There are three ways to make a veneer, namely sawing, planing and cutting. In the production of furniture, sliced ​​veneers are the most common and most important.
Planed veneers are also used on the edges of doors, strips and solid wood strips (ie thick veneers). The thickness of the veneer is from 0.2mm to 5mm, which requires the veneer to be firm, dimensionally stable and easy to process. Although the thickness of the veneers supplied on the market can be as small as 0.2 mm, such thin veneers are generally not recommended because they may increase the cost of painting, thereby offsetting material savings.
Board preparation
In this respect, the control of moisture content is very important, and the water content should be controlled within 8-11%. If the moisture content of the veneer is too high, for example 15-16%, it will be troublesome in the next step of processing. On the other hand, if the water content is too low, cracks may occur and problems occur during cutting.
The veneer should be stored flat, the temperature and humidity of the storage should be controlled, and dust should be avoided. If it is to be stored for a long period of time, it is best to cover it with a film on the outside of the veneer bundle and to prevent sun exposure. This rule even applies to the case where the board has been stacked.
When cutting a board into a certain size, it should be carried out in a clean environment, and the chips should be removed in time to ensure that the board does not have cracks.
Sometimes it is necessary to splicing the veneers or slanting the veneers (note: a length of the joint, which can be single beveled or two beveled scissor), which can be stitched by wire ( Thread splicing, such as the German Kuper process, paper taping or vertical splicing (note: edge splicing, or "straight spelling", that is, two veneers stitched along the longitudinal edges, seams and textures The direction is the same).
Glue line splicing is the most common method used. In addition to its high splicing strength, the polyamide glue line gives the splicing a good flexibility, which means that the splicing veneer can withstand a little shrinkage and expansion. Climate change often occurs. If the thickness of the veneer is at least 0.6 mm, no post-processing is required. It is best not to use a veneer that is thinner than 0.6mm. This is because the veneer is hot pressed by the Kuper press and a thinner layer of glue is visible, which affects the appearance.
Paper splicing veneers are often used for furniture surface coverings of more complex shapes, such as the table top. This process is labor intensive and requires sanding and drying after splicing. It is a labor intensive process.
In terms of processing speed and cost, longitudinal drawing is the best process (standard urea-formaldehyde resin (UF) or milk white glue (polyvinyl acetate) can be used, but the moisture content should be controlled and the working environment Keep it clean). This process also does not require post-processing and does not require the use of glue lines.
However, the use of this process in Asia is less effective than in Europe. This is because climate change in Asia is relatively large. If the polyamide glue line in the Kuper process is not used, then there is no adaptability for shrinkage and expansion. The shrinkage and expansion may cause the seam at the end of the veneer to crack.
The process after the veneer splicing is glued and pressurized, or at room temperature or high temperature. The storage period between cutting into size and splicing should be as short as possible, because during this period of time, changes in the moisture content of the veneer will cause cracks. It is recommended that the cutting, splicing, and flat coating be carried out as quickly and continuously as possible.
Plane overlay on the press
Generally, the flat coating is carried out on a hot press or a cold press, and the glue used is urea-formaldehyde resin or milk white glue. Mixtures of these two types of glue can be used for several special veneers such as birch or veneers that are raised on the back. The balance veneer or balance paper must be applied to the back of the substrate board (plain board) to prevent warpage of the board. If warpage occurs, the next step of processing such as edge sealing will cause problems.
Urea-formaldehyde resin glue is the most commonly used glue in flat coating. The reason is:
First, in all types of rubber, the open aging time of the glue (Note: the open aging time refers to the time after the surface of the adherend is glued to the time of exposure to the air before lamination). This longer period of time is very beneficial for more manual work in Asian factories, such as manual coating rolls used in Asian factories.
Solvent-based colorants or varnishes reduce the adhesion. Urea-formaldehyde resin is a cross-linking type of glue that is more resistant to this chemical than white glue, which is commonly used in antique furniture. in production. In the case of using a white glue, the glue layer often has a problem of delamination.
Because of its wide applicability, it has a good effect on film compression, but care must be taken not to contact it with solvent-based chemicals.
For birch veneers (used in Europe in the past two years), it is possible to use a mixture of urea-formaldehyde resin and milky white glue, which has the adhesive properties of these two types of glue, and the thermoplastic white glue makes the adhesive layer more adaptable. Large, and urea-formaldehyde resin adhesives have good durability to solvent-based chemicals.
Urea-formaldehyde resin adhesive (UF)
Urea-formaldehyde resin adhesives are used in a wide range of applications in about 90% of factories in Southeast Asia. The actual open aging time varies widely, from 1 minute on automated production lines to 15 minutes on manual operations.
Factors affecting the open time of urea-formaldehyde resin glue are: temperature, humidity, water addition, ventilation, and properties of the substrate. For example, in the case of high temperature and low humidity, the ventilation time is reduced to prevent the glue from curing too fast. The same effect can be achieved by reducing the amount of glue applied or by using a highly absorbent substrate such as particle board.
Modern urea-formaldehyde resin adhesives are often supplied in powder form (containing gums, curing agents and special ingredients), which are mixed with water to form a gel. The required open aging time can be controlled according to the amount of water added (the more water is used, the longer the aging time is).
Modern rubber types are much better than the three-component or four-component glue used in the original device. The latter is cumbersome to operate because the ratio of different components needs to be mastered to avoid delamination. Some boards are in Layering occurs only months after lamination. These rubber types must also be spared because of their shorter pot life. (Note: pot life, the pot life refers to the time when the formulated adhesive can maintain its usable performance)
If it is aged and then pressurized, the adhesive is already dry and there is not enough adhesive strength. A common cause of pre-cured glue drying is that the worker should open the fan in the vicinity for cooling, and the fan should not blow directly against the glue surface, which can reduce this problem. In order to avoid the wrong ratio, the easiest way is to use a modern one-component glue.
Another problem is that the worker always puts the plain plate into the applicator at the same position when applying the glue (usually in the middle of the applicator), which makes this part of the applicator roll dry, so that the glue is applied. Not enough. When applying glue, the board should be evenly distributed in all parts of the rubberizing roller to ensure sufficient and uniform coating.
The temperature and pressure during hot pressing must also be sufficiently high. The typical urea-formaldehyde resin hot pressing parameters are:
Pressurize at 120 ° C for 1 minute; or
80-100 ° C, pressurization for 3 minutes; or
Pressurize at 60 ° C for 4-5 minutes.
The pressing pressure depends on the tree species of the wood, and the applied pressure is usually 1.5-3 Kg/cm2. If the pressure is too high, it will produce a transparent rubber (note: the rubber or its components pass through the surface material of the artificial board, which causes the surface of the board to have rubber spots or discoloration), and may also cause cracks in the veneer.
For high resin content tree species such as pine, birch, and teak, it is best to use a low temperature, long hot press time process. High temperatures activate the oil or resin in the wood, causing partial delamination. For example, pine, the temperature of hot pressing should be about 50 °C.
The hot platen should be cleaned regularly to remove the remaining glue drops on the plate. The uneven platen surface will not cause the pressure on the parts of the plate to be inconsistent after the press is closed.
The surface of the substrate and veneer should be kept clean and free from spots or smudges. Workers are not allowed to touch the substrate or veneer with oily hands.
The glue is the glue that seeps out the surface of the veneer and makes the surface bright. The main reasons are:
â— The board is too thin;
â—The veneer is a porous material;
â— excessive water in the glue;
â—The amount of glue applied is too large;
â—The pressure and temperature are too high
â— The surface of the substrate is closed or the density is too high.
The use of veneers with a thickness of 0.2-0.3 mm is the most common cause of penetration. When such a thin veneer is used, waste is produced because the veneers are too thin to be sanded. It can't stand the glue layer. It must be remembered that the veneer used must be at least 0.6 mm thick to avoid the above quality problems.
When using a veneer wood veneer, some milky white glue can be infiltrated into the urea-formaldehyde resin to prevent penetration. This is because the milky white glue is a thermoplastic glue that makes the mixing time of the glue shorter. It is recommended that the proportion of unsaturated PVAC glue in the mixed rubber is 5-10%.
Urea-formaldehyde resin adhesives are not suitable for use in board-sealed or high-density boards. For example, when using water-resistant sheets, another type of glue is required because the sheets cannot absorb moisture from the urea-formaldehyde resin. When hot pressed, unabsorbed moisture will permeate from the surface of the veneer.
Cracks in veneers, especially cracks in hard veneers, are caused by many reasons:
â— The glue layer is too hard;
â—The pressure or temperature is too high;
â— The density of the board is very high;
â— The board is too wet.
When a veneer is used to form a veneer, the glue layer is often less than ideal. However, if 10-40% of the appropriate PVAC is infiltrated into the urea-formaldehyde resin, the flexibility of the adhesive layer is increased, and a good effect is obtained.
When the temperature of the press is too high, moisture will quickly escape from the veneer and cracks may occur. This situation is even more serious when the moisture content of the veneer itself is high.
Edge banding
After the veneer is veneered, it is usually edged and sanded and surface treated.
Thick veneer edge sealing generally does not require further processing. Solid wood edge sealing is very common in Southeast Asia, especially in the furniture industry in the Philippines, but the moisture content of the edge band must be low enough, about 7-11%. Higher water content creates cracks and shrinkage, and delamination due to steam generated by the interaction of hot melt adhesives.
Thin veneers are supplied in rolls, the back can be made of fluffy or back-fleece, the back fluffing is used for softforming, and the back is lint-free for straight edge sealing.
Unsaturated hot melt adhesives are commonly used for edge sealing. The advantage of this process is that they have good wetting properties, high strength and good heat resistance. Heat resistance is important because the product usually has to pass through a drying chamber where the temperature is 50-60 ° C, or later through a UV-curing line, where the temperature is 70-80 ° C.
Another benefit of using unsaturated hot melt adhesives is that their adhesive layers are invisible to the naked eye and meet aesthetic requirements. Finally, with the same adhesive effect, the amount of unsaturated hot melt adhesive is less than that of saturated hot melt adhesive, which reduces the cost, although the price of unsaturated hot melt adhesive is slightly higher than that of saturated hot melt adhesive.
Planed veneers are also used on the edges of doors, strips and solid wood strips (ie thick veneers). The thickness of the veneer is from 0.2mm to 5mm, which requires the veneer to be firm, dimensionally stable and easy to process. Although the thickness of the veneers supplied on the market can be as small as 0.2 mm, such thin veneers are generally not recommended because they may increase the cost of painting, thereby offsetting material savings.
Board preparation
In this respect, the control of moisture content is very important, and the water content should be controlled within 8-11%. If the moisture content of the veneer is too high, for example 15-16%, it will be troublesome in the next step of processing. On the other hand, if the water content is too low, cracks may occur and problems occur during cutting.
The veneer should be stored flat, the temperature and humidity of the storage should be controlled, and dust should be avoided. If it is to be stored for a long period of time, it is best to cover it with a film on the outside of the veneer bundle and to prevent sun exposure. This rule even applies to the case where the board has been stacked.
When cutting a board into a certain size, it should be carried out in a clean environment, and the chips should be removed in time to ensure that the board does not have cracks.
Sometimes it is necessary to splicing the veneers or slanting the veneers (note: a length of the joint, which can be single beveled or two beveled scissor), which can be stitched by wire ( Thread splicing, such as the German Kuper process, paper taping or vertical splicing (note: edge splicing, or "straight spelling", that is, two veneers stitched along the longitudinal edges, seams and textures The direction is the same).
Glue line splicing is the most common method used. In addition to its high splicing strength, the polyamide glue line gives the splicing a good flexibility, which means that the splicing veneer can withstand a little shrinkage and expansion. Climate change often occurs. If the thickness of the veneer is at least 0.6 mm, no post-processing is required. It is best not to use a veneer that is thinner than 0.6mm. This is because the veneer is hot pressed by the Kuper press and a thinner layer of glue is visible, which affects the appearance.
Paper splicing veneers are often used for furniture surface coverings of more complex shapes, such as the table top. This process is labor intensive and requires sanding and drying after splicing. It is a labor intensive process.
In terms of processing speed and cost, longitudinal drawing is the best process (standard urea-formaldehyde resin (UF) or milk white glue (polyvinyl acetate) can be used, but the moisture content should be controlled and the working environment Keep it clean). This process also does not require post-processing and does not require the use of glue lines.
However, the use of this process in Asia is less effective than in Europe. This is because climate change in Asia is relatively large. If the polyamide glue line in the Kuper process is not used, then there is no adaptability for shrinkage and expansion. The shrinkage and expansion may cause the seam at the end of the veneer to crack.
The process after the veneer splicing is glued and pressurized, or at room temperature or high temperature. The storage period between cutting into size and splicing should be as short as possible, because during this period of time, changes in the moisture content of the veneer will cause cracks. It is recommended that the cutting, splicing, and flat coating be carried out as quickly and continuously as possible.
Plane overlay on the press
Generally, the flat coating is carried out on a hot press or a cold press, and the glue used is urea-formaldehyde resin or milk white glue. Mixtures of these two types of glue can be used for several special veneers such as birch or veneers that are raised on the back. The balance veneer or balance paper must be applied to the back of the substrate board (plain board) to prevent warpage of the board. If warpage occurs, the next step of processing such as edge sealing will cause problems.
Urea-formaldehyde resin glue is the most commonly used glue in flat coating. The reason is:
First, in all types of rubber, the open aging time of the glue (Note: the open aging time refers to the time after the surface of the adherend is glued to the time of exposure to the air before lamination). This longer period of time is very beneficial for more manual work in Asian factories, such as manual coating rolls used in Asian factories.
Solvent-based colorants or varnishes reduce the adhesion. Urea-formaldehyde resin is a cross-linking type of glue that is more resistant to this chemical than white glue, which is commonly used in antique furniture. in production. In the case of using a white glue, the glue layer often has a problem of delamination.
Because of its wide applicability, it has a good effect on film compression, but care must be taken not to contact it with solvent-based chemicals.
For birch veneers (used in Europe in the past two years), it is possible to use a mixture of urea-formaldehyde resin and milky white glue, which has the adhesive properties of these two types of glue, and the thermoplastic white glue makes the adhesive layer more adaptable. Large, and urea-formaldehyde resin adhesives have good durability to solvent-based chemicals.
Urea-formaldehyde resin adhesive (UF)
Urea-formaldehyde resin adhesives are used in a wide range of applications in about 90% of factories in Southeast Asia. The actual open aging time varies widely, from 1 minute on automated production lines to 15 minutes on manual operations.
Factors affecting the open time of urea-formaldehyde resin glue are: temperature, humidity, water addition, ventilation, and properties of the substrate. For example, in the case of high temperature and low humidity, the ventilation time is reduced to prevent the glue from curing too fast. The same effect can be achieved by reducing the amount of glue applied or by using a highly absorbent substrate such as particle board.
Modern urea-formaldehyde resin adhesives are often supplied in powder form (containing gums, curing agents and special ingredients), which are mixed with water to form a gel. The required open aging time can be controlled according to the amount of water added (the more water is used, the longer the aging time is).
Modern rubber types are much better than the three-component or four-component glue used in the original device. The latter is cumbersome to operate because the ratio of different components needs to be mastered to avoid delamination. Some boards are in Layering occurs only months after lamination. These rubber types must also be spared because of their shorter pot life. (Note: pot life, the pot life refers to the time when the formulated adhesive can maintain its usable performance)
The correct use of urea-formaldehyde resin adhesives The problems that arise in the use of urea-formaldehyde resin adhesives are mainly cracking of the layered, transparent and veneer. The reasons for stratification are: â— After the aging time, press again; â— Improper mixing ratio of various components; â— The amount of glue applied is too low; â— insufficient pressure or temperature; â— Excessive temperature is used for wood with high resin content; â—The thickness of the plate is not uniform, the platen is uneven or the glue is uneven; â— There are dust, oil or other solvents that cause separation on the plain board or veneer.
If it is aged and then pressurized, the adhesive is already dry and there is not enough adhesive strength. A common cause of pre-cured glue drying is that the worker should open the fan in the vicinity for cooling, and the fan should not blow directly against the glue surface, which can reduce this problem. In order to avoid the wrong ratio, the easiest way is to use a modern one-component glue.
Another problem is that the worker always puts the plain plate into the applicator at the same position when applying the glue (usually in the middle of the applicator), which makes this part of the applicator roll dry, so that the glue is applied. Not enough. When applying glue, the board should be evenly distributed in all parts of the rubberizing roller to ensure sufficient and uniform coating.
The temperature and pressure during hot pressing must also be sufficiently high. The typical urea-formaldehyde resin hot pressing parameters are:
Pressurize at 120 ° C for 1 minute; or
80-100 ° C, pressurization for 3 minutes; or
Pressurize at 60 ° C for 4-5 minutes.
The pressing pressure depends on the tree species of the wood, and the applied pressure is usually 1.5-3 Kg/cm2. If the pressure is too high, it will produce a transparent rubber (note: the rubber or its components pass through the surface material of the artificial board, which causes the surface of the board to have rubber spots or discoloration), and may also cause cracks in the veneer.
For high resin content tree species such as pine, birch, and teak, it is best to use a low temperature, long hot press time process. High temperatures activate the oil or resin in the wood, causing partial delamination. For example, pine, the temperature of hot pressing should be about 50 °C.
The hot platen should be cleaned regularly to remove the remaining glue drops on the plate. The uneven platen surface will not cause the pressure on the parts of the plate to be inconsistent after the press is closed.
The surface of the substrate and veneer should be kept clean and free from spots or smudges. Workers are not allowed to touch the substrate or veneer with oily hands.
The glue is the glue that seeps out the surface of the veneer and makes the surface bright. The main reasons are:
â— The board is too thin;
â—The veneer is a porous material;
â— excessive water in the glue;
â—The amount of glue applied is too large;
â—The pressure and temperature are too high
â— The surface of the substrate is closed or the density is too high.
The use of veneers with a thickness of 0.2-0.3 mm is the most common cause of penetration. When such a thin veneer is used, waste is produced because the veneers are too thin to be sanded. It can't stand the glue layer. It must be remembered that the veneer used must be at least 0.6 mm thick to avoid the above quality problems.
When using a veneer wood veneer, some milky white glue can be infiltrated into the urea-formaldehyde resin to prevent penetration. This is because the milky white glue is a thermoplastic glue that makes the mixing time of the glue shorter. It is recommended that the proportion of unsaturated PVAC glue in the mixed rubber is 5-10%.
Urea-formaldehyde resin adhesives are not suitable for use in board-sealed or high-density boards. For example, when using water-resistant sheets, another type of glue is required because the sheets cannot absorb moisture from the urea-formaldehyde resin. When hot pressed, unabsorbed moisture will permeate from the surface of the veneer.
Cracks in veneers, especially cracks in hard veneers, are caused by many reasons:
â— The glue layer is too hard;
â—The pressure or temperature is too high;
â— The density of the board is very high;
â— The board is too wet.
When a veneer is used to form a veneer, the glue layer is often less than ideal. However, if 10-40% of the appropriate PVAC is infiltrated into the urea-formaldehyde resin, the flexibility of the adhesive layer is increased, and a good effect is obtained.
When the temperature of the press is too high, moisture will quickly escape from the veneer and cracks may occur. This situation is even more serious when the moisture content of the veneer itself is high.
Edge banding
After the veneer is veneered, it is usually edged and sanded and surface treated.
Thick veneer edge sealing generally does not require further processing. Solid wood edge sealing is very common in Southeast Asia, especially in the furniture industry in the Philippines, but the moisture content of the edge band must be low enough, about 7-11%. Higher water content creates cracks and shrinkage, and delamination due to steam generated by the interaction of hot melt adhesives.
Thin veneers are supplied in rolls, the back can be made of fluffy or back-fleece, the back fluffing is used for softforming, and the back is lint-free for straight edge sealing.
Unsaturated hot melt adhesives are commonly used for edge sealing. The advantage of this process is that they have good wetting properties, high strength and good heat resistance. Heat resistance is important because the product usually has to pass through a drying chamber where the temperature is 50-60 ° C, or later through a UV-curing line, where the temperature is 70-80 ° C.
Another benefit of using unsaturated hot melt adhesives is that their adhesive layers are invisible to the naked eye and meet aesthetic requirements. Finally, with the same adhesive effect, the amount of unsaturated hot melt adhesive is less than that of saturated hot melt adhesive, which reduces the cost, although the price of unsaturated hot melt adhesive is slightly higher than that of saturated hot melt adhesive.
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