3D Printing Infill Explained: Strength vs Cost vs Weight

Infill is the honeycomb-looking structure hiding inside almost every 3D print. Most parts are not solid plastic, they are a thin shell wrapped around an internal lattice. That lattice is infill, and choosing it well is one of the easiest ways to balance strength, cost, and weight. Here is how it actually works.

What infill is and why it exists

Printing a part fully solid wastes material and time, and surprisingly does not add much usable strength for most parts. Instead, the slicer fills the interior with a repeating pattern at a chosen density. A part at 20 percent infill is roughly 80 percent air on the inside, held together by a strong outer shell.

Two settings control most of the behavior:

• Infill percentage is how dense the internal lattice is, from 0 percent (hollow) to 100 percent (solid).
• Infill pattern is the shape of the lattice, which affects strength direction, print speed, and material use.

How infill affects the three tradeoffs

More infill means more material and more print time, which means more cost. It also adds weight and strength, up to a point. The relationship is not linear, so the table below is the practical version.

The key insight: going from 20 to 40 percent infill adds real strength, but going from 80 to 100 percent adds a lot of cost for diminishing returns. For most parts, the shell (wall count) matters more than cranking infill toward solid.

Walls often matter more than infill

If a part feels flimsy, the answer is frequently more perimeters (shells), not more infill. Adding a third or fourth wall thickens the strong outer skin and usually improves strength and surface quality more efficiently than pushing infill higher. We tune both together.

Choosing an infill pattern

Patterns trade off speed, strength, and direction:

1. Grid is the fast default, strong in two directions, fine for most parts.
2. Gyroid is a smooth 3D wave that is strong in all directions and prints quickly, great for functional parts that flex or take load from multiple angles.
3. Cubic / triangles offer good multi-directional strength.
4. Honeycomb is very strong but slower to print.
5. Lightning is an ultra-fast, ultra-light pattern meant only to support the top surfaces of display models.

For most engineering parts we reach for grid or gyroid. For pure display pieces, lightning or low-percentage grid keeps cost down.

Practical recommendations by use

• Decorative or display: 5 to 15 percent, lightning or grid. Save material, nobody is loading it.
• Everyday functional parts: 15 to 25 percent grid or gyroid. This is our default for a reason.
• Brackets, mounts, tools under load: 30 to 50 percent gyroid, plus extra walls.
• Impact or high-stress parts: 50 to 80 percent, and consider a tougher material like PETG or nylon rather than just maxing infill.
• Very small or thin parts: often near 100 percent, simply because there is no room for a lattice.

A note on weight

For drones, RC parts, or anything where grams matter, infill is your main lever after material choice. Dropping from 30 to 15 percent infill with a smart pattern can shave meaningful weight while keeping the part usable. We can target a weight or a strength goal, not just a number.

Let us pick the right numbers for you

You do not have to specify infill at all. When you send us a part, just tell us how it will be used, will it hold weight, take impact, sit on a shelf, or fly, and we will choose the infill percentage, pattern, and wall count that hit your goal without overspending on plastic and machine time. Not sure what your part needs? Send a photo or sketch along with your quote request or upload your file, and we will recommend settings from our shop here in the Greater Houston area. Custom 3D printing, built layer by layer in Texas.