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In this series, “Letters from the Fab Academy,” Shawn Wallace, member of AS220, the Providence, RI community arts space, shares his experiences with the Fab Academy, a distributed learning collaborative, built on the infrastructure of the Fab Lab network. — Gareth

Mold making and casting

By Shawn Wallace

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Noah Bedford from the Providence Fab Lab made this flexible spider coupling for use in one of the machines built in the lab.

This week’s topic, mold making and casting, came as a welcome change from the previous two-week session (embedded programming), which was creating a bit of anxiety here at the Fab Academy. The materials and processes for molding and casting are easy to learn and yield satisfying results; programming microcontrollers can be satisfying in its own way, but is a more intractable topic to learn in two weeks. The assignment this time around was to model an object, machine a positive mold, then cast a flexible urethane mold that could be used to make multiple objects in a variety of materials.

Here are a few samples of the work created in the class:

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Food-friendly silicone molds can be made to cast edibles, like these candies, modeled by Susanna Tesconi in Barcelona.

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It is easy to embed electronics into cast parts. Here Anna France modeled a Tardis in Sketchup which will have a high-intensity LED cast into the roof.

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Victor Freundt put together a great slideshow detailing how he modeled and molded this press-fit puzzle.

For my final project, a pinball machine themed on an opera whose libretto is written by monkeys on typewriters, I want to have a central chorus pit with five or six monkeys on typewriters. This seems like a perfect application for the molding and casting unit. My wife (Jill Colinan) is making the monkeys, so I thought I’d cast the typewriters for this assignment.

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I wanted to base the design of the typewriter on this drawing (inset) that my friend Sue Riddle drew, almost a decade ago. I know I have a larger version around somewhere, but this was the only image of the drawing I could find (everything was much smaller on the Internet back in the twentieth century). I modeled the thing in Sketchup but ran into a problem where the STL plugin we are using only outputs ASCII STLs, where the stl2png.py script we are using only accepts binary STLs. Rather than muck with the plug-in code, I decided that the mold was simple enough to draw each projection in 2D using Inkscape. (Note that we have since solved this ASCII/binary problem by first bringing the ASCII STL into MeshLab).

I sketched out what the mold projections might look like and mapped the gray values “by hand.” Note that this is a three-piece mold; left, right, and one for the angled keyboard.

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I drew the three projections in Inkscape and brought them into cad.py for tracing. The cad.py script accepts grayscale PNGs, where each gray value is used to determine the depth at that point. I used an 1/8″ endmill bit for the left and right sides of the mold. The typewriter keyboard required a finer bit to get around the keys, so I did that in two passes; one with the 1/8″ bit and one with a 1/16″ bit for the detail. The details could not be more that 1/4″ deep due to the difference in size between the tool tip and the shank.

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After milling from the machinable wax, I cast a mold using the two-part 121 Smooth-On rubber urethane. I realized it would be difficult to align the keyboard mold within the two side molds. Rather than re-do the mold with a key for the third mold, I cast a strong magnet into the piece so that it could be pulled into place with another magnet on the outside.

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I had to break the wax to get the molds out:

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and the magnet hack seemed to work well:

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I cast the first couple of typewriters in Hyrdo-Stone, which did not require a mold release. I later tried to cast in plastic, with mixed results. We’re still trying to figure out a “soap scum” mold release method that works well without leaving bubbles in the material. Unfortunately, I didn’t choose a great location for the fill hole, which made it difficult to fill the keys of the keyboard all the way up to the top without bubbles.

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When you’re done with the machinable wax, it can all be melted back down and cast into a new block. The wax melts at about 230 degrees and vaporizes at about 575 degrees. The method that worked best for me was to bake the wax at 350 degrees in an old bread pan for twenty minutes or so, then pour it into a project box that was about the same size as the original block. The block on the left was heated at 250 degrees, and cooled down too quickly as it was poured:

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Next Week: 3D printing and scanning

More:

From MAKE magazine:

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MAKE Volume 21 is the Desktop Manufacturing issue, with how-to articles on making three-dimensional parts using inexpensive computer-controlled manufacturing equipment. Both additive (RepRap, CandyFab) and subtractive (Lumenlab Micro CNC) systems are covered. Also in this issue: instructions for making a cigar box guitar, building your own CNC for under $800, running a mini electric bike with a cordless drill, making a magic photo cube, and tons more. If you’re a subscriber, you may have your issue in hand already, and can access the Digital Edition. Otherwise, you can pick up MAKE 21 in the Maker Shed or look for it on newsstands near you!

Gareth Branwyn

Gareth Branwyn is a freelancer writer and the former Editorial Director of Maker Media. He is the author or editor of a dozen books on technology, DIY, and geek culture, including the first book about the web (Mosaic Quick Tour) and the Absolute Beginner’s Guide to Building Robots. He is currently working on a best-of collection of his writing, called Borg Like Me.


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