Block Investment PLA burnout

Discussion in 'Lost PLA casting' started by Rotarysmp, Dec 19, 2020.

  1. Rotarysmp

    Rotarysmp Silver

    I managed to get a bag of Cetus Unicast.

    Suprisingly difficult to obtain investment here. Anyway, I did a first (complete failure) casting today. I'll put the details in a video and upload it, once I recast the part.

    I don't have a controllable burn out oven. I also failed to look up the recommended burn out schedule until after trying it.

    Did air dry over night, then about 2 hours in the kitchen oven at 100°C, followed by a 2 1/2 hours in my foundry furnace with the little 1/2" bunsen burner at about 2 psi running rich (candling). After that, I increased it over and hour to about 35psi, and max temp. By the end of that time the mould was glowing slighly red.

    Without a controllable burn out oven, is there anything I could do better (after the initial 150°C step in the kitchen oven) to burn out this sort of block investment?
  2. Rotarysmp

    Rotarysmp Silver

    The mould was a mess of cracks so there is a lot of flashing. P1160545.jpg P1160546.jpg
  3. Billy Elmore

    Billy Elmore Silver

    So you used 3D printer PLA and used it in a lost foam type application? Please add more details. I am very curious about this method. I was just asking if this was possible the other day and have been thinking about trying it.
  4. DavidF

    DavidF Administrator Staff Member Banner Member

    Lost pla is treated the same way lost wax would be.
    Billy Elmore likes this.
  5. Rotarysmp

    Rotarysmp Silver

    Correct. Not lost foam. You don't pour the molten metal onto the plastic. You enclose the plastic model in a plaster block, burn it out, and then pour the molten metal into the void. Same investment casting process (lost wax) use for millennia.
  6. Billy Elmore

    Billy Elmore Silver

    Watched a video on it last night from MyFordBoy. Damn cool! My mind has been racing since!
  7. Billy Elmore

    Billy Elmore Silver

    I would have thought the PLA would have left some residue or something that would affect the metal, but I was shocked at how clean the investment came out of the oven. Super cool!
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Tobho Mott and Billy Elmore like this.
  9. Billy Elmore

    Billy Elmore Silver

  10. Manitoba Man

    Manitoba Man Copper

    What is the story on the yellow Sea Kerr, tracked machine in your photo? Bob.
  11. DavidF

    DavidF Administrator Staff Member Banner Member

    My first cad modeling project that I did learning the program. Was hoping to build it one day and install a metal detector on it to hunt lost jewelry off the beach in shallow <20' water.
    sea1001.jpg SEA4001.jpg
    dtsh likes this.
  12. dtsh

    dtsh Silver

    Ammusingly, I was at the beach swimming with my dog in about 8' waters when my car and home keys came out of my pocket and I saw them slip out of sight. I dove and dove trying to find them, but eventually gave up.
    About a month later I get a call from a pet store, not the one I go to but another of the chain, seems someone had somehow found my keys and dropped them off at the store because I had a tag from the store on there. The store looked up the tag and got my number, totally unexpected.
  13. ddmckee54

    ddmckee54 Silver Banner Member


    What were your printer settings? It sounds like your printed object expanded during the burnout cycle and cracked the mold. Maybe too many perimeters and/or too much infill?

    I know it's a delicate balance. The printed object has got to be strong enough to survive the molding process, but yet the object can't be so tough that it cracks the mold when the object expands during the burnout cycle.

  14. Fulmen

    Fulmen Silver

    I'm with Don, it would be useful to post the print specs. I wonder if the type of infill has an effect as well. The gyroid has no straight walls, and long straight sections are often more sensitive to thermal expansion. With PLA I would keep the shell at 2 layers max. If you need extra bridging capacity at the top, try increasing or some type of gradual infill.
  15. Rotarysmp

    Rotarysmp Silver

    Since my last unsuccessful attempts at lost PLA block investment casting, I have addressed the following:

    - Uncontrolled burnout in the furnace --> built a burn out oven:

    - Flasks oxiding to rust during burn out --> bought a stainless tube as a flask
    - Poured too early with cold metal --> did yoga until I became a more patient person *
    - Half-arsed vacuum hand pump --> a friend lent me a 3ph industrial pump.

    So how effective were these changes... see for yourself:

    *Okay, I just put the pot back in for a bit more super heating after skimming. My immersion thermometer should work at bronze temps, but the battery was flat. I bought a new battery for the next attempt.
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  16. Peedee

    Peedee Silver

    That made me LOL
    Rotarysmp likes this.
  17. Rotarysmp

    Rotarysmp Silver

    Just a question, just before pulling the block from the oven, I blew out the cavity with compressed air. A small cloud of powder came out. Is this good practice and I am removing loose investment and ash, or is this bad practice which just loosens investment and makes defects in the knook and crannys?
  18. Rotarysmp

    Rotarysmp Silver

    I redid George (George IV). The good news was that clear coat burns out cleanly without ash causing defects. The bad news is that I was not careful enough in preping the model, and there were still some unseal layer striations which investment crept into.

    There is also a sand casting in there, which I ddi while the furnace was heating up, and the little way wipers, which were a fail.

    Jason likes this.
  19. Jason

    Jason Gold

    You can blow it out, but go easy on it. I saw the puff too. The investment companies have some pretty specific recommendations when using their stuff. I don't know if the distilled water thing is really req'd but I do it anyways. Probably depends on your local water. Mixing is another biggie. Put a glove on and use your hand to break down the lumps and gently mix it. Especially if you are not going to vac chamber your investment. Can't remember if you skipped that step. You know the drill, proper procedures yield consistent results.;)
  20. Rotarysmp

    Rotarysmp Silver

    Thanks. Yeah, I tried not to blast it with full pneumatic Px. I haven't used demineralised water, but the Vienna water supply is pretty soft water anyway.

    I added water to plaster last time. I can see it will be better to reverse that and put in the water first. The powder is not lumpy. I did vacuum it, but maybe not for long enough.

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