Defects of Chinese Plywood and Their Causes: Structural Defects and Appearance Defects
Glue strength defect
Low bonding strength The bonding strength of plywood is lower than the specified value of the standard. The reason is that the quality of adhesive is poor; 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 content, poor veneer quality, improper aging time, etc.
Low wood breaking rate The wood breaking rate of plywood is lower than the specified value of the standard. 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 veneer is low, the quality is poor, and the wood breaking rate is low if the aging time is improper.
Bubbling or glue opening bubbling refers to that when the pressure is reduced, the steam in the slab destroys the structure of the plywood, forming raised bubbles on the board surface, sometimes accompanied by a loud noise. If the slab is completely damaged in the adhesive layer, it can be regarded as glue opening. Sometimes it is difficult to separate the two defects exactly. This kind of defect is caused by too fast pressure reduction and insufficient hot pressing time; The moisture content of veneer is too high or the gluing amount is too large; There are blank spots or inclusions and dirt on the veneer when gluing; The hot pressing temperature of pine plywood with poor air permeability is too high.
The edges and corners of the plywood are not glued together, resulting in the phenomenon of glue opening. The reasons for this defect are: the aging time of the glue coated veneer is too long, and the corners have dried up and lost their activity; The corners are short of glue; The quality of corn starch is low, and the bonding strength is poor; The veneer has poor quality and uneven thickness; The edges and corners of the slabs in each single compartment are not aligned, and the slabs are placed askew and pressed unevenly during loading; The pressure is insufficient, the pressure plate is deformed and the surface temperature of the pressure plate is uneven due to the wear of the edge corners of the pressure plate.
Structural defects
Plywood warping Plywood warping deformation in production. If the board is uneven and placed horizontally, its warping degree can be observed through diagonal lines. The reasons are:; The moisture content of table and back plate is inconsistent; The veneer collocation of different tree species is unreasonable and violates the principle of symmetry; The veneer has twist lines; Individual hot pressing plate temperature is not enough; Uneven stacking place, etc.
The core board edges of the core board of the core board lamination joint plywood are overlapped together or leave a large gap, and there are obvious streaks and unevenness on the board surface. The reason for this defect is that the reserved gap is too large or too small when manually arranging the cores with parts; The core plate is misaligned during plate installation; Parts are uneven, with cracks or ruffles.
The size of the surface back and core plate of the plywood with missing edges and less meat is insufficient, resulting in missing edges. The reason is that “one side and one end are flush” is not achieved during gluing and assembling; The specification of the watch and back plate is small and the layout is not correct; The edges between the plywood are stuck or the backing plate is stuck.
Appearance defect
The glue penetration adhesive passes through the plywood and has glue marks on the board surface. The reasons are:; The glue solution is too thin; Too much glue; The crack on the back of veneer is too deep; The moisture content of veneer is too high; Insufficient aging time; The slab is too large, etc.
The color of the plywood surface is abnormal. The reasons are: the alkalinity or acidity of glue is too strong, and the tannin content of wood is high; The wet veneer is not dried in time in summer, and the surface has mildew; Water leakage of hot pressing plate, etc.
