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Infill is the internal structure printed inside a 3D model. Instead of filling every part completely with plastic, a slicer creates a repeating pattern inside the outer walls. This saves material and print time while still supporting the top surfaces and giving the object useful strength.
Infill is one of the most important settings in FDM printing, and also one of the most misunderstood. More infill does not always mean a meaningfully stronger part.
How Infill Works
A typical FDM print has three main areas:
- Walls (perimeters): the outer shell of the part.
- Top and bottom layers: the solid skins that close the shape.
- Infill: the internal structure between them.
Infill density is expressed as a percentage. At 0%, a part is hollow inside its shell. At 100%, the inside is solid. You set this in your slicer, which is part of the wider 3D printing software workflow.
What Infill Percentage Should You Use?
Guides differ slightly, but the following are sensible starting points for common plastics.
| Use | Typical density | Notes |
|---|---|---|
| Display model | 5% to 15% | Enough to support the top surfaces |
| General-purpose print | 15% to 25% | A strong default for most parts |
| Functional part | 25% to 40% | Combine with extra walls |
| Heavily loaded part | 40% and above | Test it, as gains become smaller |
| Vase-mode object | 0% | A single continuous wall |
Always test parts that have real functional requirements. These figures are starting points, not engineering guarantees.
Why Walls Often Matter More Than Infill
When a part bends or twists, much of the stress is carried by its outer surfaces. That is why adding walls is often a more efficient way to gain strength than raising infill.
A reliable starting point for a functional part is 3 to 4 walls with 15% to 25% infill. Many practical guides report that the benefit of additional infill falls away beyond about 30% to 40%, while material use and print time continue to rise.
Professional Note
Before raising infill, consider part orientation. FDM parts are generally weakest between layers, so aligning layers sensibly with the expected load can improve a part more than any density change.
Common Infill Patterns
| Pattern | Strengths | Good for |
|---|---|---|
| Grid | Simple and quick | General parts |
| Lines | Fast and light | Models with few strength needs |
| Triangles | Good in-plane stiffness | Parts with flat loads |
| Cubic | Strength in several directions | Functional parts |
| Gyroid | Balanced behavior in all directions | Reliable general and functional use |
| Lightning | Very low material use | Display models only |
Gyroid is an excellent default when you are unsure, because its curved structure performs consistently in several directions and avoids repeated nozzle crossings. Grid is a sound choice when print speed is the priority.
How Infill Affects Time, Cost, and Weight
Higher infill means more plastic and more print time, which increases the cost of a 3D print. The effect is strongest in large parts, where the interior volume is much greater than the surface.
If a part needs weight in its base, such as a stand, adding localized density or a metal insert can be better than making the entire model dense.
Common Infill Problems
- Top surface has gaps or sagging: infill may be too low. Increase the top layers or the density slightly.
- Infill shows through walls: add a wall, or check the overlap setting.
- Print takes too long: reduce density, switch patterns, or use adaptive infill where your slicer offers it.
- Part breaks along layers: this usually points to orientation or layer adhesion rather than infill.
Does Resin Printing Use Infill?
Resin parts are usually printed solid or hollowed with drainage holes rather than using an FDM-style pattern. Some software can add internal lattice supports to hollow models. Learn more in our resin vs filament guide.
Frequently Asked Questions
Is 100% infill the strongest?
It generally provides the most material, but it greatly increases time and weight, and it can introduce heat-related warping on some parts. Extra walls and improved orientation are often better value.
What infill is best for PLA?
For everyday parts, 15% to 25% is a sensible starting point. For stronger parts, use more walls and consider 25% to 40%.
Is 20% infill strong enough?
For many household objects and light-duty parts, yes. Anything load-bearing should be tested in real conditions, and printed parts should not be used where failure could cause injury.