a typical plant layout is provided to illustrate to the readers the processes involved.
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.
Figure 2. Plywood production – A simplified process flow
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.
Debarking of the logs then takes place so as to facilitate the lathe operator’s task and to remove the dirt and debris which would otherwise prove detrimental to the lathe knife, whereupon the logs are cut to length to fit the lathe, which is normally 240-270 cm.
Nowadays, in almost all cases, plywood veneer is rotary cut, in that the peeler block is rotated around its axis in a lathe, whilst a continous veneer sheet is cut by a knife mounted parallel to the block’s axis.
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.
The assembly of the plywood prior to pressing entails the jointing of the narrow strips of veneer, which are edge-glued so as to make sheets of the required size. Glue is then applied to the inner plies or core, which in turn, are laid between the outer veneers ready for bonding. This operation accounts for a large share of the manual labour employed in the production process.
Although hand roller spreaders is a widely used method of glue application, developments in alternative systems have led to the adoption of curtain coaters, extruders, spray booths, etc., each with their own following.
Once the veneers are laid-up as assembly plywood sheets, they are fed into hydraulic presses so as to bring the veneer into direct contact with the adhesive, where the application of heat cures the glue.
The departure from single opening cold presses towards multiple-opening hot presses, with between 5 to 25 daylights and operating at platen temperatures in the order of 80-180°C, has considerably reduced the overall pressing cycle time and increased press capacity. Heating of the platens is generally by hot water or steam, although thermic oil is used when pressing at higher temperatures.
Cold pre-pressing, at comparatively low pressures, is not being incorporated in the more recent production lines. This is largely due to the fact that veneer stuck together is easier to handle and load into the hot-press, added to which the ply’s reduced thickness allows for smaller daylight openings in the hot press resulting in an overall reduction in loading and hot pressing 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.
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