Production process and quality control of environmentally friendly veneer curved wood furniture
2019-08-15 06:04:00
Veneered and curved wood furniture is one of the more popular furniture products in the world. Compared with solid wood and panel furniture, curved wood furniture has advantages in many aspects. For example, a multi-surface anisotropic member can be conveniently produced using a three-way or multi-directional press; since the member is laminated by a plurality of veneers, the best raw material can be used on the surface, and the defective raw material can be placed on the core. Layer, which improves the quality of the product and the utilization of wood; due to the stratification of the veneer, the static bending strength and elastic modulus of the member are far better than the solid wood and wood-based panels of the same tree species, so it can be compared The small size acts as a load-bearing part of the furniture, and the furniture made with it also has advantages in terms of comfort and fatigue resistance. With the increasing living standards of the people and the increasing brand awareness of furniture manufacturers, environmentally-friendly furniture will become the mainstream of the market. The problem of free formaldehyde has always been an important factor restricting the development of curved wood furniture due to the large amount of urea-formaldehyde resin adhesive used in the bending of wood. With the introduction of some advanced technologies and production processes, China's environmentally-friendly curved wood furniture has made great progress. The following briefly introduces the production process and quality control of low-formaldehyde veneer curved wood furniture.
1 veneer curved wood furniture production process
2 Main factors affecting product quality (related to gluing)
2.1 Board
The solid wood veneer is the basic raw material for the curved wood. The veneer requires uniform thickness and the error is generally controlled at ±0.1mm.
When the raw materials enter the plant, the moisture content of the veneers should be determined in time, and the suitable water content is 6-10%; the difference in moisture content between the plates should not exceed ±2%. The moisture content of the veneer should be measured every 4 hours on the production line. If the moisture content is found to be greater than 12%, the veneer must be removed. The moisture content of the veneer is too high, which may cause problems such as incomplete curing of the resin, opening of the product, an increase in the amount of formaldehyde released, and unstable product size.
The water content is more obvious for the manufacturer using the high-frequency press. In addition to causing the above defects, the curing speed of the adhesive is also lowered, and in severe cases, the product is bubbled or broken down, thereby causing the factory to reject the product. increase rapidly. Therefore, for manufacturers using high-frequency presses, the control of moisture content of veneers should be more stringent. It is recommended to control the moisture content within the range of 4-8%.
2.2 Adhesive selection
Most of the adhesives used for veneer curved wood furniture are urea-formaldehyde resin adhesives. Urea-formaldehyde resin adhesive has the advantages of good bonding force, fast curing speed, no color, easy preparation, and low cost. The performance of the adhesive is one of the important factors related to the inherent quality of curved wood furniture. The properties of the glue include strength grade, water resistance, aging resistance, active period, curing speed, operability, and the like. The process of synthesizing urea-formaldehyde resin by reaction of urea with formaldehyde is very complicated, and the performance and quality of different glues are greatly different. Therefore, the selection of high-quality adhesive is the key to ensure the quality of veneer bent wood. When selecting an adhesive, it is first necessary to know the end use of the product, the required testing standards, and the transportation, storage, and use conditions. After rigorous testing, the performance of the adhesive can be determined after comparing the properties.
2.3 Glue amount
In the production of curved wood, we recommend a coating amount of 11Os 1O g/m2, which can increase the amount of glue for veneers with poor quality and deep cracks. The requirements for the production line are: the amount of glue applied is measured once at the beginning of each shift, and once every four hours (experienced operators can basically detect small changes in the amount of glue with long-term work experience). The amount and the uniformity of the rubber layer vary greatly. The glue applicator should be adjusted in time to restore the correct amount of glue. If the amount of glue applied is too high, the release of free formaldehyde in the product will exceed the standard, and the product will be brittle and the production cost will increase. Too low a coating amount will result in a failure to meet the required bonding strength.
2.4 aging time
In the production of curved wood, the aging time depends on the viscosity and solids content of the mixed glue. A certain closed aging time is beneficial to the bonding strength, but in principle it is not too long, and should be paid more attention in the high temperature and dry season. It is recommended that the shorter the aging time, the better, and the closed aging time is less than 15 minutes. Closed aging test should be done at different seasons. If the rubber layer is dry within the closed aging time, it indicates that the rubber layer has been pre-cured, which will greatly reduce the bonding strength. The closed aging time should be shortened appropriately.
2.5 hot pressing
Hot pressing is a key link in the production of curved wood. The production process and parameters of the hot pressing and high-frequency hot pressing of the ordinary hot plate are introduced according to the different heating methods.
2.5.1 Hot pressing pressure
The hot pressing pressure is first determined before hot pressing, and the hot pressing pressure depends on the density of the adherend, the surface smoothness, the viscosity of the mixed glue, the thickness tolerance, and the paving time. The normal pressure is 0.5-1.6 MPa (about 5-16 Kg/cm2). Increasing the hot pressing pressure within a certain range will significantly increase the bonding strength, but too high pressure will make the production more difficult (difficulty in exhausting during hot pressing), and at the same time reduce the utilization rate of wood, resulting in an increase in production cost; On the contrary, if the pressure is insufficient, it is difficult to ensure sufficient pressure during the bonding to make the bonding surface close contact, which is easy to cause the product to open, especially for the high density material; the viscosity of the rubber should be increased appropriately. pressure. Generally, low-density tree species (density less than 0.4g/cm3) should be at a pressure of about 0.6 to 1.0 MPa, while trees with a high density (density of more than 0.4 g/cm3) should be at a pressure of about 1.0 to 1.6 MPa. So choosing the right pressure is very important.
2.5.2 Hot pressing temperature and hot pressing time
The hot pressing temperature can be determined according to the hot pressing parameters of the glue. On the contrary, the formulation of the adhesive can be adjusted according to the parameters of the hot press to adapt to the working parameters of the hot press.
(1) Hot pressing plate hot pressing
The hot pressing temperature of the hot pressing of the hot plate is generally not lower than 75 ° C, otherwise the production efficiency will be lowered. The temperature uniformity should be checked after changing the hot pressing parameters or adjusting the press. The temperature at different positions on the surface of the product should be detected immediately after the board is released. Too low or uneven temperature will cause the glue layer to open. The pressurization time depends on factors such as temperature, curing speed of the glue, density of the adherend, and the like. When hot pressing is applied, the pressing time is related to the temperature of the glue line. It is necessary to allow time for heat to be transferred from the hot plate. The heat penetration time varies according to the density and humidity of the wood and the distance from the farthest glue line. Table 1 shows the hot pressing time required for low and medium density wood (for reference only).
Additional time (min) corresponding to the distance of the farthest glue line per mm of the pressurizing temperature
Less than 5mm 5-10mm is greater than 10mm
90 1.1 1.3 1.5
100 1.0 1.2 1.4
110 0.9 1.2 1.3
Note: The farthest glue line distance refers to the distance from the curved wood surface layer to the middle layer.
(2) high frequency heating hot pressing
High frequency heating depends on many factors such as press frequency, voltage, slab moisture content, amount of glue applied, ambient temperature and electrical properties and size of the adherend. The high-frequency hot pressing in the production of curved wood is generally divided into two stages, the first stage is the high-frequency heating stage, and the second stage is the pressure-maintaining cooling stage. High-frequency heating generally takes a short time, while holding pressure takes a long time and depends on the area of ​​the adherend and the ambient temperature.
The determination of the high-frequency heating time is generally based on the determination of the adhesive parameters to determine a number of equivalent test pieces after the adhesive formulation, and the performance of the test piece is tested according to different high-frequency heating time under the condition that other parameters are unchanged, and the high-frequency in the qualified test piece The short heating time is the high frequency heating time of the test piece of this specification. The high-frequency heating time is proportional to the area of ​​the test piece and the amount of glue applied. The high-frequency heating time should also change when the high-frequency power, voltage, slab moisture content, ambient temperature and tree species change.
The determination of the holding pressure cooling time, because the high temperature heating causes the temperature of the test piece to start from the internal glue line, the temperature inside the test piece is obviously higher than the external temperature. When the internal temperature is higher than 100 如, if the pressure is suddenly relieved, try The instantaneous release of water vapor inside the piece causes the test piece to bubble open. The holding time is determined according to the internal temperature reduction speed. The internal temperature is reduced to less than 100 å³å¯ to end the holding pressure. For products with small size or water vapor, the pressure-preserving cooling stage can be omitted. In addition, the pressure holding stage is also a stage for equalizing the temperature. Since the temperature outside the test piece (the part in contact with the mold) is low, the heat is easily absorbed by the mold, and the test piece needs a certain time to absorb the internal heat to supplement the lost heat. Therefore, the glue on the outside of the test piece is cured completely, requiring a relatively long curing time. Manufacturers generally use a set of high-frequency equipment to configure two sets of press systems for products that require pressure-maintaining cooling, which saves energy and doubles production efficiency.
2.6 Formaldehyde emission
The amount of formaldehyde released from a product using a urea-formaldehyde adhesive depends on many factors. The most important factors are the formulation of the glue, the tree species, the amount of glue applied, the humidity of the glue to be glued, the paving time and the pressurization conditions. In addition, the modifier can effectively reduce the amount of formaldehyde released. Therefore, the choice of high-quality adhesives and advanced production processes is the key to producing environmentally-friendly curved wood furniture.
There are some issues to be aware of when choosing an adhesive. In order to reduce free formaldehyde and production costs, some adhesive suppliers usually provide some low molar ratio urea-formaldehyde resin. If the molar ratio is too low, the strength, water resistance and aging resistance of the adhesive will be lowered, and the service life of the product will be reduced. As the aging resistance of the adhesive is reduced, the formaldehyde in the adhesive will be gradually released from the product, which will damage the consumer's interests. Although the addition of the modifier of the same kind of adhesive can reduce the free formaldehyde, the bonding strength and water resistance of the product are also Will decline, the principle of adding modifiers must be carried out under the premise of ensuring the physical and mechanical properties of the product.
1 veneer curved wood furniture production process
2 Main factors affecting product quality (related to gluing)
2.1 Board
The solid wood veneer is the basic raw material for the curved wood. The veneer requires uniform thickness and the error is generally controlled at ±0.1mm.
When the raw materials enter the plant, the moisture content of the veneers should be determined in time, and the suitable water content is 6-10%; the difference in moisture content between the plates should not exceed ±2%. The moisture content of the veneer should be measured every 4 hours on the production line. If the moisture content is found to be greater than 12%, the veneer must be removed. The moisture content of the veneer is too high, which may cause problems such as incomplete curing of the resin, opening of the product, an increase in the amount of formaldehyde released, and unstable product size.
The water content is more obvious for the manufacturer using the high-frequency press. In addition to causing the above defects, the curing speed of the adhesive is also lowered, and in severe cases, the product is bubbled or broken down, thereby causing the factory to reject the product. increase rapidly. Therefore, for manufacturers using high-frequency presses, the control of moisture content of veneers should be more stringent. It is recommended to control the moisture content within the range of 4-8%.
2.2 Adhesive selection
Most of the adhesives used for veneer curved wood furniture are urea-formaldehyde resin adhesives. Urea-formaldehyde resin adhesive has the advantages of good bonding force, fast curing speed, no color, easy preparation, and low cost. The performance of the adhesive is one of the important factors related to the inherent quality of curved wood furniture. The properties of the glue include strength grade, water resistance, aging resistance, active period, curing speed, operability, and the like. The process of synthesizing urea-formaldehyde resin by reaction of urea with formaldehyde is very complicated, and the performance and quality of different glues are greatly different. Therefore, the selection of high-quality adhesive is the key to ensure the quality of veneer bent wood. When selecting an adhesive, it is first necessary to know the end use of the product, the required testing standards, and the transportation, storage, and use conditions. After rigorous testing, the performance of the adhesive can be determined after comparing the properties.
2.3 Glue amount
In the production of curved wood, we recommend a coating amount of 11Os 1O g/m2, which can increase the amount of glue for veneers with poor quality and deep cracks. The requirements for the production line are: the amount of glue applied is measured once at the beginning of each shift, and once every four hours (experienced operators can basically detect small changes in the amount of glue with long-term work experience). The amount and the uniformity of the rubber layer vary greatly. The glue applicator should be adjusted in time to restore the correct amount of glue. If the amount of glue applied is too high, the release of free formaldehyde in the product will exceed the standard, and the product will be brittle and the production cost will increase. Too low a coating amount will result in a failure to meet the required bonding strength.
2.4 aging time
In the production of curved wood, the aging time depends on the viscosity and solids content of the mixed glue. A certain closed aging time is beneficial to the bonding strength, but in principle it is not too long, and should be paid more attention in the high temperature and dry season. It is recommended that the shorter the aging time, the better, and the closed aging time is less than 15 minutes. Closed aging test should be done at different seasons. If the rubber layer is dry within the closed aging time, it indicates that the rubber layer has been pre-cured, which will greatly reduce the bonding strength. The closed aging time should be shortened appropriately.
2.5 hot pressing
Hot pressing is a key link in the production of curved wood. The production process and parameters of the hot pressing and high-frequency hot pressing of the ordinary hot plate are introduced according to the different heating methods.
2.5.1 Hot pressing pressure
The hot pressing pressure is first determined before hot pressing, and the hot pressing pressure depends on the density of the adherend, the surface smoothness, the viscosity of the mixed glue, the thickness tolerance, and the paving time. The normal pressure is 0.5-1.6 MPa (about 5-16 Kg/cm2). Increasing the hot pressing pressure within a certain range will significantly increase the bonding strength, but too high pressure will make the production more difficult (difficulty in exhausting during hot pressing), and at the same time reduce the utilization rate of wood, resulting in an increase in production cost; On the contrary, if the pressure is insufficient, it is difficult to ensure sufficient pressure during the bonding to make the bonding surface close contact, which is easy to cause the product to open, especially for the high density material; the viscosity of the rubber should be increased appropriately. pressure. Generally, low-density tree species (density less than 0.4g/cm3) should be at a pressure of about 0.6 to 1.0 MPa, while trees with a high density (density of more than 0.4 g/cm3) should be at a pressure of about 1.0 to 1.6 MPa. So choosing the right pressure is very important.
2.5.2 Hot pressing temperature and hot pressing time
The hot pressing temperature can be determined according to the hot pressing parameters of the glue. On the contrary, the formulation of the adhesive can be adjusted according to the parameters of the hot press to adapt to the working parameters of the hot press.
(1) Hot pressing plate hot pressing
The hot pressing temperature of the hot pressing of the hot plate is generally not lower than 75 ° C, otherwise the production efficiency will be lowered. The temperature uniformity should be checked after changing the hot pressing parameters or adjusting the press. The temperature at different positions on the surface of the product should be detected immediately after the board is released. Too low or uneven temperature will cause the glue layer to open. The pressurization time depends on factors such as temperature, curing speed of the glue, density of the adherend, and the like. When hot pressing is applied, the pressing time is related to the temperature of the glue line. It is necessary to allow time for heat to be transferred from the hot plate. The heat penetration time varies according to the density and humidity of the wood and the distance from the farthest glue line. Table 1 shows the hot pressing time required for low and medium density wood (for reference only).
Additional time (min) corresponding to the distance of the farthest glue line per mm of the pressurizing temperature
Less than 5mm 5-10mm is greater than 10mm
90 1.1 1.3 1.5
100 1.0 1.2 1.4
110 0.9 1.2 1.3
Note: The farthest glue line distance refers to the distance from the curved wood surface layer to the middle layer.
(2) high frequency heating hot pressing
High frequency heating depends on many factors such as press frequency, voltage, slab moisture content, amount of glue applied, ambient temperature and electrical properties and size of the adherend. The high-frequency hot pressing in the production of curved wood is generally divided into two stages, the first stage is the high-frequency heating stage, and the second stage is the pressure-maintaining cooling stage. High-frequency heating generally takes a short time, while holding pressure takes a long time and depends on the area of ​​the adherend and the ambient temperature.
The determination of the high-frequency heating time is generally based on the determination of the adhesive parameters to determine a number of equivalent test pieces after the adhesive formulation, and the performance of the test piece is tested according to different high-frequency heating time under the condition that other parameters are unchanged, and the high-frequency in the qualified test piece The short heating time is the high frequency heating time of the test piece of this specification. The high-frequency heating time is proportional to the area of ​​the test piece and the amount of glue applied. The high-frequency heating time should also change when the high-frequency power, voltage, slab moisture content, ambient temperature and tree species change.
The determination of the holding pressure cooling time, because the high temperature heating causes the temperature of the test piece to start from the internal glue line, the temperature inside the test piece is obviously higher than the external temperature. When the internal temperature is higher than 100 如, if the pressure is suddenly relieved, try The instantaneous release of water vapor inside the piece causes the test piece to bubble open. The holding time is determined according to the internal temperature reduction speed. The internal temperature is reduced to less than 100 å³å¯ to end the holding pressure. For products with small size or water vapor, the pressure-preserving cooling stage can be omitted. In addition, the pressure holding stage is also a stage for equalizing the temperature. Since the temperature outside the test piece (the part in contact with the mold) is low, the heat is easily absorbed by the mold, and the test piece needs a certain time to absorb the internal heat to supplement the lost heat. Therefore, the glue on the outside of the test piece is cured completely, requiring a relatively long curing time. Manufacturers generally use a set of high-frequency equipment to configure two sets of press systems for products that require pressure-maintaining cooling, which saves energy and doubles production efficiency.
2.6 Formaldehyde emission
The amount of formaldehyde released from a product using a urea-formaldehyde adhesive depends on many factors. The most important factors are the formulation of the glue, the tree species, the amount of glue applied, the humidity of the glue to be glued, the paving time and the pressurization conditions. In addition, the modifier can effectively reduce the amount of formaldehyde released. Therefore, the choice of high-quality adhesives and advanced production processes is the key to producing environmentally-friendly curved wood furniture.
There are some issues to be aware of when choosing an adhesive. In order to reduce free formaldehyde and production costs, some adhesive suppliers usually provide some low molar ratio urea-formaldehyde resin. If the molar ratio is too low, the strength, water resistance and aging resistance of the adhesive will be lowered, and the service life of the product will be reduced. As the aging resistance of the adhesive is reduced, the formaldehyde in the adhesive will be gradually released from the product, which will damage the consumer's interests. Although the addition of the modifier of the same kind of adhesive can reduce the free formaldehyde, the bonding strength and water resistance of the product are also Will decline, the principle of adding modifiers must be carried out under the premise of ensuring the physical and mechanical properties of the product.
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