Turtle Sense casting instructions for Smart Sensor and connectors

Overview

The Smart Sensor circuit board is coated in epoxy and then cast into a 1.5" diameter sphere of quick setting polyurethane resin. The back end of both mating halves of the Molex connectors are also cast in the same polyurethane resin, in order to prevent flexing of the wires between the end of the cable jacket and the Molex connector. Flexible silicone molds were used for the casting in order to make it easy to produce a quantity of sensors and connectors.

Materials used

• Tap Plastics "Platinum" silicone two part mold making material
• Tap Plastics "Easy Cast" two part epoxy resin
• Tap Plastics "Quik-Cast" two part polyurethane casting resin
• Pol-Ease 2300 spray release agent (optional)
• Small plastic mixing cups and wood stirring sticks
• 91% Isopropyl (rubbing) alcohol
• 5 minute epoxy
• Short lengths (< 3"). of 1/4" diameter, 1/2" diameter, and 3/4" diameter wood dowels
• Vaseline
• A plastic ping-pong ball
• A plastic drinking cup or round food container between 2.5" to 3.5" diameter. The cup or container should have a slight taper. This will be used to make the sensor mold.

Making the spherical Smart Sensor mold

1. Cut a length of 1/2" wood dowel about 1" long.
2. With a Dremel tool or X-acto knife, hollow out one end of the dowel slightly so that the perimeter of the dowel fits well against the ping pong ball. This dowel will provide a filling hole for the mold.
3. With the ping pong ball centered side to side in the plastic container and about 1/2"-1" above the bottom of the container, measure the distance from the ping pong ball to the side of the container.
4. Cut a length of 1/4" dowel the same length as the measurement in the previous step. This dowel will provide a slot in the mold to hold the cat5e cable. Hollow out one end of the dowel the same way as the 1/2" dowel.
5. Using 5 minute epoxy, glue the hollowed out end of the 1/2" dowel to the "north pole" of the ping pong ball, and glue the hollowed out end of the 1/4" dowel on the "equator" of the ping pong ball. The epoxy doesn't stick very well to the ping pong ball, so handle it gently after it hardens.
6. Coat the two dowels lightly with vaseline.
7. With the ping pong ball placed about 1/2"-1" above the bottom of the container, estimate the volume of silicone needed to fill the mold up to the "equator" of the ping pong ball. A good way to do this is to use rice to fill the mold to that point, then empty out the rice and measure it with a measuring cup.
8. Mix up that volume of silicone mold material and pour it into the container. Be sure to mix the silicone thoroughly.
9. Gently place the ping pong ball assembly into the container, and push the ball down into the silicone up to the "equator". You may need to secure the top dowel with a piece of tape to keep the ball from floating up. Make sure that the container is on a level surface, that the top dowel is vertical, and that the side dowel is horizontal and halfway into the silicone.
10. When the silicone hardens in a few hours, smear a thin layer of vaseline on the surface of the silicone, but avoid getting any on the ping pong ball.