Sep 30, 2026 · 25 min · 10 segments
Every 3D print is mostly air on the inside, and one number in the slicer decides how much. In this episode we pull apart the infill percentage: what it is, why printed parts are not solid, and what…
So to start, if you've ever watched a standard desktop 3D printer work, it's a process called FDM or fused deposition modeling.
Which sounds super technical, but-
It does, but it's really just a highly precise robotic hot glue gun.
I mean, it basically just squeezes melted plastic through this microscopic nozzle, and we're talking less than half a millimeter wide.
Wow, that is tiny.
Yeah.
And it draws your object one incredibly thin layer at a time, usually just a fifth of a millimeter per layer.
But the fascinating part isn't the outer shell.
It's the, uh, microscopic decisions the software makes on the inside.
Let's unpack the anatomy of that because, you know, looking at a finished part, you just naturally assume it's a solid block of plastic all the way through.
Exactly.
Okay.
Yeah.
So the printer draws solid outlines to form the outer walls, like tracing the edge maybe three or four times, and then it prints a completely solid top and bottom skin.
Right.
But the inside, that space you never actually see, is filled with this sparse web-like pattern, and that internal web is the infill.
So it's not solid at all.
No, not at all.
So if you set your infill to, say, 20%, the interior of your object is literally one-fifth plastic and four-fifths empty air.
Okay, wait.
[laughs] If I am paying for a physical object, making it mostly air feels, I don't know, a bit like a cheat.
Like, why wouldn't I just want it completely solid?
Well, we're actually just doing exactly what nature does.
So to start, if you've ever watched a standard desktop 3D printer work, it's a process called FDM or fused deposition modeling.
Which sounds super technical, but-
It does, but it's really just a highly precise robotic hot glue gun.
I mean, it basically just squeezes melted plastic through this microscopic nozzle, and we're talking less than half a millimeter wide.
Wow, that is tiny.
Yeah.
And it draws your object one incredibly thin layer at a time, usually just a fifth of a millimeter per layer.
But the fascinating part isn't the outer shell.
It's the, uh, microscopic decisions the software makes on the inside.
Let's unpack the anatomy of that because, you know, looking at a finished part, you just naturally assume it's a solid block of plastic all the way through.
Exactly.
Okay.
Yeah.
So the printer draws solid outlines to form the outer walls, like tracing the edge maybe three or four times, and then it prints a completely solid top and bottom skin.
Right.
But the inside, that space you never actually see, is filled with this sparse web-like pattern, and that internal web is the infill.
So it's not solid at all.
No, not at all.
So if you set your infill to, say, 20%, the interior of your object is literally one-fifth plastic and four-fifths empty air.
Okay, wait.
[laughs] If I am paying for a physical object, making it mostly air feels, I don't know, a bit like a cheat.
Like, why wouldn't I just want it completely solid?
Well, we're actually just doing exactly what nature does.
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