I’m All Ears: How Mechanical Engineer Rebecca Celsi Uses 3D Printing to Create Silicone Costume Prosthetics

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Meet the author — Rebecca Celsi joined Formlabs in 2022 straight out of earning her bachelor’s degree in mechanical engineering at UCLA. She has been working on the next generation of Formlabs’ SLA 3D printers, contributing mechanical design work to several subsystems including optics and resin mixing. Before starting at Formlabs, Rebecca managed a student robotics team at UCLA under ASME (the American Society of Mechanical Engineers) and was a staff member at the UCLA Engineering Makerspace. She has always enjoyed throwing herself into creative projects in media ranging from gingerbread to sheet metal.

Halloween and Engineering Go Hand-in-Hand

If there’s one thing I look forward to every year, it is the Halloween season, which provides an endless creative opportunity in the form of my annual costume project. Over the past few years, I have used these projects as a creative outlet as well as a chance to learn a new skill or process. I’ve learned how to laser cut cardboard, sew a bodysuit, make a mask with modeling clay, and 3D print the joints for an extra pair of legs. These skills have allowed me to embody a cactus, a spider, and even a building (Royce Hall from my alma mater UCLA).

Photos of (left) laser-cut cardboard “Royce Hall” Costume from 2018, and (right) Cactus bodysuit from 2022.

Photos of my spider costume from 2019: (left) the mask and (right) the full body. The mask was a mixed-media creation of fabric, clay, paint, and peacock feathers. The extra legs were laser-cut cardboard with 3D printed hinge joints. The abdomen was made from a half-deflated beach ball.

The Concept

This year, I knew I wanted to go even bigger and more ambitious! But Halloween was going to be on a Monday, and I accepted that I would have to wear my costume throughout a normal work day. Formlabs has a forgiving dress code (especially on Halloween with the annual costume contest, which promises many vibrant and creative creations!), but trying to make it through the day in my spider costume, for example, may have been a little too challenging. I decided I wanted to make something ambitious, yet subtle; plus I wanted to tackle learning some new skills I had been thinking about. The concept came to me in a flash: originally expressed simply as “a bald cap covered in ears.” The idea consumed me.

After a few failed attempts to sketch out the idea, I decided the only way to visualize the idea was in 3D. So I pulled out some oven-bake clay and produced a mockup bust that I used to plan out a three-dimensional, organic-looking arrangement of ears across my head. I wanted to create an effect similar to an unfolding pattern of scales or feathers; something that would read as “natural” despite the distressingly un-natural nature of the project. The next step was planning out how I would actually make this happen. I wanted to create an effect akin to high-end movie monster makeup. I’d been interested in these kinds of practical effects for a while, ever since watching the TV competition show Face Off as a teenager. But I had never made a prosthetic piece of any kind before. The process usually involves creating a plaster cast of a hand-sculpted clay original, cleaning out the mold with a pressure hose, then casting the final piece in foam rubber or silicone. This seemed excessive, in both time and equipment required. Plus, I figured I would use Formlings’ unlimited printing perk, so I opted to 3D print the molds, which cut out several steps of the traditional process.

Part 1: The 3D Model

3D printing the molds allowed me to 3D model my design rather than sculpt it by hand. I learned how to use Zbrush (a professional 3D sculpting software) to create a model of an ear. Since I was designing a headpiece with eighteen nearly-identical ears, I could then duplicate the ear and place copies where I wanted them over one of Zbrush’s default head models.

A video of the completed 3D mockup of the full design in Zbrush; the final product would have 20 ears (including my real ones).

I still needed a way to mold this design, which meant I needed to create a 3D model of the negative space created by the ears. This was the most challenging part of the project. In Zbrush, I extended the outer surface of each ear to create a “shell” around the model, then subtracted the desired final positive model (containing all the ears and the outer surface of the head), leaving a model for molding the bald cap and ears. Since the ear is a complex and undercut shape, I knew I had to mold every ear individually from a two-part mold in order to be able to remove the molded part afterwards. Thus eighteen ears turned into thirty-six mold halves, all of which had to be connected together at the end to create the single-piece cap.

Screenshots of the final 3D mold design in Zbrush, with each ear molded from 2 parts. Each mold section includes flanged edges that can be bolted or clamped together.

Part 2: Pressing “Print”

Now I had thirty-six parts that, all together, made up a single mold for the ears and bald cap. I just had to print them all. I decided to use the Fuse 1+ 30W to print all the parts with Nylon 12 SLS powder. After packing the models, all 36 parts fit inside a single Fuse build volume and could print within a single day.

Screen capture before using the PreForm packing algorithm. Red outlines indicate parts that are overlapping in the build volume.

Screen capture after using the PreForm packing algorithm. All 36 parts were packed with a minimum 2mm gap between itself and the next closest surface, and I still ended up using less than a full build volume!

Part 3: Silicone Molding

After the parts were printed, they were put into the Fuse Blast to be automatically cleaned and polished. This resulted in printed parts with a smooth semi-gloss surface finish, and also removed any powder that could have gotten stuck in the molded silicone later. I also coated each part using a glossy acrylic spray, to further seal and smooth the surface.

Photo of post-processed 3D-printed parts before an acrylic coat was applied, test-assembled with some nuts and bolts to show the final shape of the cap.

Next was the casting step — a nerve-wracking step because I only had enough silicone to do everything once (plus, it was the weekend before Halloween). I assembled the molds for each ear and placed them on a tray propped up with tinfoil. Each ear had to be molded individually, with the opening facing upward, to prevent the liquid silicone from running out of the molds.

I then assembled the entire cap together and “painted” on the final layer of silicone to create a thin, flexible layer for the base of the bald cap.

Photos of (left) before and (right) after adding the silicone base layer that serves as the bald cap with all the ears attached to it

Then… the moment of truth…

Photo taken during the process of demolding the silicone ears.

The final product looked great, but it was missing a truly lifelike touch. I used silicone-based pigments, thinned with solvent, to add touches of red, blue, and yellow to the ears.

Photos of the final cast silicone product before (left) and after (right) it was painted to appear more life-like. This was the moment I began to be truly repulsed by what I had created.

Part 4: The Big Reveal

The day arrived. I used some plastic wrap to smooth down and cover my hair, then applied the cast cap and sealed down the edges with some skin-safe silicone. My colleague Elena Huang took glamor shots of me in costume.

Glamor shots taken by fellow Formling Elena Huang on the rooftop of Formlabs in Somerville, MA.

It wasn’t until I got to work and was encountered by the first inquisitive coworkers that the perfect title for the costume occurred to me. “I’m all ears!” I would respond when someone asked me what on Earth I was dressed as. Overall, I learned so much working on this and making something that would have been totally inaccessible to me– professional prosthetics–achievable through 3D printing. Plus, I won the vote of fellow Formlings at the annual Formlabs Halloween costume contest against some very creative 3D-printed competition!

So all I need to figure out now is how I can top this next year. Got ideas? I’m all ears.

What’s next?

There are so many other topics we want to write about and share. What else do you want to hear about? Email us at insideFLblog@formlabs.com. Looking forward to hearing from you and sharing more from behind the scenes at Formlabs.