I created my model in onshape then exported it to stl file then imported it to simplify 3d to convert to gcode.

However my model didn't go any in fill, it just continue to print layer after layer in the same fasion as the 1st layer.

Correct me if am wrong infill is used so that the middle of your model isn't completely solid hence saving on filament.

Is there something special I need to do in onshape or simplify 3d for it to use infill? To me it looked like it was just filling it up with pla

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I don't use either of the two piece of software, but the first things to pop to mind:

  • Inspect the slicing preview: it should be self-evident if your GCODE is being generated correctly. In my slicer the infill is red and you can discern the typical pattern within:

slicing preview

  • Check your settings: your shell thickness should be something reasonable (like 0.8mm / 2 shells) and your infill should also be below 100% if you don't want to print it solid. For maximum strength, it doesn't help go much over 60%, typical values for light-duty parts are 20% and 30%.

  • Check your STL file: your mesh should be a closed surface, a "shell". Or the slicer won't be able to know what is "inside" and "outside". Many slicers verify this for you automatically and have a built-in utility to attempt to "repair" a broken mesh. I use slic3r Prusa Edition and this information is visualised at the bottom right:

info box from slic3r PE

  • Let the print finish: the wording of your question is ambiguous, but it sounds like you may have stopped the printer before the part finished. Certain combination of settings can be deceitful. For example, a layer height of 0.05mm and a shell thickness of 2mm means you will see the printer making 40 (forty!) layers of solid printing, before starting to create the infill.

  • Try another slicer: in case your slicer went berserk because of a bug (unlikely but possible), this should fix it.

On an unrelated note: the use of infill has a lot of different reasons besides "saving filament", for example:

  • relative to an empty print, even a very light infill (5%) provides a lot of added rigidity,
  • relative to a solid print, infill reduces weight and relief the tensions that are trapped in the FDM/FFF printing process
  • certain infill patterns allow to provide different responses to stressed in different directions (like for example a crash box that needs to give way in one direction but bear a load in another, or a wing that need to flex on its length but not on its chord)
  • infill provide support for top layers and other concave structures that may otherwise be non-printable
  • ...
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The infill portion of your model is configured during the Simplify3D process. After loading your STL file into S3D, edit the process and examine the Infill tab and Infill slider. You'll see a percentage indicator, as well as an extruder selection (left or right, if you have two) to be used for the infill. There are other options within the configuration that would have little to no effect on the problem you are experiencing.

Once you have checked and corrected the settings as needed, used the prepare-to-print option and press the play button for a preview to see the infill being printed before you send it to the printer.

100% infill is impractical, and one can create strong models with as low as 20% infill.

Infill is used to save filament, as you suggest, but it is also used to provide support for top layers on areas that are not vertical. Sometimes, one would use a higher infill figure to provide for smoother top surfaces although increasing the layer count for top/bottom can accomplish that.

Check your top/bottom layer figure in the layers tab to ensure you have a reasonable figure. Three or four layers are good for cosmetic reasons, more if you need additional strength. Anything higher or absurdly high would cause some of the trouble you describe.

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