Hartnell, June (ed.) / The Wisconsin engineer
Volume 49, Number 6 (February 1945)
Watson, Ralph A.
Wings via mother's pie plate, p. 8
Wings P A LTHOUGH the facts are not yet widely known, it may be said that Wisconsin industries are letting no dust collect in their research labs. Two notable examples of pioneering in the development of new and better prod- ucts are the low pressure laminates being developed by the Consolidated Water Power and Paper Company of Wisconsin Rapids and the resin faced plywood recently put on the market by the Kimberly-Clark Corporation of Neenah. Both of these companies have large research staffs assigned to the search for new uses for paper and other wood products. The laminates are of paper or fabric impregnated with phenolic or urea resins. The layers are precut roughly to final shape by standard dies and shears before the resin is applied. Forming is done over low cost molds and the resin is cured by heat at low pressures. Some have strength weight ratios equal to metal. Though they were first developed as a substitute for the scarce light aircraft metals, these laminates are now used for many purposes including binocular cases by the Navy, safety hats for miners and construction workers, electrical parts (principally housings), jettison gasoline tanks by the Army Air Forces, and even airplane wing tips. Many other uses are being studied. The Coast Guard is testing channel buoys made of laminates. The saving of 350 lbs. weight giving greater freeboard with conse- quent increased visibility, coupled with a similar saving in cost, is very convincing. Machine gun ammunition boxes made of such laminates illustrate its resistance to abrasion particularly well. Samples tested fed 1200 continuous rounds of ammunition without jamming. Metal doesn't match that performance. Laminates now used in the aircraft industry equal and exceed the strength and stiffness of aircraft aluminum on a weight basis. Impact strengths of 20 to 25 foot pounds are obtained as compared with 2 foot pounds for most molded plastics. Fatigue tests of 10 to 60 million flexes result in no deformation and virtually no fatigue. They compare favorably with other plastics in chemical and electrical properties. Aircraft with fuselage, wing and tail sections built of low pressure laminates have been as- sembled and are now being tested. Top tensile strength of the molded laminates is more than 80,000 pounds per Via Mother's ie Plate -Ralph A. Watson, m&m'46 square inch obtained with fiberglass fabric. Most tests, however, run about 40,000 to 50,000 pounds per square inch. Cotton fabrics show low strengths, averaging from 9,000 to 12,000 pounds per square inch, while some paper laminates equal the strength attained by those of fiberglass fabric. Perhaps the most significant feature to be noted when considering the use of the laminates is their low cost and ease of fabrication. It is said that "they mold as easily as dropping a wet handkerchief over a pie tin". Large and small, simple and complex shapes are easily formed over inexpensive, easy to tool-up dies of wood, Kirksite, Form- rite, cast aluminum, concrete, sheet metal, cast iron, or even molded laminates themselves. The forming tech- niques used in production are the autoclave, hot press, metal to metal, or variations of these. Perhaps the method most widely known is the autoclave. The layers of resin- impregnated paper or fabric are placed between the mold and an inflated rubber bag before the entire assembly is sealed in the autoclave. Steam, hot air, or even hot water is forced into the autoclave under pressure to cure the resin in the laminate. Curing takes only 15 to 30 minutes after which the cured shape is removed and set aside to cool. The autoclave has the advantage of being able to form large complex shapes in one operation thus saving assembly costs. The simple "hot press" principal is used for molding smaller parts. The press consists of a heated lower die of aluminum or Kirksite and a rubber bag which is inflated as the upper die. The resin coated layers are put into the lower die and the upper die lowered. Air or water is then introduced into the rubber bag forcing the impregnated layers to conform to the contour of the heated lower die. The resin cures in five to nine minutes depending on the thickness of the material. When using a method utilizing a rubber bag as one side of the die, only that part of the laminate which is in contact with the formed die is given a smooth finish. A third method of low pressure laminating, that of us- ing presses with heated metal male and female dies, similar to the conventional compression molding of plastics, gives both sides of the finished shape a smooth finish. (please turn to page 13) THHE WISCONSIN ENGINEER 8
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