Slicer settings that matter

A slicer is the program that cuts your 3D model into layers and turns each layer into toolpaths for the printer. It exposes several hundred settings, and beginners either leave them all at default or change ten at once and learn nothing. In practice about ten settings decide almost everything: how strong the part is, how long it takes, how much plastic it uses and how good it looks. This lesson explains those ten and, more importantly, why each one behaves the way it does.

Layer height and line width

Layer height is the thickness of each slice. A useful range is 25 to 75 percent of the nozzle diameter, so 0.1 to 0.3 mm for a 0.4 mm nozzle. Thin layers hide the stair-stepping on curves, but print time grows roughly in proportion to the layer count: a 0.1 mm layer takes about twice as long as 0.2 mm. For robot parts that nobody will admire, 0.2 mm is a good default, and 0.28 mm is fine for drafts.

Line width is how wide the extruded line is. Slicers default to about the nozzle diameter, or slightly more, such as 0.42 to 0.45 mm. Wider lines overlap neighbours better and make stronger walls. The nozzle squashes the plastic into a flattened oval, so the line is wider than the layer is tall.

These two numbers fix how much plastic the printer must push per second, called the volumetric flow:

flow = line width x layer height x speed = 0.4 mm x 0.2 mm x 60 mm/s = 4.8 mm3/s

A common hotend melts roughly 10 to 15 mm3/s of PLA before the plastic stops melting fast enough and the extruder under-extrudes or clicks. Raising the layer height to 0.3 mm and the speed to 100 mm/s would demand 12 mm3/s and hit that wall. This is why fast, thick prints need a bigger nozzle or a high-flow hotend, and why speed is never a free setting.

Walls, top and bottom, and infill

Walls (also called perimeters) are the outer shell. With a 0.4 mm line, three walls make a shell 1.2 mm thick. Walls matter more than infill for strength, because when a part bends, the outer material is farthest from the centre and carries the most stress. A robot bracket with 3 to 4 walls and 20 percent infill is usually stronger than one with 2 walls and 60 percent infill.

Top and bottom layers close the part. Use enough that infill does not show through: with 0.2 mm layers, 4 to 5 layers give 0.8 to 1.0 mm.

Infill is the internal structure. Percentage controls density, and pattern controls how the load is spread. Grid and lines are quick. Cubic and gyroid are three-dimensional patterns with no weak direction, so they hold up under loads from any side, and gyroid also prints without sharp turns. Typical choices are 15 to 20 percent for covers and 30 to 40 percent for parts that take screws or loads. Above about 50 percent you pay a lot of plastic for little extra strength. If one area needs to be solid, such as around a screw boss, add walls there rather than raising infill everywhere.

Supports, temperature, speed and cooling

Supports are sacrificial scaffolding for overhangs beyond about 45 to 50 degrees from vertical. They cost plastic and time and leave a rough surface where they touch, so Lesson 3 shows how to design them away. When you need them, tree supports touch the part at fewer points and are easier to remove.

Temperature. Too cold, and layers fuse poorly and the part splits along layer lines. Too hot, and the plastic oozes, strings and sags on overhangs. Print a temperature tower for each new spool: it steps the nozzle down by 5 degrees every few millimetres of height, and you pick the best-looking section.

Print speed. Slower is more precise and gives each layer time to bond. 50 to 60 mm/s for PLA and 40 to 50 mm/s for PETG is a safe start. Outer walls should be slower than infill because they set the surface quality.

Cooling. A part fan solidifies the plastic quickly so overhangs and bridges hold their shape. PLA wants close to 100 percent after the first few layers. PETG bonds worse when over-cooled, so 30 to 50 percent works better.

Recommended starting values

Treat the table as a first guess and adjust it using the spool manufacturer's printed range, a temperature tower and your own test parts.

How much plastic will it use?

The slicer reports the plastic volume for the sliced part, after walls and infill. To turn volume into something you can buy, you need the cross-section of the filament, which is a circle of diameter 1.75 mm:

area = pi x (1.75 / 2)^2 = 2.405 mm2 = 0.02405 cm2

length = volume / area, mass = volume x density, cost = mass x price per kg

Worked example. A motor bracket sliced in PLA uses 18 cm3. PLA has a density of about 1.24 g/cm3 and a spool costs 20 USD per kilogram.

• Length: 18 / 0.02405 = 748 cm, which is 7.48 m
• Mass: 18 x 1.24 = 22.3 g
• Cost: 22.3 / 1000 x 20 = 0.45 USD

A 1 kg spool therefore holds about 1000 / 1.24 = 806 cm3, which is roughly 335 m of filament. The same volume in PETG (density 1.27) weighs 22.9 g, because PETG is denser, and in ABS (1.04) it weighs only 18.7 g.

Try your own part below. Change the volume, the density or the price and run it.

filament_cost.py

ResetRun

import math def filament_use(volume_cm3, density, diameter_mm=1.75, price_per_kg=20.0): # cross-section of the filament in cm2 (radius in cm = diameter / 20) radius_cm = diameter_mm / 20.0 area_cm2 = math.pi * radius_cm ** 2 length_m = volume_cm3 / area_cm2 / 100.0 mass_g = volume_cm3 * density cost = mass_g / 1000.0 * price_per_kg return length_m, mass_g, cost volume = 18.0 # plastic volume reported by the slicer, in cm3 materials = {"PLA": 1.24, "PETG": 1.27, "ABS": 1.04, "TPU": 1.21} for name, density in materials.items(): length_m, mass_g, cost = filament_use(volume, density) print(f"{name}: {length_m:.2f} m, {mass_g:.2f} g, {cost:.2f} USD")

The length is the same for every material, since it depends only on volume and the filament diameter. Mass and cost change with density. The printout for PLA reads 7.48 m, 22.32 g and 0.45 USD, matching the hand calculation.