Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Patent
1987-08-31
1991-10-22
Thibodeau, Paul J.
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
144132, 156 94, 4285371, 428543, B32B 1900
Patent
active
050594729
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a multi-ply wood product. The expression "multi-ply wood product" is here mainly meant to include plywood and laminated veneer lumber.
Plywood industries all over the world have in recent years been compelled to face the fact that it is becoming increasingly difficult to obtain wood of an appropriate quality. The supply of birch wood for wood plies has decreased to such an extent that it is today difficult to maintain the traditionally high quality of plywood; birch wood is very well suited for the production of plies as it is compact and possesses a high wear resistance. Due to the decreased supply it has become necessary to replace birch with other kinds of wood, e.g. softwood, to an increasing degree. Wood plies made of softwood cause problems both for the manufacturer and the user of plywood. Softwood plies have a coarser surface than birch plies. They fracture more easily, and the grossgrained pieces of wood at knot points do not properly stay in place. The traditional production processes of plywood can utilize only about 25 per cent of the obtained raw material. The rest requires new special measures in order to maintain a reasonable quality.
Deterioration in the quality of plywood has caused reactions everywhere in the world. Plywood industries in e.g. Sweden, the Soviet Union and France have made earnest appeals for improvement in this matter. For example, it has been found out that plywood is today more liable to fracture than previously. Neither is it suited for various purposes as well as previously. Plywood causes problems e.g. in the molds of concrete casting. It does not withstand the strains caused by the concrete flow or the vibration of concrete but is broken so that freezing water gets into the cracks, and fractures are formed. Nor does it absorb water and, consequently, water remains between the concrete and the mold, and the formed air bubbles leave marks in the hardened concrete. It has also been found that the impact strength of plywood is lower than previously.
Various efforts have been made to solve the problems. Superficial defects, such as recesses and cracks, affecting the appearance of the plywood, have been evened with a filler. This may be acceptable if the defects are small and not too numerous. As compared with the surrounding wood surface, a defect nevertheless forms an area of discontinued homogeneity which may be the root of problems, depending on the use. The filler does not eliminate the fracture effect caused by the defect; furthermore, the filler often comes off partially or wholly so that moisture gets in between the plies, and a stripping process is started. Defects caused by knots have been repaired by means of a circular piece of homogeneous woodply which is fitted accurately in a hole made round the knot and fastened in the woodply beneath in the single pressing of the plywood sheet. After the pressing, the sheet is ground. With respect to the fracture effect, this way of repairing is better, because the defect is confined within smoothly curving contours. However, the circular piece does not always stay in place. When the plies are apart from each other before the pressing step, the piece is glued in the ply only at the edges thereof. When handling the ply, the piece is often displaced or comes off completely. A further problem arises in that an unnecessarily accurate control is required to provide the circular piece with similar shrinking and swelling properties as the surrounding wood ply. It often happens that the circular piece is curved or cracked due to the greater swelling thereof, or comes off at the edges thereof due to greater shrinking. In general, all the repairs are carried out manually. In view of the manufacturing costs, this is a drawback. The repairs do not notably improve the wear resistance of the plywood, nor is its resistance to weather improved at the repaired points. The appearance, either, is not improved to any greater degree by the local repairings. Thus, it is obvious that this kind of problems are fur
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Algars Rainer
Hautaniemi Erkki
Jarvela Pentti
Le Bell Jean
Pirhonen Jussi
Oy Partek Ab
Thibodeau Paul J.
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