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  1. #1

    How to properly sinter a stainless steel part?

    Hello,

    I am experimenting with some exotic filament that I purchased. I am using the stainless steel filament that Markforged sells for their MetalX printer. The filament is very delicate but after heating my enclosure I have successfully been able to print a part. I guess this would be called a green part. The problem is sintering it....

    Currently I have only done one experiment where I put the completed part into a metal casting foundry I built for casting aluminum to try and burn out the PLA binder. I slowly heated the part over the course of 15minutes to around 600 deg F. While heating it slowly smoked a bit and then stopped. At this point I turned up the foundry to 1200 deg F and left it in there for maybe 15 more minutes. I removed the part to see what I had done to it and it basically just fell to dust. Can anyone explain what happened here? Did I basically just burn out the binder and have a part made of dust? Is there some process I missing by allowing it to be exposed to air? I know the real sintering ovens they sell fill a chamber with some type of gas and then bake the part for hours. Does anyone have any ideas for how I could sinter the part correctly?

    Thanks!

  2. #2
    Engineer-in-Training
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    Do you have a pic of the part after printing? Did it need supports to print? If so, the Markforged website says to leave them on while sintering, "When the binder melts away, there is a period during which the part is effectively a sandcastle. The supports are critical to keeping overhang features from falling over during this step of the sintering process."
    https://markforged.com/resources/blog/metal-3d-printing


    When I sinter an oddly-shaped piece of silver clay, I support it on a bed of vermiculite. After sintering, the piece looks solid, but is actually porous. This makes it brittle, so you need to treat it more gently than a solid piece. I imagine that the Markforged stainless steel would also be somewhat porous after sintering.

  3. #3
    Here is a pic...not much to see though. There are no support structures or support filament. Its solid stainless filament.

    20201101_072808.jpg

  4. #4
    Engineer-in-Training
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    I just noticed that you sintered at 1200 degrees Farenheit. That is way too low to sinter steel. The Markforged sintering oven heats to over 1300 degrees Celsius (over 2,300 degrees Farenheit), and their sintering process takes over 24 hours.

  5. #5
    gotcha...yeah I kind of figured that I just burnt out all the binder and was left with powder due to the way it disintegrated after picking it up. I will try again at a higher temp.

  6. #6
    OK I made a second attempt at sintering a part. I printed another 1"x1"x1/8" part. This time I printed it solid infill. I think I may have heated it up too much during the filament burn off process because the part caught on fire. I ended the experiment here. I think I am done with the experiments at this point because my foundry is not able to heat up enough. Before now I was only hitting temps around 1300degree F for melting aluminum. I have some new element wire on order now which should be able to get me closer to 2000 degree F.I will report back when I do more with this.

  7. #7
    Super Moderator curious aardvark's Avatar
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    the desktop metal/markforge process is THREEFOLD.

    1) print
    2) dissolve the majority of the support
    3) sinter.

    Given that both the washing and sintering machines both cost more than $10,000 - I would be impressed if you can replicate the process with 3rd party equipment.

    There is something in the 'washing' process that removes most of the packing plastic, while leaving the metal still stuck together with 'something' and ready for sintering.
    Presumably if you could just sinter the raw filament - they would do so.

    https://markforged.com/resources/lea...turing-process

    Sintering is done under a mix of gasses.

    I would guess that unless you remove most of the packing - then the metal particles won;t be close enough to actually sinter together - probably why the parts keep falling to dust.

    Oh and there's a 17% overall shrinkage rate. So - presumably - some shrinkage occurs in the washing/solvent process.
    Last edited by curious aardvark; 11-08-2020 at 09:18 AM.

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