This article describes some of the common problems and defects which could be found in plywood manufacturing. The defects emerge either in the core or on the surfaces.
Table of Content
1. Gluing defects
1.1 Weak Bonding
The bonding strength of plywood is lower than the standard value. The reason is that the quality of the adhesive is not good; the hot pressing conditions are not well controlled, such as low hot pressing temperature, insufficient pressure or short hot pressing time, low veneer moisture content, insufficient glue, poor veneer quality, and improper aging time Wait.
1.2 Low wood failure rate
The wood failure rate of plywood is lower than the standard value. The reason is that the bonding strength is low, and the quality of the adhesive used does not meet the requirements. Sometimes the hot pressing temperature is low, the pressure is insufficient or the hot pressing time is short, the moisture content of the veneer is low, the quality is poor, and the improper aging time will make the wood breakage rate low.
1.3 Bubbling or Delamination
Bubbling means that the steam in the veneer gaps destroys the structure of the plywood when the pressure is reduced, forming raised bubbles on the surface of the board, sometimes accompanied by a loud sound. If the failure of the slab is entirely in the glue layer, it can be regarded as glue opening. Sometimes two kinds of defects are difficult to separate exactly. The reasons for this kind of defects are that the pressure drop is too fast and the hot pressing time is insufficient; the moisture content of the veneer is too high or the amount of glue applied is too large; there are blank spots or inclusions on the veneer during glue application, sticking stains; pine with poor air permeability The hot pressing temperature of the plywood is too high.
1.4 Delamination at corners
The corners of the plywood are not glued together, and the glue is split. The reasons for this defect are: the glued veneer has dried up before hot pressing. the corners are lack of glue; the quality of the glue is low, and the bonding strength is poor; the quality of the veneer is poor and the thickness is uneven; The edges and corners of the slabs in every single compartment are not aligned, and the slabs are placed skewed and unevenly compressed when the plate is installed; the corners of the pressing plate are worn and the pressure is insufficient, the pressing plate is deformed, and the temperature of the pressing plate surface is uneven.
2. Structure Defects
2.1 Plywood warping
Bad plywood warps and deforms. If the board is placed horizontally, the degree of warping can be observed through the diagonal. The reason is that the moisture content of the surface and the backboard is inconsistent; the veneer of different tree species is unreasonably matched and violates the principle of symmetry; the veneer has torsion patterns; the temperature of individual hot press plates is not enough; the stacking place is uneven.
2.2 The core board is separated from the seam.
The edge strips of the core board of the plywood are stacked together or separated from a large gap, and obvious streaks and unevenness appear on the board surface. The reasons for this defect are: the reserved gap is too large or too small when manually discharging the core with the parts; the core plate is misaligned when the plate is installed; the parts are uneven, there are cracks or ruffles, etc.
2.3 Missing edges
The plywood case back and core board are insufficient in size, and there are missing edges. The reasons are: the glued assembly of blanks did not achieve “one side and one end”; the surface and back panels were small in size and arranged incorrectly; the edges of the plywood were stuck or the backing board was glued.
2.4 Too High Moisture Content
The moisture content of the board should not be higher than 12%. Any plywood that exceeds this limit should be picked out of the shipments.
3. Appearance defects
3.1 Adhesive penetration
The adhesive penetrates the plywood, and the defect of glue marks appears on the surface of the board. The reasons are:
- Glue is too thick
- Cracks on the back of the veneer are too deep
- Moisture content of the veneer is too high.
Surface of the plywood has an abnormal color. The reasons are:
- Glue is too alkaline or acidic, and the wood contains high tannins;
- Wet veneer is not dried in time in summer, and the surface of the veneer is moldy
- Hot press board leaks.
3.3 Face Veneer Splits
Short size splits can be fixed with putty. Long splits can not be fixed and should be rejected.
3.4 Face Veneer Delamination
3.5 Rubbish & Imprint on Surface
The inner and outer defects of the board should be found out in the quality control process at the plywood manufacturing factory. The problematic boards are either fixed or rejected.