Page 503 - Fluid-Structure Interactions Slender Structure and Axial Flow (Volume 1)

EXPERIMENTAL METHODS FOR ELASTOMER PIPES 473

the aid of an electric drill, and then poured into the syringe, typically filling &-; of
its volume. Then the top of the injector is connected to a vacuum pump, capable of
generating a pressure of 0.1 atm approximately, ‘to boil off trapped air in the viscous
mixture (of the consistency of bread dough), but not low enough to reach the boiling
point of the silicone rubber itself; hence the piston in the injector must be leak-proof. Air
is trapped not only by the folding of the mixture during pouring, but also in the form
of small bubbles trapped during mixing, which cannot rise to the surface fast enough.
The vacuum is applied and held long enough for the mixture to expand, filling half or
two-thirds of the injector volume, allowing the larger bubbles to burst and the mixture to
collapse. This cycle of (a) application of the vacuum, (b) holding it, and (c) releasing it
gradually has to be repeated - perhaps up to 50 times - until application of the vacuum
results in no noticeable change in volume.
The ‘working time’ available before the mixture begins to set varies from one silicone
rubber to another, but it is typically 1-2 hours. Room-temperature curing takes about
72 hours, but in a temperature-controlled oven at 160°C this can be accelerated to 1 hour.
Once the mixture is de-aerated, it is injected into the lower end of the mould slowly,
so as to rise in it at no more than 0.5mds. The mould and injector are arranged in a
vertical configuration and, usually, remain so connected during curing.
Extracting the casting from the mould is perhaps the most challenging aspect of the
manufacturing process. Even with the mould-release agent, the casting does not simply
slide off the central core, because of the vacuum that needs to be broken between the
surfaces. An effective way is to put the pipe, with the core in it, on a long V-block and
then apply compressed air (at no more than - 140psi or 1 MPa) at one end, to slightly
expand and lift the pipe off the core; a little water lubrication helps to then draw the core
out from the other end. In the case of a split core, the first half is removed in this way,
but the second one has to be painstahngly eased out mechanically, by tapping it with a
smaller rod carefully, so as not to damage the bond between the metal strip and the pipe.
Lower-quality, but easier to manufacture pipes and cylinders may be cast in glass tubes
which, after curing, are broken and the core removed in the manner just described. The
weakness here is the imperfect uniformity and straightness of the glass tubes.
For cantilevered pipe experiments, it is best to make the free end ‘square’ to the long
pipe-axis at manufacture. If cutting a piece of the free end becomes necessary, however, it
should be done with great care. A good way is to sandwich the pipe between a close-fitting
rod inside and a shorter pipe outside with a square-cut end, then to slice the elastomer
pipe with a sharp razor, slowly and with minimum local deformation.

D.2 SHORT PIPES, SHELLS AND CYLINDERS
For short pipes and shells it is more important than for other pipes that, in the experiments
with flow, the transition from the metal supporting structure upstream to the flexible pipe
be smooth and as disturbance-free as possible. Hence, in such cases an upstream adapter
is actually cast integral to the elastomer pipe; in the experiments, the adapter is then
screwed directly into the fluid-supply piping.
For obvious reasons, cylinders are the easiest to cast, unless they are instrumented;
instrumented cylinders will be discussed separately in Volume 2. Finally, conical cylinders
and pipes (such as those in Section 4.1) present no special difficulties.