of particleboard, and the dosage should not exceed 8%, otherwise the product strength will be reduced. In addition, besides bagasse and hemp stalk, non-wood materials have been paid more and more attention. , has long been used in the production of wood-based panels, and extended to the stems and seed coats of many plants.
The manufacturing process of various wood-based panels includes the following five main processes.
Cutting technology should be applied to the treatment of raw materials and the final processing of products, such as the rotary cutting and planing of veneer, the cutting of sawdust and shavings, the grinding and separation of fibers, and the sawing and sanding in the final processing. Cutting wood into units with different shapes and recombining them into various boards in a certain way can improve some properties of wood, such as anisotropy, heterogeneity, wet swelling and dry shrinkage. The mechanical strength of the plate composed of large units is higher, and the uniformity of the plate composed of small units is better. Accurately controlling the thickness error of rotary cutting veneer can improve the yield by 2 ~ 3%. The cutting shape of wood shavings affects all physical and mechanical properties of particleboard. Fiber morphology is also closely related to the strength of fiberboard. The final sawing and grinding of plates will also affect the specifications and quality of products.
Drying includes veneer drying, wood shavings drying, fiber drying in dry fiberboard process, and heat treatment of wet fiberboard. Drying technology and process control are different from finished product drying. The process control of finished product drying is based on the relative humidity of drying medium, and attention must be paid to prevent drying stress; The drying of flaky and granular materials for wood-based panels is carried out under the conditions of relatively high temperature, high speed and continuity, and the heating stage immediately shifts to the deceleration drying stage. Veneer and shavings are thin and have large surface area, and the influence of drying stress is little or non-existent. In addition, in the process of cutting, the wood structure is relaxed to varying degrees, and the resistance of water diffusion is small, so the law of water diffusion in wood is meaningless to veneer and shavings.
The dry heat source is mainly steam or combustion gas. The energy consumption of infrared drying is too high, and it needs 5500 ~ 1 8000 kilojoules per kilogram of water evaporated. And steam drying only needs 4200 ~ 5000 kilojoules. High-frequency drying has the advantages of fast drying speed and uniform final moisture content when the moisture content of the dried material is high, but the drying cost is too high. It is advantageous if it is used in combination with steam to realize compound heating. Vacuum drying is not only expensive but also inefficient. When steam is used as the heat source, for every evaporation of 1 kg of water, veneer drying 1.75 ~ 2 kg of steam, wood shavings drying 1.8 kg of steam and flexible fiberboard blank drying 1.6 ~ 1.8 kg of steam are required.
Sizing includes veneer gluing, wood shavings and fiber sizing. Veneer gluing still adopts traditional roller gluing in Europe, and many plywood factories in the United States have switched to glue spraying since the 1970s. China Plywood Factory also uses rollers to glue. This glue spraying method is suitable for the whole medium plate and automatic blank assembly process. Wood shavings and fiber sizing mainly use glue spraying.
In the late 1970s, some countries in Europe and America studied the non-adhesive bonding technology. The progress is to activate lignin molecules and use lignin to bond under certain conditions. Or using hemicellulose in wood or other materials, after treatment, it is converted into cementing material for gluing. In the early 1980s, Canada successfully manufactured adhesive-free particleboard from steam residue. Domestic research institutes and universities are also conducting research on non-adhesive bonding technology, and have achieved initial results.
Molding and pressing plywood assembly and particleboard fiberboard blank molding and pressing belong to the molding process of wood-based panel manufacturing. The study of wood structure in wood science reveals that the arrangement of wood fibers in natural wood is hierarchical and directional, so it can withstand a certain limit of external forces exerted by nature on wood. The evolution of wood-based panel manufacturing technology is undoubtedly influenced by this understanding: the level of particleboard and fiberboard blank has changed from single layer to three-layer and multi-layer structure; The arrangement of shavings and fibers in the slab also tends to be from random to directional; However, the fiber directions of adjacent layers of plywood are perpendicular to each other, which improves the anisotropy of wood under natural growth conditions and improves the dimensional stability.
Pressurization is divided into preloading and hot pressing. When the dunnage-free system is used, the plate must be pre-pressed. It can prevent the slab from being damaged when it is pushed into the hot press. Hot pressing process is the key process to determine the production capacity and output of enterprises. The commonly used hot pressing equipment in wood-based panel industry is mainly multi-layer hot pressing machine. In addition, single-layer large-format hot press and continuous hot press are gradually adopted. Single-layer hot press is often used in particleboard factory, which can realize the combined heating of high frequency and steam in the manufacture of medium density fiberboard, which is beneficial to shorten the pressurization period and improve the uniformity of product cross-sectional density.
After the final processed board is discharged from the hot press, it has to go through the stages of cooling and moisture balance, that is, sawing and sanding. Hard fiberboard needs heat treatment and humidity control. In the past, the sawing of plate edges was cold sawing, but now it is also hot sawing, but it must not be hot sanding, because hot sanding will damage the surface quality of finished products. According to the use requirements, some plates need special treatment such as dipping, painting, covering and edge sealing.