Logs of suitable dimension and quality for slicing and peeling are generally sorted in the log-yard upon arrival, according to size and species. Handling may be by heavy lift trucks, derricks or cranes, all of which are sized to cater for the logs' dimensions and weight.
Logs of suitable dimension and quality for slicing and peeling are generally sorted in the log-yard upon arrival, according to size and species. Handling may be by heavy lift trucks, derricks or cranes, all of which are sized to cater for the logs' dimensions and weight.
Before peeling, the majority of timbers need to be conditioned so as to soften the wood in order to facilitate peeling and to produce an acceptable quality of veneer. Conditioning involves the exposure of the peeler blocks to both heat and moisture by way of soaking in hot water vats or exposed to live steam or hot water sprays.
Before peeling, the majority of timbers need to be conditioned so as to soften the wood in order to facilitate peeling and to produce an acceptable quality of veneer. Conditioning involves the exposure of the peeler blocks to both heat and moisture by way of soaking in hot water vats or exposed to live steam or hot water sprays.
The veneer sheet is then wound on spools, or led to a multi-tray system, so as to provide storage and surge capacity in the event of fluctuations in the veneer feed from the lathe; speeds of both storage systems are generally synchronized to that of the lathe.
The green veneer is then clipped to size, either manually or by high-speed knives, graded and stored in piles ready for drying. Any defects, such as knots and splits, are then cut out of the sheet.
The drying of veneer, to between two and ten percent moisture content, is to aid the gluing process during the manufacture of the plywood. Depending on the location and sophistication of the plywood mill, the veneer sheets may either be left outside to dry in the air or kiln-dried. Kiln-drying involves the drying of stacked veneer in batches or the continuous drying of sheets which are mechanically conveyed either on a continous belt or roller system through the length of the dryer. Obviously a controlled drying environment, with minimal handling, will result in a more uniformly dried veneer, with the least amount of damage
Veneer drying accounts for some 70 percent of the thermal energy consumed in plywood production and approximately 60 percent of the mill's total energy requirement. For this reason new and improved drying systems are being constantly developed, as well as the manner in which they are heated.
Dryer heating may be by the indirect use of steam or thermic oil, or direct firing with the temperature being controlled by the regulation of the fresh-air make-up. Although drying temperatures of between 90-160°C may be considered normal, increased temperatures of some 175°C are being used on certain species in order to reduce the overall drying time.
Primary finishing, which entails the trimming, sanding and upgrading of the plywood after pressing, is undertaken so as to enhance the marketability of the product. It is carried out at either separate work stations, or, in the case of modern mills, as a combined operation in a continuous semi-automatic line.
Trimming saws cut the plywood boards to the required size, which are then sanded in machines fitted with wide-belt or drum sanders so as to obtain the desired surface smoothness. Damage or imperfections to the face veneers are then manually repaired by plugging and the application of patches.
Plywood is produced in a wide range of sizes and thicknesses, although the sizes most commonly produced are 1220 x 2440 mm together with 1830 x 3050 mm and 915 x 915 mm sized panels. Thicknesses may range from 3-25 mm, with the number of plies being between three for boards up to 7.5 mm thick, to five or more plies for thicker varieties.