How many walls does a 3D print need? More isn't always better

Last time, in Myth Busted number one, we found that walls beat infill. So more walls must always be better, right? Ten walls, twelve walls, why not? Let's run the numbers, and find where extra walls stop paying.

So you set wall loops to ten. Now the print takes longer, it eats more filament, and that thin clip on the side? Exactly as strong as before.

Pause and guess. The same 20 millimeter bar as last time. Going from eight walls to ten: how much stiffer does it get? A: twice as stiff. B: about a quarter. C: less than ten percent.

It's C. Only about 8 percent stiffer, for 14 percent more plastic. And in this simple model, bending strength grows by the same percentage, because the bar's outer size stays the same. Let's see why.

In Myth Busted number one, we saw that bending stress is highest at the outer skin. Your first wall sits right at that skin. Every next wall sits a little further in, closer to the quiet middle.

So each wall buys less than the one before. On this bar, the second wall adds about 12 points of stiffness. The tenth adds about 3. But every wall still costs plastic.

Put it side by side. From two walls to four: 48 percent stiffer, for 44 percent more plastic. A good deal. From eight to ten: 8 percent stiffer, for 14 percent more plastic. Now you pay more than you get.

And that's a chunky 20 millimeter bar. Real parts are full of thin bits: ribs, tabs, clips. That's today's free model, a wall comb. Five fins, from 1 millimeter up to 10.

One wall loop is a line about 0.42 millimeters wide, and every loop goes on both sides. So a 2 millimeter fin is full after about 2 loops. A 3 millimeter fin after about 3. Set ten, and nothing changes. There's no room left.

Only the thick block keeps changing. It takes about 11 loops before it's full. So on most parts, extra walls only land in the sections that were already thick.

If your part splits cleanly between two layers, the weak spot is the bond. Walls help a bit there too, because there's more plastic in the layer. But the bond itself doesn't get any stronger. A hotter nozzle, less part cooling, or rotating the part so the load runs along the layers usually helps more.

So how many do you need? As a rule of thumb: two for parts that only need to look good. Three to four for functional parts. Five or six when it really carries load. Past that, check the math first.

In Bambu Studio, that's wall loops. PrusaSlicer calls it perimeters, Cura calls it wall line count. Change it, then scroll through a layer in the preview before you print.

In short: each extra wall adds less than the one before, thin parts fill up after a few loops, and layer splits need a better bond first.

The wall comb is free. Slice it with two walls and with six, and compare. Tell me in the comments: how many walls do you run by default? Next time on Myth Busted: is PLA really weak? Subscribe so you catch it. I'm Spoolbert, see you in the next one.