Most FDM parts are not solid. Inside the outer walls sits a lattice called infill, and how much you use has a big effect on strength, weight, print time and cost. More is not always better.
Density and pattern
Infill density is usually given as a percentage. A display model might need only a little to hold its shape, while a load-bearing part benefits from more. Beyond a certain point, though, extra infill adds time and material for diminishing returns, since most strength comes from the perimeters.
Pattern matters too. Grid and gyroid patterns spread load evenly and print efficiently, which is why they are common defaults for functional work.
Choosing sensibly
For a strong part, it is often better to add a wall or two than to crank the infill to full. We tune walls and infill together to hit the strength you need without wasting material.
Not sure what a part needs? Send it through with a quote and we will recommend settings to suit.