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MOHAI Maker Days: Making the Mold

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 it'd be a solid plastic medallion in any color they wanted with their design embossed on top. This repository contains all the software and other files that went into making the event, in case anyone wants to reproduce it.

Here are some photos of the process and the event.

First, a participant would draw a design on a sheet of paper. We scanned the paper and turned the design into a 3D model -- a 40mm-diameter circular medallion with the drawn design embossed on top. Next, we printed the medallion face on a 3D printer. To save time, the body of the medallion was pre-printed. At the event, we only printed the top layer with the embossed design on it and snapped it into the pre-printed body.

Then we helped participants measure out the proper amounts of a fast-curing silicone compound and pour it over the 3D-printed medallion. Ten minutes later, the silicone cured into a negative of the medallion -- a mold.

The last step was to mix together a two-part resin formula. We had a dozen different colorants on hand that could be combined to make the medallion almost any color. We'd pour the liquid resin into the silicone mold. After a couple of minutes of curing you've got yourself a solid medallion ready to attach to a lanyard, also available at the booth.

The entire process took roughly a half-hour from beginning to end. I chose the fastest-curing silicone and resin that Smooth-On makes so people could get instant results.

Detailed Process

We set up four stations at the event: drawing, printing, molding and casting.

The process below 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 pipline running.


  • Before the event I first generated 4 QR codes using the Linux 'qrencode' program.

  • In Inkscape, I produced an instruction sheet with a circle to draw in, four QR codes in the corners and instructions to only draw in red ink.

  • Print or photocopy enough instruction sheets depending on how many participants you expect.

  • Instruct participants to draw their design of choice using a red marker inside the circle without getting any marks outside the circle.

Make sure the red markers you provide are good, thick magic markers; don't use 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, make sure participants draw with thick, bold strokes; fine detail won't be reproduced by most 3D printers.


  • Take a photo of the completed sheet with a phone, wait for it to pop up in the Google Photos web interface (hit reload), and hit shift-d to download the photo to local disk.

  • Run my Python scanner script that looks for the most-recently-written file in the Chrome downloads folder, rotates it according to EXIF data, detects the QR codes, crops, filters out everything but red pixels, scales the image to the medallion size (using the QR code distance data), and feeds it to the 'potrace' program that produces an SVG out of a raster image.

  • In FreeCAD, run my macro that opens a template medallion face, imports the SVG produced in the previous step, extrudes it, and fuses it with the base. After adjusting the position manually if necessary, export the fused object to STL.

  • Slice the STL with your favorite slicer (e.g. PrusaSlicer), drag the gcode file to OctoPrint and print. (I have a Prusa i3 Mk3.)

The conversion process from paper to STL took two to three minutes. The bottleneck was the printer: it took about 8 minutes to print a medallion face, plus a couple minutes extra to wait for the cooling, cleaning, and reheating cycle between prints. If I were doing this event again, I'd definitely want more than one printer.


  • Snap the printed medallion face onto one of the reusable preprinted bases which has the body of the medallion and a built-in mold box.

  • Spray mold release lightly.

  • Measure out 30g of Part A and 30g of Part B of Smooth-On Body Double Fast. The cartridge applicator is much faster and more convenient (5 squirts) than measuring it manually out of the bottles using a scale. Don't use the mixing nozzle with the cartridge because its large volume leads to a lot of wasted silicone; just dispense into a cup and stir with a popsicle stick.

  • Mix! Mix quickly because you only have 30 seconds of pot life on this stuff!

  • Pour into the mold.

  • Wait 10 minutes or so for it to cure.

  • Once cured, carefully remove the mold from the box. I found that a dental pick was a good choice of tool for this. Insert the pick parallel to the box wall and rotate it 90 degrees to bite into the silicone, then pull up.

  • The custom-printed medallion face can be given to the participant as another souvenir. Reuse the mold box with its integrated medallion body template for the next participant.


  • Measure out 10g of Part A and 9g of Part B of Smooth-On Smooth Cast 300Q using eyedroppers

  • Color as desired using tiny drops of Smooth-On So-Strong

  • Combine the two halves of the resin compound and stir quickly

  • Pour into mold

  • Wait a few minutes for it to cure, remove, and put on a lanyard in your choice of color!

Packing List

Here's my packing list, which might be useful if you're trying to set up a similar event:

Drawing and Scanning Station

  • Template sheets

  • Red magic markers with thick tips

  • Laptop with software preloaded and tested (e.g. make sure you have all the Python modules you need)

  • Laptop Charger

  • USB mouse

  • Phone with internet connection

  • Phone charger

  • Power strips

3D Printing Station

  • One or more 3D printers

  • OctoPi or similar control software

  • SD card reader to fix OctoPi wifi connection issues

  • Extra SD card

  • Isopropyl alcohol with wipes to clean printer bed

  • Scraper, needlenose pliers

  • Extra PLA

Mold Making Station

  • At least a half-dozen pre-printed medallion bases (with integrated mold boxes)

  • Smooth-On Body Double Fast, Parts A and B, 400mL cartridges; 1 pair of cartridges per 5 molds (link)

  • Body Double dispenser (link)

  • Mold release spray such as Ease-Release 200

  • Dental pick (to remove molds from boxes)

  • Shop towels

  • Popsicle sticks for mixing

  • 4oz plastic containers as mixing vessels

Casting Station

  • Smooth-On Smooth-Cast 300Q (link)

  • Smooth-On So-Strong colorants, 2oz set with built-in droppers (link)

  • A selection of colored lanyards, e.g. these

  • Several gram scales (GDEALER DS1 is decent and only $12 on Amazon)

  • 2oz plastic containers and popsicle sticks for mixing

  • More shop towels

  • Disposable plastic long-nose eyedroppers or pipettes, e.g. these