3D printing infills: patterns, strength and settings explained

Most 3D prints do not need to be solid. For everyday parts, 15–25% infill gives the top layers enough support without wasting filament.

Start with gyroid or grid if you are unsure. For a strong functional part, increase the wall count before pushing infill much beyond 30%. Walls usually contribute more useful strength than a dense interior.

Infill still matters, though. Its pattern, direction and density affect print time, weight, top-surface quality and how a part handles force. The right choice depends on what the part must actually do.

What infill does inside a print

Infill creates an internal framework between the outer walls. It supports top surfaces and stops larger parts feeling hollow or crushing easily.

Your slicer generates this framework automatically. You normally choose a percentage and pattern, while settings such as line width and infill speed control how it prints.

A higher percentage places the lines closer together. It does not scale strength in a simple way. A part at 40% infill is not automatically twice as strong as the same part at 20%.

The outer walls take much of the load in many designs. Layer direction also matters because FDM parts are usually weaker between layers than along an extrusion line. More infill cannot fully correct a poor print orientation.

Which infill pattern should you use?

Gyroid is a good general choice. It creates a continuous, curved structure with support in several directions. It also avoids many same-layer line crossings, which can make printing smoother on a well-tuned machine.

Grid is simple and strong for many workshop parts. It prints lines in two directions on each layer, but the nozzle may cross previous lines at their intersections. You might hear knocking if the material builds up at those crossings.

Cubic infill gives support in three dimensions. It works well for functional parts that may receive force from different directions. It can take longer than basic line patterns, depending on the slicer and model.

Lines or rectilinear infill prints quickly and uses material efficiently. It suits visual models, prototypes and parts that do not need equal strength in every direction.

Lightning infill branches upwards only where the top surface needs support. It saves a lot of material in decorative models, but it adds little resistance to crushing or bending. Do not use it for loaded brackets or structural parts.

Honeycomb looks strong, but it often adds unnecessary movement and print time. Modern patterns such as gyroid or cubic usually give a better balance for routine printing.

What infill percentage is enough?

Use the lowest density that supports the top layers and survives the expected load. These are sensible starting points rather than fixed rules:

• 5–10% for decorative models and quick shape checks.
• 15–20% for enclosures, toys and lightly loaded parts.
• 20–30% for brackets, mounts and general workshop components.
• 30–50% for parts facing higher compression or repeated handling.
• 80–100% only where a particular region genuinely needs to be dense.

Going from 20% to 100% can add hours and a large amount of material. It can also increase shrinkage, warping and heat build-up in small areas. A completely solid print is not always the strongest or cleanest result.

If only one section needs extra support, use a modifier mesh or local infill setting. For example, you can add dense infill around a bolt hole while leaving the rest at 20%. That is usually better than making the whole model heavy.

Infill or more walls for stronger parts?

Add walls first when a part must resist bending, impact or tension. Three to five walls often improve a functional print more than jumping from 20% to 60% infill.

Wall thickness should match the nozzle and line width. With a 0.4 mm nozzle and roughly 0.4–0.45 mm extrusion width, four walls produce about 1.6–1.8 mm of outer structure. Your slicer preview shows the actual result.

Top and bottom thickness matter too. Thin top surfaces can sag between infill lines or expose the pattern underneath. Aim for enough solid layers to produce around 0.8–1.2 mm of top thickness as a starting point. Flexible materials and wide infill gaps may need more.

For screw holes, clips and brackets, design also beats brute force. Add fillets around sharp corners, place holes away from thin edges and orient layers across the expected load where possible.

How to set infill in your slicer

You can make a useful first test without changing every advanced option.

1. Set infill to 20% and choose gyroid, cubic or grid. 2. Use at least three walls for a functional part. 3. Set enough top layers to create roughly 1 mm of solid material. 4. Slice the model and inspect the layer preview. 5. Check that infill supports broad top surfaces and narrow internal ledges. 6. Confirm the estimated time and filament use before printing. 7. Print one part, then test it in the direction it will be loaded.

Do not judge strength by squeezing the part in your hand if it will carry a shelf, motor or moving assembly. Test it under a controlled version of the real load. Keep clear of anything that could fall, snap or expose live wiring.

Changing firmware, motion limits or manufacturer profiles is rarely necessary just to improve infill. Those changes can affect reliability and warranty support. Stay within the printer maker’s stated temperature and speed limits unless you understand the consequences.

Why infill prints badly

Gaps and broken infill lines often come from printing too quickly. Infill speeds in default profiles can be much higher than wall speeds. The hot end may not melt material fast enough, particularly with a larger nozzle or wide extrusion.

Reduce infill speed and check the manufacturer’s temperature range for your filament. If the extruder clicks, do not keep raising temperature blindly. Check for a partial nozzle blockage, tangled spool, slipping drive gear or excessive volumetric flow.

If the nozzle scrapes across grid infill, inspect the sliced path first. Grid lines overlap by design. Slight over-extrusion, poor cooling or a loose motion system can make the raised intersections worse. Gyroid may reduce crossings, but it will not fix loose belts or an incorrect flow setting.

Weak infill that does not join the walls can leave a part feeling hollow and brittle. Check the infill-to-wall overlap setting and extrusion calibration. Too little overlap leaves gaps; too much can mark the outer surface.

Pillowing on top surfaces usually means the infill is too sparse, the top shell is too thin or cooling is poor. Add top layers before filling the whole model with dense infill.

Keep hands away from hot ends, heated beds and moving axes while testing. Isolate mains power before opening a printer’s electronics enclosure. Power supplies can retain dangerous voltage, and exposed boards are vulnerable to static discharge.

How infill changes cost and print time

More infill means more filament and usually a longer print. The exact increase depends on the model, pattern, nozzle size, layer height and machine profile, so there is no honest flat UK price for an infill percentage.

A small component may show little difference between 15% and 30%. A large enclosure can gain several hours. Slice both versions and compare the software’s material weight and time estimate.

For a printing quote, the final model matters more than the percentage alone. Orientation, support material, wall count and finishing can outweigh the infill cost. We would need the file, material choice and intended use before confirming a price.

When to mail it in

If your printer is working, changing infill is a safe DIY job. Run a small test and inspect the slicer preview before committing to a long print. If you need a finished part instead, send us the STL, 3MF or STEP file through our contact page, along with dimensions, material preference and what the part must withstand. We only provide 3D-printing services; we do not repair printers or electronics. You can also see how our mail-in printing service works. We will check whether the walls, orientation and infill suit the job before quoting.