parabolic surface

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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.