/ 1+31 V ' ' BAG-MOLDING Of PLYWOOD May 1943 AT st\, vae.urt65A No. R1431 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin OCD Py BR1 . KELBINK, Associate Techno". - Bag-molding of plywood and laminated veneer members probably had its origin in the vacuum- pr that was introduced in the furnitur dustry d (see fig. 1, A). While the vacuur.- | ■ depended upon atmospheric pressure and ordinarily only room temperature to ' 'he glue betv. he plies, the new^r techniques employ higher fluid » 8 sure 8 and varying degrees of heat. Misnomers, such as "plastic plywood" and "plasti aes," hav applied to structures of molded plywood that ar<= actually od bonded v. ; rthetic resin adhesive. By weight, these struc'. r otably about SO percent wood and 20 percent resin. Except for variatic: -roduct is essentially the same as flat-press plywood. Molded plywood is produced by several techniques often ref *:o sifically as the Duramold, Vidal , Aeromold, or vacuum- Other terms sometimes used in describing the technique are "ba - "autoclave -molding," or "tank-molding." Perhaps the most inclusive is tl term "fluid-pressure molding." A number of published i molding processes are listed at the end of this report. The fundamental procedure is the same for all proce: use. In principle the technique consists of attach!: ^y staples, tape, clips, or some c s, superimposed layers of a ' or sheets of glue-coated veneers to a mold of the d these into a unit structure by the application of hea fluid pres- vhrough a flexible, impermeable bar or bla . All the procerus are relatively simple and provide a means by which plywood of simple or compc rvature, and of constant or varying thickness, in any ar.- of plj be produced. Naturally, flat plywood can alro be made b; -melding, but due to the critical bag material.: required in most operations, it is recommended that the technique be limited to the production o: *ateg molded parts that can be manufactured by no oth .IB general, parts that fall in this category will hav or more of tl followin, -aoteristicS: Appreciable compound cur- iable thick- cur v roximatir. : 1S0 C whe -ss too thick to be steam bent from fla ocd; j- - ^00 large to be made pr*- iy by mating dies; qui y too small to justify ma til The princi] truction that -o f la- wood are applicable to molded plywood. Per maximu all plywood should be syi - the - r.ection symr nvolv* 'rer of pi I , r s of : on of £rain. or;,* , a syi -ly construct - laid at -espect to direotic would ' Jraum -i itabil ity « . .a .Rl ! <-31 -1- construction with alternate plies at 90° to each other Ls 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 wr inkles. Equipment The manufacture of bag-molded plywood of aircraft or boat quality requires the use of considerable equipment. Means of supplying fluid pres- sure 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 squar° inch, is used. In figure 1, C and E, the use of an autoclave in bag-mold- ing 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 recom- mended 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 con- tent of the veneer throughout the process is desirable. Adequate air con- ditioning equipment is then an additional requirement . Mimeo. No.RL^31 -2- Molds The forming of ar.y piece of bag-molded plywood requires a mold of some type. Molds, sometimes called forms, dies, or -els, are broadly classified as male or female. Male molds as illustrated in . 1 , A, B, and C are the desired shape on conve;- aces, while female molds (fig. 1, D and E) have the proper shape on concave surfac . Common mold materials are wood (solid or plywoc . 1, cast iron, or low-temperature alloys), plastic materials, and choice of mold materials will depend largely on the shape of the item to be molded, the quantity desired, availability, advantages and disadvantages of considered materials. '..ood molds are normally of the male type and have the ol ' advantage of presenting a tacking surface often necessary in tei • ily attaching the strips of veneer. I'ale wood molds may be grooved to allow th<=> insertion of stiffeners to be glued to the molded plywood in a single operation. Wood molds are prone to distort somewhat after r .eating and cooling, particularly if they are allowed to become wet. A mold may become wet as a result of leaks in the bag. The temperature penetration in wood molds is relatively slow as the mold is usually several inches thick, and the molded plywood must be heated essentially from the _de. Metal molds, usually of greater cost than those of wood, may be less expensive to use because of their long life and stability, practice, metal molds are female in type. The smoother surface of the piece being molded is on the convex side of the piece. Some attempts have been made to use concrete, cement- , casting resins in mold construction, but to date these materials have proved practicable in relatively few uses. Bags or Blan. The purpose of the bag is to provide a flexible impervious between the fluid under pressure and the mold. The piece V olded pressed between this flexible bag and the rigid surface of the mold and the full fluid pressure is applied at right angles to the surface of the bag regardless of the shape. The pressure at certain glue joints may slightly less than the full fluid pressure by the amount necessary to shape the veneer. Bags are classified as full bags or half tags (blankets), bag is a complete envelope of impervious flexible material (fig. 1 A and C) clamped shut at one end or side and having a connection, usually called a bleeder, to allow the entrapped air to escape to the atmosphere. It may be completely closed, similar in principle to a basketball bladder (fig. 1, B and D), having only a tube connection for inflation. A half bag, or •o.l o.RlVjl -3- blanket, is a sheet which normally fits the mold without v/r inkling and is sealed by some temporary means to the edges of the mold (fig. 1, E). The bleeder may be attached to the mold or to the bag. Pull bags are normally used over male molds and half bags are used on female metal molds. Most bag-molding operations at present require bags made of specially compounded natural or synthetic rubber. i. The useful life of a bag depends largely on the heating medium used, the temperature of the cycle, the size of the bag, and the care used in handling. It may be as short as 10 hours or as long as 200 hours of operation. Because of the present critical supply problem with all rubber products, attempts are being made to find suitable substitute bag materials. Some of these show promise and preliminary tests have shown that certain low-cost materials may be used for one operation. Glues It is possible to use a variety of glues for bag-molding; however, typical bag-molded parts require long assembly periods, and have definite use requirements that normally limit the choice of glues to the hot-setting resin types. Glues most generally preferred are those that are dry at the time the veneer is assembled on the mold and may be pressed at any time within 7 days after application. The molding of curved parts often requires that the flat strips of veneer slip slightly during molding. Some glues aid this slipping by act- ing as a lubricant while passing through the plastic stage when heated. This peculiar characteristic and the somewhat critical relation between temperature, time, and pressure in the curing of the glue makes close coop- eration between operator and glue supplier advisable. Specially compounded glues are available which have been developed for tag-molding.— These glues have been formulated to produce good bonds at the normal moisture con- tents and temperatures us<=d in hot-pressing plywood at fluid pressures of 30 to 100 pounds per square inch commonly used in bag-molding. —See partial list of bag material manufacturers in appendix. 2 -^See appendix for partial list of suppliers of glues for bap,-molding Mimeo. Wo. RIU3I -k- .IX Partial List of Manufacturers of Rubier Ba lis or Bags E. I. duPont deNemours & Co., Inc., Fabrikoid Division, Fairfield, Cc Firestone Rubber & Latex Products Co., Fall River, Goodyear Tire & Rubber Co., Akron, Ohio. P. F. Goodrich Co., ^0 S. Main St., Akron, Ohio. Tyer Rubber Co., 100 Railroad Ave., Andover, Voit Rubber Co., Los Angeles, Calif. -ial List of Suppliers o; I >. -Mc ' . : .. .--lu- 3 . American Cyanamid Co. Plastics Div. 30 Rockefeller I :>rk CH :elite Cor . 230 Grove St. Bloomfield, N. J. Carbide & Carbon Chemicals Corp. 30 E. U2nd St. City Casein Co. of America Bainbridge, N. Y. Catalin Co 1 Park Ay . New York City Monsanto Chemical Co. 9 s t i C 3 D i • Perkins Glue Co. La:. , Pa. Flaskon Co. , Inc. 2112-2^ Sylvan A Toledo, Ohio 3 1 s & C : al C o • . Washington Philadelphia. Shawinigan ire or . Fifth Ave. . York E. I. duPont deljemour Co. Plastics Dept. Arlington, rez Flastics d Chemicals, Inc. 1181 Walck Road North Tonawan , . Y. Lauxite Cc Lockport, N. Y. . ..-. I'ljl -r,_ Part ial List of Articles on Molded Flywood 1. ANONYMOUS. 1940. MOLDED AIRPLANES FOR DEFENSE. Med. Plastics 17(11 ) :2^-}l , 7S,gO,S2, illus. 2. 3- 4. 5- 6. 1941. FLETCHER BASIC TRAINER: AN ALL-PLYWOOD SKIN-STRESSED AIR- PLANE. Aero Digest 385(2 ): I65-I66, illus. S. 1941. NEW PLASTIC PLANE: SUMMIT AERONAUTICAL JOB PRODUCED UNDER VIDAL PROCESS FATENTS. Business Week (593) =3^-37, illus 19 U L THE MORRCP VICTORY TRAINER. Aero Digest 39 ( 6 ) : 231 , 235, illus . 1941. TfflN MOTORED PLASTIC PLYWOOD PLANE. Mod. Flastics 19(2): 52-53, 110, 112. 1942. MOLDING PLASTIC -PL WOOD. Mod. Plastics 19(11 ) :46- 49, 112,114,116, illus. 1942. MOULDED AIRCRAFT UNITS: USE OF RESIN-BONDED PLYWOOD AS A STRUCTURAL MATERIAL: A SURVEY OF PROGRESS. Aircraft Prod. 4(4):312-315, illus. 1942. VICTORY TRAINER. Mod. Flastics 19(5):42-43, illus. 9. BARNES, JOHN S. 1943. MAKING PLYWOOD WITH MULTIDIRECTIONAL. PRESSURE. Mech. Engin, 65(l):17-20, illus. 10. CHRISTIAN, PAUL 1942. AIRPLANES AND BATHTUBS, COOKED TO ORDER. Sat. Evening Fost 215(3):12-13,36,39, illus. 11. FAIRCHILD, SHERMAN M. 1943.. DETAILS CF DURAMOLB FABRICATION. Aero Digest 42(2):232, 235, illus. 12. HA"*THORNE, RANDOLFH 1941. MOLDING THE LANGLSY AIRPLANE: A NEW .PLASTIC BONDED PLYWOOD PLANE. Aviation 40(11 ) 175-76, 154,156, illus. Mimeo. No. R1431 -6- i art ial List oT Ar I - - iaeci Flywood (continued) . L. J. •2. DESK 3NSID1 DIONS HIRES. Aviation 4l(ll):llU-117, V4C; (l2):lUt- ,il] . Ik. 1 19^2. THE VIDAL PROCESS . . Aviation 1*1(10): 12if-127, . Lllus, 15. .'.. . . 19^1. DEVELOPMENT OF A PLASTIC MOLDED AIRPL . . (1): 44-^5, 1*10,1^, iiiu . lb. FERRY, THOMAS D. 194-1. AIRCRAl .6(8): 53-58, &2,&,&i , ill is. 17. ■;;. FLE PRESSURE IN ■ ted at , ' .'ood In. • - ... Er. - • . pr. -28, 19^3. ial List of led Flywood . 1. ALUTARD, . A. 19^2. AHCRAF" STRUCTURE. . S. ... 2. .V. E. 19^1. AIRCRAFT TING STRUCTURE. (U. S. ■ ,2RS,1; 3. TEAGUE, MONROE . 1937. FLUID ] ■-. S. Fatent No. - . ,290). k. VERHEY, V.ILL I AM 19^0. METHOD OF : . .3. Fatent No. 2, 223, 5^7). 5. VIDAI , EUGENE L. . . J. 19^2. STRUCT - ES. . S. No. 2,276,00*0. Mimeo. No. RIU3I -7- DRAIN CYUNDER JO VACUUM BA6 HEAT AND PRESSURE FLUID STEEL SHELL HEAT AND PRESSURE FLUID BLEEDER (OPEN TO ATMOSPHERE) CYLINDER B SUPPORTS CARRIA6E HEAT AND PRESSURE FLUID BAND TO SEAL BAG TO MOLD BLEEDER(OPEN TO ATMOSPHERE) CARRIAGE BA6 HEAT AND PRESSURE FLUID PLYWOOD DRAIN ZU 41399 r Figure 1. — Five methods of forming "bag-molded plywood, UNIVERSITY OF FLORIDA 3 1262 08924 5566