Infill Explained: Why Your Print Isn't Solid Inside (and Why That's Good)

Pick up a 3D printed figurine and it feels lighter than it looks. Cut it open and you find a honeycomb, not solid plastic. That internal structure is called infill, and far from being a shortcut, it is one of the cleverest things about 3D printing. This guide explains what infill is, why almost nothing is printed solid, and how the percentage changes strength, weight and cost.

What infill is

An FDM printer builds each object as a shell (the outer walls, plus solid top and bottom layers) and fills the space between the walls with a repeating pattern. The density of that pattern is the infill percentage:

• 0% is hollow. Only the walls exist; vases print this way.
• 15 to 20% is the everyday default for décor and models.
• 40 to 60% is for parts that carry load or get screwed together.
• 100% is solid plastic, used rarely and for specific reasons.

The walls do most of the work. A print with three or four perimeters and 15% infill is surprisingly strong because the outer shell, not the core, resists most bending and impact.

Why solid is usually a bad idea

Printing an object at 100% infill sounds like the "premium" option, but it typically makes the part worse:

• It takes far longer. Filling every cubic millimetre can triple print time compared with 20% infill.
• It uses much more material. More plastic, higher price, heavier object to ship.
• It warps and cracks more. Solid masses of plastic shrink unevenly as they cool, which pulls corners up and splits layers. A lattice absorbs that stress.
• The strength gain is small. Going from 20% to 100% infill can double the material but adds far less than double the strength for most shapes. Adding one more wall perimeter often gives more strength per gram.

So when a studio prints your model at 15% infill, it is not cutting corners; it is choosing the setting that produces a better part.

Infill percentage and what it means in practice

Infill	Feels like	Typical use	Effect on price
0 to 5%	Very light, hollow	Vases, lampshades, large display shells	Lowest
10 to 20%	Light but solid-feeling	Figurines, book nooks, organisers, décor	Standard
30 to 40%	Noticeably heavier, rigid	Brackets, phone stands, tool holders	Moderate increase
50 to 70%	Dense, hefty	Load-bearing clips, gears, screw bosses	Higher
100%	Solid	Small mechanical parts, items that get drilled or tapped	Highest

Because material and machine time both climb with infill, this setting is one of the five factors behind every quote. Our post on how weight affects price shows how a heavier setting adds up.

Infill patterns

The pattern matters as much as the percentage.

Grid and lines. Fast to print, fine for decorative parts. Strong in two directions, weaker in the third.

Gyroid. A wavy, three-dimensional lattice that is equally strong in all directions and lets liquid drain. It is the modern default for functional parts and also looks beautiful when printed translucent.

Cubic and cubic subdivision. Stacked tilted cubes; good all-round strength and a smooth top surface.

Honeycomb. Strong and rigid but slow. Often used for parts that must not flex.

Lightning and support-only. Extremely sparse; only exists to hold up the top surface. Ideal for big display pieces where weight and speed matter and nobody will squeeze the part.

For a flexible TPU part, a sparse gyroid infill makes the object squishier; for a rigid one, denser cubic makes it stiffer. Same material, different feel.

When you should ask for more infill

Tell the studio if your object will be:

• Screwed or bolted. Screw threads need dense material to bite into, at least around the holes.
• Load-bearing. Shelf brackets, hooks, clamps and anything holding weight should be 40% or more, with extra walls.
• Drilled, sanded through or machined after printing. Sanding can break into sparse infill and leave holes. A thicker shell or higher infill prevents that.
• Weighted for feel. A chess piece or trophy base sometimes benefits from higher infill just so it feels substantial. See our post on how strong 3D printed parts really are for the wall-versus-infill trade-off.

For everything else, the default is right. Fantasy Lab 3D sets infill and wall count per object, so a book nook wall, a chess knight and a drone mount each get their own settings rather than one generic number.

Ready to order?

You do not need to specify infill yourself; just tell us what the part is for and whether it will be handled, loaded or painted. Reach out through the contact section or DM us on Instagram @fantasylab3d.almaty. The price depends on size, material and settings, so request a quote and we will explain what we chose and why.