Plywood Manufacture

Plywood Manufacture
Each peeler log is conveyed to a barker where it is rotated against a steel claw that strips the bark from
the log. The debarked block is moved to the veneering lathe and centred in the chucks. As the log is
rotated, a steel blade peels a conƟnuous sheet of veneer. A large diameter log may yield more than a
kilometre (half mile) of veneer.
As veneer is produced, it is directed to semi-automaƟc clippers that cut it into desired widths. The
clipper blade can also be actuated manually to remove defecƟve pieces of veneer. Next, the veneer
moves to the sorƟng area where it is graded and separated into stacks of veneer originaƟng from either
the heartwood or sapwood sectors of the tree. This segregaƟon is necessary because the heartwood
and sapwood contain very different moisture contents and therefore require different drying Ɵmes.
Steam or gas-heated ovens are used to dry the veneer to a moisture content of about 5 percent. The
speed of passage of veneer through the drying chamber depends on the thickness of the veneer and
whether it is of heartwood or sapwood origin.
AŌer drying, electronic moisture detectors are used to verify the moisture content of all pieces. Pieces
not meeƟng dryness requirements are returned for further drying.
Next, the veneer is graded. In some types of plywood, narrow strips are then edge trimmed and edge
glued to form a conƟnuous sheet. In other types, only the face plies are made from full sheets, and the
interior plies are comprised of loose lain strips placed edge to edge.
Substandard face veneer with oversize imperfecƟons is channelled to patching machines where the
imperfecƟons are neatly replaced with sound wooden patches. Sound face and interior veneer moves to
the glue spreader or automaƟc layup line. There the veneers are uniformly coated with phenolformaldehyde
resin glue and laid at right angles to the adjacent face and back veneers.
AŌer layup, the veneer sandwiches go to the hot press, the key operaƟon in the manufacturing process.
Here, depending upon the thickness of the plywood panel, one or more sandwiches are loaded intoeach
press opening. The press is then hydraulically closed and the panels subjected to a temperature of 150°C
(300°F) and a pressure of 1.38 MPa (200 psi) which cures the glue.
AŌer removal from the hot press, trim saws cut the plywood panels to the required dimensions, usually
1220mm x 2440mm (4 x 8 Ō). Panel edges and ends are trimmed in consecuƟve operaƟons.
Panels are then graded as sheathing or selected for further finishing. Panels to be produced as sanded
grades pass to a sander where faces and backs are sanded smooth simultaneously. Any minor
imperfecƟons remaining in the face and back veneers are repaired with wood inlays or syntheƟc filler
before the panels are finally graded. Plywood is also manufactured with overlays which improve the
appearance and durability of the panel.
The manufacturing process for plywood results in a final moisture content of about 4%. During
installaƟon, a 3 mm (1/8 in) gap should be leŌ between panels to allow for expansion during service.
Tongue and Groove Plywood
Tongue and groove plywood is usually a Sheathing or Select grade panel with a factory-machined tongue
along one of the long edges and a groove along the other. Where diaphragm acƟon is not an overriding
concern, the tongue and groove profiles can be used to dispense with the need for blocking by
maintaining abuƫng panels in the same plane, thus ensuring the effecƟve transfer of loads across joints.
Tongue and groove plywood panels are usually manufactured in thicknesses of 12.5 mm (1/2 in) or
more. Some tongue and groove profiles are designed to buƩ at the Ɵp of the tongue to leave a 0.8 mm
(1/32 in) gap on the face and underside of the joint between panels to allow for swelling caused by
exposure to wet condiƟons before a building’s weather membrane has been applied. The tongue and
groove lock together to form a beƩer rain barrier from rain during construcƟon.

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