Danny Griffin HTMAA 2023

Week 06 :: Molding and Casting

How do we design a production process for reproducible elements?

1. Group Training

We started the week by reviewing safety data sheets for the casting compounds we'll be trying this week, followed by a demo in the shop using some prior years' molds.

SDS Review Notes: SDS for OOMOO SILICONE RUBBER

Section 1: Indicates that this SDS is relevant for both the "OOMOO" series which we'll be starting with, but also with others such as the "SORTA-CLEAR" series.
Section 2: Not a hazardous substance or mixture according to OSHA.
Section 4: First Aid Measures: fresh air in case of inhalation, water in case of eye contact, water+soap in case of skin contact.
Section 7: Handling and Storage Handling Precautions: Wash hands after use. Storage Requirements: Keep container(s) tightly closed and propery labeled. Store in cool, dry, well ventilated place away from heat, direct sunlight, strong oxidizers, and any incompatibles. Although this is slightly generic text, it seems to be highlighting something important which is that as long as the material is properly labeled, and stored without contamination, it's fairly non-hazardous.
Section 8: PPE Safety glasses and appropriate ventilation are recommended.
Section 10: Stability and Reactivity Chemical Incompatibilities: Strong bases, and acids.

Responsive image — I didn't realize that Part A and Part B had different sections initially. Both are labelled for general use as "Silicone Elastomer," and comparing "Section 9 - Physical and Chemical Properties," there are no differences listed.

The technical overview included in the box instructions includes several criteria which help with choosing a product for your design purposes. In particular, pot life, cure time, and shore hardness are all important considerations to the success of the mold.

Preparation for the casting prioritizes a clean workspace. Lots of brown paper on the table, paper cups and wooden stirring sticks are prepared in advance.

After the two parts are measured by volume, they are incorporated together by stirring. Remember to stir with horizontal motion only to not incorporate air bubbles.

Pouring in a narrow drizzle limits opportunities for air to enter the mixture.

The final step in bubble prevention is to vibrate the mold, or in our case, tap it against the worksurface. With sufficient vibration, the bubbles float to the top and exit the mold.

2. Design a Mold

This week, I wanted to explore an idea I've been curious about for a while - how do I get binding cane (AKA rattan) to take on a highly specific geometry?

Excerpt from toolpathing setup with Jen O'Brien, showing specific paths imported to cut as close as possible to the positive of the cane. There were no overhangs in the geometry, but there were sharp corners. We decided to do a specific path using a 1/8" ball to incorporate a fillet into the surface at these locations.

Milling on the Intelitek.

Manual finishing. The visual quality of the smooth surface is not important to the function of this mold, but I was worred that all of the sharp edges left by the rough finishing might cause issues with demolding. I decided to wetsand with 400 grit to remove these unnecessary textures.

Casting with "Sorta-clear 37." Note: this rubber was past its expiration date, and both sides of the two-part liquid mixture were highly viscous. I don't think there would be so many bubbles normally, but I had to mix forcefully to get the two parts to incorporate together.

Demolding was a clean process. I waited 4 hours for the Sorta-clear to cure, and then I was able to pinch it and remove from the mold with no issue.

Not pictured in these steps, I picked up some 1/8" masonite from the scrap pile at the shop, and lasercut a housing for the two parts. (see below)

3. Steambending Test

The reason for choosing Sorta-clear 37 and masonite for the materials was that they're both hard and hold up well to wear. Rattan springs back when compressed, so I was expecting this jig needed to be resilient.

The process for using the cast mold was to soak the rattan in boiling water until it was saturated and pliable. Then, I would manually press the rattan into the channels of the mold.

As expected, the rattan does not want to stay in this shape. Using lots of rubberbands, it can be secured tightly into the channels.

If a suitable amount of time has passed for the moisture to evaporate, the rattan seems to hold its shape well once the rubber bands are removed. Repeating the process so far, the result seems decent after 8 hours, but better after 24 hours.

The biggest discrepancy in the intended geometry is at the tip of the loop, where the channel is more shallow due to the limitation of having no underhangs in the 3-axis milling process.

At the start of this week, I was skeptical about the relevance of casting now that 3D printers are so prevalent. However, I really enjoyed the degree of customization in this process, especially in the mix design, or fine-tuning the material performance of your mold and cast. The repeatability of the process feels quite rewarding, where it takes a fair bit of setup to create the first object, but each subsequent one you produce gets incrementally simpler.

Casting was not part of my plan for the final project, but now I'm quite curious to try more.