Wooden objects obtened by bonded wood - Molded plywood for chair and other articles

In our homes, it's common to come across molded plywood without even knowing it. They are mainly used to equip seats and armchairs for the seat, back and sometimes armrest sections. For this same application, we find them as a support when they are upholstered (foam, fabric, leather, etc.). They are also used to make bed slats, sports boards ( skateboard ) and various curved parts for fittings or special applications (horse saddles, harp cases...).

Plywood is characterized by a number of features. It's strong, often complexly curved, easy to fasten and has an attractive final cost compared with systems using steel or plastic.

The wood species used must have good mechanical strength, flexibility and availability. In Europe, beech is ideal for this application.

The flawless log is sliced into veneers by peeling. Since it is too rigid, it must be plasticized with steam or hot water. Like all wood, it must then be dried to a moisture content close to the equilibrium moisture content required for indoor use. The composition of plywood requires an odd number of veneers for balance. The total number depends on the mechanical strength required.

Veneers are glued on both sides. Every other veneer is glued with a roller gluer. The glue is urea-formaldehyde or vinyl-based. The gluing machine is equipped with a mixer and cooler to handle highly reactive glues.

The glued veneers are placed in an electrically or high-frequency heated mold. The mold is clamped in a platen press. As the mold is closed, the veneers deform and take on the shape of the mold. The pressure used is important because, with wood, variations in thickness and structural defects must be compensated for by plastic deformation. Only hydraulic presses can achieve the necessary pressures. The pressing time must be adapted to allow the glue to harden at the heart of the plywood. To save time, parts are demolded before they have fully hardened. The parts must be stabilized before the next cutting and drilling operations.

After stabilization, the parts are trimmed because, for reasons of quality and ease of manufacture, they are larger in width and length than necessary.

Finishing consists of trimming the parts, sanding them, fitting the elements that will be used for fastening and then applying finishes such as varnish and paint. Upholstered seats are glued with foam and fabric.

The aim of this article is to provide sufficient information on the manufacture of molded plywood to help specifiers choose this product, which allows the use of wood and offers certain technical advantages. In addition to the technician confronted with its...