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What am I doing wrong with Lost PLA investment casting (aluminum)?

I've tried several times to get lost PLA investment casting to work, but it never turns out great.  In the photos you see an attempt at casting some 3D printed pulleys.  Interestingly enough, the one furthest from the sprue turned out the best, but there are still pieces missing and the details aren't as good as they could be. I used 6061 scraps for the melt.  I've tried adding a bit of boraxo/boraxo+salt.  I've tried without it. The mold was made by using 325 mesh silica and plaster of paris about 45 to 55% respectively.  This produced far better details than with play sand, but still not great. After casting the mold I let it sit for 12 hours or so, and then baked it for 2 hours at 350 F, and then 2 hours at 500 F (upside-down), and then put it in the propane BBQ grill for 2 or 3 hours at max heat.  I then blew out the ash using a can compressed air.  In this photo I even kept the mold heated until just before pouring, and then I put it into the furnace until the top level of aluminum re-melted. You can see that I have several vents (2 for each pulley) and a very large sprue.  The sprue fills quickly before anything comes out of the vents. Once the aluminum melts I set a timer for 3 minutes and allow it to continue to heat before pouring. Any advice is welcome from those with experience.  Also any photos or videos of how you've made it work would be great! The pulley's I've printed are from

Question by als_liahona    |  last reply