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
• A small nylon cable tie, about 5" or less.
• 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
• 75cc white pharmaceutical HDPE round bottles with screw top lids.
• 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 assembly 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. Make a small wood stick about 1/4" x 1/4" x 5", and coat it with a thin layer of vaseline.
9. Stick the wood piece vertically against the side of the plastic container. This piece will be left in place when the mold is done, and serves as a key to align the top and bottom halves of the mold.
10. Mix up that volume of silicone mold material and pour it into the container. Be sure to mix the silicone thoroughly.
11. 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.
12. 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.
13. Mix another batch of silicone, enough to make the top half of the mold. It should come up about 1/2" over the top of the ping pong ball.
14. When it's dry in a few hours, gently remove the top half and pull out the ping pong ball assembly, but leave the vertical wood stick on the side of the container. Leave the bottom half of the mold in the plastic container.
15. Cut a vertical slot in the side of the container from the top of the container and going down to slightly below the bottom of the 1/4" dowel. Make it a little wider than the 1/4" dowel. This provides a place for the cable to come into the mold.
16. The mold is now ready to use.

Making the mold for the Molex connectors

1. Take one of the 75cc plastic bottles (with cap) and cut off the top 7/8" of the bottle (just below the threaded neck)with an X-acto knife.
2. Take a piece of 3/4" dowel about 1" long, and coat it with a thin coat of vaseline.
3. Place the dowel on the inside of the bottle cap and piece of bottle. Make sure it is centered.
4. Mix up about 10cc of silicone and pour it into the space between the bottle and the dowel.
5. When the silicone is dry in a few hours, remove the dowel and unscrew the cap.
6. The mold is now ready to use.

Coating the Smart Sensor with epoxy

1. Check the orientation of the Cat5e cable to the Smart Sensor PCB. The axis of the cable should be in line with the mid point of the PCB. Adjust if necessary.
2. Clean the last 2"-3" of the cable with a tissue soaked in isopropyl alcohol.
3. Secure a small cable tie around the cable about 1/16"-1/8" from the end of the stripped jacket, and clip off the end of the cable tie.
4. Mix a quantity of the Easy Cast epoxy, and dip the sensor board into it. Use the mixing stick to apply epoxy to the last 1"-1.5" of the cable.
5. Support the cable further back, and allow the excess epoxy to drip off. Don't disturb it until it dries, about 24 hours. Keeping it warm helps the drying time.

Casting the Smart Sensor

1. Spray a tiny amount of Pol-Ease mold release onto both halves of the mold cavity (optional - this prolongs the life of the mold and makes it easier to remove the casting).
2. Place the bottom half of the mold on a level surface, and insert the sensor into the cavity.
3. Adjust the position of the cable so that the sensor PCB is centered in the cavity, with no corners touching the mold.
4. Support the cable so that it is level and well fitted into the channel in the mold. A Panavise is a good way to hold the cable in place.
5. A good way to check that the upper corners will not touch the upper half of the mold is to cut a 1.5" diameter semicircle out of the edge of a piece of stiff paper, and hold the paper over the lower half of the mold.
6. Place the top half of the mold over the sensor, and check that it is snug against the lower half.
7. Mix 40cc of the Quik-Cast resin. Mix it thoroughly for 1.5 minutes, and no later than 2 minutes from the time you start mixing, begin evenly pouring it into the hole in the top of the mold. Fill the mold up to about 1/2" into the filling hole. There may be some small amount of leakage between the two halves of the mold. It will be too thick to pour at about 4 minutes, so work quickly.
8. At about 8 minutes, the casting will be fairly firm, and can be removed from the mold.
9. With an X-acto knife, trim off any flash that leaked out the middle of the mold, and trim off the plug of material at the top of the sphere.
10. At about 15 minutes the epoxy will be very hard.