Analysis and Control of Common Quality Defects in Plywood
Currently, most plywood manufacturers in China do not produce veneers (face and core veneers) or adhesives themselves but purchase them externally. Therefore, this article omits the production processes from log cutting to veneer peeling and adhesive preparation. Figure 1 shows the plywood production process flowchart.
2. Analysis and Control of Common Quality Defects in Plywood
2.1 Overlapping and Gapping
Overlapping refers to the phenomenon where adjacent core veneers (or two parts of a split veneer) within the same layer overlap. Gapping (also called core separation) occurs when core veneers or adjacent spliced core veneers in the same layer separate. These issues are common in plywood production and should be analyzed and controlled from the following aspects:
1) Core assembly issues: Overlapping, misalignment, or improperly sized gaps during core layup. To address this, operators should receive better training and develop a stronger sense of responsibility to ensure precise veneer alignment.
2) Movement during handling: Core veneers may shift during loading or transportation. To prevent this, glued veneers should be rested before assembly, followed by pre-pressing to bond the layers. Handling should be done gently to avoid displacement.
3) Warped or uneven veneers: Severely warped or uneven core veneers should undergo softening and flattening before assembly, with edges trimmed for straightness.
2.2 Low Bonding Strength or Delamination
Bonding strength refers to the firmness of adhesion between veneers in plywood, a critical quality indicator. Causes and solutions for weak bonding or delamination include:
1) Poor adhesive quality: Adhesives that are substandard, expired, or too diluted should be rejected. If viscosity is too low, flour can be added as a thickener.
2) Improper glue application: Insufficient or uneven glue spread. The amount should be adjusted based on veneer species and thickness, ensuring even coverage.
3) Incorrect veneer moisture content: Core veneers should be checked before gluing, with moisture content controlled within the required range (typically 8%–14%).
4) Improper glue open time: If too long, the glue dries before pressing; if too short, a continuous film doesn’t form. Optimal open time is 15–30 minutes, adjusted for climate.
5) Defective veneers: Deep tear-outs or poor peeling quality. Strict quality control in veneer selection and trimming is essential.
6) Incorrect hot-pressing parameters: Insufficient pressure, low temperature, or short pressing time. Parameters should be optimized for the product, with adjustments to temperature, pressure, and duration.
7) Faulty hot-pressing equipment: Issues like inadequate pressure or uneven heating. Regular maintenance ensures proper function.
2.3 Blistering and Local Delamination
Blistering refers to surface bulges caused by trapped gas, while local delamination is partial adhesive failure. Solutions include:
1) High or uneven moisture content: Strictly control drying to 8%–14%. Excess moisture promotes blistering.
2) Glue application flaws: Uncoated spots or contaminants. Ensure even glue spread, manual touch-ups for missed areas, and clean veneers.
3) Improper hot-pressing: Inadequate time or uneven temperature. Optimize parameters for resin curing.
4) Rapid pressure release: Slow decompression prevents steam-induced blistering.
2.4 Glue Leakage
Glue leakage occurs when adhesive seeps through face veneers, staining the surface. Control measures:
1) Face veneer quality: Avoid overly thin or cracked veneers; repair large cracks.
2) Adhesive viscosity: Ensure proper thickness.
3) Excessive hot-pressing pressure: Adjust to prevent glue squeeze-out.
2.5 Warping
Warping (curvature at edges or center) results from internal stresses. Solutions:
1) Asymmetric plywood structure: Follow odd-layer rule with balanced species, thickness, and moisture in symmetrical layers.
2) Uneven veneer moisture: Pre-dry naturally before machine drying; monitor dryer temperatures for uniform moisture.
3) Hot-pressing setup: Ensure even upper/lower platen temperatures.
2.6 Surface Indentations
Indentations are localized depressions caused by external factors. Prevention:
1) Uneven or contaminated caul plates: Clean or replace plates before pressing.
2) Debris on veneers: Inspect and clean surfaces before pressing.
2.7 Edge Damage or Delamination
Edge damage involves missing or damaged corners/edges due to mechanical or handling errors. Edge delamination is adhesive failure at edges. Solutions:
1) Core veneer defects: Ensure purchased veneers meet length requirements; trim damaged edges.
2) Assembly alignment: Use a right-angle reference edge for uniform layup.
3) Hot-press loading misalignment: Align veneers properly in the press for even pressure.
4) Equipment issues: Replace worn caul plates; ensure even heat distribution in platens.
5) Glue application flaws: Avoid insufficient glue at edges.
2.8 Excessive Formaldehyde Emission
Formaldehyde emission occurs when urea-formaldehyde adhesives release gas during production, storage, or use. Control methods:
1) High-formaldehyde adhesives: Test each batch; reject non-compliant adhesives.
2) Excessive/uneven glue spread: Optimize application to minimize excess.
