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Veneer decoration is a crucial process in panel furniture production. Veneer materials are used to decorate furniture surfaces, concealing surface imperfections and enhancing the substrate's surface resistance to wear, heat, water, and corrosion. They also improve and enhance the material's strength and dimensional stability. However, due to varying production methods across various companies, quality issues often arise during the processing and use of veneer decoration. This article proposes solutions to these issues.
1 Veneer decoration quality requirements
Veneers made of veneer and other materials must not exhibit debonding, bubbling, or obvious glue seepage. The veneers must be tightly assembled, smooth, and have similar grain. There must be no dents, pitting, cracks, scratches, chipping, or edges. National standards specify requirements for furniture veneer application: Companies must strictly adhere to these standards in their production, and internal company standards are often stricter than national standards.
2 Veneer processing technology
Before applying the finishing material, the plain board is sanded to a fixed thickness or surface to ensure that the thickness and surface roughness meet the veneer quality requirements; glue is applied to one or both sides of the sanded plain board surface; then the decorative material is laid on the glued surface and pressurized and cured on a cold press or a hot press to ensure that the finishing material and the base material are tightly adhered together; finally, the veneered board is trimmed to complete the veneer work.
Veneer is typically applied using a dry-applied process. This involves first applying adhesive to the substrate, allowing it to cure, and then applying the veneer according to the design. Alternatively, a wet-applied process is used. This involves directly gluing the veneer to the substrate without drying it (to a moisture content of 30% or higher). Glue is applied to the substrate surface, and then the pre-cut and stitched veneer blanks are placed on top. The veneer is then pressed in a cold or hot press. After pressing, the veneer quality is inspected and treated accordingly. Finally, the edges of the panels are trimmed and sanded.
3 Main problems and causes in veneer production
Whether it is thin wood veneer or decorative paper veneer, some quality problems will occur during the processing. The main reasons are as follows.
3.1.0 Base Material
The smoothness and flatness of the substrate surface, the bonding strength of the inner surface, and the uniformity of thickness all affect the quality of veneer.
3.1.1 Substrate surface quality
The appearance, quality, and physical and chemical properties of the artificial board used in panel furniture must meet the latest national standards. Small areas of glue spots, paraffin spots, oil stains, and other impurities often appear on the surface of the board, which can affect the penetration of the adhesive, preventing the material from gluing properly and causing problems such as debonding and bubbling.
3.1.2 Substrate thickness deviation
The national standard for man-made panels stipulates a thickness tolerance of ±0.2 mm. We tested the thickness of a certain brand of E1-grade panels and found that nearly 10% of the panels exceeded the national standard. This can lead to various quality issues for thinner finishing materials.
3.2 Finishing materials
Overdrying the impregnating paper can cause white spots; uneven thickness can cause discoloration. Excessively thin veneer, excessively large ducts due to the material's structure, or excessive moisture content can cause glue to bleed through the surface of the adhesive. High moisture content in the veneer can cause black glue seams, gaps, or overlaps at the veneer joints, sometimes causing the entire board to warp and deform. Excessively thin veneer can cause the base color to show through.
3.2.1 Moisture content of decorative materials
Moisture content is a key factor influencing veneer quality. It influences the viscosity and wettability of the adhesive used during veneer application, as well as the apparent physical properties of the decorative material. Veneer, a thin, wood-based veneer material with the characteristics of a rare species, typically has a moisture content of 8-15%. A low moisture content results in slow heat transfer and brittleness. However, the moisture content of veneer should not be too high. Excessive moisture content can lead to drying shrinkage and warping of the board surface. The moisture content of decorative paper also affects veneer quality. Processed paper is typically stored at a temperature of 25°C and a humidity of 55-65%. If the decorative paper is too dry, white spots will appear, while if it is too damp, wrinkles will form.
3.2.2 Thickness of decorative materials
Veneer typically has a thickness ranging from 0.3mm to 1.5mm. When the substrate surface is flat, the veneer can be thinner; when it's uneven, the thickness is thicker. Furniture companies typically use veneer with a thickness of around 0.4mm to 0.6mm, which meets both the veneer process requirements and the decorative effect of solid wood. If the veneer is too thin, or if the duct is too large, the glue and the underlying color will show through. The decorative paper used must also be of a certain thickness, otherwise the underlying color will show through. Uneven thickness can also cause discoloration.
3.3.0 Veneer Process
The veneer process includes the process methods, standardized operations, processing conditions, etc., which is the key link to ensure the quality of veneer.
3.3.1 Process
Sanding the raw boards to a fixed thickness is a crucial process for ensuring uniform thickness and improving surface quality. Improper handling or omission of this step can lead to veneer defects that cannot be remedied in later steps. The gluing process involves evenly applying a defined amount of glue to the substrate surface. Excessive glue can make the layer hard and brittle; insufficient or uneven application can result in localized gaps and blistering. When assembling the blanks, the top and back surfaces must be glued and applied symmetrically. Otherwise, uneven stress on the substrate surface can occur, leading to warping of the board.
3.3.2 Processing conditions
During the veneering process, good processing conditions must be ensured, such as a clean environment, appropriate humidity, temperature, and pressure. Before applying glue, the board surface must be kept clean. After applying glue, dust particles must be prevented from falling on the board surface, otherwise, there will be granular protrusions on the board surface.
3.3.3 Standardized operations
Veneer processing requires strict adherence to process standards. This is especially true during the base material coating process. The adhesive is first prepared according to a specific ratio, stirred evenly, and then added to the coating machine using a specialized container. In practice, workers often add the base adhesive directly to the rubber roller, then add modifying materials like lignin powder, and continue mixing on the coating machine. This can result in uneven mixing, creating large particles, damage to the coating machine, and defects such as localized adhesive gaps and granular protrusions after coating.
4 Measures and methods to solve the quality of veneer
4.1 Treatment of adhesive penetration or cracks on decorative stickers
If there is glue seepage or cracks on the veneer surface, increase the thickness of the veneer or insert a layer of paper between the veneer and the base material for cushioning. Also, the moisture content of the veneer should not be too high. Also, appropriately reduce the hot pressing temperature and pressure. After hot pressing, stack the boards face-to-face to minimize moisture evaporation. Adjust the viscosity and amount of adhesive applied, and appropriately increase the adhesive solids content.
4.2 Treatment of grease or stains on the surface of decorative stickers and treatment of transparent base color
When the surface of the decorative sticker is contaminated by oil, fat, or wax, it can be dissolved and wiped off with an organic solvent. Common solvents include alcohol, ether, benzene, and acetone. Alternatively, it can be scrubbed with weakly alkaline 1% sodium hydroxide or sodium bicarbonate, and then wiped off with clean water. Most of the color spots on the surface of the decorative sticker are caused by the tannins, pigments, and iron ions in the wood. They can be wiped off with hydrogen peroxide or 5% oxalic acid. When the base color is transparent, use a slightly thicker veneer or paper of the same color as the veneer to stick on the substrate first, and then apply the veneer. Another method is to color the substrate first and then apply the veneer. To color the substrate, use a paint finishing method to directly color the substrate, or add a small amount of colorant to the adhesive and color it while applying the glue.
4.3 Blistering and debonding occur in a small area
Debonding or blistering areas should be deflated before gluing. Use a sharp cutter to cut the veneer at the blister along the wood grain to deflate the air. Use a thick-tipped syringe to inject strong glue into the blister, then flatten it with an iron. For larger blistering areas, use a sharp blade to cut the veneer at an angle along the wood grain, scraping away any remaining glue. Select a similar veneer, trim it to the desired size, and re-glue it with strong glue. However, be sure to bevel the edges of the veneer to ensure that the repaired veneer contacts the slope of the original facing veneer, leaving no seams and a smooth, even surface.
4.4 Treatment of large-area bubbling and degumming
When large-scale bubbling and debonding occur, and the number of workpieces is large, repair alone cannot fundamentally solve the problem. It is necessary to find out the cause, analyze and discuss, and find corresponding measures.
First, tear off the veneer material from the substrate surface and observe the adhesion of the veneer material, substrate and adhesive at the blister. If there is almost no adhesive left on the substrate surface, it means that the substrate is not compatible with the adhesive used. You can replace or modify the adhesive, or use a high-quality artificial board, or sand the substrate surface to obtain better surface quality and thus improve the surface bonding strength. If there are many substrate debris on the torn veneer material, it means that the internal bonding strength of the substrate does not meet the strength requirements of the adhesive veneer material. The simplest solution is to use a substrate that meets the E1 or E0 grade. However, due to the tight construction schedule and the fact that existing raw materials cannot be wasted, it is impossible to replace the adhesive or substrate. The following is a method to solve the problem from a technological perspective using existing materials.
The process of treating the substrate surface: it has a good effect on solving the bonding strength of veneer.
After sanding and cleaning the substrate surface, spray a low-solids, low-viscosity sealer (approximately 8-10 seconds). The substrate preparation process is as follows: Due to its rapid drying speed, thick applications are not recommended. Generally, a single spray application of 60-90g/m2 is required. After drying thoroughly, sand gently and then apply the adhesive and finishing material. The sealer has excellent viscosity and penetrating properties, penetrating deep into the substrate to form a film, providing a sealant and preventing moisture absorption and dissipation. It also acts as a binder for substrate debris, enhancing the internal bond strength of the substrate surface. However, the compatibility of the sealer with the adhesive and topcoat should be considered. "Debao" primer provides excellent sealing results. In short, spraying a sealer is an effective solution for addressing insufficient internal bond strength or surface porosity.
4.5 The surface of the adhesive tape is uneven
In severe cases, the veneer can be planed off and the surface refinished. The main cause is large thickness variations in the substrate. By sanding the substrate to reduce these variations and create a smoother curve, the veneer process can generally meet the requirements. Furthermore, the surface of the plain wood-based panel contains a high wax content, which can hinder the infiltration of the glue and reduce the bonding strength.