Molds for plastic gamepieces
Preview final product
We used the machines to create molds for a couple of handsize gamepieces, custom-made for use on a multitouch table. One of the final products and the actual application look like this:
Creating molds for plastic gamepieces
The sub-project described here is about creating the molds for some custom plastic gamepieces, which involves both the milling machines at Fablab Amsterdam
BZT: http://fablab.waag.org/node/792
Modela: http://fablab.waag.org/node/797
Please note that for the final result, also a vacuforming machine was
used (shown on the right), which is currently not yet available at Fablab Amsterdam. We used several materials, like Pet G and Polystyrene of various thickness. Don't forget you need to create you models with a slope because of the draft angles - otherwise release of the formed plastic and the mold is very difficult.
Design
One of the first sketches of the shapes for the gamepieces:
A render preview of one of the final designs:
Materials
Before we created the molds with the machines at the Fablab, we had already made them by hand from clay and plain MDF, so we already had a good idea for the right scale. These models were actually also used for vacuforming, but have a high
risk of wearing out after just a few rounds because of fragility of the
material.
So for the final molds to be good enough for a medium production series, we decided to try something new, recommended by Mike of the Fablab Amsterdam: Chemical Wood (Obomodulan, or actually polyurethane: http://fablab.waag.org/node/789).
This material is not really cheap, and you have to buy a big block at once from the Dutch supplier, but definitely worth it's price - while it's very strong, you can finalize it really neat with sanding paper and other manual tools as well. And you can choose the right density for your application from their range of 13 types - I chose type Grey 850.
Milling process
Settings used varied heavily per model, due to very different complexity and shapes:
Roughing:
Chemical Wood preset @ 5 mm Square
XY-Speed: 5-7 mm/s
Z-Speed: 0.2 mm/s
Spindle: 6500 rpm
Cutting-in: 1.3-1.5 mm
Path Interval: 2.0-2.5 mm
Finish Margin: 0.1 mm
Path Type: Contour Lines - Up Cut
Note: Path Type mode can really influence total milling time. Compare the estimated times in the preview to see which one works best for you. I found Contour Lines (Up Cut) and Spiral (Up Cut) quite fast.
Finishing:
Chemical Wood preset @ 3 mm Square
XY-Speed: 10-12 mm/s
Z-Speed: 2.8-3.2 mm/s
Spindle: 6500 rpm
Cutting-in: 0.5 mm
Path Interval: 1.0-1.5 mm
Finish Margin: 0.1 mm
Path Type: Contour Lines - Up Cut
Tips for (rapid) prototyping with milling machines
Tip 1: try to avoid using the milling machines for less complex models in very strong materials, as doing this by hand is most of the time way faster. Even if you're sure about the exact cutting settings and don't have to do any test runs, the milling process itself will take several hours (about 8 hrs in this case).
Only one of these models was actually really hard to create by hand, because of the organic shapes of the different layers. If you are able to think of any other way to produce your shapes, I recommend considering the amount of time you want to spend on getting a perfect model from the milling machine, as it seems not too rare you will encounter some problems with alignment (fix your material really good!) over a couple of hours.
Tip 2: I stronly recommend testing your model first on
polystyrene foam before using the Chemical Wood, because of the
relatively long time the milling will take on a tough material like
this.
Tip 3: You can also save a considerable amount of time by setting a partial area or depth for the finishing process in the Modela software application!
Tip 4: The BZT milling machine is a lot faster than the Modela, but also a lot stronger and inaccurate. If you're in a hurry I really recommend trying this one, but be aware to check the alignment of the origin at least one time during your job. Cause if it skips, check the picture below to see what might happen ;-)
Final word: Of course, after the molds were done, we had to finish them by polishing and sanding. Then we needed to make some preparations for using them in the thermoforming process, like drilling holes for the vacuum. Here you see the final molds we used and the extruded plastic shape below.
