cause setting. Once they have been set, they are waterproof and rotproof. Ürea resins have similar characteristics but are not as strong as the phenol resins, nor can they be subjected to the extremes of combined heat and moisture against which the phenol resins are impregnable. Urea resins set either in hot presses or under pressure alone when kept in a hot storage room. Figure 29.-Plywood press. After panels have been glucd, they are baled and placed under pressure to insure close contact between various plies. Pressure is held constant until the glue has set. Note massive construction of press and careful equalization of pressure on panels. The introduction of both these phenol and urea resins into the plywood field has broadened the market to include outdoor use, or installation under conditions of humidity and temperature which would destroy any other glue lines. The effectiveness of the waterproofing properties of urea glues is dependent upon the percentage of flour used. Resinous glues do not support any type of bacterial deterioration. As a consequence, the use of resin-bonded plywood in humid climates is being greatly extended. PLYWOOD ASSEMBLY Before being assembled into a plywood panel, the veneer cores, cross-bandings, and faces are dried down to a uniform moisture content and ironed flat. Lumber cores are also dried down in kilns to a uniform moisture content and then planed to uniform thickness. Cold glues are distributed from the hopper of the glue spreader to parallel rollers between which the sheets are fed. All contacting faces receive a uniform coat of glue. These glued sheets are laid up in bales and placed under pressure to insure close contact between the various plies. Pressure is held constant, by clamps with turnbuckles, until the glue has set. The time required for setting varies with the glue used and with the size of the panel. After the glue has set, the plywoood panels are taken to drying rooms equipped with temperature control or mechanical ventilation, or both, to remove (through evaporation) the moisture introduced in the panels by the glue. The panels are then trimmed to width and length and are drum-sanded to remove the veneer tape and to smooth the panel surfaces. A second sanding, belt-sanding, is resorted to by manufacturers if an exceedingly smooth finish is required. After belt-sanding and thorough inspection, the plywood is ready for fabrication and for finish, without any further preparation, except a final sanding after all cabinet work or installation work has been completed. The handling of plywood assembly when resin glues are used is somewhat complicated by the unique character of the adhesive. Resins rely for their adhesive quality on the chemical change effected by simultaneous application of heat and pressure, instead of by the evaporation which effects the drying and setting of glues. The adhesive is applied either in thin sheets or with a glue spreader (depending on the type), and the individual assemblies are placed in compartments of the hot plate press whose platens are steam or electrically heated. Under heat and pressure the resin first flows and then hardens. The resulting glue line is insoluble in water and is proof against bacteriological deterioration. Hardwood plywood panels made by this process-particularly those made with phenolic resin-are proof even against continued boiling. One of the latest developments of this type of panel is for the planking and decking of small boats, particularly of the high-speed racing types. STANDARD SIZES Standard panel thicknesses run from % inch by eighths to 13/16 inch, for either three-ply or five-ply. Standard plywood construction, in order of expense, may be summarized as follows: Low cost. % or 4 inch thick, three-ply, rotary core. Increased permanency. Special constructions for bending purposes are frequently produced in two-ply, but this construction sacrifices some of the normal characteristics of plywood. Thinner panels from three-ply (%2 inch) and thicker panels (up to 7 inches) are manufactured for special purposes. Such constructions are designed by engineers to meet unusual requirements.8 8 Data on the strength of plywood are available on request to the Forest Products Laboratory, Forest Service, Madison, Wis. USES OF HARDWOOD PLYWOOD Modern American architecture makes steadily increasing demands on hardwood plywood. Whether the design calls for broad, flush expanses in the ultramodern mode or for the more ornate paneling of conservative architectural style, architect and designer come more and more to rely on hardwood plywood panels as a medium of expression. Paneled walls and ceilings of hardwood plywood give an appealing warmth. Whether in the staid precincts of a courtroom, the gay rotunda of a hotel lounge, the rich simplicity of an auditorium, the restful efficiency of an executive's office, or the quiet ease of a residential living room, the architect can find in American hardwood plywood a color and figure exactly in keeping with the atmosphere of the installation. Matched panels with face veneers cut from the same flitch permit the architect to employ a repetition of pattern through a series of chambers and corridors, or to produce an arresting effect in salon or foyer. Whatever the mood, the restful beauty of fine wood is forever present. The natural figure and grain with which hardwood species have been endowed is as permanent as the wall itself. Many woods take on, with age, a mellow tone which adds to, rather than detracts from, their beauty and color. The designer may specify a thin panel, as a measure of economy, where the installation will not be subjected to too great strain, or a thick panel where his work is to stand as an enduring monument to his taste and ingenuity. Modern manufacturing methods enable him to develop curved surfaces almost without limit. Columns with tremendous structural strength, but without sacrifice of beauty, are within the range of his calculations. Hardwood panels can be used by the designer to interpret and supplement the lines of his sketch. Panels of proper proportion lend height to walls when applied vertically; panels applied horizontally lend width. Veneers with wide-striped figures lend spaciousness; narrow stripes concentrate attention; large matched figures break up the monotony of broad wall expanses; small matched figures add a delicate touch to smaller surfaces. Fancy faces-burls, crotches, and stumpwood-are sometimes pretentious and attention-compelling. Whatever the effect, hardwood panels present more than a surface appeal. The human eye looks into the wood and perceives a depth of design and coloring which even the most consummate artist cannot approach with brush and paint. Nature's own incomparable handicraft supplements the work of the designer who relies on hardwood for the interpretation of his ideas Wherever hardwood plywood is installed, it introduces a new note of economy and practicability. Maintenance costs, whether in residential project or public place, are minimized. Surfaces not subject to abrasion require nothing more than an occasional wiping to remove accumulated dust. Other surfaces need little more than a light waxing to restore their original brilliancy and beauty. The abrasion resistance of many hardwoods is in itself a guaranty against most of the marring to which walls are prey. The construction of the hardwood panels protects them against cracking and splitting. |