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Plywood production process

Storage of raw material

A simple, cheap and effective way of preserving wood is storing it in water or sprinkling the material kept on the ground with water. Such storing protects the raw material against mildew and insects, as well as fractures of wood.

 

Hydrothermal treatment

To prepare the wood for peeling, it must be softened in order to cut it into smooth veneer of even thickness. In our plant we soak the wood in hot water (wood digestion) or expose it to steam (wood steaming). Wood steaming is conducted in long concrete pools in form of logs, wood digestion in a short boiler in form of cutlogs. Hydrothermal treatment is conducted in the temperature of 40-70°C for 8-48 hours.

 

 

Wood peeling of cutlogs

Moist, warm and barked cutlogs are rotary cut into thin layers called veneers. A cutlog is axially fitted in the machine and it starts to rotate; a knife with a termination bar makes progressive movement in relation to the cutlog. Thus, the knife peels, into the centre, on Archimedes’ spiral, a layer of veneer in the form of an endless band. Standard thickness of final veneer is 1.5 mm, 2.0 mm and 2.5 mm.

 

Drying

Both sheets and pieces of veneer are dried before gluing in roll-conveyor dryers. Air temperature in the dryers oscillates between 160-185°C. Veneer moisture before drying is high 30-120%, while moisture after drying should not exceed 6%. The degree of drying is regulated by the change of speed of rolls in dryers. Drying time depends on the thickness and type of wood and usually is from 3 to 10 minutes.

Forming and pressing

Glue is applied by means of 2- or 4- roller glue spreaders with metal rollers covered or not covered with rubber. Glue is applied on both sides of middle veneers. The insert is completed in such a way that wood fibres of adjacent layers are staggered by 90°. Depending on plywood types and veneer thickness, glue application is 130-200 g/m3 of veneer area. Before hot pressing there is preliminary cold pressing which is to lower the insert height. Inserts with urea glue should be glued in a hot press immediately after the preliminary pressing, while those with phenol glue can wait for gluing for a few hours or even days. Prepared inserts are pressed in a hydraulic press at 125-130˚C (depending on glue type); about 1 minute per 1mm of plywood thickness; at the pressure of 1,1-1,5 MPa for soft species and 1,5-2,0 MPa for hard species.

 

Finishing treatment

After previous seasoning the glued plywood sheets are given final sizes by edging. Cutting precision +/- 1 mm. Veneer losses on the surface, splits and holes are puttied. After repair the plywood sheets are sanded by means of a wide-belt sander using proper endless belts. Plywood sanding has a double task, i.e. calibrating thickness and smoothing surfaces.

 

Sorting

Before placing plywood in a warehouse for ready products, it goes through classification and quality control. Plywood quality is controlled in respect of appearance, dimensions, moisture, and gluing. The first two features are controlled during sorting, remaining two are tested in the laboratory. During sorting the appearance of each side of sheet is evaluated, according to requirements. After classifying to a particular quality class, the sheet is stamped in the left upper corner of plywood or on the narrow plain.

Packing and storing

Plywood panels are stored in a warehouse in which relative air humidity does not exceed 80%. Sheets of plywood are piled up on pallets that are bigger than the sheets. Each pile should consist of one plywood type (moisture resistant or weather-boilproof), kind, format, thickness and quality class. The storage places should provide protection against excessive humidity and other adverse influence. For easy identification and finding, each packet of plywood is described on an identification card.