Molded plywood
As plywood is called two - or three-dimensionally deformed products from multiple plies , with heat and pressure in a pressing tool glued be. The veneer layers are arranged crosswise at an angle of 90 degrees. Molded plywood is used for flat furniture components such as seats , backrests and seat shells . If you arrange the veneer layers in the same direction , you are dealing with molded plywood . It is used for armrests and chair frames . After pressing the blanks , these are machined finish. A special feature is the possibility of producing different contour variants from the blanks . Due to its high strength and low weight, molded plywood is particularly suitable for interior fittings , seating furniture , bed slats , skateboards and vehicle construction .
History of molded plywood
It was an American named Isaak Cole who was the first to patent a process for producing molded wood in 1874 . He designed a chair made of layer glued wooden strips.
From 1830, the famous furniture designer Michael Thonet was already experimenting with gluing layers of veneer. He could only produce two-dimensional deformations with it . He stopped these attempts and developed the bentwood process to produce three-dimensional molded wood components from solid wood . In 1931 the Gebrüder Thonet company brought the first tubular steel cantilever chair onto the market with the model S43 , in which the designer Mart Stam used three-dimensional molded plywood parts as the seat and back , in order to make the chair very comfortable and appear light without upholstery . It was the Finn Alvar Aalto who was the first to use the natural spring effect of the material for his model "Piamio" in the 1930s . Ten years later, the architects and designers Charles Eames and Eero Saarinen won a furniture design competition in New York with a three-dimensionally shaped shell . Charles Eames then developed the spectacular “plywood furniture collection” with his wife Ray for the Herman Miller company (USA). In 1952 , Arne Jacobsen presented a chair with the unusual name “ Ant ”, which later became very successful, in Europe . The seat and back are self-supporting with a narrow waist . These classics, which have sold millions of times, are still in production today.
Terms
Molded plywood
Term for molded parts made of at least three thin veneers glued to one another , rotated by 90 degrees in the direction of the grain . This creates a barrier between the individual layers , the swelling and shrinking is reduced and more strength is created in the surface. The properties are used in flat molded parts such as seats , backrests and chair shells . The veneer qualities can be combined for visible or upholstered molded plywood parts. The inner layers are made from cheaper peeled veneers . The outer layers can be made of peeled veneers , exotic wood - sliced veneers or laminates are elected.
Molded plywood
Molded plywood is the name given to molded parts in which the grain runs in the same direction. To increase the flexural strength and to reduce the swelling and shrinking behavior , molded plywood is blocked off with some transverse veneers for certain applications . Molded plywood is used for chair frames , cantilever side parts and armrests . By inserted plywood gusset (triangle), the can timber connection simultaneously with pressing (see Fig.). The thickness of the molded parts can be varied by using sanded inner veneers. The tensile strength is significantly higher than that of molded plywood.
Manufacturing
The manufacturing process is divided into three steps. First the peeled veneer is produced, then the molded plywood or molded plywood blanks are pressed from it, which are then processed. At the end there are ready-to-install furniture or interior components .
Veneer production
Beech wood is used in Europe . It is available in large quantities from sustainably managed forests . In Northern and Eastern Europe , birch wood is often used . In North America comes hard maple (maple) are used. The trunk wood is only harvested in winter because of the dormancy of the sap and then has to be irrigated in the storage area or stored under water . For veneer production attenuates to the tribes in Dämpfgruben at 85 degrees Celsius. This makes the wood supple . The next station is the log division and debarking . The lengths of the log sections depend on the veneer dimensions required for the orders . In the peeling machine , the peeling knife is guided against the rotating trunk section with high pressure . The result is a coherent veneer band that is cut into the required widths. Veneers with a thickness of 0.6 mm to 2.3 mm can be produced. The still wet veneers run through the drying process . Immediately afterwards, the peeled veneer is automatically sorted according to optical quality. A distinction is to use veneers for visible and after those with branches and cracks , which are überpolstert as moldings later.
Compression molding
If the veneer is ready for an order, a pressing tool is installed in a press . The pressing tools can be constructed in two parts (for seats , backrests and shells ) or in several parts for z. B. U-shaped molded parts that also require pressure from the side . The pressing begins with gluing the veneers on glue rollers . As glue is a urea - resin with a hardener for the use of during pressing thermoset cures. A so-called veneer package is then placed in the press tool, which is heated to 100 degrees Celsius . The press is closed. The pressing pressure is approx. 25 N / cm². The pressing time depends on the thickness of the molded parts. The thicker the molded part, the longer it is pressed. Bowls take approx. 5 minutes, thicker side parts sometimes 20 minutes. An exception is high-frequency gluing , in which the glue joint is heated using a capacitor field , which significantly reduces the pressing time . This method is therefore suitable for very thick molded parts. Since veneer is a natural product , certain limits must be observed when deforming it . The smallest radius depends on the veneer thickness used and is 12 mm at a 90 degree angle. A tapering of the molded parts is possible. The possibility of three-dimensional deformation is hardly limited for upholstery parts . For exposed wood , slight deformations up to the appearance of cracks or compression of the veneer are possible. What can be deformed with solid cardboard can also be made from molded plywood, as both behave similarly. These limits can be exceeded by using special 3D veneers , which, however, are more complex to manufacture and are therefore associated with additional costs.
Machining
The three-dimensionally shaped molded plywood blanks for seats , shells and backrests are more axis CNC - milling contour milled and, if necessary drilled . The variety of variants is very high, since only new milling programs are written for other contours . After sanding the edges and installing the attachment fittings are ready for painting .
Molded laminated wood blanks for chair frames and components are separated by computer control , sanded and further processed with self-setting CNC milling machines. Special connection techniques such as screw , dowel , finger joint , spring or mortise and tenon connections can be used to produce ready- to -install components or complete frames .
surfaces
The surfaces of veneer plywood and laminated veneer lumber can be designed in a variety of ways by pressing top layers made of fine wood sliced veneers such as oak , maple , cherry , ash , walnut and other woods. Colored melamine films offer further design options ; Decorative films , digital prints and decorative laminate boards ( HPL / CPL ) and of course staining and painting , either transparent or opaque . Modern water-based paint systems are used .
properties
Molded plywood and molded plywood are very resilient compared to other wood-based materials such as medium-density fiberboard or chipboard . This gives them a wide range of uses.
strength
The tensile strength , depending on the type of wood at 110-135 N / cm². She cares z. B. with molded plywood cantilever chairs for the high load capacity and the pleasant swing behavior.
Compressive strength
The types of wood used for molded wood have compressive strength between 50 and 62 N / cm². It ensures long-lasting furniture that can withstand extreme loads, such as B. occur in schools , withstand.
Weather resistance
Beech wood is actually not suitable for outdoor use, as it is decomposed very quickly by wood-destroying fungi . But through a chemical or thermal change it is possible to produce molded plywood and molded laminated timber to be weather-resistant → wood modification .
Flame retardant
By incorporating salts in the production process which will flammability of molded wood reached. This is an important aspect, particularly when it comes to interior fittings and when used in seats for trains .
literature
- Chris Lefteri: Wood. AVedition, 2003, ISBN 3-929638-74-6
- Andreas Kalweit, Christof Paul, Sascha Peters: Handbook for Technical Product Design. Springer-Verlag, Berlin Heidelberg 2006, ISBN 978-3-540-21416-8 .
- The Becker Formholz Compendium 3rd edition, Fritz Becker KG, Brakel 2014.
swell
- Wood lexicon. DRW-Verlag 2003, ISBN 3-87181-355-9 , p. 405.
- Wolfgang Fiwek: bending wood Vincentz Network GmbH & Co. KG, Hanover 2011, ISBN 978-3-86630-952-4 , pp. 137-139.