FLUID-PRESSURE MOLDING of PLYWOOD Information Reviewed and Reaffirmed
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K A FOREST PRODUCTS LIBRARY FOREST RESEARCH LABORATORY OREGON STATE UNIVERSITY FLUID-PRESSURE MOLDING OF PLYWOOD Information Reviewed and Reaffirmed July 1953 No. 81624 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5,Wisconsin In Cooperation with the University of Wisconsin FLUID-PRESSURE MOLDING OF PLYWOOD* By BRUCE G. HEEBINK, Engineer Forest Products Laboratory, Forest Service U. S. Department of Agriculture The molding of plywood by means of fluid pressure applied through flexible bags or blankets of some impermeable material (bag molding) has found appli- cation in making plywood parts of various degrees of curvature. In size, these parts may vary from a small aircraft fairing to a complete boat hull. Typical parts include all combinations of single and compound curvature, cylinders, paraboloids, portions of a sphere -- in short, any curved piece for which a mold can be made, and later separated from the finished product. Bag-molded aircraft parts, such as fuselages, wing fillets, and fairings, are reported to offer improved performance characteristics as a result of the superiority of the stiffness-weight ratio of molded plywood to that of metal and the smooth ripple- and rivet-free surfaces presented to the air stream. Bag molding of plywood and laminated veneer members probably had its origin in the vacuum-bag process that was introduced in the furniture industry several years ago (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 veneers bonded with synthetic resin adhesive. By weight, these structures are prob- ably about 80 percent wood and 20 percent resin adhesive, Except for varia- tions in shape, the product is essentially the same as flat-press plywood. Methods of Molding Molded plywood is produced by several techniques which are 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," Perhaps the most inclusive is the term "fluid-pressure molding." Five general methods are shown in figure 1. *This report supersedes Rept. 1347 and Rept. R1431. **Maintained at Madison, Wis., in cooperation with the University of Wisconsin. Report No. 81624 -1- Agriculture-Madison 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 con- stant or varying thickness, in any arrangement of plies can be produced. Plat plywood can also be made by bag molding, but flat plywood can be produced more economically by other methods so that the bag-molding technique is largely limited in use to the production of 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 curvature; 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. , The application of heat to the glue during the pressure period is the basic difference between present-day bag-molding methods and the older vacuum-bag process. The newer processes, therefore, permit the use of thermosetting and thermoplastic glues with long assembly periods. Most of these newer processes use pressures within the range of 30 to 100 pounds per square inch instead of only the atmospheric pressure used in earlier work. Figure 1 illustrates diagrammatically the processes in use at present: A, the vacuum-bag process, B, C, D, and E, techniques employing higher pressures and heat. There are numerous combinations of these five means of supplying heat and pressure. Most of the variations in methods in current use, as well as suggested possible techniques not yet in commercial use, are presented in table 1, which gives the practical limitations of pressures and temperatures, and suggests bag and mold materials for each method. The first three methods listed in table 1 are used most widely ardat present probably produce over 90 percent of the bag-molded plywood of aircraft or boat quality. All three methods require the use of bag materials that are steam-, water-, and air-tight under the conditions of temperature and pressure used.. Perhaps 95 percent of the bag-molded plywood now being produced requires bags of natural or synthetic rubber. Cellophane bags have been used, but only experimentally, in these three methods. Full utilisation of rubber bag materials requires that bags be used to maximum life. Under these conditions, leaks developing from deterioration of the bag during the repeated application of heat and handling are, unavoid- able. Steam, steam-air mixtures, and water as heating and pressure mediums, when used with a leaky bag, cause considerable damage to the part being molded and often to the mold itself. This difficulty has prompted many users of the bag-molding technique to experiment with other fluids, such as air or inert gas, in their search for a more satisfactory pressure and heating medium. Gases have very low specific heats and using them to trans- fer heat to the part limits the rate of production. Furthermore, tests have Report No. 31624 -2- shown that hot air is particularly damaging to all rubber bags, consequently these heating mediums are most logically used in combination with cellophane or some other substitute bag material that is cheaper than rubber. Bag leaks should be avoided regardless of the pressure fluid used. Pre- liminary tests have shown that even relatively small leaks can cause as much as 20 percent reduction in net pressure applied on the glue joints. The pressure loss depends upon the size of the leak and the restriction under the bleeder fitting. When using a gaseous fluid as a pressure medium, the heating time can be greatly decreased if the mold is heated directly by steam or electricity. Heat is thus transferred to the glue lines by conduction rather than by con- vection. In this method, heat travels from the mold to the plywood and the bag stays relatively cool, thus broadening the selection of usable bag materials. Several methods listed in table 1 (6, 7, 10, and 11) offer these advantages and should be investigated carefully, particularly when it is desired to form a large number of parts successively on the same mold. In some cases (10 and 11, table 1) the mold is built to withstand internal pressure, therefore, no autoclave is required. All the methods thus far discussed anticipate the use of pressures consider- ably in excess of atmospheric. It is possible under certain limited condi- tions to produce bag-molded plywood of durable quality using only a vacuum within the bag to develop the external pressure of the atmosphere. However, by this method, good results are obtained only with thin, flat veneers, on relatively flat molds, with a limited number of glues. Methods 12, 13, and 14 in table 1 might be used for this purpose. Low-temperature-setting glues with sufficiently low curing temperatures might also be used with the vacuum-bag process in a warm room in producing small parts that can be assembled in a relatively short time. Equipment for Fluid-pressure Molding Molds The forming of any piece of fluid-pressure-molded plywood requires a mold of some type. Molds, sometimes called forms, dies, or mandrels, are broadly classified as male or female. Hale molds as illustrated in figure 1, A, B, and C are the desired shape on convex surfaces, while female molds (fig. 1, D and E) have the proper shape on concave surfaces. Common mold materials are wood (solid or plywood), metal (steel, cast iron, or low-temperature alloys), plastic materials, and cements. The choice of mold materials will depend largely on the shape of the item to be molded, the quantity desired, and the availability, advantages, and disadvantages of the materials considered. Report No. 81624 -3- Wood olds Wood moles are commonly made of softwood lumber, such as Western white pine or sugar nine, cut approximately to the contour of the mold and glued and nailed together. The rough shape, usually not more than 6 inches thick at any point, is then marked into stations -- a procedure very similar to that used in defining the shape of a boat -- and worked down by plane and sander to the desired shape minus an allowance for a hardwood skin. This hardwood skin (often of birch veneer) is bonded directly to the mold by bag molding and is later worked carefully down to the exact contour at each station. Attention is given to the direction of the grain in both the skin and the mold -ororaer so that the maximum cross-banding effect is obtained. Wood moldsomndistorted somewhat by repeated exposure to moisture and heat. A leaky bag results in damage to a wood mold and the piece being molded is usually ruined: Overheating the mold will hasten the distortion and necessitate early recontouring or other repair.