Assignments Goals

• Design a 3D mould, machine it, and cast parts from it

Week's Lectures

Mold design

Before starting to design my mold for this week assignment, I spent a lot of time thinking about what I would have wanted to machine. I had a lot of ideas in mind but those were too complicated to machine, because of big overhangs or fragile points.

Then I looked into making a ice mold and other food molds, but the material we have in our FabLab is not food compliant. So in the end I decided to design something like a keychain and ended up by falling back on a Star Wars Stormtrooper Helmet 3D model.

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I downloaded the model from Thingiverse site (original link HERE) and started to modify it to prepare a mold, made by two halves, with centering spheres. The first step was to split the helmet in 2. I tried to use Rhino to perform this operation but the original 3D mesh file had a lot of problems and I'm not quite accustomed to use well Rhino yet. So I imported the 3D mesh in SketchUp and split the helmet quite easily using boolean operations.

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After that, I created the mold box, placing the two halves horizontally, I added the hemi-spheres, and the duct through which cast the object.

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Work files links

CAM and Machining

To prepare the machining on the Roland Modela MDX-40A, I used Vectric Aspire CAM software. I already used it for 2D cutting in the "Make Something Big" assignment week, so I wanted to try and learn the same software for 3D tooling.

Aspire is really user friendly, and after watching a couple of tutorials from the official site, I was quite comfortable in using it once again.

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After setting the initial parameters, such as job dimension and material thickness, I set up the mills parameter, including diameter, pass depth, stepover and feed rate. In the images there's a recap of the mills we have in our FabLab, with the parameters I set.

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The next step was to generate the toolpaths to machine the material I choose, molding wax. I decided to create the mold in three passes: one roughing pass and two finishing passes, with different mills. The roughing pass was programmed to use the 6mm end mill, the first finishing pass to use a 4mm ball nose mill and the last finishing pass to use a 1.5mm end mill.

Thanks to the simulation feature of Vectric, I could see the possible result of the machining.

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The result, after a LOT of time, was not good as I expected, but I found the cause: the 1.5mm end mill I used for the final smoothing was old and wrecked! So for the second half of the mold I decided to use the 1mm ball nose mill for the final smoothing instead of the 1.5 end mill. The result was really good in this case! What a relief...

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After that I wanted to try to use the big mill we have in the FabLab (the one used in "make something big" assignment), so I created a mold of a medal sized FabAcademy Logo, and the result was really good!

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Work files links

Silicone mold

After machining the mold out of the wax, I filled the two halves and the medal with liquid silicone [5% hardener solution] and placed them in a pressure cooker, modified ad hoc to create vacuum. That technique made possible to get rid of possible air bubbles. It's a simple procedure that lead to a better result. Thanks Stefano Paradiso for this useful insight!

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The curing process took almost a night, and when the silicone was fully hardened, it was easy to remove it from the wax. The coin was almost perfect, and the two halves of the stormtrooper helmet was quite good. The left side was worse than the other, but that was due to the bad mill in the machining, as I explained before.

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Plaster and Resin Cast

Once the silicone molds were ready, I build a simple press made with wood and four bolts to keep the two halves tightly together, so that I could slowly pour the resing inside.

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Plaster has quick hardening time, so after about 20 minutes the part was ready. The result was a little bit disappointing, because I couldn't reach all the parts of the helmet correctly. Hint for the next time: place the pouring duct in the most high position of the mold so that hair bubbles can exit easily! Moreover the difference between the two machined wax parts was clear, and maybe the halves coupling was a bit off despite the centering spheres.

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The second run was with transparent resin. The result was way better than the one in plaster, but the resin needs a really long time tu cure completely: after 2 days in the mold it still was somehow soft.

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Assignments Outcomes

• Design appropriate objects within the limitations of 3 axis machining
• Demonstrate workflows used in mould design, construction and casting

Have you:

explained how you made your files for machining
shown how you made your mould and cast the parts
described problems and how you fixed them
included your design files and ‘hero shot’ photos of the mould and the final object