Joined: April 30 2013 Location: United States Posts: 3
Posted: May 01 2013 at 3:40am | IP Logged
Inspired by the "lost PLA metalcasting" project, I decided to try "lost ABS", and it works! The basic idea is:<br>
Make your part in some soft, melty substance like 3D printed plastic.
Surround the part with a castable "investment" (I used plaster of paris).
Burn out the part, leaving a part-shaped hole in the investment.
Pour in metal, filling the hole and making a part.<br>
Here are some castable AK FCG sets, with 3D printed hammer and disconnector (at low infill), and a wax gear as a control. The vertical fill/vent tubes are just straws, which I hotglued onto the cast parts. The plastic, straws, and hotglue will all melt away. The can and plaster will remain. <br><br><br>The steel dogfood cans are my "casting flasks", which keeps the soft plaster from cracking during burnout. <br><br>I mixed up some really thin plaster of paris, with some sand added for high-temperature strength. I also tried adding concrete, but I think the sulfates in the plaster kept it from curing, so I can't recommend it! You can also just buy commercial casting investment (jewelers and dentists use it).<br><br>Caution: plastic floats! I had to weigh down the parts to keep them from floating out of the investment. I let the plaster cure for about three days on the radiator, then set them inside my boiler's firebox for a few hours to burn out all the plastic. Here's the flask after burnout, ready for casting:<br><br><br><br>Note the carbonized remnants of the tall plastic sprue. Clearly I don't have good temperature control in the boiler firebox, since only one side is white. Evidently burnout is supposed to hit 1350F, basically glowing a dim orange; I wasn't patient enough to let it get that hot, and didn't know if my ghetto investment would survive anyway. I've tried this without the steel can, and cheapo plaster investment tends to crack up if it's not securely surrounded.<br><br>Next I poured in molten metal. I didn't know what was going to work, so I tried some low-melting zamak (90% zinc, 10% aluminum) and my standard die-cast aluminum (which includes some silicon for fluidity). I can't melt steel yet, and you need ceramic investment to take the heat, but evidently this process scales to steel too. <br><br>The moment of truth: breaking open the flasks.<br><br><br>They both worked! And they worked *great*!<br><br><br>The detail is really incredible, especially in the zamak. You can see every 0.2mm filament layer. You can see the smoother spots where I plugged holes with a swipe of hotglue. You can see where tiny air bubbles clung to the original part. I'm impressed!<br><br><br>The 5mm disconnector hole cast fine, even in aluminum. This hole is as-cast!<br><br><br>Here's a comparison with my "best" sand cast hammer on the left, and my "first" investment cast hammer on the right. The investment cast hammer has way better small feature detail, although the sand gives a smoother finish.<br> <br><br>Overall:<br> &nbs p;- Detail is *amazing*, clearly showing the 0.2mm 3D printed layers.<br> - Everything filled 100%, and out of three tries in two materials (zamak and aluminum), all three produced usable castings. This is way better than sand casting, which still fails more often than it works for me.<br> - Holes, undercuts, and overlapping parts seem to be castable without issues. This is also a huge advantage over sandcasting.<br><br>There were some drawbacks:<br> - The tiny 1/8" spring hole in the disconnector was filled with metal. I don't know if the plaster failed to fill the hole, or if it broke off and the little plaster slug floated off somewhere.<br> - A few tiny air bubbles left holes in the plaster on the bottom side of each part, which then filled with metal. These are easy enough to scrape off as a postprocess, but I should vacuum debubble the investment; just tapping the sides wasn't enough.<br> - A few carbonized fragments of burned ABS still remained on the top sides of each object. Evidently you're supposed to flip the mold over halfway through burnout to let these burn off, or blow air through the mold, or something.<br> - The overall process has lots of steps and waiting. You print the parts (1 hour), glue them up (10 minutes), mix the investment (10 minutes), dunk and let cure for several days, burn out for several hours, melt metal, pour metal, let cool, and finally break out the parts. <br><br>But when you see gleaming metal parts appear all shiny and perfect, it's worth it!<br><div style="display: none; visibility: ; opacity: 0;" id="lb"><div style="display: none;" id="lbCenter"><div id="lbImage"><div style="display: none;" id="lbBottomContainer"><div id="lbBottom"><div id="lbCaption"><div id="lbNumber"><div style="clear: both;">
wow nice work! im about to do casting myself and some things i pick up to get a better finish is to use some fine sand (then strain the fine sand to powder if you want) and put 10% bentonight clay mix in it. then coat the part with drywall mix and it will come out alot less pitted like.
Joined: August 13 2012 Location: United States Posts: 326
Posted: May 02 2013 at 1:42pm | IP Logged
Thats an incredible undertaking and thanks for posting that work. I appreciate the effort you went through. I think there is a huge potential in precision casting parts. S&W has been doing it for about 15 years will good success, even for their revolver barrels.
On the other hand for small parts which method is more time and cost effective: casting or 3d printing vs. CNC milling. Right now I think CNC milling is the best route to follow. I have a friend who makes precison parts for medical equipment on a series of CNC lathes and mills and some of these parts are hard to see with the naked eye and I doubt they could be cast or printed. Small gun action parts I think would be in the same catagory as easier to mill than to cast as no secondary cleanup work is needed.
I don't think the problem is with the technology as to printing or casting but rather with the materials we use for parts. Once somebody comes up with a high strength alloy for printing or casting then it's a whole new ball game.
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