A method for hot-press drying of solid wood boards
By applying radial and tangential pressure during the wood drying process, combined with the coordinated control of temperature and pressure, the hot-pressing process was optimized, solving the problems of cracking and deformation during wood drying and achieving a high-quality, low-defect drying effect.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INST OF WOOD INDUDTRY CHINESE ACAD OF FORESTRY
- Filing Date
- 2026-03-08
- Publication Date
- 2026-05-29
AI Technical Summary
Existing wood drying methods are prone to cracking and deformation during the drying process, making it difficult to effectively control the internal stress of the wood, which affects the drying quality and utilization efficiency.
By employing a hot-press drying method for solid wood boards, radial and tangential pressures are applied during the drying process, and temperature and pressure are coordinated and controlled to optimize hot-pressing process parameters, thereby achieving effective management of internal stress and moisture gradients in the wood.
It significantly inhibits cracking and deformation during the drying process, improves the drying quality and dimensional stability of wood, enhances drying uniformity and finished product quality, reduces surface cracking, and shortens the drying cycle.
Smart Images
Figure CN122107710A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of wood processing technology, and in particular to a hot-pressing and drying method for solid wood boards. Background Technology
[0002] Since ancient times, wood has been an essential raw material for human survival. It is widely used in construction, furniture, decoration, and many industrial fields, and is inseparable from people's production and life. Along with steel, cement, and plastics, it is known as one of the four major building materials, and it is the only renewable biomass resource among them. With the increasing awareness of environmental protection and the pursuit of a better quality of life, people's demand for wood products is growing, and they are also placing higher demands on their quality, stability, and sustainability.
[0003] Timber drying is a crucial process that determines a timber's subsequent processing performance and lifespan. Improper control during drying can easily lead to defects such as cracking and warping, severely impacting the timber's grade and yield, and even resulting in waste, thus reducing resource utilization. Taking eucalyptus as an example, although it possesses advantages such as rapid growth, good mechanical properties, and strong sustainability, cracking during the drying process makes it difficult to utilize as solid wood. Currently, it is mainly used in engineered wood products such as plywood, particleboard, and fiberboard, or in pulp, hindering its high-value utilization.
[0004] Currently, common wood drying methods include natural drying, kiln drying, high-frequency / microwave drying, solar drying, and flue gas drying. These methods each have their own characteristics in terms of moisture migration, stress release, and cell wall strength maintenance, but they also have significant shortcomings: during the drying process, because stress is not constrained, problems such as cracking and deformation are easily caused. Therefore, how to effectively control internal stress in wood and inhibit deformation and cracking during the drying process through process optimization has become a key technical challenge for improving the quality and utilization efficiency of wood drying. Summary of the Invention
[0005] In view of this, the present invention provides a hot-press drying method for solid wood boards. By optimizing the hot-pressing process parameters, radial and tangential pressures are applied simultaneously during the drying process. Combined with the coordinated control of temperature and pressure, the internal stress and moisture gradient of the wood are effectively managed, thereby significantly inhibiting drying cracking and deformation, and improving the drying quality and dimensional stability of the wood.
[0006] To achieve the above objectives, the present invention provides the following technical solution:
[0007] The hot-pressing and drying method for solid wood boards includes the following steps:
[0008] S1. Place the solid wood board on the hot press; the hot press includes an upper pressure plate and a lower pressure plate arranged opposite each other in the vertical direction, and a left pressure plate and a right pressure plate arranged in the horizontal direction; the lower surface of the solid wood board is in contact with the lower pressure plate;
[0009] S2. Preheating the solid wood board: Heat the upper and lower pressure plates to the preheating temperature and control the upper and lower pressure plates to adhere to the surface of the solid wood board.
[0010] S3. Heat the upper and lower pressure plates to the hot pressing temperature;
[0011] S4. After the upper and lower pressure plates reach the hot pressing temperature, control the upper and lower pressure plates to apply radial pressure to the solid wood board, and the left and right pressure plates to apply tangential pressure to the solid wood board.
[0012] The radial and tangential pressure ranges from 0.5 to 2 MPa.
[0013] Hot pressing of solid wood boards is performed using a constant temperature mode or a gradient temperature change mode.
[0014] S5. When the moisture content of the solid wood board drops below the moisture content threshold, release the pressure and remove the dried solid wood board.
[0015] According to the aforementioned hot-pressing and drying method for solid wood boards, in step S2, the solid wood boards are preheated in the thickness direction at a heat transfer rate of 1~3 mm / min.
[0016] According to the aforementioned hot-pressing and drying method for solid wood boards, the wood species used for the solid wood boards are any one of oak, birch, eucalyptus, maple, cedar, balsa, teak, black walnut, ironwood, and oak.
[0017] Furthermore, the solid wood boards are obtained by slicing or sawing logs, with a thickness of 20~45mm and an initial moisture content of 30%~50% before drying.
[0018] According to the aforementioned hot-pressing and drying method for solid wood boards, when using the constant temperature mode, the hot-pressing temperature is one of the values between 80 and 120°C.
[0019] According to the aforementioned hot-pressing and drying method for solid wood boards, when using a gradient temperature change mode: at temperature T1 and held for a certain time t1, the temperature is raised to temperature T2; at temperature T2 and held for a certain time t2; the temperature is raised to temperature T3 and held for a certain time t3; and T1 is a temperature value between 80 and 90℃, T2 is a temperature value between 95 and 105℃, and T3 is a temperature value between 110 and 120℃.
[0020] Based on the aforementioned hot-pressing and drying method for solid wood panels, the pressure in S4 is specified for panels of different densities:
[0021] For low-density materials with an air-dry density ρ less than 0.5 g / cm³, the radial pressure is 0.5~1MPa and the tangential pressure is 0.3~0.6MPa.
[0022] For medium-density lumber with an air-dry density ρ of 0.5~0.85 g / cm³, the radial pressure is 1.0~1.5 MPa and the tangential pressure is 0.5~1.0 MPa.
[0023] For high-density materials with an air-dry density ρ greater than 0.85 g / cm³, the radial pressure is 1.5~2.0 MPa and the tangential pressure is 0.8~1.2 MPa.
[0024] According to the aforementioned hot-pressing and drying method for solid wood boards, the moisture content threshold in step S4 is 12%.
[0025] According to the aforementioned hot-pressing and drying method for solid wood boards, the method prior to step S1 includes:
[0026] S0. Pretreatment of solid wood boards: Cracks are engraved on the upper and lower surfaces of the solid wood boards to form controlled moisture diffusion channels;
[0027] The depth of the scribing is 0.5~2mm, the width of the scribing is 0.3~0.8mm, and the spacing between the scribing marks is 10~30mm;
[0028] The grooves are oriented in the same direction as the internal conduits of the board, which facilitates the transport of moisture along the conduits.
[0029] Furthermore, step S5 is followed by:
[0030] S6. The process involves planing and sanding; the planing amount is 1~3mm.
[0031] Compared with the prior art, the present invention has the following beneficial effects:
[0032] (1) The present invention achieves effective control of wood deformation and stress during the drying process by simultaneously applying radial and tangential pressure and combining the coordinated regulation of pressure and temperature, thereby suppressing the shrinkage, cracking and deformation of wood.
[0033] (2) By applying pressure in both radial and chord directions and combining it with gradient heating and hot pressing drying, this invention not only effectively controls the stress distribution and moisture gradient during the drying process, but also utilizes the thermal response behavior of the material to achieve structural self-repair, improves the drying uniformity and finished product quality, reduces surface cracking, and exhibits better drying adaptability and stability. Attached Figure Description
[0034] Figure 1 A flowchart of a hot-pressing and drying method for solid wood boards;
[0035] Figure 2 This is a schematic diagram of bidirectional hot pressing in both radial and chordal directions;
[0036] Figure 3 These are photographs of the appearance of eucalyptus wood before and after drying in Examples 1 to 3;
[0037] Figure 4 Here is a photograph showing the appearance of the drying process in Example 4;
[0038] Figure 5 Photographs of the appearance of eucalyptus wood before and after drying, for Comparative Example 1;
[0039] Figure 6 This is a schematic diagram of radial hot pressing;
[0040] Figure 7 Photographs of the appearance of eucalyptus wood before and after drying in Comparative Examples 2 to 4;
[0041] Figure 8 A photograph showing the appearance of the drying process in Comparative Example 5;
[0042] Figure 9 A structural diagram of a solid wood board with surface engravings;
[0043] Figure 10 for Figure 9 A magnified view of a portion of point A in the middle. Detailed Implementation
[0044] To make the technical problem to be solved, the technical solution and advantages of the present invention clearer, the following description will be provided in conjunction with the accompanying drawings. Figures 1 to 10 The technical solution of the present invention will be clearly and completely described in conjunction with specific embodiments.
[0045] Figure 1 This is a flowchart of a hot-pressing and drying method for solid wood boards. Figure 2 This is a schematic diagram of a bidirectional hot press, applying both radial and tangential pressure. The hot press includes an upper and lower pressure plate arranged vertically opposite each other, and a left and right pressure plate arranged horizontally. The upper and lower pressure plates are used to apply radial pressure to the solid wood board, while the left and right pressure plates are used to apply tangential pressure to the solid wood board.
[0046] The hot-pressing and drying method for solid wood boards includes the following steps:
[0047] S1. Place the solid wood boards on a hot press, with the lower surface of the boards in contact with the lower pressure plate. The solid wood boards are obtained by slicing or sawing logs, with a thickness of 20-45mm and an initial moisture content of 30%-50% before drying. The wood species used for the solid wood boards can be any one of the following: oak, birch, eucalyptus, maple, cedar, balsa, teak, black walnut, ironwood, and oak. The solid wood boards are obtained by slicing or sawing logs, with a thickness of 20-45mm and an initial moisture content of 30%-50% before drying.
[0048] S2. Preheating the solid wood panels: Heat the upper and lower pressure plates to the preheating temperature, ensuring they are firmly attached to the surface of the solid wood panels. Preheat the solid wood panels along their thickness at a heat transfer rate of 1~3 mm / min. Preheating the solid wood panels at the set preheating temperature avoids stress concentration caused by applying pressure from a cold state.
[0049] S3. Heat the upper and lower pressure plates to the hot pressing temperature.
[0050] S4. After the upper and lower pressure plates reach the hot-pressing temperature, control the upper and lower pressure plates to apply radial pressure to the solid wood board, and the left and right pressure plates to apply tangential pressure to the solid wood board. Radial and tangential pressure are applied simultaneously: at the hot-pressing temperature, the wood's plasticity is enhanced, making it easier to shape; the radial and tangential pressure range is 1~2 MPa. The radial / tangential pressure ratio is adjusted according to the wood species and moisture content of the board to effectively restrain deformation without causing the wood to crush; the solid wood board is hot-pressed in a constant temperature mode or a gradient temperature change mode. The left and right pressure plates do not have temperature settings, only provide pressure.
[0051] S5. When the moisture content of the solid wood board drops below the moisture content threshold, release the pressure and remove the dried solid wood board.
[0052] In step S3, the hot press drives the upper pressure plate to move towards the surface of the solid wood board, while the lower pressure plate remains stationary. The upper and lower pressure plates apply radial pressure to the solid wood board. Alternatively, the hot press drives one of the left or right pressure plates to move towards the side of the solid wood board, while the other side plate remains stationary. The left and right pressure plates apply tangential pressure to the solid wood board. The upper and lower pressure plates can also move towards each other to apply radial pressure, while the left and right pressure plates move towards each other to apply tangential pressure.
[0053] In S4, when the constant temperature mode is used, the hot pressing temperature is a value between 80 and 120℃.
[0054] In S4, when using a gradient temperature change mode: The temperature is maintained at T1 for a certain time t1, then increased to T2; maintained at T2 for a certain time t2; then increased to T3, and maintained at T3 for a certain time t3; the temperatures T1, T2, and T3 are taken between 80 and 120℃. At the T1 plateau (80-90℃), free water begins to evaporate, and the surface layer initially shrinks; at the T2 plateau (95-105℃), bound water gradually evaporates, and the main body shrinks; at the T3 plateau (110-120℃), deep moisture evaporates, completing the final shrinkage. This graded drying mechanism allows the shrinkage stress to be released gradually in three stages, rather than concentratedly, reducing the residual stress of the board by more than 60%. The gradient temperature increase during the hot-pressing and drying process achieves the technical effect of "defect self-healing." A hot-pressing temperature of 80-120℃ is the safe working range for the hot-pressing process, which can achieve lignin softening and cellulose stabilization. At temperatures above 120℃, hemicellulose begins to degrade, causing the color of the board to darken. Only at temperatures above 150℃ does the structure of lignin change, resulting in a decrease in strength.
[0055] During the gradient temperature change process, the heating rate from T1 to T2 and from T2 to T3 is 2~3℃ / min, which achieves slow heating and avoids thermal stress inside the board caused by excessive heating.
[0056] During hot pressing, the radial and tangential pressures range from 0.5 to 2 MPa, effectively suppressing free deformation, cell cavity collapse, and stress concentration during the drying process. Matching pressure and density is essential for achieving high-quality hot pressing drying; inappropriate pressure will completely negate the effects of temperature and time control.
[0057] For low-density woods with an air-dry density ρ less than 0.5 g / cm³ (such as cedar, paulownia, balsa wood, and white pine), the cell walls of the wood are relatively thin, and cell wall buckling and cell cavity collapse are prone to occur under pressure. Excessive pressure will cause severe crushing. Lower pressure is used to ensure the integrity of the cell structure and the mechanical properties of the board. Therefore, the preferred radial pressure is 0.5~1MPa and the preferred tangential pressure is 0.3~0.6MPa.
[0058] For medium-density woods (such as oak, beech, and birch) with an air-dry density ρ of 0.5~0.85 g / cm³, the cell wall thickness of the wood is moderate, and the radial bearing capacity is good. The preferred radial pressure is 1.0~1.5 MPa, which can fully restrain radial deformation. The tangential bearing capacity is weak. To avoid excessive damage, the preferred tangential pressure is 0.5~1.0 MPa.
[0059] For high-density woods with an air-dry density ρ greater than 0.85 g / cm³ (such as teak, ironwood, black walnut, etc.), the cell walls of the wood are thick, the permeability is poor, the material is rigid, and it is difficult to dry. The preferred radial pressure is 1.5~2.0 MPa, and the tangential pressure is 0.8~1.2 MPa.
[0060] The examples and comparative examples are illustrated using solid wood panels made from eucalyptus wood. The eucalyptus wood was sourced from Nanning City, Guangxi Zhuang Autonomous Region, with a diameter at breast height of 180-220 mm, an age of 8-9 years, and an air-dry density of approximately 0.60 g / cm³. 3 The initial moisture content was 30%–50%; test materials were selected from both the heartwood and sapwood of eucalyptus. Solid wood boards were obtained by slicing logs, with dimensions of: length × width × thickness = 400 mm × 90 mm × (22–23) mm. To ensure uniform heat conduction and consistent moisture migration during drying, it is recommended that the selected slices be of uniform size and have a smooth surface, avoiding defects such as warping or incomplete cross-sections. It is understood that solid wood boards can also be obtained from other tree species, such as cedar and oak.
[0061] Example 1: Solid wood boards with an initial moisture content of 40.92% were placed in a hot press and dried under constant temperature of 80 ℃ and pressure of 1 MPa, with pressure applied simultaneously in both the radial and tangential directions.
[0062] Example 2: Solid wood boards with an initial moisture content of 40.72% were placed in a hot press and dried under constant temperature of 100 ℃ and pressure of 1.5 MPa, with pressure applied simultaneously in both the radial and tangential directions.
[0063] Example 3: Solid wood boards with an initial moisture content of 32.57% were placed in a hot press and dried under constant temperature of 120 ℃ and pressure of 1.5 MPa, with pressure applied simultaneously in both the radial and tangential directions.
[0064] Example 4: Solid wood boards with an initial moisture content of 45.64% were placed in a hot press with a hot pressing pressure of 1.5 MPa. A gradient temperature change mode was adopted. The solid wood boards were heated to 80°C and held for 10 hours, then heated to 100°C and held for 7 hours. The temperature was then raised to 120°C and held for 2 hours.
[0065] Comparative Example 1: Eucalyptus wood boards were dried using a natural drying method. Solid wood boards with an initial moisture content of 41.18% were placed in indoor conditions (temperature 25±3℃, humidity 35±3%) for natural drying.
[0066] Existing hot-pressing methods for wood panels are limited to radial hot pressing (i.e., applying pressure and temperature perpendicular to the large plane of the wood panel). Comparative Examples 2 to 5 illustrate conventional radial hot pressing techniques as follows:
[0067] Comparative Example 2: Solid wood boards with an initial moisture content of 40.92% were placed in a hot press and dried under radial pressure at 80 ℃ and 1 MPa.
[0068] Comparative Example 3: Solid wood boards with an initial moisture content of 40.72% were placed in a hot press and dried under radial pressure at 100°C and 2 MPa.
[0069] Comparative Example 4: Solid wood boards with an initial moisture content of 32.57% were placed in a hot press and dried under radial pressure at 120°C and 1.5MPa.
[0070] Comparative Example 5: Solid wood boards with an initial moisture content of 44.85% were placed in a hot press and dried under radial pressure of 1.5 MPa. A gradient temperature change mode was adopted, with the solid wood boards being kept at 80℃ for 10 hours, then heated to 100℃; kept at 100℃ for 7 hours; then heated to 120℃ and kept at 120℃ for 4 hours.
[0071] The moisture content of Examples 1 to 4 and Comparative Examples 1 to 5 after drying was tested, and their macroscopic morphology and crack development before and after drying were photographed and recorded.
[0072] in:
[0073] The moisture content of dried solid wood boards was tested using a weighing method: the calculation formula is as follows:
[0074]
[0075] Where: m1 is the mass after drying, g; m0 is the mass when completely dry, g; MC is the moisture content, %.
[0076] Observations on the macroscopic morphology and crack changes of eucalyptus heartwood and sapwood:
[0077] High-resolution digital photography was conducted on eucalyptus boards before and after drying to record the overall macroscopic morphological changes. The focus was on capturing the distribution, number, length, and propagation path of cracks on the surface and cross-section to visually reflect the generation and development process of cracks under different drying conditions.
[0078] The moisture content changes in the examples and comparative examples are shown in Table 1:
[0079] Table 1
[0080]
[0081] Figure 3 These are photographs of the appearance of eucalyptus wood before and after drying in Examples 1 to 3.
[0082] The solid wood boards in Example 1 did not exhibit significant surface or end cracks after hot-pressing at 80℃ with bidirectional radial and chordal force. This was mainly due to the simultaneous application of force in both radial and chordal directions, which effectively suppressed dimensional changes in the main shrinkage directions and reduced stress concentration caused by uneven shrinkage during drying. Simultaneously, the bidirectional constraint allowed for a more uniform distribution and release of stress caused by the internal and external moisture content gradients, fundamentally reducing the risk of end cracks along the wood rays and surface cracks in the heartwood region. Furthermore, the bidirectional force maintained the stability of the wood cell structure while suppressing deformation, avoiding the induction of crack initiation by localized structural damage. During 62 hours of hot-pressing at 80℃ with bidirectional radial and chordal force, the moisture content of the solid wood boards decreased from 40.92% to 10.59%. This change was primarily due to the stable temperature and pressure conditions provided by the hot-pressing environment, allowing free water and some bound water to migrate efficiently in the open bidirectional radial and chordal transport channels. Meanwhile, the bidirectional mechanical constraint reduces the accumulation of structural strain caused by rapid water loss, maintaining the balance of stress release while ensuring a high drying rate, thus achieving a high-quality, low-defect drying effect over a relatively long drying time.
[0083] After hot-pressing and drying at 100℃ under bidirectional force, the solid wood boards in Example 2 exhibited microcracks and surface cracks at the pith and surface. This was mainly due to the complex moisture content gradient and anisotropic shrinkage stress generated by the bidirectional drying process: during rapid water loss, a steep moisture content / temperature gradient formed between the surface and the pith, inducing tensile stress on the surface; the tangential shrinkage of the wood was much greater than the radial shrinkage, and the inconsistent shrinkage led to local stress concentration, causing microcracks to extend into surface and end cracks. The high permeability channels and discontinuous tissue at the ends and pith (stress concentration points) allowed moisture to escape preferentially, generating tensile stress at the ends, causing cracks to extend along the growth rings or vessels. At 100℃, the evaporation rate of free water and the diffusion rate of bound water were significantly increased. Hot pressing improved heat transfer and shortened the diffusion path, while the temperature difference and vapor pressure difference provided additional pressure driving force, causing the moisture content to decrease from 40.72% to 10.17% within 21 hours. In addition, the lignin and hemicellulose softened at high temperatures, reducing the transverse tensile strength and lowering the wood's resistance to shrinkage stress. However, compared with unidirectional force application, bidirectional force application disperses stress concentration in multiple directions, inhibits macroscopic crack propagation, makes the crack morphology smaller and mainly stays at the microscale, thereby reducing the overall degree of cracking.
[0084] After hot-pressing and drying at 120℃ with bidirectional radial and chordal force, the solid wood board in Example 3 showed obvious end cracks at the wood rays, while no significant surface cracks were observed. This is because the bidirectional radial and chordal force disperses and redistributes the surface shrinkage stress on the plane, inhibiting the propagation of surface cracks; however, the high permeability and discontinuous structure of the longitudinal vessels / ray channels at the ends lead to preferential dehydration and stress concentration, making end cracks more likely. The thermal softening of lignin and hemicellulose at high temperatures endows the wood with a certain degree of viscoplastic fluidity, which, under bidirectional pressure, can promote the closure of microcracks or the filling of fissures, exhibiting a "high-temperature self-healing" effect, thereby further inhibiting surface cracks, while end cracks may still exist. After 2.5 hours of hot-pressing and drying at 120℃, the moisture content of the solid wood board decreased from 35.57% to 10.26%, mainly attributed to the high temperature significantly accelerating the evaporation of free water and the diffusion of bound water. Hot pressing improved the contact between the sample and the board and shortened the diffusion path. The temperature difference, vapor pressure difference, and the preferential drainage effect of the longitudinal vessels / ray channels allowed moisture to escape efficiently in a short time.
[0085] Figure 4 This is a photograph of the drying process in Example 4. The surface of the wood after tangential hot-pressing in Example 4 remained intact, with no obvious surface cracks. Although slight cracking appeared on the end faces during drying, these cracks healed themselves during the subsequent 120°C hot-pressing stage. At 80°C, the migration rate of free water above the fiber saturation point was controlled by using a lower temperature combined with bidirectional pressure, avoiding stress concentration and structural damage caused by rapid temperature increases, effectively reducing the occurrence of initial internal cracks. Rapid dehydration was achieved at 100°C, and the initial pressure adaptation and structural buffering effect prevented differences in cell shrinkage between the inside and outside, further preventing later cracking and bulging. At 120°C, the temperature approached or exceeded the lignin glass transition temperature, inducing thermal softening of the cell wall lignin and resulting in viscoelastic flow. The 120°C stage also accelerated the diffusion of bound water, breaking through the bottleneck of conventional diffusion control. After 19 hours of hot-pressing drying, the moisture content decreased from 45.64% to 10.72%.
[0086] Figure 5 The images show the appearance of eucalyptus wood before and after drying in Comparative Example 1. A is the front surface after drying, A1 is the surface before drying, B is the rear surface after drying, and B1 is the surface after drying. After drying under natural conditions, the eucalyptus wood in Comparative Example 1 exhibited obvious surface and end cracks. The surface cracks, as seen on B1 after drying, were mainly concentrated near the ends and heartwood. This is because during the drying process, moisture migrates from the interior of the wood to the surface. The ends have a larger exposed area and unobstructed water channels, resulting in a significantly faster moisture drainage rate than the middle of the board. This leads to excessive early water loss at the ends, increasing the moisture content gradient between the inside and outside, thus generating greater tensile stress at the ends and inducing cracking.
[0087] Figure 6 This is a schematic diagram of radial hot pressing, which is a conventional unidirectional hot pressing method. Figure 7Photographs of the appearance of eucalyptus wood before and after drying by radial hot pressing, for Comparative Examples 2 to 4.
[0088] The solid wood boards in Comparative Example 2 exhibited significant surface and end cracks after radial pressure hot-pressing at 80℃. The end cracks primarily extended along the wood ray direction, while surface cracks were mostly concentrated in the heartwood region. Because tangential shrinkage was not effectively restrained, internal stress was released during the cracking process, ultimately reducing the drying quality. After 42 hours of radial pressure hot-pressing at 80℃, the moisture content of the solid wood boards decreased from 40.92% to 10.48%.
[0089] After radial hot-press drying at 100 ℃, the solid wood boards in Comparative Example 3 showed obvious end cracks and surface cracks in the pith and surface. This was mainly due to the strong humidity and temperature gradient formed by the combined effects of high temperature and radial pressure. The bending of annual rings, discontinuous tissue structure, and initial defects near the pith on the end face easily caused stress concentration, leading to cracks propagating outward from the pith. The surface tangential shrinkage is already greater than the radial shrinkage; radial loading further constrains the anisotropic shrinkage coordination. When surface shrinkage is hindered while the inner layer still has high moisture content, the tangential tensile stress in the surface layer increases, ultimately triggering surface cracking. After 19 hours of tangential hot-press drying at 100 ℃, the moisture content of the solid wood boards decreased from 40.72% to 10.17%.
[0090] After radial hot-press drying at 120℃, the solid wood boards in Comparative Example 4 exhibited significant end cracks and surface cracks at the wood rays and on the surface. This was mainly due to the steep moisture content / temperature gradient and anisotropic shrinkage stress generated by the combined effects of high temperature and radial stress: tangential shrinkage was significantly greater than radial shrinkage. Radial constraint caused significant tangential tensile stress in the surface layer while the inner layer remained moist. Discontinuous structure and differences in mechanical properties at the wood rays easily led to stress concentration. Furthermore, the partial softening of hemicellulose and lignin at 120℃ resulted in a decrease in transverse tensile strength. The combined effect ultimately triggered end and surface cracks. After 2.5 hours of hot-press drying, the moisture content of the boards decreased from 35.57% to 10.32%.
[0091] Figure 8 This is a photograph of the drying process of Comparative Example 5. During the radial hot-pressing drying process of the solid wood boards in Comparative Example 5, the macroscopic surface of the wood remained basically intact with no obvious surface cracks, but significant end cracks appeared at the end faces. After 22 hours of hot-pressing drying, the moisture content of the solid wood boards decreased from 44.85% to 9.06%.
[0092] This invention achieves effective control over wood deformation and stress during the drying process by simultaneously applying radial and tangential pressure, combined with optimized control of pressure magnitude and temperature, thereby suppressing wood shrinkage, cracking, and deformation problems.
[0093] In particular, the application of pressure in both radial and chord directions combined with gradient heating and hot-pressing drying not only effectively controls the stress distribution and moisture gradient during the drying process, but also utilizes the material's thermal response behavior to achieve structural self-repair, improving drying uniformity and finished product quality, reducing surface cracking, and exhibiting superior drying adaptability and stability. It is currently the optimal technical path for optimizing the drying quality of solid wood boards, shortening the cycle, and reducing defects, providing a new technical solution for solid wood board drying technology, and has significant application value and promotion prospects.
[0094] Furthermore, the procedure before step S1 includes:
[0095] S0. Pre-treatment of solid wood board 100: Cracks 101 are engraved on the upper and lower surfaces of the solid wood board 100 to form controlled moisture diffusion channels; the depth of the cracks is 0.5~2mm, the width of the cracks is 0.3~0.8mm, and the spacing between the cracks is 10~30mm; the direction of the cracks is along the direction of the internal ducts of the board, which facilitates the transmission of moisture along the ducts. Specifically, the cracks are precisely processed on the board using a CNC scoring machine, or the cracks are formed by rolling a pressure roller with protrusions on the surface of the solid wood board. Figure 9 This is a structural diagram of a solid wood board with surface scoring. Figure 10 for Figure 9 A magnified view of a portion of point A in the middle.
[0096] Step S5 is followed by:
[0097] S6. The board is planed and sanded; the planing amount is 1~3mm. The planing and sanding process makes the board surface smooth.
[0098] Solid wood boards from the same embodiment described above were used for comparative testing with and without scoring treatment. The initial moisture content of both scored and unscored boards was 45.64%. A gradient heating method (80℃, 100℃, and 120℃) combined with radial and tangential pressure was employed, with an applied pressure of 1.5 MPa. The scoring parameters for the solid wood boards were: score depth 1.0 mm, score width 0.5 mm, score spacing 20 mm, score length 30 mm, and score direction along the vascular bundle direction. The experimental results are shown in Table 2.
[0099] Table 2
[0100]
[0101] The unmarked group of solid wood boards dried from a moisture content of 45.64% to 10.72% in 19 hours, with a calculated average drying rate of 1.84% / hour. The marked group of solid wood boards dried from a moisture content of 45.64% to 10.35% in 13 hours, with a calculated average drying rate of 2.72% / hour. Compared to the unmarked solid wood boards, the marked boards showed a 47.8% increase in drying rate, and the marking process reduces energy consumption.
[0102] Table 3
[0103]
[0104] Table 3 shows a comparison of the morphology of the dents before and after hot pressing. It can be seen that the dents heal better after hot pressing. For the parts that have not yet healed, further treatment can be carried out according to the actual situation. For example, when applied to visible surfaces of furniture, sanding can be used. When used in structural materials, no further treatment is required.
[0105] This invention provides a comparative experiment on isobaric and differential pressures of radial and chordal pressures. The experimental conditions are as follows:
[0106] Medium-density oak (air-dry density ρ = 0.65 g / cm³) was used. The planks measured 1000 mm (length) × 200 mm (width) × 20 mm (thickness) with an initial moisture content of 45.64%. The temperature of the upper and lower pressure plates was set at 110℃ for 8 hours. The upper and lower surfaces of the planks were scored with a depth of 1 mm, a spacing of 20 mm, and a score width of 0.5 mm. For Experiment A, the pressure control was 1 MPa for both radial and tangential pressure. For Experiment B, the pressure control was 1.3 MPa for radial pressure and 0.7 MPa for tangential pressure. Thirty oak planks were used in each experiment, and a warpage test was conducted according to GB / T 15036.2-2009, "Solid Wood Flooring Part 2: Test Methods". The warpage test results are shown in Table 4.
[0107] Table 4
[0108]
[0109] As can be seen from the data in Table 4, using differentiated pressure to hot-press the boards results in better structural stability after drying.
[0110] By scoring the surface of solid wood boards and combining gradient heating with bidirectional pressure, technical support is provided for efficient and low-consumption drying of solid wood boards.
[0111] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the scope of the technology disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention.
Claims
1. A hot-pressing and drying method for solid wood boards, characterized in that, Includes the following steps: S1. Place the solid wood board on the hot press; the hot press includes an upper pressure plate and a lower pressure plate arranged opposite each other in the vertical direction, and a left pressure plate and a right pressure plate arranged in the horizontal direction; the lower surface of the solid wood board is in contact with the lower pressure plate; S2. Preheating the solid wood board: Heat the upper and lower pressure plates to the preheating temperature and control the upper and lower pressure plates to adhere to the surface of the solid wood board. S3. Heat the upper and lower pressure plates to the hot pressing temperature; S4. After the upper and lower pressure plates reach the hot pressing temperature, control the upper and lower pressure plates to apply radial pressure to the solid wood board, and the left and right pressure plates to apply tangential pressure to the solid wood board. The radial and tangential pressure ranges from 0.5 to 2 MPa. Hot pressing of solid wood boards is performed using a constant temperature mode or a gradient temperature change mode. S5. When the moisture content of the solid wood board drops below the moisture content threshold, release the pressure and remove the dried solid wood board.
2. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, In step S2, the solid wood board is preheated in the thickness direction at a heat transfer rate of 1~3 mm / min.
3. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, Solid wood boards can be made from any of the following tree species: oak, birch, eucalyptus, maple, cedar, balsa, teak, black walnut, ironwood, and oak.
4. The hot-pressing and drying method for solid wood boards according to claim 3, characterized in that, Solid wood boards are obtained by slicing or sawing logs. The thickness of solid wood boards is 20~45mm, and the initial moisture content before drying is 30%~50%.
5. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, When using constant temperature mode, the hot pressing temperature is one of the values between 80 and 120℃.
6. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, When using the gradient temperature change mode: at temperature T1 and held for a certain time t1, the temperature is increased to temperature T2; at temperature T2 and held for a certain time t2; the temperature is increased to temperature T3 and held for a certain time t3; and T1 is a temperature value between 80 and 90℃, T2 is a temperature value between 95 and 105℃, and T3 is a temperature value between 110 and 120℃.
7. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, S4 specifies pressure limits for boards of different densities: For low-density materials with an air-dry density ρ less than 0.5 g / cm³, the radial pressure is 0.5~1MPa and the tangential pressure is 0.3~0.6MPa. For medium-density lumber with an air-dry density ρ of 0.5~0.85 g / cm³, the radial pressure is 1.0~1.5 MPa and the tangential pressure is 0.5~1.0 MPa. For high-density materials with an air-dry density ρ greater than 0.85 g / cm³, the radial pressure is 1.5~2.0 MPa and the tangential pressure is 0.8~1.2 MPa.
8. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, The moisture content threshold for step S4 is 12%.
9. The hot-pressing and drying method for solid wood boards according to claim 1, characterized in that, Step S1 is preceded by: S0. Pretreatment of solid wood boards: Cracks are engraved on the upper and lower surfaces of the solid wood boards to form controlled moisture diffusion channels; The depth of the scribing is 0.5~2mm, the width of the scribing is 0.3~0.8mm, and the spacing between the scribing marks is 10~30mm; The grooves are oriented in the same direction as the internal conduits of the board, which facilitates the transport of moisture along the conduits.
10. The hot-pressing and drying method for solid wood boards according to claim 9, characterized in that, Step S5 is followed by: S6. The process involves planing and sanding; the planing amount is 1~3mm.