UNITED STATES DEPARTMENT OF AGRICULTURE FOREST PRODUCTS LABORATORY In Cooperation with the University of Wisconsin BAG-MOLDING OF PLYWOOD By BRUCE G. HEEB INK, Associate Technologist Bag-molding of plywood and laminated vene er members probably had its origin in the vacuum-bag process that was introduced in the furniture industry several years ago (see fig. 1, A). While the vacuum-bag process depended upon atmospheric pressure and ordinarily only room temperature to set the glue between the plies, the newer techniques employ higher fluid pressures and varying degrees of heat. Misnomers, such as "plastic plywood" and "plastic planes," have been applied to structures of molded plywood that are actually made from wood bonded with synthetic resin adhesive. By weight, these structures are probably about 80 percent wood and 20 percent resin. Except for variations in shape, the product is essentially the same as flat-press plywood. Molded plywood is produced by several techniques often referred to specifically as the Duramold, Vidal, Aeromold, or vacuum-bag processes. Other terms sometimes used in describing the technique are "bag-molding, "autoclave-molding," or "tank-molding." or 'tank-molding." Perhaps the most inclusive is the term "fluid-pressure molding." A number of published descriptions of bagmolding processes are listed at the end of this report. The fundamental procedure is the same for all processes in common use. In principle the technique consists of attaching temporarily by staples, tape, clips, or some other means, superimposed layers of strips or sheets of glue-coated veneers to a mold of the desired shape, and molding these into a unit structure by the application of heat and fluid pressure through a flexible, impermeable bag or blanket. All the processes are relatively simple and provide a means by which plywood of simple or compound curvature, and of constant or varying thickness, in any arrangement of plies can be produced. Naturally, flat plywood can also be made by bag-molding, but due to the critical bag materials required in most operations, it is recommended that the technique be limited to the production of strategic molded parts that can be manufactured by no other practical means. In general, parts that fall in this category will have one or more of the following characteristics: Appreciable compound curvatures; variable thickness; single curvature bends approximating or exceeding 180° when pieces are too thick to be steam bent from flat plywood; parts too large to be made practicably by mating dies; quantity too small to justify mating dies. In The same principles of balanced construction that apply to flat plywood are applicable to molded plywood. For maximum resistance to warping all plywood should be symmetrical about the center plane of thickness. this connection symmetry involves species, number of plies, thickness of plies, and direction of grain. In theory, a symmetrically constructed panel with alternate plies laid at 90°, with respect to direction of grain, would have maximum dimensional stability. In practice, however, a construction with alternate plies at 90° to each other is often impossible in pieces of pronounced compound curvature. It is likewise impossible to make flat strips of veneer conform to appreciable compound curvature before the application of fluid pressure. Fluid pressure forces the veneer to conform to the surface of the mold; therefore, the greater the compound curvature at right angles to the grain of the strips, the narrower the strips must be to avoid wrinkles. Equipment The manufacture of bag-molded plywood of aircraft or boat quality requires the use of considerable equipment. Means of supplying fluid pressure and heat must be provided. Normally, an autoclave or cylindrical pressure tank 3- to 12-foot diameter and 10 to 60 feet in length, which will withstand an internal working pressure of 30 to 120 pounds per square inch, is used. In figure 1, C and E, the use of an autoclave in bag-molding is illustrated diagrammatically. Occasionally, the means of supplying heat and pressure is combined with the mold as illustrated in figure 1, B and D. A mold capable of withstanding the desired internal fluid pressure is then required and the bag is inflated by the pressure fluid. All bag-molding of aircraft or boat quality plywood requires heat. This can often be supplied most economically by steam, either directly or indirectly by heating water or air, hence a boiler or an adequate supply of steam from an existing steam line is required. When air is used for a steam-air mixture, or to provide pressure on hot water, or as a combined heating-pressure medium, a compressor and receiver are required. The size and capacity of the compressor and the steam generator depend entirely upon the size and number of autoclaves or units being operated. Obviously, to charge a pressure tank of perhaps 1,000 cubic feet capacity to working pressure and temperature in about 10 minutes requires a large-sized boiler and compressor. A vacuum pump may be used either to induce air pressure or to check the bag over an assembly for leaks. Vacuum alone produces insufficient pressure for most bag-molding operations and therefore is used only for single curvature veneering operations in furniture work and is not recommended for aircraft plywood. Any operation involving the use of veneer presupposes the use of Some of the ordinary veneer trimming tools, such as a saw, clipper, shaper or router, as well as a glue spreader, and veneer conditioning racks. In some bag-molding operations careful control of the moisture content of the veneer throughout the process is desirable. Adequate air conditioning equipment is then an additional requirement. |