PHYSICAL PROPERTIES OF WOOD qc,quality control,quality assurance,inspection,checking,sourcing,factory auditing,import,export,manufacturing

PHYSICAL PROPERTIES OF WOOD Generally, the first information about a species is obtained by a wood taxonomist or wood anatomist. Working with herbarium material and small wood samples, he classifies the species and describes its structure. This information is valuable for screening species to be considered for use as veneer. Such information is often available from libraries or by contacting Federal and State wood research laboratories or wood technology departments of forestry schools throughout the world. Physical properties of wood of interest to potential veneer producers include specific gravity, moisture content, permeability, shrinkage, extraneous cell contents, figure, odor, and cell size, type, and distribution. (Values for individual species are given in Appendix II, ‘Thysical Properties of U.S. Woods for Veneer.”) Specific Gravity Specific gravity or density is easily obtained and is often one of the first properties known about a species. As indicated in table 1, it can be used as a general guide in screening woods for use as veneer. For example, a wood with moderately low specific gravity is preferred for use as core and crossbands of decorative plywood. Detailed information is available about the variation in specific gravity of many species, and additional data are being collected for other species. Information on the specific gravity of wood species can prove commercially valuable. For one example, knowledge of specific gravity for the various pines proved important in founding the southern pine plywood industry. When this industry started, the question was asked if all species of southern pine could be used and still make a product that could be marketed in the same strength category as Douglas-fir for structural softwood plywood. (Species are placed in various groups for use as structural plywood primarily on the basis of stiffness and strength; in general, the strength of wood is related to specific gravity.) Based on the recorded strength values and specific gravity records, the major southern pines—loblolly, longleaf, shortleaf, and slash pine—were permitted to be marketed in the same category as West Coast Douglas-fir. The minor southern pines, which have lower specific gravities, did not meet these requirements. Thus, while not foolproof, specific gravity can be used to quickly screen new species for tentative classification. While most species can be cut into veneer by suitable manipulation of the cutting conditions, it is more difficult to cut wood at the two extremes of the range of specific gravity. Very lightweight species tend to cut with a fuzzy surface. Dense species require more power to cut and tend to develop deep cracks in the veneer as it passes over the knife. Basswood, with a specific gravity (based on green volume and ovendry weight) of about 0.32, is toward the low end of the range for species that are successfully cut into veneer. Hickory, with about 0.65, is near the high end. Still, a valuable species like rosewood, specific gravity of 0.75, can be successfully sliced into face veneer, but this requires suitable heating and limiting the cutting to thin veneer.

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