how to make [almost] anything mas.863.12   work about
molding + casting week_5  


The focus of this week was to develop a workflow creating custom molds. Using the 3d software package of your choice to create a .stl, then milled on the Modela, and cast with rubber & hydrostone.


The 3d model I developed for this week was a parametric truncated tetrahedron modeled in rhino 5 w/ grasshopper and python.

The six sided faces have a recursive mesh patterning and some mesh refinement was done with Geomagic.

The smoothed mesh faces were populated back onto the final parametric component.

rendered module

assembly of modules

A lot of care was taken to develop a precise digital model of the mold. The most important aspect is to always check the exact dimensions of the stock you will be working with before you start modeling (especially if not all components of your mold are parametric). The size of the wax sized ened up measuring 0.3" less than what I had anticipated in my final mold layout. While the module itself was parametric I ended up parting the piece manually in rhino and then developing, the pour and vent holes, mold indexing, and rubber mold boundaries also by hand modeling. In the future it would be great to build a quick grasshopper tool that takes your object takes into consideration the size of your tool, offsets an amount + safety and then creates the pour and positive and negative keys for the mold. I ended up re-locating my parts within the new stock limits and ended up with very little room around the top and bottom faces of my part, not to mention this was all repositioned manually. Luckily or unluckily there was a long wait for the modella.

I was sure to change my rhino units to mm and export the final .stl as mm as well. I then imported the .stl to the fab modules, set the view to top, and manually reset the z heights to 0 and the negative value representing the bottom-most cut. We later found out there was a tweak to be made with the fab modules so that the z heights are interpreted from your .stl. Above are the settings for the rough pass.

The modella settings for the finish pass. In hopes to save time I gently reduced the overlap which is similar to the step-over in other cam softwares to 0.8 (though I could have gone further.)

Preview of the .ps paths from the fab modules.

Beginning the roughing / xy path with the modella. The was was fixtured to an aluminum plate this week using hot glue around the edges.

A close up of the roughing passes.

This is the last shot between a series of difficulties. Somewhere approximately 2/3rds of the way through the roughing pass it became much harder for the 1/8" bit to remove the chips from the depth of the mold. At one point I heard the hum of the mill begin to slow to a strain and eventually dull to a stop. I paused the job in hopes that I could pick back up and when pressing view I found the bit fell out of the collet completely. At this point I attempted to the best I could reposition the bit and resume the job but the paths did not seem to be at all correct in comparison to where the job was stopped. At this point I cut my losses, re-zeroed the tool as best as possible and restarted the job milling air until I got back down to the piece. This happened two more times with the 3rd time ending in the collette bottoming out on the top of the stock due to my fears of the bit coming loose. So total time spent milling approximately 12 hours with probably 6+ hours milling 'air' and I was not able to complete even the roughing pass. In future iterations of the fab modules it would be great to add a feature that would allow you to pause and/or resume a job a bit more easily. It would be also be wise on the design side to allow a larger area to the edges of the mold bounds to get the chips out as you are milling.

After failing to produce a finished wax mold, rubber mold, or plaster part I decided to seek the help of Andrew in the arch shop to set up the job on the small shopbot. Tool paths were generated in mastercam which is quite complex for the novice but very powerful. The torque of the shopbot allowed a for a roughing pass with a 1/4" in about 15 minutes and a finishing pass with a 1/8" ball end mill in about 45 minutes. As a rule of thumb I would say actual machining times are about double that of the software runtime estimate. Either way it is great to have some concept of how long you will be on the mill.

The second portion of the roughing pass. Notice a lot of standing chips. It might not be a bad idea to pause the job a few times while running wax to remove excess chips especially during the finishing pass.

Here is the finished second pass at the wax mold. Notice as mentioned above when milling wax the heat from the bit spinning through already cut chips can cause them to re-melt and stick to themselves and the mold walls. I had to spend a little bit of time cleaning the mold walls before I was ready to pour the rubber.

Prepping the smooth-on pmc 121 flexible casting rubber mixed 1:1 (80g per part). Be sure to use gloves as you can feel a little bit of irritation if this stuff touches your skin. Also be sure to stir the darker part for about 3 minutes prior to combining with the other part. Combine and then stir again for another 3 or so minutes taking care not to introduce air bubbles to the mix. It is also a good idea to vibrate the mix to work any air bubbles up to the top and then pour into your molds in a thin but steady stream.

Read the smooth-on directions carefully. The blue rubber cured in only a few hours but the darker rubber took 16 hrs or overnight for a proper cure. I returned the next day and the parts were ready to de-mold. I did not use release agent since the molds were rubber and I was sure to take extra care when removing them from the wax. If you are careful your wax mold should be able to produce another set of rubber molds if you needed to do a large production run of parts.

De-molded unit.

Finished units.
  -- jared laucks -- © 2006-2012 --