The Main Causes and Solutions of Warpage and Cracking of Solid Panels and Laminated Timbers

　　The Main Causes and Solutions of Warpage and Cracking of Solid Panels and Laminated Timbers

 

　　1, solid wood panels and Glulam production process

 

  Now the production of solid wood panels and glulam is different depending on the equipment used, and its production process is not consistent. The general production process is:

 

  1.1 wood panel production process

 

Dry sawn timber→selected material/workbench→cross-section/cross-cut saw→double-sided planer/double-sided planer→sliced/multi-saw (single saw)→slab/workbench→glue/gluer → jigsaw/matching machine → moisture content balance/drying kiln → sanding/shaving machine → inspection/workbench → solid wood panel

 

  1.2 Glulam production

 

  Dry sawn timber→selected material/workbench→cross-section/cross-cut saw→duplex planer/double-sided planer→sliced/multi-saw (single saw)→cut off (defective defect)/cut saws→milled teeth/ Tie machine → gluing/gluing machine → lengthening / lengthening machine → four-sided planing/planing machine (fingers) → finger joint production line/automatic finger joint machine fittings/workbench → coating/gluing Machine→splicing/splicing machine→balance of moisture content/drying kiln→sanding/sanding machine→inspection/workbench→glue

 

  2. After the wood is dry, it is released and strictly controls the moisture content.

 

  For the production of solid wood panels or glulam wood, the drying benchmark must be strictly controlled to ensure that the wood moisture content of the entire wood pile is uniform. This is the primary factor for ensuring that wood panels or glulam panels do not exhibit warping and cracking.

 

  2.1 The wood after drying

 

  Wood drying for solid wood panels or laminated timber must be strictly controlled in its final moisture content, and at the same time ensure that the wood moisture content deviation of the entire kiln is controlled to the minimum range, that is, the drying benchmark for the moisture balance phase of the wood drying stage is strictly controlled. After the wood is dried out of the kiln, it must be placed in a dry shed for aging. Chen Fang's main purpose: First, to further balance the moisture content of wood. During the drying process of wood, the upper and lower parts of the wood pile, the inner and the outer parts, and the moisture content inside and outside of each piece of wood are different. The appropriate time of aging can make the wood moisture content tend to be consistent. The second is to release the stress in the wood. As the wood is in the drying process, the change of the moisture causes the size and volume of the wood to change in each direction, so that the wood generates internal stress, 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 aging time should not be less than 1 month.

 

  2.2 Strictly control the moisture content of solid wood narrow plates and finger joints

 

  Before the narrow board of solid wood or glulam panels, the final moisture content must be strictly controlled. Generally, the water content of solid wood for furniture use is required to be 8-12%, but the specific moisture content also depends on the equilibrium water content corresponding to the place where the product is used. . When splicing solid wood narrow plates or finger joints, the narrowness of the neighboring slats, the wood moisture content deviation must be controlled within 2%; if possible, control within 1% is better. To control the moisture content of the wood in the puzzle, it is mainly to carry out the correct balance of moisture content in the late drying of the wood. If not, it is necessary to balance the forced moisture content before the panel to ensure that the moisture content of the wood tends to be consistent.

 

  3, solid wood panels and glulam splicing process conditions

 

  3.1 Strictly control the specifications of solid wood narrow plates and finger joints

 

  Solid wood narrow plates and finger joints can be used in different splicing structures to produce the desired width of the sheet. However, in the current production, there are mainly many forms of flattening. The narrow width of solid wood or finger-joint materials should generally be controlled within 60mm, but it also depends on the thickness of solid wood panels or glulam panels. If the thickness of solid wood panels or glulam panels is less than 20mm, the width should be reduced. When the thickness is greater than 30mm, its width can be appropriately increased. The chord and radial texture of adjacent narrow slats should be properly matched when splicing to avoid excessive chord-wise slats.

 

  3.2 Raw material processing for splicing

 

  Solid wood narrow plates or finger joints should be surface-machined using a planer or precision edging saw so that the roughness of the glued surface is not greater than Rmax=200-300 μm. 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 applied will increase, and the excessive amount of rubber will greatly reduce the strength of the glue. This is mainly due to the fact that the adhesive changes from a liquid to a solid when it is glued to form an internal stress. This internal stress happens to coincide with that of the glue. The pilgrims are the opposite.

 

  3.3 reasonable splicing pressure

 

  When solid wood narrow plates or finger joint materials are used to synthesize solid wood panels or glulam, it is mainly the use of a periodic type and continuous type of jigs for pressure gluing. The general bonding pressure depends on the hardness of the wood species, usually the pressure of the glue direction is 0.7-0.8MPa. In order to prevent lifting or bulging of glue pieces when glued, a certain pressure is often applied to the front surface of the glue pieces (the side of the glue direction), and the pressure is generally 0.1-0.2 MPa. After the splicing, the residual stress in the plate needs to be released for subsequent machining. The aging time is determined according to the specifications of the solid wood panel or glulam, 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 for solid wood panels or laminated timber

 

  Forced moisture content balance is the rapid balance of wood moisture content and reduce internal stress. Forced moisture balance treatment is usually performed when processing into solid wood panels and glulam. If the wood is not dried well, it must be carried out in the raw material before the finished product, but which method should be adopted should be based on the actual production of the company.

 

  4.1 The balance of forced moisture content of solid wood narrow board or small material

 

  If the wood is not handled well in the post-drying stage, that is, if the reference conditions for the moisture balance stage are not well controlled, the wood must also enforce the forced moisture content balance of the solid wood narrow board and the finger-jointed small materials in the drying kiln. In the second drying process, the dry bulb temperature in the kiln should be controlled at 50-60°C, and the dry and wet bulb temperature difference should correspond to the equilibrium moisture content in the kiln (Table 1, Table 2). The equilibrium moisture content in the kiln The corresponding relationship can greatly shorten the post-drying time of the wood, and to a certain extent reduce the warpage and cracking of the finished product. However, there are also some problems. First, after the forced moisture content of wood is treated, the moisture content of the entire wood can be controlled to the lowest level (ideally, the deviation of the moisture content of the entire wood can be controlled at about 1%). However, due to mandatory processing, it takes a certain amount of time to carry out subsequent processing, and a large amount of aged timber will also be affected by the environmental humidity, which will change the moisture content of the wood and reduce the amount of finished products after splicing; second, there will be more production. The process increases the cost, and at the same time, it pays less attention to the post-processing of the wood drying in the production, so that the moisture content deviation increases, and sometimes the final moisture content of the wood cannot be reached even after the forced moisture content balance treatment.

 

  4.2 Forced moisture content balance of solid wood panels or glulam

 

  The manufactured solid wood panel or glulam is subjected to forced moisture content balance treatment, and stricter control of the process is required. The forced moisture content balance for solid wood panels or laminated timber should be stacked in accordance with the requirements of wood drying. When the moisture content of multiple wood piles is balanced together, the heel of each stock pile should not be less than 100 mm. The stack is required to be squared and to achieve "three straight" and "one side pressure", that is, the top of the pile and the two sides are straight, and the bead is straight and pressed on the stack. The process conditions for 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°C, and the temperature should not be too high, otherwise it is easy to crack. The wet bulb temperature should be based on the final moisture content of the wood. The process conditions in the kiln can be referenced in Table 1. In the entire forced moisture balance stage, according to the dry and wet bulb temperature difference required to properly steaming, to maintain dry and wet bulb temperature difference, and at the same time, the style and hourly according to the thickness of the wood. For 30mm hard hardwood, the processing time is generally 72 hours. For example, if the thickness of wood is 25mm, the dry bulb temperature and wet bulb temperature are 50-52°C and 45-47°C respectively, and the treatment time is 72 hours. The final wood moisture content is 12%, and the forced moisture content is balanced. After that, the moisture content of solid wood panels or glulam is uniform. After the treatment time is reached, open the moisture-discharging hole, start the fan, and start cooling. The cooling rate should not be too fast. Generally, the cooling rate is 3-6°C per hour to prevent wood cracking. When it is reduced to the room temperature, it can be out of the kiln, and it can be further processed after 24 hours of aging. The forced moisture content balance treatment at this stage can effectively prevent the warping and cracking of the solid wood panels and glulam, and at the same time, the quality problems caused by the incorrect control of the process conditions before the panels can be found, and the flow can be prevented from flowing into the next production process.

 

  In short, there are many reasons for the warping and cracking of solid wood panels or laminated materials. In addition to the above, it also involves the selection of adhesives, but the main reason is the wood moisture content when splicing, so strict control of wood moisture content, and take effective technology The measure is to completely avoid warping and cracking of solid wood panels and glulam.