From the perspective of the production of solid wood panels and laminated timber, all production companies pay more attention to the quality of products than before. With the continuous improvement of the craftsmanship, the quality of the products has also been greatly improved. However, due to the different production methods of different enterprises and different process levels, solid wood panels or laminated timbers are the main raw materials for the production of solid wood products. Some problems are exposed in the production and finished products. The outstanding thing is that if the production process conditions are not well controlled, It is prone to warpage, and severe cracking may occur. Even if the above problems do not occur in production, warping or cracking often occurs during the user's use. How to solve this problem is a common concern of enterprise technicians and managers. Based on years of research and production practices, this paper discusses the quality problems that arise in the production of solid wood panels and laminated timber.
1 Solid wood panel and glulam production process
Now the production of solid wood panels and laminated timber is not consistent according to the equipment used. The general production process is:
1.1 solid wood panel production process
Dry sawn timberâ†’Material/workbenchâ†’Cross-section/cross-section sawâ†’double-sided planing/double-sided planingâ†’longitudinal/multi-blade saw (single-saw)â†’with panel/workbenchâ†’gluing/gluing machine â†’Plate/Plate machineâ†’Water content balance/drying kilnâ†’sanding/sanding machineâ†’inspection/workbenchâ†’solid wood panel
1.2 integrated production
Dry sawn timberâ†’Material/workbenchâ†’Cross-section/cross-section sawâ†’double-sided planing/double-sided planingâ†’longitudinal/multi-blade saw (single-saw)â†’cut (defect)/cut sawâ†’milling/ Iron gear machine â†’ glue / glue machine â†’ length / length machine â†’ four plane planer / four planer â†’ (refer to the material) â†’ finger joint production line / automatic finger joint machine
Fitting/Working Tableâ†’Gluing/Gluing Machineâ†’Plate/Plate Machineâ†’Water Content Balance/Drying Kilnâ†’Sanding/Scraping Machineâ†’Inspection/Workbenchâ†’Glulam
2 After the wood is dried, it is aged and the moisture content is strictly controlled.
For the production of solid wood panels or laminated timber, the drying standard must be strictly controlled to ensure that the moisture content of the wood is uniform throughout the pile, which is the primary factor to ensure that the solid wood panels or laminated timber are free from warpage and cracking.
2.1 After the wood is dried
Wood drying for solid wood panels or laminated timbers must be strictly controlled to the final moisture content, while ensuring that the wood moisture content deviation of the whole kiln is controlled to a minimum extent, that is, the drying benchmark for the moisture balance phase of the wood drying phase is strictly controlled. After the wood has been dried out of the kiln, it must be placed in a dry shed for aging. The main purpose of Chen Fang: First, 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, and the moisture content of the inner and outer parts of each piece of wood are different. The appropriate time can be used to make the moisture content of the wood tend to be consistent. Second, the stress in the wood can be released. As the wood changes during the drying process, the change in the size and volume of the wood in all directions causes the internal stress of the wood, and the wood is kept for a certain period of time, which can effectively reduce the internal stress. When the wood is dried and used for the production of solid wood products, especially furniture, the time for the release should be no less than one month.
2.2 Strictly control the moisture content of solid wood narrow plates and finger joints
Before the solid wood narrow board or laminated board is assembled, the final moisture content must be strictly controlled. Generally, the moisture content of the solid wood for furniture is required to be 8-12%, but the specific moisture content is also determined according to the equilibrium moisture content corresponding to the place of use of the product. . When the solid wood narrow board or finger joint material is spliced, the adjacent board narrow strips, the wood moisture content deviation must be controlled within 2%; if possible, the control is better within 1%. Control the moisture content of the wood in the panel, mainly in the correct moisture content balance treatment in the late stage of wood drying. Otherwise, it is necessary to balance the forced water content before the board to ensure that the moisture content of the wood tends to be consistent.
3 splicing process conditions of solid wood panels and laminated timber
3.1 Strictly control the specifications of solid wood narrow plates and finger joints
Solid wood narrow plates and finger joints can be combined into different splicing structures to form plates of the required width. However, at present, there are mainly more forms in the production. The width of solid wood narrow plates or finger joints should generally be controlled within 60mm, but it depends on the thickness of solid wood panels or laminated timber. If the thickness of solid wood panels or laminated timber is less than 20mm, the width should be reduced; When the thickness is more than 30 mm, the width can be appropriately increased. When splicing, the chord and radial texture of adjacent narrow slats should be properly matched to avoid excessive chordwise slats.
3.2 Raw material processing for splicing
Solid wood narrow plates or finger joints should be surface treated with planer or precision trimming saw so that the roughness of the bonded surface is not more than Rmax=200-300Î¼m. The amount of glue applied depends on the surface finish of the glued surface. Generally, the amount of glue applied should be controlled at 150-180 g/m2. If the surface roughness is too large, the amount of glue will increase, and the excess rubber will greatly reduce the bonding strength. This is mainly because the adhesive changes from liquid to solid during bonding, forming internal stress, which is just when bonding. Glued countries are the opposite.
3.3 Reasonable splicing glue pressure
When solid wood narrow board or finger joint glue is used to synthesize solid wood panels or laminated materials, it is mainly used for pressure bonding by periodic and continuous type of jigsaw machine. Generally, the bonding pressure depends on the softness and hardness of the wood species, and the pressing pressure in the usual bonding direction is 0.7-0.8 MPa. In order to prevent the lifting or bulging of the glue piece when it is glued, a certain pressure is often applied to the front side (the side of the gluing direction) of the rubber piece, and the pressure is generally 0.1-0.2 MPa. The residual stress in the spliced â€‹â€‹plate needs to be released in order to carry out subsequent machining. The aging time is determined according to the specifications of the solid wood panel or the laminated timber, and the natural aging time is usually not less than 7 days and nights.
4 Forced moisture content balance of raw materials and finished products produced by solid wood panels or laminated timber
The mandatory moisture content balance is to quickly balance the moisture content of the wood and reduce the internal stress. Forced moisture balance treatment, usually when processed into solid wood panels and laminated timber. If the wood is not well dried, it must be done in the raw materials before the finished product, but which method should be used depends on the actual production of the enterprise.
4.1 Forced water content balance of solid wood narrow plate or small material square material
If the wood is not treated well in the later stages of drying, that is, the baseline conditions in the water-water balance phase are not well controlled, the wood must also have a forced moisture balance in the solid kiln for the solid wood narrow plate and the finger-joined small material square, also known as Secondary drying treatment, at this time the dry bulb temperature in the kiln should be controlled at 50-60 Â° C, the dry and wet bulb temperature difference corresponds to the equilibrium moisture content in the kiln (Table 1, Table 2), the equilibrium moisture content in the kiln Corresponding relationship, which can greatly shorten the post-drying time of the wood drying, and reduce the warpage and cracking of the finished product to a certain extent. However, there are some problems. First, after the forced moisture content of the wood is treated, 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 can be controlled to about 1%) However, due to the forced treatment, it takes a certain period of time to carry out subsequent processing. A large amount of aged wood will be affected by the environmental humidity, which will change the moisture content of the wood and reduce the finished product after splicing. Second, one more production. The process increases the cost, and at the same time, the post-treatment of wood drying is less paid attention to in the production, so that the water content deviation is increased, and sometimes the final moisture content of the wood cannot be achieved even after the forced water content balance treatment.
4.2 Forced water content balance of solid wood panels or laminated timber
The solid wood panels or laminated materials produced are subjected to forced water content balance treatment, and the control of the process is more strict. The mandatory moisture content balance for solid wood panels or laminated timber shall be piled up according to the requirements of wood drying. When multiple water piles are balanced together, the spacing between each pile shall not be less than 100mm. The pile requires squareness and â€œthree flat straightâ€ and â€œone side pressureâ€, that is, the upper side and the two sides of the pile are straight, and the bead is straight and pressed on the top of the pile. The process conditions for the forced moisture content balance of solid wood panels or laminated timber are as follows: The temperature of the dry bulb in the kiln should be controlled at about 50 Â°C, 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. The process conditions in the kiln can be referred to Table 1. In the entire forced water content balance stage, the steam is appropriately sprayed according to the requirements of the dry and wet bulb temperature difference, and the dry and wet bulb temperature difference is maintained, and the starting style is determined according to the thickness of the wood per hour.
For hard hardwood with a thickness of 30 mm, the treatment time is generally 72 hours. For example, when the thickness of the wood is 25mm, the dry bulb temperature and the wet bulb temperature are 50-52Â°C and 45-47Â°C, respectively, and the treatment time is 72 hours. The final wood moisture content is 12%, and the forced water content balance is obtained. After that, the moisture content of the solid wood panels or laminated timber is uniform. After the treatment time is reached, open the wetted hole, start the fan, start to cool down, and the cooling rate should not be too fast. The general cooling speed is 3-6 Â°C per hour to prevent cracking of the wood. When it is lowered 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 the warpage and cracking of the solid wood panels and the laminated materials, and at the same time, it can be found that the quality problems caused by the control of the process conditions before the slabs are prevented from flowing into the next production process.
In short, there are many reasons for the warpage and cracking of solid wood panels or laminated materials. In addition to the above, the selection of adhesives is also involved, but the main reason is the moisture content of wood during splicing. Therefore, the moisture content of wood is strictly controlled and an effective process is adopted. The measure is to completely avoid the warpage and cracking of solid wood panels and laminated timber.
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