Analysis of Factors Affecting Surface Waviness of Blockboard (1)
2024-04-21 14:09:09
In the national standard for wood-based panels, blockboard has an indicator called surface waviness, which is not available for wood-based panels such as plywood, hardboard, MDF and particleboard. A special indicator of the surface waviness of a blockboard is determined by its special structure. In general, the smaller the basic constituent unit of the artificial board, the smaller the surface waviness or unevenness. For example, the constituent elements of the fiberboard are elongated fibers, the particleboard is shaved and polished, and the plywood is a thin flat veneer. These structural units are easily plastically deformed at high temperature and hot pressing, and the board surface is generally In the case, the flatness is high and the waviness is small. The core of the blockboard is made of wood saw with a certain length, width and thickness, and the unit is large. Under the same hot pressing process, it is difficult to achieve permanent plastic deformation. Due to the influence of various factors such as the material type, material content, water content, processing precision and processing size of the core strip, the unevenness of the board surface often occurs. This may be one of the reasons why the blockboards produced by many domestic manufacturers have been difficult to meet the high-end furniture materials. Based on the research and production practice of blockboard in recent years, the author puts forward some opinions on the causes and prevention methods of the surface waviness of blockboard, for the reference of the blockboard researchers and manufacturers.
The influence of the middle plate is a common veneer between the core of the blockboard and the back of the watchboard, and its texture is perpendicular to the texture of the core strip and the back panel. The main function of the middle plate is to increase the transverse static bending strength of the plate, and at the same time play an important role in the surface waviness of the plate.
1.1 Influence of the thickness of the medium plate Due to the increasingly tight source of the material of the watch plate, the front and back plates of the plywood and the blockboard have become increasingly thin. At present, the thickness of the back sheet at home and abroad is generally between 0.7mm and 1mm, and the thinnest is even 0.5mm. The intermediate plate naturally becomes a necessary buffer layer between the core sheets of such a thin surface plate and the wooden strips. Under normal circumstances, the core sheets assembled by the core strips will appear on the surface of the pressed sheet due to the inconsistent water content, the different hardness of the wood strip, the different shrinkage ratio of the core material, the dimensional error of the core strip processing, and the like. Local unevenness. The middle plate acts as a buffer layer to compensate for these unevenness and the surface in contact with the steel platen is still flattened. The pressed blockboard is soaked in hot water, and the middle plate and the core plate are layered, and the concave (or convex) portion of the middle plate is exactly opposite to the convex (or concave) portion of the core plate. If you match, you can prove this. Therefore, one of the important structures must also have a certain thickness. If the thickness is too thin, the purpose of buffering is not achieved. If the thickness is too large, it is not only unnecessary, but also loses the characteristics of the board which mainly uses the core strip as the main raw material. According to the production practice, when the thickness of the back sheet is 0.7 mm to 1 mm, the thickness of the middle plate should be between 1.5 mm and 2.2 mm. When the back sheet is thick, the middle plate may be appropriately thinned, but it is preferably not less than 1.5 mm.
1.2 Influence of the processing quality of the middle plate Since the back of the watch is thin, the quality of the middle plate is not high, and its defects are easy to be expressed on the surface. The first is the roughness of the middle plate. If the surface grooves and cracks of the middle plate are large, the thin back plate cannot cover and buffer these defects. As a result, the surface of the groove and the crack will appear to be consistent with the plate defect. Groove. Secondly, the surface of the middle plate is not flat or called "the ruffled edge". Due to the excessive surface area, the "flange edge" overlaps and forms no overlap when hot pressing. As a result, a laminated core phenomenon occurs, and convex stripes appear on the surface of the plate. Once again, the thickness of the plate is uneven or the material of the knot is soft and hard. After the hot pressing is completed, the surface is not flat. It is not obvious. After a period of time, due to the rebound of the wood in the middle plate, the surface will be uneven. . Therefore, the processing of the middle plate should be smooth and flat, the cracks should not be too large, and the excessive thrift should also be repaired and trimmed to reduce the unevenness of the surface of the plate.
1.2 Influence of medium plate types Most of the domestic board of the blockboard manufacturers do not use the whole piece at present, which is basically a multi-piece piece, generally two, three and four. Between these panels, if the material of the veneers is not consistent, the shrinkage ratio and compression ratio are also different. After hot pressing, there will be unevenness between the plate and the middle plate. As a result, at their joints, there is a distinct streak on the surface of the sheet. When using a ruler to detect this part, it can be measured that the sides of the stripe are uneven. Therefore, the material of the middle plate should be consistent. Of course, this means in a piece of blockboard.
2 core strip effect
For blockboards, the quality of the core strip is the main quality of the panel, which depends to a large extent on the quality of the core strip material. The density, hardness, drying characteristics, compressibility, shrinkage, equilibrium moisture content and processability of the core material directly or indirectly affect the appearance quality, dimensional tolerance and mechanical properties of the blockboard. For the surface waviness of the blockboard, the influence of the core strip size, processing accuracy, water content, material type and material are large.
2.1 Influence of core strip size
According to the previous analysis, the surface flatness of the wood-based panel is related to the size of the basic unit constituting the panel. That is to say, under the condition that the processing precision is constant, the smaller the size of the core strip, the smaller the surface waviness of the blockboard will be. However, the smaller the core strip size, the lower the utilization rate of the wood, the lower the processing efficiency, and the higher the requirements for the machined equipment. In addition, the size of the core strip is too small, which also affects the flattening of the core sheets. On the other hand, due to the shortcomings of the wood itself, such as bending deformation, shrinkage and swelling, the larger the size of the core strip, the more uneven the surface of the board. For example, the shape of the core strip in the width of the tile and the bow in the length will cause local stress inside the blockboard, causing ripples on the board surface. The larger the width of the core strip, the longer the length, and the thinner the thickness, the easier it is. Therefore, when designing the size of the core strip, various factors should be considered, including the material of the core strip and the deformation characteristics of the material. Strictly speaking, the method of material production has an effect on the deformation of the core strip. The deformation and contraction of the diaphragm section and the string section are different, and the flatness of the surface of the sheet has different effects. However, in the production, it is impossible to separate the slab from the chord. As long as the size of the core is controlled well, the effect can be reduced to the extent that the quality of the slab is not affected. For the specific size of the core strip, there have been reports that the width should be 2.5 to 3 times the thickness. For example, a 12 mm thick core strip has a width of 30 to 36 mm. According to the author's experience, the size of the core strip is first determined by the material. For the masson pine, poplar, lotus, water melon, etc. used in the southern core, the width of the core strip can be 4 to 6 times its thickness. For wood with small deformation such as fir and cedar, it can even be more than 6 times its thickness. For wood with large deformation such as Metasequoia and Liquidambar, the width should be a small multiple of the thickness, such as 3 to 4 times. The length of the core strip is generally between 500mm and 1000mm, and the wood with large deformation takes a low value, and vice versa.
2.2 Influence of core strip processing accuracy
The core board of the blockboard has a side glue and a side glue. For the process of side-sealing into a core board and then sanding or planing, the influence of the processing precision of the core strip is small. For the core plate not to be side-stitched, the processing precision of the core strip, especially the thickness, obviously affects the flatness of the surface. The uneven thickness of the core strip leaves high and low unevenness on the surface of the sheet. This may not be obvious after the hot press has just been completed. However, when the sheet is left for a certain period of time, different degrees of rebound will occur due to the compression of the uneven thickness of the core strip, and the surface will be uneven. Therefore, the processing of the core strip must control the thickness tolerance. In general, the thickness tolerance of the machining should be controlled between ±0.3mm and ±0.5mm. When the core strip material is soft, take the high limit, and vice versa.
2.3 Influence of core strip moisture content
The water content of the core strip is generally required to be between 8% and 12%, and its effect is mainly due to the inconsistent water content between the core strip and the core strip. When the moisture content of the core strips in a certain board differs greatly, the shrinkage under high temperature and high pressure will have a big difference. The core strip with high water content shrinks greatly, forming a partial depression on the surface of the sheet, causing unevenness. This phenomenon generally occurs when the partial core strip moisture content is higher than 20%.
2.4 Influence of core strips
According to the above analysis, the size of the core strip has a certain relationship with the tree species used in the core strip. In addition, the density, hardness and cross-sectional drying factor of the blockboard core material have a certain influence on the surface waviness of the blockboard. On the one hand, due to various factors such as local conditions, tree age, heartwood and other factors, the density, hardness and dry shrinkage coefficient of the same tree species and even the same tree are different, and the core strips in the blockboard are also Different compression ratios and shrinkage rates are produced, resulting in a certain effect on the surface ripple of the blockboard. But this effect is often small or even difficult to detect. On the other hand, the density, hardness and shrinkage of the various woods often vary greatly. If the woods with large differences are made into core strips and mixed together to produce blockboard, it will obviously be Surface quality has a big impact. From the analysis of a test result, the blockboard core strips are preferably made of wood having a density of 0.4 to 0.6 g/cm3 and a hard or slightly soft material. The blockboard of such woods has a lower surface ripple when used as a core strip. degree.
The influence of the middle plate is a common veneer between the core of the blockboard and the back of the watchboard, and its texture is perpendicular to the texture of the core strip and the back panel. The main function of the middle plate is to increase the transverse static bending strength of the plate, and at the same time play an important role in the surface waviness of the plate.
1.1 Influence of the thickness of the medium plate Due to the increasingly tight source of the material of the watch plate, the front and back plates of the plywood and the blockboard have become increasingly thin. At present, the thickness of the back sheet at home and abroad is generally between 0.7mm and 1mm, and the thinnest is even 0.5mm. The intermediate plate naturally becomes a necessary buffer layer between the core sheets of such a thin surface plate and the wooden strips. Under normal circumstances, the core sheets assembled by the core strips will appear on the surface of the pressed sheet due to the inconsistent water content, the different hardness of the wood strip, the different shrinkage ratio of the core material, the dimensional error of the core strip processing, and the like. Local unevenness. The middle plate acts as a buffer layer to compensate for these unevenness and the surface in contact with the steel platen is still flattened. The pressed blockboard is soaked in hot water, and the middle plate and the core plate are layered, and the concave (or convex) portion of the middle plate is exactly opposite to the convex (or concave) portion of the core plate. If you match, you can prove this. Therefore, one of the important structures must also have a certain thickness. If the thickness is too thin, the purpose of buffering is not achieved. If the thickness is too large, it is not only unnecessary, but also loses the characteristics of the board which mainly uses the core strip as the main raw material. According to the production practice, when the thickness of the back sheet is 0.7 mm to 1 mm, the thickness of the middle plate should be between 1.5 mm and 2.2 mm. When the back sheet is thick, the middle plate may be appropriately thinned, but it is preferably not less than 1.5 mm.
1.2 Influence of the processing quality of the middle plate Since the back of the watch is thin, the quality of the middle plate is not high, and its defects are easy to be expressed on the surface. The first is the roughness of the middle plate. If the surface grooves and cracks of the middle plate are large, the thin back plate cannot cover and buffer these defects. As a result, the surface of the groove and the crack will appear to be consistent with the plate defect. Groove. Secondly, the surface of the middle plate is not flat or called "the ruffled edge". Due to the excessive surface area, the "flange edge" overlaps and forms no overlap when hot pressing. As a result, a laminated core phenomenon occurs, and convex stripes appear on the surface of the plate. Once again, the thickness of the plate is uneven or the material of the knot is soft and hard. After the hot pressing is completed, the surface is not flat. It is not obvious. After a period of time, due to the rebound of the wood in the middle plate, the surface will be uneven. . Therefore, the processing of the middle plate should be smooth and flat, the cracks should not be too large, and the excessive thrift should also be repaired and trimmed to reduce the unevenness of the surface of the plate.
1.2 Influence of medium plate types Most of the domestic board of the blockboard manufacturers do not use the whole piece at present, which is basically a multi-piece piece, generally two, three and four. Between these panels, if the material of the veneers is not consistent, the shrinkage ratio and compression ratio are also different. After hot pressing, there will be unevenness between the plate and the middle plate. As a result, at their joints, there is a distinct streak on the surface of the sheet. When using a ruler to detect this part, it can be measured that the sides of the stripe are uneven. Therefore, the material of the middle plate should be consistent. Of course, this means in a piece of blockboard.
2 core strip effect
For blockboards, the quality of the core strip is the main quality of the panel, which depends to a large extent on the quality of the core strip material. The density, hardness, drying characteristics, compressibility, shrinkage, equilibrium moisture content and processability of the core material directly or indirectly affect the appearance quality, dimensional tolerance and mechanical properties of the blockboard. For the surface waviness of the blockboard, the influence of the core strip size, processing accuracy, water content, material type and material are large.
2.1 Influence of core strip size
According to the previous analysis, the surface flatness of the wood-based panel is related to the size of the basic unit constituting the panel. That is to say, under the condition that the processing precision is constant, the smaller the size of the core strip, the smaller the surface waviness of the blockboard will be. However, the smaller the core strip size, the lower the utilization rate of the wood, the lower the processing efficiency, and the higher the requirements for the machined equipment. In addition, the size of the core strip is too small, which also affects the flattening of the core sheets. On the other hand, due to the shortcomings of the wood itself, such as bending deformation, shrinkage and swelling, the larger the size of the core strip, the more uneven the surface of the board. For example, the shape of the core strip in the width of the tile and the bow in the length will cause local stress inside the blockboard, causing ripples on the board surface. The larger the width of the core strip, the longer the length, and the thinner the thickness, the easier it is. Therefore, when designing the size of the core strip, various factors should be considered, including the material of the core strip and the deformation characteristics of the material. Strictly speaking, the method of material production has an effect on the deformation of the core strip. The deformation and contraction of the diaphragm section and the string section are different, and the flatness of the surface of the sheet has different effects. However, in the production, it is impossible to separate the slab from the chord. As long as the size of the core is controlled well, the effect can be reduced to the extent that the quality of the slab is not affected. For the specific size of the core strip, there have been reports that the width should be 2.5 to 3 times the thickness. For example, a 12 mm thick core strip has a width of 30 to 36 mm. According to the author's experience, the size of the core strip is first determined by the material. For the masson pine, poplar, lotus, water melon, etc. used in the southern core, the width of the core strip can be 4 to 6 times its thickness. For wood with small deformation such as fir and cedar, it can even be more than 6 times its thickness. For wood with large deformation such as Metasequoia and Liquidambar, the width should be a small multiple of the thickness, such as 3 to 4 times. The length of the core strip is generally between 500mm and 1000mm, and the wood with large deformation takes a low value, and vice versa.
2.2 Influence of core strip processing accuracy
The core board of the blockboard has a side glue and a side glue. For the process of side-sealing into a core board and then sanding or planing, the influence of the processing precision of the core strip is small. For the core plate not to be side-stitched, the processing precision of the core strip, especially the thickness, obviously affects the flatness of the surface. The uneven thickness of the core strip leaves high and low unevenness on the surface of the sheet. This may not be obvious after the hot press has just been completed. However, when the sheet is left for a certain period of time, different degrees of rebound will occur due to the compression of the uneven thickness of the core strip, and the surface will be uneven. Therefore, the processing of the core strip must control the thickness tolerance. In general, the thickness tolerance of the machining should be controlled between ±0.3mm and ±0.5mm. When the core strip material is soft, take the high limit, and vice versa.
2.3 Influence of core strip moisture content
The water content of the core strip is generally required to be between 8% and 12%, and its effect is mainly due to the inconsistent water content between the core strip and the core strip. When the moisture content of the core strips in a certain board differs greatly, the shrinkage under high temperature and high pressure will have a big difference. The core strip with high water content shrinks greatly, forming a partial depression on the surface of the sheet, causing unevenness. This phenomenon generally occurs when the partial core strip moisture content is higher than 20%.
2.4 Influence of core strips
According to the above analysis, the size of the core strip has a certain relationship with the tree species used in the core strip. In addition, the density, hardness and cross-sectional drying factor of the blockboard core material have a certain influence on the surface waviness of the blockboard. On the one hand, due to various factors such as local conditions, tree age, heartwood and other factors, the density, hardness and dry shrinkage coefficient of the same tree species and even the same tree are different, and the core strips in the blockboard are also Different compression ratios and shrinkage rates are produced, resulting in a certain effect on the surface ripple of the blockboard. But this effect is often small or even difficult to detect. On the other hand, the density, hardness and shrinkage of the various woods often vary greatly. If the woods with large differences are made into core strips and mixed together to produce blockboard, it will obviously be Surface quality has a big impact. From the analysis of a test result, the blockboard core strips are preferably made of wood having a density of 0.4 to 0.6 g/cm3 and a hard or slightly soft material. The blockboard of such woods has a lower surface ripple when used as a core strip. degree.
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