This week I attempted to cast a gear in hard resin. The gear is piece of a differential gear system (at least 14+ parts total) that I'm working on:
Eventually I'd like to make this mechanism in metal, currently all the parts are 3D printed on an Objet machine from a white resin. The resin parts are too flexible for this application and they warp over time.
I already had my model ready in Inventor, so I went into the part file and added some registration geometry. I sliced the model right through the middle of the round spokes of the gear, this left me with two sides that have no overhangs - they can be cut precisely with a three axis toolpath.
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I added some spheres and one cylinder as registration marks. To make the registration negatives I had to extrude a block of material from my slice plane and cut the shapes into it.
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This process of preparing my stl for two part molding was fairly tedius, but also very algorithmic. I tried looking around to see if there was a more automated process for this and couldn't find anything. If I were to do a lot of this I think I would write some kind of script.
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I also added some air/pouring vents to one half of the model. I attached these vents to the flat part of the model so that they could easily be sanded down later, and since they are not on the teeth, a small imperfection will not affect the model functionally.
I did all my CAM in PartWorks3D. Installing that software on my own machine saved me a ton of time here, because I was quickly able to make changes in my model (change the size of registration marks, air vents, etc) and then re-load it in Partworks. I split my original piece of machinable wax in two to make the two molds, and since each piece was less than half an inch thick, I had to make sure I was efficient in the Z-depth. I also created a few new tools for myself ('end mill 1/8" rough', 'end mill 1/16" finish') with all my feeds and speeds, so that is saved for me later. I played around with a few different tools in Partworks to find something that was the best mix of speed and precision for this part. I ended up using a 1/8" end mill for roughing along the y axis and then I planned to go over it with the 1/16" end mill twice (once along 45 degrees and another pass perpendicular to that), but I only ended up running one pass with the 1/16" mill to save time. It turns out the time estimations from Partworks are very low.
In the shopbot I first faced off my two pieces of cut wax.
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Then I ran my toolpaths - one roughing and one finishing path for each side. Each piece of wax took about an hour to mill.
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I used duct tape to add some extra walls to my mold and I wrapped tape around each of the air holes to try to bring them out to a wider funnel shape (obviously I couldn't mill them that way on a three axis machine). I poured OOMOO silicone rubber in my wax positives.
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I was really happy with how my molds came out. There were some very tiny air bubbles in the tiny corners of the gear teeth, but they shouldn't affect my model.
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Unfortunately, My duct tape strategy with the air vents was not too successful. I think the tape moved around once the OOMOO was poured in - only one vent was successful (I used it to pour). I used some coffee stirrers to open up the remaining vents so that air could easily pass through.
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I used tape to clamp my molds together during pouring. I poured the Smooth-On Urethane "Liquid Plastic" resin.
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My results were not spectacular. I tried pouring twice and both times had about three or four large bubbles that disrupted the cast significantly - next time I would make the vent holes much larger. I might even try putting vents on some of the internal spokes. On my second attempt I poured the first half by itself, then taped on the second half and poured the rest, but I didn't have significantly different results. Maybe throwing the mold into vacuum or a minute after pouring would have sucked some of the bubbles out (though we found that too much vacuuming will cure the resin). I also had a lot of extra material at the seam of my molds - I think more clamping would have prevented that.
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Direct comparison to 3D printed part.
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