Veneer sorting is one of the most important operations of the technological process, which determines the varietal yield of finished plywood. The grade of the veneer sheet is determined by the sorter depending on the totality of sheet defects (defects in raw materials and production). The number of possible combinations of defects is very large, besides, individual defects are sometimes difficult to determine with sufficient accuracy. The task of the sorter is to be able to give a correct overall assessment of the quality of the veneer sheet so that there are no sharp deviations in one grade.
There are certain rules for the selection of veneer depending on the types of plywood. Two sides (faces) are distinguished in each sheet of plywood: the front and back, while the front side is usually of better quality than the back; and the inner veneer (middle) is of lower quality than the plywood face.
Acceptable wood defects and manufacturing defects are clearly defined for each veneer grade. Variety-forming defects for veneer are knots of various types and sizes, end cracks, abnormal colors and rot (false heart, incipient deterioration, flecks and bluestain), inbarks, bark patches, i.e. wood defects caused by natural conditions. Production defects that occur during the production process include roughness of the sawn surface, quagginess, cracks, etc.
Plyterra Group plywood is divided into the following grades depending on wood defects:
- В (1) grade is almost without defects, only a few sound knots with a diameter of up to 15 mm are allowed;
- BB (2) grade allows patches on the surface of the sheet. Knots and open defects are repaired with veneer inserts;
- CР (3) grade allows partly accrete, single and loose knots with a diameter of no more than 6 mm in the amount of 10 pcs. per 1 m2 of the sheet surface;
- C (4) grade allows many manufacturing defects, a strong gluing is guaranteed.