From the point of view of the production of solid wood panels and laminated materials, each manufacturer pays more attention to the quality of products than before. With the continuous improvement of the panel technology, the product quality has also been greatly improved. However, due to the different production methods and technological levels of various enterprises, solid wood panels or laminated materials, as the main raw materials for the production of solid wood products, have exposed some problems in the production and finished products. What is prominent is that if the production process conditions are not well controlled, it is easy to warp and crack in serious cases. Even if the above problems do not occur in production, warping or cracking often occurs during the use of users. How to solve this problem is a common concern of enterprise technicians and managers.
Main Causes and Solutions of Warping and Cracking of Solid Wood Panelling and Glulam
1. Production process of solid wood splicing and integrated timber
Now the production of solid wood plate and integrated timber according to the use of different equipment, the production process is not consistent, the general production process is:
Production Process of 1.1 Solid Wood Plate
Drying sawn timber → material selection/workbench → cross section/cross section saw → double-sided planing/double-sided planing → longitudinal section/multi-blade saw (single-blade saw) → board matching/workbench → gluing/gluing machine → board splicing/board splicing machine → water content balance/drying kiln → sanding/sand planing machine → inspection/workbench → solid wood board splicing
1.2 integrated timber production
Drying sawn timber → material selection/workbench → cross section/cross section saw → double-sided planing/double-sided planing → longitudinal section/multi-blade saw (single-blade saw) → cutting (defect removal)/cutting saw → milling/iron tooth machine → gluing/gluing machine → extension/extension machine → four-sided planing light/four-sided planing machine → drying finger joint production line/automatic finger joint board/workbench → glue board coating machine/water ratio machine/sand planer → inspection/workbench → laminated material
2. After the wood is dried, the moisture content shall be strictly controlled.
For the production of solid wood panels or laminated wood, the drying benchmark must be strictly controlled to ensure that the moisture content of the wood in the whole pile is uniform, which is the primary factor to ensure that the solid wood panels or laminated wood do not warp and crack.
2.1 the aging of wood after drying
The final moisture content of wood used for solid wood splicing or integrated wood drying must be strictly controlled, and at the same time, the deviation of the moisture content of wood in the whole kiln must be controlled to the minimum range, that is, the drying benchmark of the moisture content balance stage in the later stage of wood drying must be strictly controlled. After the wood is dried out of the kiln, it must be placed in the dry material shed for aging. The main purpose of Chen Fang: one is to further balance the moisture content in the wood. During the drying process of wood, the upper and lower parts of the pile, the inner and outer parts of the pile, and the inner and outer moisture content of each piece of wood are different. The appropriate time of aging can make the moisture content of the wood tend to be consistent. The second is to release the stress in the wood. As the wood in the drying process, the change of moisture leads to the change of the size and volume of the wood in all directions, which makes the wood produce internal stress, and the wood keeps a certain period of time, which can effectively reduce the internal stress. When the wood is used for solid wood products, especially furniture production, the stale time should be not less than 1 month.
2.2 strictly control the moisture content of solid wood narrow plates and finger bonding materials
The final moisture content of solid wood narrow board or integrated board must be strictly controlled. The moisture content of solid wood for general furniture is required to be 8-12%, but the specific moisture content depends on the equilibrium moisture content corresponding to the place where the product is used. When splicing solid wood narrow plates or finger joints, the moisture content deviation of adjacent narrow plates and wood must be controlled within 2%; If possible, it is better to control within 1%. Control of the moisture content of the wood, mainly in the late drying of the wood for the correct moisture content balance treatment, whether it is necessary to force the moisture content balance before the plate, to ensure that the moisture content of the wood tends to be consistent.
3. The splicing process conditions of solid wood plate and glued lumber
3.1 strictly control the specifications of solid wood narrow plate and finger
Solid wood narrow plate and finger bonding material can be used in different splicing structure form, the required width of the plate. But at present in the production is mainly in the form of flat fight more. The width of solid wood narrow plate or finger bonding material should generally be controlled within 60mm, but it also depends on the thickness of solid wood plate or integrated material. If the thickness of solid wood plate or integrated material is less than 20mm, its width should be reduced. When the thickness is greater than 30mm, its width can be appropriately increased. When splicing, the chord and radial texture of adjacent narrow slats should be properly matched to avoid excessive chord slats.
3.2 raw material processing for splicing
Solid wood narrow plate or finger bonding material shall be surface processed by planer or precision trimming saw, so that the roughness of bonding surface shall not be greater than Rmax = 200-300μm. The amount of glue applied depends on the surface finish of the gluing surface. Generally, the amount of glue applied should be controlled at 150-180g/m2. If the surface roughness is too large, the amount of glue will increase, and excessive glue will greatly reduce the gluing strength. This is mainly due to the fact that the adhesive changes from liquid to solid during gluing, forming internal stress, which is exactly the opposite of the gluing country during gluing.
3.3 reasonable splicing pressure
When solid wood narrow plate or finger bonding glue is used to synthesize solid wood splicing or integrated wood, periodic and continuous splicing machines are mainly used for pressure bonding. The general gluing pressure depends on the hardness of wood species, and the pressing country in the gluing direction is usually 0.7-0.8MPa. In order to prevent the glue parts from lifting or bulging when gluing, a certain pressure is often applied to the front of the glue parts (the side of the gluing direction), and the pressure is generally 0.1-0.2MPa. The residual stress in the panel after splicing needs to be released before subsequent mechanical processing. The aging time is determined according to the specifications of the solid wood panel or the integrated material, and the natural aging time is usually not less than 7 days and nights.
4 Compulsory moisture content balance of raw materials and finished products for solid wood splicing or integrated timber production
The forced moisture content balance benefit is to quickly balance the wood moisture content and reduce the internal stress. The forced water content balance treatment is usually carried out when processing into solid wood panels and integrated materials. If the wood does not dry well, it must be carried out in the raw materials before the finished product is made, but which method is used should be determined according to the actual production situation of the enterprise.
4.1 of forced moisture content balance of solid wood narrow plate or small square material
If the wood is not treated well in the later stage of drying, that is, the reference condition of the moisture content balance stage is not well controlled, the wood must also be forced to balance the moisture content of the solid wood narrow plate and the small square material connected by fingers in the drying kiln, which is also called secondary drying treatment. At this time, the dry bulb temperature in the kiln should be controlled at 50-60 ℃, and the temperature difference between dry bulb and wet bulb should correspond to the equilibrium moisture content in the kiln (Table 1), the equilibrium moisture content in the kiln corresponds to each other, which can greatly shorten the aging time of the late drying of wood and reduce the warping and cracking of the finished product to a certain extent. However, there are also some problems. First, after the forced moisture content treatment of wood, although the moisture content deviation of the whole wood can be controlled to the lowest range (ideally, the moisture content deviation of the whole pile of wood can be controlled to about 1%), due to the forced treatment, it takes a certain period of time for subsequent processing, and a large amount of stale wood will also be affected by environmental humidity, the moisture content of wood changes, reduce the amount of finished products after splicing; two is more than a production process, increased costs, while the production of wood drying in the late treatment more attention, so that the moisture content deviation increased, sometimes even after forced moisture content balance treatment can not reach the final moisture content of wood.
4.2 the forced moisture content balance of solid wood panels or glulam
Made of solid wood or integrated wood for forced moisture content balance treatment, in the process control requirements are more stringent. The forced moisture content balance used for solid wood splicing or glulam shall be stacked according to the requirements of wood drying. When the moisture content balance is carried out for multiple piles together, the heel of each pile shall not be less than 100mm, and each pile shall be square, and "three straight" and "one side pressure" shall be achieved, I .e. the top and two sides of the pile shall be straight, and the battens shall be straight and pressed on the top of the pile. The process conditions for the forced moisture content balance of solid wood panels or glulam are as follows: the dry bulb temperature in the kiln should be controlled at about 50 ℃, and the temperature should not be too high, otherwise it is easy to crack. The wet bulb temperature should be determined according to the final moisture content of the wood. At this time, the process conditions in the kiln can refer to Table 1. In the whole forced moisture content balance stage, according to the requirements of dry and wet bulb temperature difference, properly spray steam, maintain the dry and wet bulb temperature difference, and start the style every hour according to the thickness of the wood. For a 30mm thick hardwood, the treatment time is typically 72 hours. For example, when the northeast elm is treated and the wood thickness is 25mm, the dry bulb temperature and wet bulb temperature are 50-52 ℃ and 45-47 ℃ respectively, and the treatment time is 72 hours, the moisture content of the final wood is 12%. After the forced moisture content balance, the moisture content of the solid wood splicing or glulam is uniform. After the treatment time is reached, open the wet hole, start the fan, and start the cooling. The cooling speed should not be too fast. The general cooling speed is 3-6 ℃ per hour to prevent wood cracking. When it is reduced to the indoor temperature, it can be discharged from the kiln and placed for 24 hours before further processing. The forced moisture content balance treatment at this stage can effectively prevent warping and cracking of solid wood panels and integrated materials, and at the same time, quality problems caused by errors in process condition control before panel splicing can be found to avoid flowing into the next production process.
In short, there are many reasons for warping and cracking of solid wood panels or laminated materials. In addition to the above, it also involves the selection of adhesives. However, the main reason is the moisture content of wood during splicing. Therefore, strict control of the moisture content of wood and effective technological measures can completely avoid warping and cracking of solid wood panels and laminated materials.
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