Slicer basics: layer height, walls, infill and speed, and what each one trades

The slicer is the program that turns a 3D model into layer-by-layer nozzle moves (G-code). It has hundreds of settings, but fewer than ten decide almost all of a part’s look, strength and print time. This lesson goes through them one at a time and says what each one trades for what, because in FDM nothing comes free.

Setting names follow PrusaSlicer, OrcaSlicer and Bambu Studio, which use similar terms. Cura names a few things differently but means the same. Numbers assume a 0.4 mm nozzle.

Layer height

The single most visible setting. Thin layers give a smoother surface, fainter layer lines, cleaner curves and steeper overhangs. Thick layers print fast.

The rule both Prusa and OrcaSlicer use: layer height between 20% and 80% of the nozzle diameter. For a 0.4 mm nozzle that is 0.08-0.32 mm. Below that the extrusion gets inconsistent; above it the layers bond poorly. Prusa adds that going under 0.10 mm buys little quality for a lot of time.

Layer height (0.4 nozzle)	Use for	Trade
0.12 mm	Figurines, curved surfaces that must look good	Nearly double the time of 0.2
0.20 mm	General parts; the default in almost every profile	The balance point
0.28 mm	Boxes, brackets, parts nobody looks at	Obvious layer lines, worse overhangs

The first layer is usually set a little thicker than the rest, say 0.2-0.25 mm, to hide small bed imperfections (see lesson 9).

Line width

A 0.4 mm nozzle does not lay a 0.4 mm line. Slicers make it slightly wider so neighbouring lines overlap and fuse. PrusaSlicer defaults to 1.125 × the nozzle (0.45 mm) and recommends about 110%. OrcaSlicer gives ranges per line type: outer wall 105-120%, inner wall 120% or more, top surface 100-105%, and warns that past about 150% you get over-extrusion blobs.

Wider lines mean denser, stronger walls, but they demand more plastic per second (volumetric flow) and lose fine detail. Lines narrower than the nozzle bond poorly.

Walls

Walls (perimeters, wall loops) are the outline lines of each layer. Prusa states it plainly: a part’s strength comes mostly from its perimeters, not its infill. Prusa profiles use at least 2 walls; load-bearing parts use 3-4.

Real wall thickness is roughly wall count × line width (roughly, because adjacent lines overlap a little: two 0.45 mm lines at 0.2 mm layers come to about 0.86 mm, not 0.90). That is why models should have walls that are multiples of the line width, such as 0.9, 1.35 or 1.8 mm for 0.45 mm lines.

Top and bottom layers

The solid layers that close the part. Think of them as a thickness, not a count, because 3 top layers at 0.3 mm equal 9 at 0.1 mm. Prusa and OrcaSlicer both recommend at least 3, and if the top is too thin you see dimples following the infill pattern beneath; the sparser the infill, the worse the sag. Newer slicers have a “shell thickness” field in mm and work out the layer count for you.

Infill

Percentage. Prusa says most models print fine at 10-15% and that you “rarely need infill higher than 30%”. Infill adds resistance to compression, weight and support for the top layers, but it adds far less bending strength than walls, and it costs time and filament in direct proportion.

Pattern. From OrcaSlicer’s comparison table and Prusa’s descriptions:

Pattern	Strength	Time	Good for	Worth knowing
Grid	High in every direction	Medium	General parts that need to be strong	Lines cross within one layer; the nozzle bumps over crossings, is noisy, and some filaments fail there
Rectilinear	Medium-low	Fast	Parts under no load	Alternates direction each layer; weak vertical bonding
Gyroid	High, isotropic	Slower than grid	Parts loaded from several directions	Curved lines, no crossings, quiet; slow to slice
Cubic / adaptive cubic	High	Medium	Large parts	Adaptive thins out towards the centre, saving material and time
Lightning	Low	Fastest	Prototypes, decorative parts	Exists only to hold up the top surface; not structural
Honeycomb	High	Slowest	Visible infill	Double walls around each cell; the most material and the slowest pattern in this table

If in doubt, gyroid or cubic at 15% is a safe start for a general part.

Speed

Slicers set speed per line type. The principle from OrcaSlicer: the outer wall is slowest because it is the surface people see; inner walls faster; infill faster still; the top surface as slow as the outer wall for a smooth finish; the first layer slowest of all.

The real limit is not the mm/s figure but how much plastic the hotend can melt per second (max volumetric speed), which equals line width × layer height × speed. Raise any one and another has to fall. That gets its own lesson, number 18.

Worked example: one box lid, one setting at a time

A box lid 60 × 40 × 20 mm, 0.4 mm nozzle, 0.45 mm lines. Defaults: 0.2 mm layers, 2 walls, 4 top/bottom layers, 15% grid infill.

1. Layer count. 20 ÷ 0.2 = 100 layers. At 0.28 mm it becomes 20 ÷ 0.28 ≈ 72 layers, about 28% fewer, and time drops by roughly the same share, with more visible lines. At 0.12 mm it is 167 layers, about 67% more.
2. Walls. 2 × 0.45 ≈ 0.9 mm. If the lid snaps on tightly and gets prised off often, go to 3 walls ≈ 1.35 mm. Strength rises clearly; the extra time applies to the walls only.
3. Top/bottom. 4 × 0.2 = 0.8 mm. If you switch to 0.12 mm layers and leave the count at 4, the top is only 0.48 mm thick and will dimple over the infill. Use 7 layers (0.84 mm), or set shell thickness to 0.8 mm and let the slicer count.
4. Infill. The lid is only 20 mm tall and carries no load; 15% is enough. Going to 40% nearly triples the infill’s filament and time, while bending strength barely changes. What you gain is resistance to being pressed from above, which a lid does not need.
5. Pattern. Swapping grid for gyroid makes it quieter with no crossings, at about the same time.

Verdict for this part: keep 0.2 mm layers, go to 3 walls, 4 top/bottom at 0.2 (0.8 mm), 15% gyroid. You pay extra time on the walls only and get a lid that survives being prised off.

Common mistakes

• Raising infill to 50-100% for strength when walls are what you needed. It costs filament and time and warps more readily.
• Lowering layer height and forgetting the top/bottom count, so the top thins out and dimples into a grid pattern.
• Designing a 1.0 mm wall for 0.45 mm lines: two lines plus a 0.1 mm gap.
• Layer height above 80% of the nozzle: weak layer bonding, splits under load.
• Speeding up every line type equally. The outer wall should always be slower.
• Using grid with a filament that fails when the nozzle bumps the crossings. Switch to gyroid or cubic.

Settings sorted, but the part still lets go of the bed or lifts at the corners? The next lesson is about exactly that.

Sources

• Layers and perimeters, Prusa Knowledge Base
• Infill, Prusa Knowledge Base
• Creating profiles for different nozzles, Prusa Knowledge Base
• Line width, OrcaSlicer wiki
• Infill patterns, OrcaSlicer wiki