This page is available at https://courses.ideate.cmu.edu/16-480/s2022/ref/text/hardware/procedure.html

Silicone Rubber Procedure

Overall concerns:

• The two parts of the silicone needs to be mixed in a precise ratio measured using a digital scale.

• The mixing and pouring process needs to be completed within the pot life duration.

• The material is viscous, so air bubbles should be minimized.

• Each silicone material bottle must be kept clean and uncontaminated. As a rule, one mixing stick is used per bottle and discarded, and the bottle threads are wiped before closing with the cap.

Materials

Material	Density (g/cc)	Pot Life (min)	Cure Time (hours)
Ecoflex 00-31	1.07 grams/cc	40 min	4 hours
Ecoflex 00-45	1.06 grams/cc	45 min	4 hours
Dragon Skin 10	1.07 grams/cc	20 min	5 hours
Dragon Skin 20	1.08 grams/cc	25 min	4 hours

Small Batch Mixing Protocol (Non-Vacuum)

The following notes are intended for very small test batches, e.g., a single part in an open mold. The details will vary slightly depending on chosen material.

Planning

1. Use your CAD system to calculate the part volume (final rubber volume).

2. Look up the density (specific gravity) for your chosen material.

3. Calculate the total batch size: mass = volume * density, with an allowance of a few grams lost to the container.

4. Calculate the Part A and Part B masses. For all of our silicones, each is half the batch mass.

5. Note the pot life and cure time of your chosen material.

Preparation

1. Write your name, date, etc. on a new entry in the log sheet.

2. Gather materials and equipment:

   • two-part silicone of the selected hardness

   • three wooden mixing sticks

   • one small disposable container

   • digital scale

   • stopwatch or timer (e.g. your phone)

   • two small pieces of paper towel (for wiping the bottle threads)

   • nitrile gloves (available from IDeATe Lending or the laser cutter room)

   • brown craft paper on the table under the mold to catch drips (roll is on cart).

   • move a trash can within reach

Mixing and Pouring

1. Place the clean, empty container on the scale and note the tare weight, then zero the scale.

2. Measuring and adding Part A.

   1. Open Part A and stir with a clean stick.

   2. Add the correct mass of Part A to the clean container. For tiny amounts, it may be easiest to drizzle with the stick.

   3. Scrape the excess off the stick back into the bottle and dispose of the stick.

   4. Use the first paper towel to wipe the bottle threads and securely close.

   5. Note the final mass delivered and adjust the part B calculation if needed.

3. Measuring and adding Part B.

   1. Open Part B and stir with a clean stick.

   2. Start either your stopwatch or a countdown timer set to the pot life.

   3. Add the correct mass of Part B to the container, slowly approaching the target mass. For tiny amounts, it may be easiest to drizzle with the stick.

   4. Scrape the (clean) excess off the stick back into the bottle and dispose of the stick.

   5. Use the second paper towel to wipe the bottle threads and securely close.

4. Mixing and pouring.

   1. Using the third mixing stick, gently stir for three minutes, taking care to keep the material concentrated in the bottom, but scraping the sides to assure complete mixing.

   2. Pour material as a thin ribbon into into the lowest point of each mold, holding the container well above the mold to allow air bubbles to break.

   3. Using the mixing stick, gently work the material into the mold crevices to loosen any bubbles.

   4. Add a final drizzle if needed to fill mold to the upper edge.

   5. Stop before the pot life time is reached.

5. Curing.

   1. Place the mold(s) on paper on a level table and allow to cure undisturbed for the full cure time.

   2. Leave the mixing cup next to the mold, the residue can help indicate cure state.

Cleanup

1. Check for spilled silicone and wipe up with paper towels.

2. Return all materials and equipment to storage.

3. Put the trash can back where you found it.

Demolding

1. After the recommended cure time has elapsed, gently touch the surface to verify the rubber has fully cured.

2. Gently work the rubber part out of the mold. It may have an airtight fit, so it may be helpful to gently compress the flexible rubber in from an edge or corner to allow air to enter below the part.

3. Measure your actual part mass and enter it on the log sheet.