More Infill Doesn’t Always Mean Stronger 3D Prints: How to Prioritize Walls and Infill

Infill Is Only One Part of Strength

When an FDM-printed part feels too flexible, it’s tempting to increase the infill from, for example, 15 to 40 percent. This can help, but it doesn’t guarantee a stronger part. Infill is one of several factors that affect the result, along with outer walls, material, print orientation, and how the part is loaded.

For many models, the walls are an important part of the load-bearing structure. UltiMaker points out that in some cases, adding more wall lines can provide greater strength than a modest increase in infill. This is a useful rule of thumb—not a universal law: the size of the difference depends on the part’s shape and the load it carries. (ultimaker.com)

What Do Walls and Infill Contribute?

The walls form the shell around the part. They can be especially important when the load acts near the surface, for example when a bracket bends or a screw pulls on a wall.

The infill supports the shell from within and can help keep flat top surfaces in place. It can also help distribute forces through the model, but its effect depends on the infill pattern, density, and the part’s geometry.

That doesn’t mean walls are always better than infill. A solid block being compressed, a thin plate being bent, and a tube being twisted are subjected to different kinds of loads. So a setting that works for one part is not automatically the best choice for another.

Start with the Shape, Then Choose Slicer Settings

If you can modify the CAD model yourself, it’s often worth first asking whether the geometry can carry the load better. Ribs, rounded transitions, and locally reinforced areas can be more effective than filling the entire model with plastic. Also try to place material where the forces actually pass through the part.

Then you can adjust the slicer in this order:

1. Check the wall count. Try adding one or two wall lines and compare both print time and material usage. 2. Choose a reasonable infill percentage. Increase it gradually instead of jumping straight to 100 percent. Denser infill uses more material and takes longer to print, but it doesn’t necessarily make the part proportionally stronger. 3. Choose a pattern suited to the application. Patterns with support in multiple directions can be useful when loads vary. No pattern is best in every situation. 4. Check the print orientation. FDM parts can have different properties in different directions. If the load tries to pull the layers apart, the part may fail differently than if the load acts along the deposited lines. UltiMaker describes this anisotropy specifically for composite materials; the exact effect depends on the material and print conditions. (support.ultimaker.com)

A Simple Comparison Test

If the part is intended for practical use and the consequences of failure are minor, you can run a controlled comparison test before making your choice. Print three identical test pieces using the same material, temperature, layer height, and orientation. Change only one setting between them—such as wall count or infill—so the results are easier to interpret.

Apply the same load to each test piece and note both how much it bends and how it breaks. If you change several settings at once, you won’t know which change made the difference. However, a home test is not enough to verify a part that will support people, handle pressure, or be used in another safety-critical environment.

A Practical Starting Point

If a functional FDM part feels too weak, start by checking the wall count and orientation before substantially increasing the infill. If the part has thin shells, adding more walls may be a sensible first step; if the problem involves large flat surfaces or a more complex load, infill may also play an important role.

The best setting is therefore not the one with the highest percentage, but the one that suits the part’s shape and the load it carries—and that you have tested under relevant conditions.