Is 100% infill worth it? What the beam math says

Is one hundred percent infill really the strongest way to print a part? Everyone says so. Let's do the math, and find out what you're actually paying for.

Your part snapped. So you crank the infill to one hundred percent. Now it uses a lot more plastic, takes a lot longer, and it's not nearly as much stronger as you'd hope.

Pause and guess. A solid bar against one with six walls and fifteen percent infill. How much stiffer is the solid one? A: about four times. B: about twice. C: only about a third more.

It's C. Only about a third stiffer, for nearly double the plastic. Let's see why.

Before we touch a single setting, let's look at what happens inside a part when you load it. Most parts fail by bending, like this shelf bracket with a heavy book on it.

When a part bends, one side gets stretched and the other side gets squeezed. The stress is highest at the outer skin, and drops to almost zero in the middle.

So plastic in the middle barely helps. That's why steel beams are I-shaped, bike frames are tubes, and your bones are dense on the outside and spongy inside. Material on the outside does the work.

In a 3D print, the skin is your walls, the perimeters. The core is your infill. So the real question is: where does one extra gram of plastic do the most?

Let's compare three ways to print the same 20 millimeter bar. Bending stiffness grows with how far the material sits from the middle, so I calculated it for each recipe, treating infill as lighter material spread through the core.

One hundred percent infill: full stiffness, full plastic. Four walls with fifteen percent infill: 59 percent of the stiffness, for 41 percent of the plastic.

Six walls with fifteen percent: about 73 percent of the stiffness, for 52 percent of the plastic. Three quarters of the stiffness, for about half the plastic.

And less plastic usually means a shorter print, too.

Of course, this is simplified. Layer adhesion, the infill pattern and the direction of the load all matter. A print is weakest between its layers, so if a part snaps along a layer line, rotate it before you touch the infill.

So when does solid make sense? When you drill or tap the part, so the screw has something to bite. When it gets clamped hard. Or when it's so small it's nearly all walls anyway.

In your slicer, that means: raise the wall loops to four or six, keep sparse infill around fifteen to twenty-five percent, and for flat parts, add a few extra top and bottom layers. Those are the skin too.

In short: need more strength? Add walls first, keep infill moderate, and only go solid when the part really needs it.

Grab this shelf bracket for free and test both versions yourself. Link in the description. I'm Spoolbert, see you in the next one.