Plywood QC Inspection Checklist : Part 17 Quality defects and Grading rules
What are the common quality defects of plywood?
The common manufacturing defects of plywood are as follows:
1. The low bonding strength is mainly caused by the poor quality of the glue. If the hot pressing conditions are not well controlled, such as low temperature, insufficient pressure and too short glue pressing time, such defects will also occur. Excessive moisture content, insufficient glue content, poor veneer quality and too long or too short aging time will also reduce the bonding strength.
2. Bubbling and local delamination of the plywood ,because the glue are caused by too fast pressure-reducing speed, insufficient glue-pressing time, too high moisture content of veneer, blank spots or inclusions on the veneer, sticky dirt on veneer or too high temperature of veneer when gluing.
3. The warpage of plywood is caused by the large internal stress of plywood. The reason is the inconsistency of moisture content in the surface and back plywood, the unreasonable matching of veneers and different tree species, the twisted lines of veneers, the insufficient temperature of individual hot-pressed boards and the unequal stacking of boards.
4. Plywood edges delamination ,The reason for this defect is that the pressure is insufficient due to the wear and tear of the edges and corners of the pressing plates. The edges of the platform in each interval are not aligned. When loading the sheets, the platform are put askew and under uneven pressure. When spinning the edges of the veneer, the pressing degree is insufficient, the glue bonding is weak, the edges and corners are short of glue, the glue dries up too early, and the local temperature of the pressing plates is low
5. Penetration,The reason of permeability /Penetration is that the glue is too thin, the glue content is too large, the cracks on the back of veneer are too deep, the moisture content of veneer is too high, the aging time and pressure are too large.
6. The reasons for the core voids and overlaps of the plywood are that the reserved gap is too large or too small when the manual zero-piece core row is used, the core veneer is dislocated or overlapped when the plywood is loaded, and the edges of the veneer are uneven, etc.