Thin wood (wood veneer) decorative veneer technology and its application

Nanjing Forestry University Wu Zhihui

With the application of various wood-based panels in the production of furniture and wood products, various veneers and edge-sealing materials are required for surface decoration and edge sealing treatment. Veneer (including edge sealing) materials are available in various types according to their materials. Among them, wood is made of natural thin wood, artificial thin wood, veneer, etc.; paper type has printed decorative paper, synthetic resin impregnated paper, decorative board Etc.; plastics include polyvinyl chloride (PVC) film, polyethylene (PVE) film, polyene hydroxy (Alkorcell) film, etc.; other kinds of textiles, synthetic leather, metal foil and so on. The veneer material mainly serves both surface protection and surface decoration. Different veneer materials have different decorative effects. Among them, thin wood is a wood-based high-grade veneer material that is most commonly used in furniture manufacturing and interior decoration. With a long history of veneer veneering technology, the surface of the parts retains the excellent characteristics of wood and has the realism of natural wood grain and color tone. It is still a popular surface decoration method and is widely used in various furniture and Wood production and interior decoration.

1 classification of thin wood

Wood veneer, commonly known as "wood veneer", is a wood-like thin veneer or veneer with precious tree characteristics. There are many types of decorative thin wood (wood veneer). At present, there is no unified classification method at home and abroad. A typical representative classification method is carried out in accordance with the manufacturing method, form, thickness, tree species, and the like of the thin wood.

1.1 According to the manufacturing method

(1) Sawed veneer: A sheet-like sheet obtained by sawing a wooden or wooden board with a saw blade or a saw blade (according to the texture of the board and the direction of the sawing, there are radial thin chords and chords to the thin wood) .

(2) Sliced ​​veneer: a piece of thin wood that is cut into wood and processed by steaming and softening, and then cut into a planer according to the wood cutting texture and cutting direction. Radial thin wood chords are divided into thin wood).

(3) Rotary veneer: A continuous strip of thin wood (chord-to-thin wood) that is turned into a precision rotary cutter after steaming and softening.

(4) Semi-circular rotary veneer: a piece of thin wood that is cut into pieces by a eccentric clamping on a conventional precision rotary cutting machine or by rotary cutting on a special semicircular rotary cutting machine (according to wood The difference in the square clamping method results in radial thin wood or chord-oriented thin wood), which is a kind of spinning thin wood between the planing method and the rotary cutting method.

1.2 according to the form of thin wood

(1) Natural veneer: A thin veneer directly cut from the wood of natural precious trees or naturally grown wood.

(2) Artificial veneer: The sliced ​​veneer made from the rotary cut veneer of the common tree species, imitated by the shades of natural wood or precious tree species, and then glued into the wood in the direction of the fiber, which is called “Technology Wood”. (Technical wood) thin wood, referred to as "Technical veneer" (Technical veneer).

(3) Integrated veneer: The small square or veneer of precious tree species or common tree species (dyed) is firstly integrated into the wood and then sliced ​​into a whole piece of thin wood.

1.3 according to the thickness of the thin wood

(1) Thick wood: Thickness ≥ 0.5 mm, generally refers to ordinary thin wood with a thickness of 0.5~3 mm.

(2) Thin thin wood: 0.2 mm ≤ thickness <0.5 mm, generally refers to thin wood of 0.2~0.5 mm thick.

(3) Micro-thin wood: continuous rolled thin wood or rolled thin wood with thickness <0.2 mm, generally 0.05~0.2 mm and back-bonded with special paper or non-woven fabric.

1.4 according to the thin wood pattern

(1) Diameter-cut thin wood: Striped thin wood which is composed of wood early and late materials and which are substantially parallel to each other.

(2) Chord-cut thin wood: A generally thin mountain-shaped pattern of wood composed of early and late wood.

(3) Corrugated thin wood: Patterned thin wood produced by wavy or twisted texture, also known as the back of the piano pattern, shadow pattern, often appear in maple (Maple), birch and other tree species.

(4) Bird's eye pattern: A bird-like pattern formed by partial distortion of fibers, often found in maple (Maple), birch, and ash.

(5) Tree tumor pattern thin wood: The pattern formed by the abnormality of the local fibers caused by tree tumors, etc., often appears in walnut trees, maple wood (Maple), French paulownia, eucalyptus and other tree species.

(6) Tiger skinned thin wood: a tiger-like pattern of flake-like silver light formed by dense wood rays on the diameter cutting surface. The wood rays are spindle-shaped on the string-cut surface, often appearing in wood such as eucalyptus and beech. Ray-rich tree species.

1.5 according to thin wood species

(1) Hardwood: A thin wood made of broad-leaved or simulated broad-leaved trees, such as ash, birch, eucalyptus, cherry, walnut, and paulownia.

(2) Softwood: A thin wood made of conifer or simulated coniferous trees, such as spruce, Korean pine, Douglas fir, Masson pine, and larch.

2 characteristics of technology thin wood

The technical veneer is made of ordinary wood and is designed with computer virtual and analog technology. The simulation made by high-tech means is even better than the all-wood new surface decoration material of natural precious tree species. Often referred to as Artificial Veneer, also known as Engineering veneer. It not only maintains the properties of natural wood, but also gives new meaning.

The thin wood made of technology wood can simulate the rare and expensive natural precious tree species, and create a variety of more artistic patterns and patterns. Compared with natural thin wood, technology thin wood has the following characteristics:

(1) Rich color and variety: The technology and thin wood products are designed by computer to produce different colors and textures. The color is brighter, the texture is more solid, and the pattern is full of dynamic and vitality.

(2) High utilization rate of finished products: The technology thin wood overcomes the natural defects of natural thin wood, and the product has no natural defects such as wormhole, thrift and discoloration. Due to the regularity and consistency of the texture, the thin wood products of the technology will not produce natural thin wood products with different textures and colors due to different logs and batches.

(3) Product development potential: With the implementation of national bans and natural forest protection policies, the precious tree species available for use are decreasing, making the technology thin wood products a substitute for precious tree species decorative materials.

(4) The width of the decorative web is wide: the technical thin wood overcomes the limitation of the small diameter of the natural thin wood, and can be processed into different web sizes according to different needs.

(5) Convenient processing: easy to process and anti-corrosion, anti-mite, fireproof (flame retardant), moisture resistant and other treatments.

3 thin wood veneer technology

As mentioned above, in order to beautify the appearance of the product, improve the performance, protect the surface, and improve the strength, the high-grade furniture or wood products must be surface-finished or veneered. The veneer veneer is a high-grade decorative veneer that is attached to the surface of the substrate or the plate member with the thin wood glue with the characteristics of precious tree species, so that the surface of the component retains the excellent characteristics of the wood and has the real feeling of natural wood grain and color tone. Process. The veneer process and equipment are as follows.

3.1 Preparation of thin wood

(1) Storage of thin wood: The thin wood is highly decorative, small in thickness and easy to break, so it must be properly preserved. In order to be able to spell out a symmetrical and balanced pattern, the thin wood must be stacked and dried in a planing order, indicating the species, size and thickness of the tree. The thin wood storage room should be cool and dry, with a relative humidity of 65%, so that the moisture content of the thin wood is not less than 12%. At the same time, indoors should avoid direct sunlight and cause discoloration of the thin wood. Thin wood with a thickness of 0.2~0.3 mm or less generally does not need to be dried, and the water content should be kept at about 20%. Otherwise, the thin wood is easy to be broken and warped. In addition, it is required to be stored indoors below 5 °C, and in the winter, it should be wrapped with polyvinyl chloride film. Sealed, kept in the cold storage in the summer to avoid mold and decay.

(2) Thin wood processing: In the face of thin wood veneers, the thin wood shall be scribed, cut or sawed according to the size and texture requirements of the parts, and the defective parts such as end cracks and discoloration shall be removed and cut into the required specifications. Since the thickness of the thin wood is small, the single sheet processing is easily broken, so the thin wood is processed in a pile. When thin wood is cut, it should first be cut by horizontal stripes and then cut by straight lines. The main equipment for cutting is a heavy duty file. The edge of the thin wood after cutting should be kept straight, and there are no defects such as cracks and burrs.

(3) Thin wood selection: Because the thin wood is relatively narrow, it needs to be glued when used. The glue of the thin wood can be carried out before the adhesive tape or at the same time as the adhesive tape. Complex patterns should be manually operated, and the simple combination of mass production can be glued in the stitching machine. There are four commonly used thin wood glue forms: no paper tape (glue) glue, paper tape glue, Z-shaped glue glue and point glue glue. As shown in Figure 1.

1) glue seam glue 2) paper tape glue 3) glue line glue 4) glue glue glue

1, 2 - thin wood 3 - continuous glue joint 4 - paper tape 5 - Z-shaped glue line 6 - dot-shaped glue drops

Figure 1 Thin wood seam form

1) Manual selection: There are generally two situations.

The first is to perform hand-stitching at the same time as the thin veneer. Stick the thin wood on the substrate, overlap the two thin wood joints at the joint, press the seam position with a ruler, and then cut the two thin woods along the edge of the ruler with a sharp cutter, then connect The extra side strips cut on both sides of the seam are pulled out, so that the two thin woods can be directly glued and attached to the surface of the substrate. as shown in picture 2.

1—block paper tape 2—paper tape 3—thin wood

Figure 2 Thin wood hand-stitched seam Figure 3 Hand-made paper tape patchwork

The second is to manually stitch the veneer before the sticker. When seaming, first fix it with paper strips (blocks), then use continuous paper strips to glue, and the thin wood ends must be assembled with tape to avoid damage during handling. As shown in Figure 3. The most commonly used hand-stitched seams are hand-made thin wood mosaics. At this time, the selection of thin woods needs to determine a reasonable thin-wood tailoring process and a thin-wood mosaic scheme according to the design of the mosaic pattern. There are many kinds of thin wood mosaics. Common ones are the pattern of the pattern, the pattern of the pattern, the box pattern, the anti-box pattern (box-shaped spell), the V-shaped spell (the herringbone spell), the double V-shaped spell, the jewel pattern (diamond) Spelling, square spelling), anti-stone pattern, mat pattern (checkerboard spelling), miscellaneous pattern, vortex pattern and so on. As shown in Figure 4. The parquet work is done on a bright workbench. When assembling complex patterns, it is usually best to use a special workbench with evenly distributed holes on the table. There is a vacuum box under the table. When the machine is selected, the vacuum is applied to the veneer on the table until it is manually glued. Only stop pumping, remove the thin wood that is put together, or send it to the next process adhesive.

Shun-grain parquet, pattern mosaic, box-like mosaic, box-like mosaic

V-shaped mosaic double V-shaped parquet jewel pattern mosaic anti-gem pattern mosaic

Mat pattern mosaic pattern mosaic pattern vortex mosaic

Figure 4 handmade thin wood mosaic pattern

2) Sewing machine glue fight: So far, there is no seam-stitching machine that is completely suitable for glue-splitting thin wood. The mechanical glue of thin wood is also made by the stitching machine of single-board. The applicable scope of various stitching machines is shown in Table 1 below.

Table 1 Scope of application of various stitching machines

1 Paperless stitching machine: When there is no paper tape seaming, the side of the thin wood is coated with adhesive, and it is cured and glued under the action of the heating roller and the heating pad in the paperless stitching machine. The rubber used is a urea-formaldehyde resin or the like. This method of paperless seaming is actually based on the core board of the plywood. It is not suitable for the splicing of thin wood or thin wood.

2 There is a tape sewing machine: the tape can be attached to the surface of the veneer and removed during the sanding treatment; or a perforated tape can be attached to the back of the veneer. At this time, the tape is in the thin wood and the substrate. between. When the thin wood is thin, the paper tape is easily reflected on the surface, and the water resistance of the adhesive tape is relatively poor, which easily causes peeling or delamination between the thin wood and the substrate. Therefore, the method of surface adhesive tape is generally used, but in the method of surface finishing, in order to remove the paper tape, the surface layer is sanded or scraped off with a doctor blade, so that the surface layer thin wood is thinned, and the paper tape is thinned. The adhesive has the disadvantage of contaminating the surface and residual marks, and must be handled with great care.

3 hot melt glue line stitching machine: It is a thin glass fiber wrapped with hot melt resin (such as ethylene vinyl acetate copolymer), that is, hot melt adhesive line instead of adhesive tape, and two thin woods are spliced ​​together. During the seaming, the backs of the two thin woods are fed into the machine upwards, and under the action of the squeezing rollers, they are squeezed into each other, and the hot melt adhesive line is passed through the heater to melt the resin, and the heater swings left and right and sprays the glue. A "Z" shaped glue line is formed at the seam of the two veneers and then cured at room temperature to splicing the two veneers together. This Z-stitching machine is generally suitable for splicing thin wood with a thickness of 0.5~0.8 mm.

4 hot melt glue drop stitching machine: It is also used to replace the adhesive tape with fine glass fiber wrapped with hot melt resin, and splicing two thin woods together. During the seaming, the hot melt adhesive is heated and melted on the spot glue applicator, and the glue is applied to the joints of the two thin woods, and then the glue drops are pressed to press the two thin woods together. With this stitching machine, it is possible to splicing thin wood with a thickness of 0.4 to 1.8 mm.

3.2 thin wood adhesive tape process

Commonly used thin wood adhesive tape processes are dry and wet.

(1) Dry-sticking process: refers to the process of thinning and then sticking the thin wood. This type of adhesive method is easy to break and has a large loss, so the thickness of the thin wood is generally required to be greater than 0.4 mm. The most commonly used in the dry-laid process is the dry-laid parquet, that is, the side-stitched lace dry paste. Generally, hot melt adhesive or other suitable adhesive is first applied to the substrate, and after cooling and solidifying, the thin wood is used according to the design pattern. The irons are collaged one by one. Dry-laid thin wood has higher technical requirements, lower production efficiency, and is difficult to adapt to thicker thin wood collages. If the moisture content of the thin wood is less than 20%, the heat transfer is slow and fragile, so the moisture content of the thin wood is required to be above 20%.

(2) Wet sticking process: refers to the process of directly coating the thin wood on the substrate without drying (the moisture content is above 30%). The adhesive tape method has the advantages of simple production process, low loss of thin wood and low production cost, and is generally suitable for thin wood with a thickness of not more than 0.4 mm, and the conventionally used 0.2 to 0.3 mm thin wood can adopt this process. But pay attention to the adhesive: 1 adhesive viscosity should be large, and have a certain initial viscosity, otherwise the thin wood is easy to be misplaced or separated from the seam; 2 high moisture content thin wood will shrink when using hot-press adhesive, so the thin wood can not be tightened, should Relax and leave a shrinkage allowance, and the moisture content should be uniform. If it is too dry, spray it at any time.

The thin wood adhesive tape includes the steps of coating, blanking and pressing of the substrate.

(1) Substrate coating

For all parts that need to be glued to the veneer, the glue should be applied to the substrate on which the veneer is to be prepared. Commonly used adhesives are: urea-formaldehyde resin (UF), polyvinyl acetate emulsion (PVAc), and a mixture of these two resins (UF + PVAc) and vinyl acetate-N-methylol acrylamide emulsion (VAC) /NMA) and so on. Hot melt adhesive (vinyl acetate resin glue) is commonly used when hand-applied.

Urea-formaldehyde resin adhesive is a kind of adhesive widely used at present, with good operation performance, good water resistance and low price, but the initial viscosity is small, the permeability is strong, and the translucent and thin wood are easily misplaced. Therefore, an appropriate amount of filler (wheat flour, starch, soy flour, etc.) is generally added during the adjustment of the glue to increase the viscosity of the glue, reduce the phenomenon of penetration, and reduce the cost. In addition, it is necessary to add a suitable curing agent (such as ammonium chloride) to lower the pH to 4.8~5.0, so that the glue can be cured quickly.

Polyvinyl acetate milk white glue is a thermoplastic adhesive, which has good pre-stressing property and initial tackiness, does not penetrate the rubber, is easy to operate, and can be cold pressed or hot pressed. However, the water resistance is poor. In order to improve its water resistance, there are generally two methods. One of the most common methods is to mix it with a urea-formaldehyde resin to increase the length and improve the water resistance. The mixing ratio is generally: polyvinyl acetate milk white glue: urea-formaldehyde resin glue = 7~5:3~5, and 10~30% filler and an appropriate amount of curing agent are added. The mixing ratio should be determined according to the specific conditions. If the veneer is dried and then veneered, the milky white glue can be added less or not; if the veneer is not directly dried, the veneer should be added because the pipe is thick and easy to penetrate. . In the case of wet sticking, the commonly used ratio is: milk white glue: urea-formaldehyde glue: flour = 1:1:0.2. Another method for increasing the water resistance of polyvinyl acetate emulsion white gel is to add a crosslinking agent to copolymerize. The cross-linking agent crosslinks with vinyl acetate molecules during the adhesion or film formation of the emulsion to make it a thermosetting resin, thereby improving water resistance, heat resistance and creep resistance. A commonly used cross-linking agent is: N-methylol acrylamide, which is cross-linked with vinyl acetate to form vinyl acetate-N-methylol acrylamide (VAC/NMA) emulsion glue, which is suitable for thin wood adhesive tape. It can be cold pressed or hot pressed (hot pressing temperature is only 60 ° C). Its formula is: VAC/NMA emulsion: tin tetrachloride (50%): gypsum = 100:6:3.

When the wood-based panel is veneered with the decorative thin wood, a pigment may be appropriately added to the adhesive to cover the color of the substrate so as not to affect the decorative effect. When veneering, the substrate is generally coated on one side. The amount of adhesive applied is determined by the type of substrate and the thickness of the veneer. The surface of the veneer or plywood is coated on a single side of 110~120g/m ² (thin wood thickness is less than 0.4 mm), or 120~150g/m ² (thick wood thickness is more than 0.4 mm); The amount is 120~150g/m ² ; the amount of glue applied on the single side of the particle board is 150~200g/m ² ; the amount of glue on the single side of the fiber board is 150~160g/m ² . The amount of glue applied should not be too large, and the glue layer should be uniform. For this purpose, a four-roll coater with a squeeze roll is often used, and the rubber roll is preferably a rubber roll without grooves. In order to prevent penetration, the substrate should be aged for about 30 minutes after being glued.

(2) Matching (group) blank

In order to ensure the shape stability of the parts after the thin wood glue sticking, the symmetry principle should be adhered to when the glue is applied, that is, a thin wood should be attached to the surface of the substrate and the back surface, and the tree species, thickness, water content and fiber direction should be consistent. However, in order to save precious wood and high-quality thin wood and reduce the cost, the back side can be changed to a similar tree species to balance the stress on both sides and prevent warpage.

When the veneer is veneered, there are two cases of the blank: one is that the veneer can be directly attached to the surface of the substrate such as particleboard or fiberboard, the surface of the substrate is flat, the thickness of the thin wood can be smaller; the surface is flat. The thickness of the thin wood is preferably not less than 0.6 mm. The other is that if a thin thin wood or a thin wood veneer is used, a veneer or cardboard having a thickness of 0.6 to 1.5 mm is used as a middle plate on the surface of the substrate to ensure the flatness of the surface and increase the structural strength.

(3) Rubber pressure

The thin wood glue has two types of cold pressing and hot pressing.

Cold-formed veneer should be cold-cured urea-formaldehyde resin or polyvinyl acetate emulsion, and the slab should be aligned up and down in the cold press. For uniform pressure, a thick pad should be placed at regular intervals between the plates. When the unit pressure at cold pressing is slightly lower than that of hot pressing, it is generally 0.5~1.0MPa. Under room temperature (15~20°C), the pressing time is 4~8 hours. Summer is shorter and winter is longer. .

The veneer veneer is usually hot pressed. Hot press veneer can be pressed with a multi-layer laminator (2~15 layers) or a single laminator, and each layer of slab should be aligned in the press. When pressurizing, the pressure rise should not be too fast, so that the thin wood has a chance to stretch, but the pressure should not exceed 2 min from the pressure to the closure to prevent the adhesive layer from solidifying at the temperature of the hot plate. The thickness of the slabs in each laminate interval shall not differ by more than 0.2 to 0.3 mm. The hot pressing conditions are related to the type and thickness of the substrate, the species and thickness of the wood, and the type of the adhesive. The general hot pressing process conditions are shown in Table 2.

Table 2 Thin wood veneer hot pressing process conditions

When the thickness of the veneer is 0.2~0.3 mm, the precision of the press is required to be high. In order to make the plate surface pressure and the slab force uniform, the aluminum alloy pad can be fixed on the upper hot plate and the groove is fixed on the lower hot plate. Elastic cushioning material such as heat-resistant rubber sheets. This kind of thin wood is usually not dried and directly wetted on the surface of the substrate. In order to prevent cracks on the surface of the thin wood after hot pressing, it should be sprayed or sprayed with 5~10% formaldehyde solution before hot pressing, especially thin wood. Peripheral part.

There are two types of thin wood adhesive equipment: cold press and hot press. Cold presses have many types of pressurization, such as screw nut pressurization, pneumatic pressurization and hydraulic pressurization. There are many heating methods for hot presses, such as steam heating, hot oil heating and electric heating. The number of press plates of the hot press is single layer, multi-layer and continuous.

(4) Post processing

Immediately after hot pressing, check the quality of the thin wood glue, and use 2~3% oxalic acid solution to remove the discoloration caused by the action of tannin and iron ions, or remove the surface oil with alcohol or ether. After the veneer decorative veneer is relieved, it should be stacked for more than 24 hours, so that the stress is balanced and the internal stress is eliminated. The adhesive is fully cured, then the sanding is carried out (the belt is generally 180~240), and the wood is used to scrape. Putty and other methods to repair defects such as cracks, insect eyes, and joint holes. The putty can be prepared by using white glue and wood powder and pigment, and its color should be similar to wood color.

4 thin wood veneer defects and solutions

When using various adhesives in actual production, when manual, cold pressing or hot pressing adhesive is applied to the thin wood, the most common adhesive defects are degumming, penetration, bubbling, cracking, warping, separation, and pressing on the surface of the decorative veneer. Traces, pollution and discoloration, etc., the causes and solutions are shown in Table 3.

Table 3 Common adhesive defects and solutions for thin wood adhesive stickers

5 Standards and specifications for sliced ​​thin wood and its veneers

Classification, grading, size and dimensional tolerances, water content, surface roughness, appearance quality, etc. of sliced ​​thin wood (also known as sliced ​​veneer), and technical indicators and requirements of man-made boards after veneer veneer, respectively See the relevant provisions in "GB/T13010-2006 Sliced ​​Veneer" and "GB/T15104-2006 Decorative Veneer Veneer Panel".