Jeremy Elson, February 2019
The MOHAI, a museum in Seattle’s South Lake Union, has a monthly event called Maker Days where a local maker gives a free, hands-on, all-ages-appropriate introduction to an interesting maker skill or craft. In February of 2019, that maker was me!
I demonstrated 3D printing, mold-making, casting, and how they can be combined to make customized plastic medallions. People could draw a design on paper, and in minutes we’d turn it into a solid plastic medallion in any color they wanted with their design embossed on top.
The process took a few weeks and a lot of tinkering to develop. Eventually, I came up with a flow that could be performed in about 30 minutes. If you’d like to reproduce my work, my source code repository contains all the software and 3D models I created that went into making the event.
Step 1: Drawing and Scanning
Using Inkscape, I created a drawing template to be printed on paper. Each of the four corners contains a QR code. The instructions are to only draw in red ink. After the drawing was complete, I would take a picture of the paper with my phone and feed it into a Python script that I wrote. The script finds the four QR codes and uses their positions to scale the drawing correctly and fix perspective distortion from the camera not being perfectly parallel to the page. It then filters out everything but red marks and passes the result to a vector tracing program, leaving me with a vector (SVG) file of the drawing.
I made sure only to give out people thick red markers, not a fine red pen. I intentionally set the ratio of the radius of the circle on the instruction sheet to a magic marker’s tip width to be roughly the same as the ratio of the final finished medallion size to the minimum resolvable feature size on the Prusa MK3 printer. In other words, I wanted to make sure participants drew with thick, bold strokes; fine detail won’t be reproduced by most 3D printers.
Step 2: Modeling and Printing
Of course, it’s possible to 3D print the entire medallion without a moldmaking step at all. But this has some problems—mainly that it takes an hour to print a medallion of the size I wanted. I only have one 3D printer, and we were expecting dozens of kids to show up for a three-hour event, so that wouldn’t work. Plus, just watching a 3D printer is not a very hands-on experience.
The only unique part of each medallion is the embossed pattern on top; the bodies are identical. I realized I could speed up the process by printing several medallion bodies in advance, each with a small circular ridge at the top. This let me print only the top, customized layer of each new design and snap it into the pre-printed body. After making a mold from it, I removed the top face and reused the body for the next design. The 3D printer was still the bottleneck in the process, but it only took about 10 minutes per medallion instead of an hour.
I did all the 3D modeling in FreeCAD, which has a nice Python-based scripting system. I knew that during the event there would not be time to tinker with designs manually, so I wrote a FreeCAD script that that opens a template medallion face, imports the SVG produced in Step 1, extrudes it, and fuses it with the base. After adjusting the position manually if necessary, I could export the fused object to STL.
Step 3: Making the Mold
The main challenge with the moldmaking step was, again, that time was against us! I needed a very fast-curing silicone; having people wait 6 hours for their mold to cure just wouldn’t do. I’m a big fan of Smooth-On products, and the fastest-curing silicone in their catalog is Body Double Fast, which cures in only 7 minutes. In early tests, the most time-consuming step was carefully dispensing and weighing out equal amounts the two parts of the silicone formula. Luckily, Smooth-On sells an automatic dispenser similar to a double-barreled caulking gun that dispenses the two parts in equal measure automatically. That ended up working well.
Step 4: Casting in Resin
The last step is to mix together a two-part resin formula and pour it into the silicone mold to make the final medallion. Due to the time pressure, I picked Smooth Cast 300Q, which cures in just a couple of minutes. It’s naturally white and can be made almost any color by adding colorants. I also bought a selection of glitters, but ultimately did not bring them to the event due to the mess they make.
After a couple minutes of curing, the medallion is ready and can be attached to a lanyard!
The day of the MOHAI event was a great success! We set up four stations: drawing, scanning & printing, molding and casting. In three hours we made dozens of medallions for excited makers of all ages! We sent everyone home with both a medallion and the silicone mold it was made from so they cast more medallions of their own at home.
Despite dozens of hours of preparation and testing, the day of the event was so hectic that I barely had time to take any photos at all. But I took a few!
In case anyone else wants to replicate my event, I’ve written detailed step-by-step instructions and a packing list. My process was optimized for throughput; if you’re just making a single medallion, it’s probably easier to do it manually than to try to get my automated software pipeline running.