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Aviation History
1943
1943 - 1864.PDF
IO2 FLIGHT JULY 22ND, 1943 PLYWOOD AND PLASTICS Dut in presses with platens controlled at a temperature194-212 deg. F. and pressure should be 100-150 lb. per sq. in. With this adhesive, 5 minutes plus 1 minute per millimetreof wood is recommended as the time required for pressing which should not be commenced until 30 minutes afterthe application of the adhesive. The urea-formaldehyde adhesives consist of two ingredients, the adhesive and thehardener—usually a weak acid which acts as a catalyst and promotes rapid hardening. In general, only one type ofadhesive is marketed, but the use of hardeners to suit vary- ing conditions are specified. Widely used adhesives of this type are Aerolite and BeetleW and Beetle K for wide-gap joints, and a glue in powder form, Beetle H. Both hot and cold processes are avail-able, and for the hot process two different hardeners are supplied, one for joints which have to be waterproof andthe other for joints which must withstand the effect of boiling water. Cold-press Process Prior to application, the appropriate hardener is mixed with the adhesive and applied thinly to the wood, approxi- mately i\ lb. per 100 sq. ft. Wood surfaces should be roughened with sandpaper prior to application. Bonding is carried out in heated presses at a temperature of 90 to 100 deg. C. at a pressure of 28 lb. per sq. in. or more if necessary. With the hardener for waterproof joints the minimum time in the press is five minutes, whereas eight minutes are required for joints to withstand boiling water; one minute for each millimetre of wood being pressed should be added to the above times. With the cold press, very strong joints can be made. The adhesive has a low water content which facilitates quick drying after pressing, and moisture content of the veneer is not so vitally important with these cements. The procedure with the cold glue is to coat one side of the joint with the appropriate hardener using 1 lb. per 100 sq. ft., and allow to dry for a minimum period of thirty minutes. A very thin coating of glue is applied to the other joint face i\ to 3 lb. per 100 sq. ft. After a period of about fifteen minutes the joint faces may be clamped together at room temperatures. Pressure is maintained over a period varying with the hardener used and ranging from two to twenty-four hours. Beetle cements are manufactured by Beetle Products, Ltd., and temperatures, setting times and hardeners have been tabulated in Table 3. TABLE 3 BECTLE W CEMENT COLD PROCESS : MINIMUM SETTING TIMES Temp., Deg. F. 5060 70 Setting Time with Hardeners C.H. Orange 45 rains.30 mins. 20 mins. C.H. Yellow 5-6 hours3 hours 11 hours C.H. White 30-35 hours12 hours 4 hours BEETLE A CEMENT SETTING TIMES, RAPID AND SLOW HARDENERS Temp., Deg. F. 5060 6570 80 Setting Times with Hardeners 15 (Purple) 2 hours1| hours 1 hour\ hour 50 (Brown) 20 hours12 hours 8 hours4 hours BEETLE W CEMENT : HARDENER MIXTURES FOR HOT PRESS Class Waterproof joints .Joints to withstand boiling water ... Proportions by Weight Beetle W Cement 100 100 H.H. Liquid 10 Nil H.H.12 (Powder) Nil 111 Aerolite cements are manufactured by Aero Research, Ltd., under the direction of Dr. N. A. de Bruyne, an authority on plywood and plastics. Further reference will be made to the work of this establishment. TABLE «.—AEROLITE ADHESIVES SETTING TIME—COLD PROCESS Hardener G.B.PG.B.Q. G.B.M. Temperature, Degrees F. 50° 5-6 hours6-7 hours 10-12 houTS 60° 2£ hours3 hours 5 hours • 70° 12 hours2 hours 3J liours 80° 1 hoursl| hours 2J hours 90° 1 hour1 hour 2 hours SETTING TIME : GAP FILLING GLUE8 AT HIGH TEMPERATUSES Hardener G.B.P. ...G.B.M. ... Temperature, Degrees F. 104° 40 mins.60 mins. 140° 10 mins.15 mins. 158° C mins.8 mins. 1T6° 3 mins.4 mins. 1M° 2 mins.2| mins. 236° 1J mius.2 mins. There are a variety of Aerolite adhesives, including Aero- lite 300, 301 and 300 F, which differ mainly in their viscosity. Aerolite 300 F. is a stiff liquid designed to " stay put'' in gaps where a glue of normal viscosity would run out from the joint. Aerolite 300 is less viscous than 300 F. and can be spread more easily. Aerolite 301 is a low- viscosity glue which is exceptionally easy to rpread, but an essential point for its successful application is that it must only be used where time can be allowed for the glue to become really "tacky" before closing the joint. Hardeners for these adhesives are designated G.B.M., G.B.P. and G.B.Q., all approved for aircraft use. The method of use is somewhat similar to the previous cements described, and the period during which pressure should be applied, with the corresponding temperatures, are given in Table 4. Digressing upon the characteristics of wood veneer and its various applications, the term "plywood," it would seem, may easily become misconstrued. Plywood properly describes the material of any wooden part built up of several thicknesses with the gjrain of the veneers running in different directions, and is generally used to convey the meaning of boards built up of three or more plies and- Fig. 2. Plywood specimen (a) grain of face-ply lengthwise(b) grain of face-ply crosswise. Arrows indicate direction of grain. supplied ready for fabrication in the industry. Wood being stronger with the grain than across the grain, placing the grain of the alternate plies at right angles gives a better proportion of strength and stiffness in the two principal board directions, as shown in Fig. 2. For special purposes, the skin of a fuselage, for example, plywood is sometimes arranged with the grain of the plies at 45 deg. Though this is stronger, it is more expensive to produce. (To be continued.) In subsequent articles the production and characteristics ofwood veneer are discussed, including moisture-content control and effect.
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