Delamination currently accounts for approximately 85% of customer complaints about plywood as a sub-flooring product. It has become an urgent issue to many of our plywood members. It is estimated that by merely reducing 1% delamination in a 250 million ft2 (3/8 –in basis) plywood mill, the potential annual savings will be approximately $650,000. To help reduce plywood delamination, the key objective of this project was to develop a generic best practice checklist as a guide for manufacturing plywood.
A generic best practice checklist for manufacturing plywood was compiled with a focus on the following four key checkpoints: veneer peeling, veneer drying, panel gluing/lay-up and hot pressing. Key process variables at each checkpoint were determined as follows: peeling related veneer surface roughness and thickness variation, drying related veneer moisture content (MC) variation and surface inactivation, veneer temperature, glue coverage and dryout, and pressing time and pressure. Some technical issues were proposed to revisit as a strategy to reduce panel delamination. Among them include optimal lathe bar gap and pitch profiles, and proper knife sharpening for peeling, reduction of veneer overdry during drying, real-time adjustment of glue spread for adequate glue coverage, and use of optimum pressing time/pressure for adequate level of panel compression and glue curing. The resulting generic checklist can be modified for individual mill use.
Through literature review, pilot plant tests, and mill trials, the main causes of panel delamination were identified as: 1) glue dryout from long assembly time and high veneer temperature; 2) low panel compression, light glue spread or glue skips due to rough veneer; 3) little glue transfer due to veneer surface inactivation; 4) inadequate glue cure due to heavy glue spread, overwet veneer, sap wet spots, and short pressing time; and 5) combined effects of the above. It was found that veneer surface roughness had a significant effect on plywood gluebond quality, and excessive roughness and combined effect of veneer roughness, overdry, and glue dryout, were key causes of the low percentage wood failure. A statistical model was also developed from mill trials to predict the percentage wood failure in terms of veneer temperature, open assembly time and glue spread. The model helps establish an operating window for each key variable and adjust the gluing/layup process to reduce glue dryout. Furthermore, a practical method was developed to determine the optimum pressing parameters to achieve target gluebond quality while minimizing plywood thickness loss.