Fabricating a parabola
A 'geoid' can be constructed for
our rotating table by pouring resin in to a mold on the rotating table. The
surface of the resin takes up a parobolic shape and solidifies. If care is taken
the resulting surface can be made to be very smooth and then used in experiments
on the Coriolis force.
A simple form in to which resin is
poured can be constructed as follows. A 1/4" circular aluminum plate is
used as a base. A piece of flexible, vinyl, "base" molding (available
in building supply houses) is wrapped around the base and attached with a small
bead of 5 minute epoxy. The building materials and glue should be tested to
check that there are no adverse reactions to the resin that is poured in to the
form. The form is not treated with a mold release.
The base should be constructed of
metal and plastic or other non-moisture absorbent material because the resin is
moisture sensitive. We chose a metal base to add stiffness, preventing the
variable thickness resin from warping when curing. Measuring/mixing containers
and stirrers should also be metal or plastic.
The resin is poured into the form
while it is rotating at the desired speed. The pot life, or working time, should
be about 20 minutes, minimum. Shorter working times risk the possibility of the
resin starting to harden before it has finished flowing into the parabola shape.
Carefully measure the two parts according to resin instructions. Care should
also be taken while pouring the two part resin into measuring/mixing containers
and when pouring into the form to prevent creating and entrapping air bubbles.
Any bubbles in the resin will rise to the surface and create small cavities in
the hardened resin surface. Stir the combined parts carefully also, using the
manufacturer's guidelines. Pouring the resin in a single pour seemed to reduce
the amount of bubbles and eliminated the interaction between subsequent pours.
A total of 3,000ml of resin
(combined total of part A and part B) was enough for a 42" parabola for a
rotation rate of f = 2. This included a little extra as insurance. Strict
adherence to all manufacturer guidelines is a good investment and will prevent
wasted time and material.
Smooth-On resin was chosen because
of its availability, minimum odor and because mixing was done by volume,
eliminating the need for expensive scales. Odor is a major factor as the mixing,
pouring and curing is done in the lab and on the rotating turntable. When adding
color tint or any additive, it is possible to create problems, such as inability
to harden. Excessive amounts or incompatible additives can prevent curing. Do
sample tests when using additives. We used the manufacturer's own color tint,
using only a few drops and no other additives. The resin may be purchased on the
web at www.smooth-on.com.
Order Smooth-Cast 321, liquid plastic casting resin, slow setting off-white
resin in the gallon Unit at $58.45 each. Each unit has a gallon of part A and a
gallon of part B. Two units are required for a 42" diameter parabola and
allows extra for experimentation.
Sand the cured surface lightly with
a medium grade sandpaper to remove irregularities and finish sand with a 300 or
400 grade wet or dry sandpaper until the surface is very smooth to the touch.
Sand uniformly over the surface to preserve the parabola shape. The surface can
be used as is, or automotive primer and paint applied.