The surface of wood products is painted, veneered, carved, inlaid, branded or molded. Decoration of wood products can be done after the products are made, or when making parts and components. The raw materials of the products, such as particleboard and plywood, can also be decorated on the surface first, and then final decoration can be done after the products are made. The purpose of decorative wood products is to cover the surface of the wood products with a film with certain hardness, water resistance, weather resistance and other properties, which can prevent or weaken the wood products from sunlight, moisture, external force, fungi, insects and chemical substances, and prevent the occurrence of Warp deformation, cracking, corrosion and wear are prevented, and the service life is extended; the wood products are given a certain color, luster, texture and pattern, so that the shape, color and quality of the products are perfectly combined, giving people a beautiful and harmonious feeling.

History of Development

The wooden lacquer bowls discovered in the Hemudu Cultural Site prove that as early as 4800 BC, the Chinese people had mastered the production of natural lacquer (Jianshengqi) and wood coating. Product technology. The fragments of lacquerware unearthed from the Yin Ruins of the Shang Dynasty in Taixi Village, Gaocheng County, Hebei Province were made of carved wooden bodies coated with lacquer in relief. In the late Shang Dynasty, clam slices and clam foam were used to inlay and decorate lacquerware. During the Warring States Period, people already knew that lacquer was modified with tung oil and mixed with natural pigments to make black, red, brown, yellow, white, gold, silver and other colored lacquers. Painting with dark paint is called "lacquer painting"; using oil to paint light-colored patterns and patterns is called "oil paint". Copper or silver buckles and gold and silver wire inlays are used on the lacquerware to make the wood products exquisite and practical. The processing and construction techniques of Chinese natural lacquer have gradually spread to Japan, North Korea and Southeast Asian countries since the Tang Dynasty. Ancient Egyptians used gum arabic and egg white to make colored paints to decorate wooden products. From the end of the 19th century to the beginning of the 20th century, France, the United States and other countries successively developed shellac paint and nitrocellulose paint, which became the main paint for furniture decoration in European and American countries at that time. In 1928, phenolic resin paint was invented, and synthetic paints began to be used instead of natural resin paints. With the increasing development of the chemical industry, coatings are developing towards high solid content, no pollution, less solvent, and composite base materials. Painting technology has also developed from manual operations to mechanized and automated painting adapted to mass production, and even robots are used for painting.

Decoration technology

Including painting, veneer, inlay, engraving, molding and branding.

Painting

The process of applying paint on the surface of wood products according to process regulations to form a paint film with certain physical and chemical properties. Its characteristic is that it is not limited by the shape, size and material of the painted wood products, and can easily meet the decorative requirements and is easy to repair. The disadvantage is that the paint film will age over time and reduce its performance. According to the texture and specification requirements, coatings are mainly classified into the following categories: according to whether the paint film is transparent or not, it is divided into transparent coating, opaque coating and simulated coating; according to the hole filling condition, it is divided into hole-filling coating and exposed hole coating; according to the film's The brightness is divided into bright, semi-gloss, and matte coatings; according to the type of topcoat, it is divided into lacquer, phenolic paint, polyurethane paint, etc. Transparent coating uses transparent paint (varnish) to coat the surface of wooden products. After coating, the original wood grain can be more clearly displayed, the wood texture is stronger, and the color is pleasing to the eye. This method is mostly used for furniture, musical instruments, handicrafts, etc. made of precious wood (or veneer). Opaque finishes are used to finish wood products with opaque paints (enamels, mixed paints) containing colored pigments. The paint film completely covers the texture and color of the wood. It is often used in artificial boards and products made of wood with poor texture and tone, such as train car wall panels, doors and windows, desks and chairs. Simulation coating uses opaque paint to completely cover and fill the surface of the product, then gives it the color to be simulated, and then paints and prints wood grain, etc. The coating process varies greatly depending on the decorative quality requirements of the product, the type of base material, and the coating method.

Transparent coating

It is usually divided into three stages: wood surface preparation, finishing coating (including coating drying) and paint film modification.

① Surface preparation includes processes such as sanding, wood wool removal, dust removal, resin removal, decolorization and inlaying. The treated surface should be smooth, evenly colored, and have good paint adsorption properties.

②Finishing coatings include dyeing, hole filling, primer coating, mid-coat coating, topcoat coating and other processes. Hole filling is to fill the cut duct grooves and other cell cavities with a hole filling agent to form a flat surface and at the same time color it to clearly show the wood grain. Hole filling is not required for visible hole coating. Staining gives wood products a certain color. Dyeing with dyes or mordants. Dyeing with water-based dyes, alcohol-based dyes, etc. is more common.

Mordant dyeing uses metal salts such as ferrous sulfate or aqueous solutions of certain acids and alkalis to react chemically with tannins in the wood to cause dyeing. This method is only applicable to wood with high tannin content such as camphor and oak. The function of applying primer is to fix the color, reinforce the pore filler, form a continuous closed isolation layer under the topcoat, and reduce the consumption of topcoat. Primers include shellac varnish, amino alkyd resin primer, polyurethane wood primer, etc. Topcoat is applied to form a paint film with a certain thickness and physical and chemical properties, which plays a protective and decorative role. Often requires multiple coats. The performance of the paint film mainly depends on the type of topcoat. The coating can be dried naturally, and the drying time is longer. Artificial drying methods are used in mass production, including hot air convection drying, infrared and far-infrared drying, ultraviolet curing, etc. UV curing is suitable for photosensitive coatings, and the coating curing time can be shortened to minutes or even tens of seconds. The appropriate drying and curing method should be selected based on the type of coating, coating thickness and other conditions, and the drying procedures should be determined.

③Paint film finishing In order to obtain a paint film with good decorative properties and mirror gloss, the paint film needs to be polished or polished after the coating is cured. Polishing is to remove larger protruding parts on the surface of the paint film and reduce its unevenness. Divided into dry polishing and wet polishing. Use manual or portable sander or belt sander. The purpose of polishing is to further eliminate the slight unevenness on the surface of the paint film. Sand wax, light wax, etc. are commonly used for polishing. Roller polishing machines are more commonly used, and portable, disc, and belt polishing machines are also used.

Opaque coating

It is also divided into three stages: surface preparation, finishing coating, and paint film modification. The functions of each process are similar to those of transparent coating, but it does not require decolorization or dyeing. Putty, primer, and topcoat all use opaque paint, and the color of the topcoat forms the color of the paint film. To ensure that the paint film is smooth, it must be fully filled 1 to 2 times. When painting highly decorative handicrafts and high-end colored furniture, the paint film should be polished. Sometimes a layer of clear paint is applied to the paint film, which is called "covering".

Coating machines

Paint can be applied with hand tools or machinery. Mechanical painting is divided into two categories: ① After the liquid paint is atomized (micronized), it is sprayed and evenly adhered to the surface of the wood product, relying on the flow of paint particles to form a continuous paint film. Such as air pressure spraying (Figure 1), high-pressure airless spraying, electrostatic spraying (Figure 2), etc. This method is suitable for coating products of various shapes and sizes. ②The coating is applied to the surface of the product in a liquid state, such as shower coating (Figure 3), roller coating (Figure 4), pump coating, etc. Shower and roller coating are only suitable for coating flat surfaces and have high production efficiency.

Figure 1 Veneer

The process of pasting sheet materials with certain physical and chemical properties and finishing effects on the surface of wood products with the help of adhesive. Commonly used veneer materials include thin wood, printed decorative paper, synthetic resin impregnated paper, polyvinyl chloride film, melamine paper decorative board, metal sheet, fabric and artificial leather, etc.

Veneer

The wood is processed into large-format thin wood of 0.2 to 1.2 mm using planing, rotary cutting and other methods, presenting various textures and colors. Precious woods such as ash, teak, mahogany, and palm wood are commonly used to process thin wood. Thin wood can also be collaged into beautiful patterns according to design and aesthetic requirements (Figure 5). Artificial combined veneers and integrated veneers are also widely used in production for veneer. Artificial composite veneers are made by dyeing veneers with inconspicuous textures, then forming slabs according to certain rules, pressurizing and gluing them into wooden cubes, and then planing them into veneers. Integrated veneer is to glue wood cubes of certain specifications and materials into various patterns according to design requirements, and then slice them into veneers. Before gluing, the thin wood needs to be dried to a moisture content of 8 to 12%. However, thin wood with a thickness of about 0.2 to 0.3 mm can be wet pasted without drying. The adhesive for veneers is generally a mixture of urea-formaldehyde glue and polyvinyl acetate latex. To prevent glue penetration, 10 to 30% industrial flour is usually added to the glue. Thin wood can be veneered using cold pressing or hot pressing methods. The base material must be sanded to the required roughness before veneering. After veneer, the thin wood surface needs to be transparently painted.

Figure 2

Figure 3

Figure 4

Figure 5 Synthetic resin impregnated paper veneer

Special decorative paper is impregnated with synthetic resin, dried and hot-pressed onto the base material to form a film with certain hardness, gloss, wear resistance, etc. Commonly used resins include low-pressure melamine, phthalate diacrylate resin, phenolic resin, etc. The resin content of decorative paper is generally 100 to 150%, and the resin content of the underlying paper is about 40%.

According to different uses, paper is divided into decorative paper, surface paper, bottom paper, etc. The basis weight of decorative paper is 80 to 150 grams/square meter. In order to prevent the board from warping and bending after veneer, balance paper needs to be attached to the opposite side.

Printed decorative paper veneer

Use 20-30 g/m2 tissue paper or 80 g/m2 titanium dioxide paper with wood grain or decorative patterns printed on it. Veneer material is attached to the surface of wood products and then painted. The surface decorated with this method is more beautiful and has certain heat resistance, wear resistance and water resistance, but the wear resistance is lower than that of resin impregnated paper veneer. Generally, a mixture of urea-formaldehyde glue and polyvinyl acetate latex is applied. Use flat pressing or roller pressing to veneer at a temperature of 80 to 120°C. After veneering, apply unsaturated polyester varnish or amino resin varnish. Often used for mid- to low-end furniture decoration.

Polyvinyl chloride film veneer

The surface of the polyvinyl chloride film is printed with wood grain and embossed with grooves imitating conduit grooves. At normal temperature, use ethylene-vinyl acetate polyresin glue for flat pressing or rolling veneer. This kind of veneer material is easy to bend, corrosion-resistant, and has a soft color, but it is not heat-resistant. Therefore, it is widely used as a decoration for the casings of TVs, speakers, and cabinet furniture facades.

Melamine resin decorative board veneer

The decorative board is a paper laminate made from a slab composed of melamine resin impregnated paper and multi-layer phenolic resin impregnated paper under high temperature and high pressure conditions. . It is heat-resistant, water-resistant, wear-resistant and chemical-resistant. The board surface is smooth and printed with various wood grains or other decorative patterns. Usually, urea-formaldehyde glue is used for cold-pressed veneers, but hot-pressed veneers can also be used. It is widely used as decoration on desktops, sewing machine tabletops, and interior walls of vehicles and ships.

Inlay

The process of inlaying small wood blocks, veneers, shell flakes, metals, jade, etc. with different colors and textures into patterns and patterns on the surface of wood products. According to the inlay technique, it can be divided into digging inlay, collage, pressing inlay, sawing inlay, etc. According to the height difference between the inlay and the surface of the base material, it can be divided into relief, concave and flat shapes. The process of inlaying is: drawing design → drawing original size drawing → inlay material selection and design → pattern decomposition → material selection → marking and setting out → making inlay components and base plates → integration of inlay patterns → embedding adhesive stickers → surface modification → coating. Inlay technology is used in the production of furniture and handicrafts.

Engraving

The process of carving convex, concave or hollow patterns on the wood surface. According to its characteristics and carving methods, it is divided into relief, round carving, openwork carving, plane carving and line carving. ① Relief is divided into low relief, medium relief and high relief according to the height of the raised figure. ② The characteristic of openwork is hollowing out, including negative openwork and positive openwork. The former hollows out the pattern part. The latter carves away the parts other than the pattern on the board, leaving only the pattern on the board. ③Line carving uses curved lines of different thicknesses and depths to express patterns. ④ Round sculpture is a three-dimensional physical sculpture in three-dimensional space, also called full sculpture. The wood used for carving is best of moderate hardness and fine structure, such as boxwood, ginkgo, basswood, etc. There are two carving methods: manual or mechanical. Hand carving is labor intensive and requires considerable skills. Batch production requires the use of machinery such as upper-axis milling machines, multi-axis copy milling machines, router saws, and CNC engraving machines. Carving decoration is commonly used in furniture, architectural decoration and handicrafts.

Molding

The process of using a metal stamp to pressurize wood to cause plastic deformation of the wood to form a certain pattern and achieve a relief effect. According to the different molding materials, it is divided into solid wood molding, veneer molding, and fiber, shavings and other scrap molding. ① When molding solid wood, it is necessary to heat and soften the wood and pressurize it with a metal mold. The compression amount is about 30%. To produce furniture and building components. ② A mixture of thermoplastic and thermosetting glue should be used for veneer gluing and molding. Sprinkle water appropriately during molding to improve its plasticity. This method can be used to produce wooden lace and decorative parts. ③ Particleboard molding. When molding, the veneer should be attached. The molding temperature is about 110~160℃ and the pressure is 2.5~5.0 MPa. The scraped materials can also be used for direct molding.

Burning

Use a certain method to apply high temperature (above 150℃) to the wood according to the design requirements to change the original color before carbonization, showing yellowish brown to dark brown. Color tones, the process of forming decorative patterns. Hot stamping methods include hot stamping, hot stamping, acid etching, etc. ① Hot stamping: Use a red hot copper hollow roller or red hot template with patterns engraved on the surface to hot stamp patterns on the wood surface. ②Pyrography: Use a red-hot soldering iron to paint freely and eclectically. It is often used to decorate cabinet facades or make craft hanging screens. ③Acid etching: Use acid etching method to draw patterns.

That is, first apply a layer of paraffin wax on the surface of the processed wood. After it is dry and hard, use a knife to remove the part to be corroded, and then sprinkle concentrated sulfuric acid. After 0.5 to 2 hours, use turpentine or hot soapy water to wash away the remaining sulfuric acid and paraffin wax. Then use a brush to remove corroded wood fibers and scrub with ammonia. You can also use hydrochloric acid of a certain concentration to paint patterns on the wood surface, and then bake it with fire to make it change color for decoration.