Some thoughts about PLA printing infill.

Discussion in 'Lost PLA casting' started by garyhlucas, Dec 15, 2019.

  1. garyhlucas

    garyhlucas Silver

    The goal of infill in 3D printing is to give a part strength with as little material as needed saving both material and lots of printing time. This seems counter to what is needed for casting.

    What if a slicer program could produce say a zigzag infill specifically designed to collapse on itself so when you heat the pattern during burnout it easily collapses on itself instead of cracking the investment by expanding?

    I’d like to do lost PLA but lost foam is just so damn easy when you have a CNC mill.
  2. Patrick-C

    Patrick-C Silver

    Ideamaker has a cubic infill, which might produce that affect.
  3. DavidF

    DavidF Administrator Staff Member Banner Member

    It has more to do with the type of pla and geometry than the infill....
  4. ddmckee54

    ddmckee54 Silver

    I think that the problem has multiple causes, with infill %, and infill shape as key factors.

    You want the infill % to be as low as possible, but you still need to be able to get a decent final layer without having to use an excessive amount of top layers. I would think you'd want no more than 10-15%, probably less.

    You want the model to be able support itself and withstand a reasonable amount of handling. It doesn't need to be able to resist much crushing force so one of the weaker infill patterns would be your best choice. A rectilinear pattern is fairly weak, especially at low infill %, each layer can resist side crushing in only one dimension. Cubic infill is one of the strongest infill shapes, it's shape is designed to resist movement in all 3 dimensions.

    The part of the infill that allows the model to resist vertical crushing will be the intersections of the infill layers. With a rectilinear infill pattern, this will be a series of columns that are one filament width by one filament width in dimension. Cubic infill will give you repeating pattern of hollow triangular pyramids that are stacked vertically base to base, and tip to tip. The pyramid walls are only one filament thick, but the Cubic infill pattern will give you repeating contiguous intersecting walls throughout the model.

  5. DavidF

    DavidF Administrator Staff Member Banner Member

    Having cast parts printed with anywhere between 10 and 50% infill I can tell you for certain infill isn't the biggest problem. Geometry is.
    Print an egg (the size of an egg) with 0 infill and only 1 shell. Now on this egg add a blind hole 1/4" dia by 1/4" deep.
    Now, mentally cast it and tell me what's going to happen... ;)
  6. ddmckee54

    ddmckee54 Silver

    Depends on how you orient the blind hole when printing the egg. Any orientation other than with the blind hole on the build plate, and you're going to get a mess. Casting it would STILL give you a solid egg with a blind hole, but you SHOULD be able to print it. Maybe, if your printer's up to the task. I know my Prusa I3 clone wouldn't be up to the task, my D6 clone - maybe.

  7. DavidF

    DavidF Administrator Staff Member Banner Member

    Don, the objective of what I was trying to get people to think about had nothing to do with the actual printing part.
    But since it's been brought up, you could print the hollow egg in two halves and glue them together, or use support structures when printing it whole.

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