Accelerated Glue Setting By Arthur W. J. G. Ord-Hume, EAA 8579 "Mirador", Rose Mead, Lake Sandown.
Isle of Wight, England INTRODUCTION
Synthetic resin adhesives differ from normal glues in many ways. One of the most important is the fact that they do not set or cure from the outside inwards as, for example, by the evaporation of a volatile solution. Setting takes place evenly throughout the glue—more or less regardless of the thickness of the glue layer. With a synthetic resin adhesive, it will itself harden and set although this is a very long and slow procedure— anything up to several years at room temperature with some resins. In order to obtain a practical setting time therefore, manufacturers have formulated various types of accelerator or catalyst which immensely speeds up this process of hardening. For some types of adhesive, there is more than one type of hardener which may be used, the difference being in the resultant curing or setting time. Working at a shop temperature of 68 deg. F., setting times may be achieved of only a few hours with shuffling time (depending again on the hardener and temperature) varying between 5 and 30 minutes. The higher the temperature, the shorter the shuffling time and the shorter the setting time. The lower the temperature, the longer becomes the whole process and the greater the latitude. Using a slow hardener is the only way to obtain sufficient shuffling time for a job which takes a comparatively long while to assemble and cramp up. A vital job such as a wing beam may need to be deferred until cooler weather can provide a longer shuffling time—even with a slow hardener. However, it is clearly evident that in such a case the setting or curing time will be protracted. CLAMPING DEVICES
Q
STRIP STEEL ELEMENT
•GLUED LAMINATIONS
LEADS TO TRANSFORMER FIG.
1
Elementary application of strip heating using continuous
duplex element for heating a thick laminated former in
a conventional jig.
Also, the longer a joint takes to cure, the weaker it may become by virtue of excessive absorption of the glue line into the mating timbers. The aim, therefore, is to make the joint "slowly" and set it rapidly. The obvious way to do this is to raise the
temperature of the shop, but in practice this is very much like burning down the pig sty in order to get fried bacon. All that really needs heating is the glue-line, not the shop, the bench, the airspace and the operator. HISTORY OF HEATING
Way back before the war, radiant-heat infra-red lamps were used to set glue lines and, in 1936, a British company took out patents for methods of heating. Concurrent with this, Th. Goldschmidt A.G. of Essen, Germany, took out a patent for gluing thick pieces of wood rapidly by using an electrically-heated wire gauze in the glue line. However, it was the advent of the war which punched home the need for quick-setitng glues for assembled parts.
Quite obviously, there are many methods of heating such as lamps, furnaces, ovens and so forth. Each had its own
particular disadvantage—it was either too bulky, too inflexible in use, too costly, too uneconomic, or more often than not, it was just not efficient. The moisture content of the timber had to be preserved. The mass drying out of wood in an oven was out of the question. Working on the principles explored in the early days of glue-line heating, it was discovered that if a thin steel strip were arranged in contact with or near the glue-line, and a regulated electric current passed through, the
metal strip heated up and the heat was radiated or conducted to the glue-line. The strength properties of the glue-line thus obtained were not affected in any way. As a direct result of this and earlier work, thin stainless steel wires were actually imbedded in the glue-line when details were being assembled. After curing by passing a low-voltage, high amperage current through, the wires were cut off flush and left in. A decrease in joint strength of about 5 percent resulted with this technique, but components could be designed with sufficient glue surface to offset this deficiency. During the war, large numbers of aircraft were built in this manner including the many hundreds of troop-carrying gliders used by the British Army in the invasion of occupied France and Belgium. The famous DeHavilland "Mosquito" fighter/bomber was also construct-
ed using these techniques together with the wooden beach landing craft. Accelerated glue setting fulfilled a valuable requirement for repair schemes during wartime. Repairs were often carried out in the open air in all weathers at dispersal points. It was possible to do a major structural repair without the facilities of a hangar and using only a
mains electric lead, transformer and length of resistance wire to insure a perfect glued joint quickly made. Coupled with the use of electric blankets, gluing could be accom(Continued on next page) SPORT
AVIATION
13
ACCELERATED GLUE SETTING . . .
TABLE 1.
(Continued from preceding page)
plished extremely rapidly — one occasion on record details an aircraft which returned from a mission with a
major wooden structural member almost severed. Within eight hours, the machine was back in service! PRACTICAL STRIP HEATING
The developed technique of strip heating (also known
as low-voltage heating) involves heating the joint with a bare metal strip through which is passed a low-voltage, high-amperage electric current. The voltage is normally about 6 and seldom exceeds 12.
Strip heating is used widely in joinery shops, furniture factories and boat yards. In furniture factories, glue-
lines are heated by four different types of equipment, (a) hot presses; (b) rubber bag presses; (c) radio-frequen-
cy heaters and (d) strip heaters. The first three are essential for particular purposes but they are large and relatively expensive items of equipment designed for long production runs of one part.
250 watts/sq.ft.
Mild Steel Strip
16 SWG 0.064 in. 18 SWG 0.048 in. 20 SWG 0.036 in. 22 SWG 0.028 in. 24 SWG 0.022 in. 26 SWG
0.018 in. 28 SWG 0.0148 in.
can readily be applied to suit the needs of the amateur airplane constructor. The equipment needed is comparatively simple and cheap and may be re-used for many applications. Because low voltages are used, there is no risk of electric shock to the operator. Apart from
a step-down transformer
capable of
handling a large amperage, the only other requirement is a suitable jig for the job in hand. Fig. 1 illustrates a
simple jig for a laminated bow incorporating strip heating.
The working temperature of the strip depends on the
job in hand, but for thin plywood and veneers a temperature of about 110 deg. C. (230 deg. F.) is suitable. This is well below the temperature at which wood will scorch, but it is high enough to set the glue rapidly — setting times of .25 of a minute to 3 minutes being possible.
It will readily be appreciated that, with a scarf joint
for example, the feather edge will cure much more rapidly than the center part due to the shorter distance for
heat transfer. For scarfing thick plywood, a scarfed panel may be carefully handled after only a few minutes of strip heating, the remainder of the joint curing at room
temperature or through the transference of residual heat. On this subject of heat transfer, if the heating element is placed in direct, or near-direct contact with the glue-line (such as in a scarf joint in thin ply), setting times will be short by virtue of rapid heat transfer through only a thin layer of wood. On the whole, hardwoods and plywoods are better conductors of heat than, for example, spruce or mahogany,
0.18
119
0.19
130
0.21
141
0.20
103
0.22
113
0.24
124
0.23
89
0.26
97
0.28
106
0.27
78
0.29
85
0.31
92
0.30
69
0.32
77
0.35
83
0.33
63
0.36
69
0.39
75
0.36
58
0.40
63
0.43
67
STAINLESS STEEL
250 wotts/sq.ft. Stainless Steel
16 SWG 0.064 in.
300 wotts/sq.ft. 350 watts/sq.ft.
volts per amps per volts per amps per volts per amps per foot inch foot inch foot inch length width length width length width
TABLE 2.
Strip heating, however, is a versatile technique which
MILD STEEL
300 watts/sq.ft. 350 watts/sq.ft.
volts per amps per volts per amps per volts per amps per foot inch foot inch foot inch length width length width length width
0.31
68
0.34
74
0.37
80
0.35
58
0.39
64
0.42
69
0.41
51
0.45
56
0.49
60
0.47
45
0.51
49
0.55
53
0.022 in.
0.53
40
0.58
44
0.62
47
26 SWG 0.018 in. 28 SWG
0.58
36
0.64
39
0.69
43
0.64
33
0.70
36
0.76
30
18 SWG 0.048 in. 20 SWG 0.036 in. 22 SWG 0.028 in. 24 SWG
0.0148 in.
construction should be wood. Accelerated glue drying for the amateur airplane constructor will almost invariably be connected with producing laminated bows, formers and bulkheads. The jigs normally used for the construction of such parts may readily be adapted for strip heating. The jig must be designed so that pressures of at least 25 Ibs./sq. in. can be applied to the glue-line. Pressure can be applied by eccentric buttons on the jig, wedges and screw clamps.
but even so, wood is a bad conductor at the best of times.
The metal strip elements can be made of either mild steel or stainless steel. Stainless steel has the advantage of having a greater electrical resistance than that of mild steel and so does not require such a large current. It is.
If there are glue-lines more remote from the heating
mild steel strip should be tin-plated to prevent contact
element, then longer curing times must be allowed. A guide for the rate of heat transference is one minute per
millimeter for the distance between the element and the furthest glue-line. This rule is applicable for thicknesses
up to 6 m/m. If heating is from both sides (through a continuous or duplex element) the rule holds good for up to 12 m/m. or .5 inch. When heat is required to pass through more than 6 m/m. then the time per millimeter should be taken as one and one-half minutes. Naturally, this extra time has to be added to the setting time of the glue at the desired temperature. JIG CONSTRUCTION
Jigs wherein are embodied strip heating elements can
be manufactured very simply and the prime material of 14
FEBRUARY 1964
however, more expensive. Where possible, elements of with surplus glue causing corrosion. POWER REQUIREMENTS
The aim should be to put into the metal strip element about 250 watts per sq. ft., although if the strip is very long (such as a continuous duplex strip around the inside
and outside of a former), this should be increased to about 350 watts. If the temperature is to be kept down
for any reason, 175 watts may suffice.
To discover an appropriate transformer for a particu-
lar job, reference should be made to the Tables 1, 2 and 3. As an example, if it is required to put 300 watts per sq. ft. into a mild steel strip ly2 in. wide and 12 ft. long
using .018 in. metal, it will be found from Table 1 that
the voltage needed per foot length is 0.36. Multiply by 12 (the length of the strip in feet) gives 4.32 volts and the current will be 1.5 (width of strip in inches) x 69 (from
T=thickness in inches of the metal strip then T =
Table 2) =103 amps. Since the leads from the transformer
to the elements also consume voltage, the transformer must have an output higher than 4.32 volts. The voltage drop for various types of leads can be found in Table 3. Suppose that the leads are of 0.012 flexible conductor, it will be seen that for two five foot long leads, the voltage drop at 100 amps, (nearest to 103 amps) is 0.13. The total voltage required would therefore be 4.32 + 0.13 = 4.45.
An alternative and somewhat easier way to adjust heat simply is to use a variable resistance control or rheostat.
Never try to push a transformer above its rated out-
put, and never exceed the maximum ratings of any regulators used.
If a transformer is already at hand, it is usually possible to make up elements to match it. The formula to apply is as follows: Sjcx L J rv') — - O KA-— 2.54 x„ UB x T + where R = the optimum resistance of the circuit for the transformer. This is obtained by dividing the output voltage of the transformer by its rated amperage. R is made up of two parts, r x = the resistance of the heating strip, and r v = the resistance of the leads. S = specific resistance of the metal in ohm cm. units. For mild steel this is 20 x 10—6 and for stainless steel it is approximately 60 x 1C—° L=length in inches of the metal strip B.= width in inches of the metal strip TABLE 3.
B = L =
to rely on accurate timing for heating, and since the temperatures involved cannot damage the wood (although
excessive or prolonged heating would reduce the moisture content) the homebuilder will probably find that he need only make approximate calculations and add on a "factor of certainty" to facilitate his work. A note on the use of this equipment. The voltages are very low, the current is high. There is no danger in handling the live elements, but always disconnect the transformer leads before removing the heating strips from the jig. This is because if the live strips are put aside on the floor or bench, and if they should make contact through a nail or thin sliver of metal, the high amperage will heat this contact possibly to red heat with the consequent risk of fire. The airplane builder is offered this chance to speed his work through accelerated glue drying. There are choices of different means to this end—a portable electric reflector heater, an electric blanket, or strip heating. Each has its place and can help you to get into the air.
All you have to do is to bear in mind that these methods are there, ready for you to make use of them.
A
The voltage drop in various leads at 10 ft. length with different currents NON-FLEXIBLE
557/0.012 250 amp.
705/0.012 400 amp.
416/0.018 600 amp.
19/0.064 102 amp.
19/0.083 147 amp.
37/0.103 298 amp.
61/0.103 413 amp.
Copper 1" x VA" 800 amp.
0.00016
0.0003
0.008
0.015 0.018 0.021 0.024 0.027 0.03 0.045 0.06 0.075 0.090 0.105 0.12
490/0.0076 770/0.0076 1 130/0.0076 1680/0.0076 2200/0.0076
196/0.012 266/0.012 100 amp. 150 amp.
(ohms) per 10 ft.
S xL rx x 2.54 x T r, x 2.54 x B x T
It is thus possible to arrive at a heating strip adapted to the work and to the transformer. In actual working, it should seldom be necessary
FLEXIBLE Coble Size
S x L rx x 2.54 x B
0.0038
0.0028
0.0013
0.00107
0.00079
0.0013
0.00079
0.00026
0.19 0.23 0.27 0.30 0.34 0.38 — — — — — — —
0.14 0.17 0.20 0.22 0.25 0.28 0.42 — — — — — —
0.065 0.078 0.091 0.10 0.12 0.13 0.20 0.26 0.33 — — — —
0.053 0.064 0.075 0.086 0.096 0.11 0.16 0.21 0.27 0.32 0.37 0.43 —
0.040 0.047 0.055 0.063 0.071 0.079 0.12 0.16 0.20 0.24 0.28 0.32 0.40
0.065 0.078 0.091 0.10 0.12 0.13 — _.. — —. — —
0.040 0.047 0.055 0.063 0.071 0.079 0.12 — — — — —
0.013 0.016 0.018 0.021 0.023 0.020 0.039 0.052 0.065 0.078 — _
—
—
Current
50 60 70 80 90 100 150 200 250 300 350 400 500
DESIGN DETAILS
(Continued from page 12)
. .
I most highly recommend to the amateur and the professional aircraft designer-builder as well, to review and study FAA Technical Manual No. 103, entitled "Aircraft Design Through Service Experience." This manual shows hundreds of examples with illustrated drawings describing poor detail design as the case in point. This manual can be purchased from the Supt. of Documents, U.S. Government Printing Office, Washington 25, D.C. Purchase price is $1.25.
—
0.0096 0.011 0.013 0.014 0.016 0.024 0.032 0.04 0.048 0.056 0.064 —
0.15
My final comment and suggestion in accordance with
EAA policy, is to strongly urge its members to have qualified individuals conduct a detail design survey or "hazard analysis" of his aircraft, with the objectives of design improvement and the elimination of any potential
failures or accidents. In this connection, I wish to point
out that such design surveys are inherently more fruitful if conducted independently of the original designer, in order that full advantage can be taken of fresh viewpoints and suggestions. SPORT
AVIATION
IS
