Walls and top/bottom layers: strength and clean surfaces
are explained briefly when you hover over or tap them.
Every printed part consists of a shell and a filling. The shell is the walls on the sides and the closed layers at the top and bottom. They determine how sturdy a part is and what the surface looks like, often more than the infill inside.
What are walls and top/bottom layers?
Walls (also perimeters or wall lines) are the outline paths the printer draws in every layer along the outer contour and around holes. The outermost one is the outer wall, which is the one you see later.
Top and bottom layers are the fully filled layers. The lower ones close the base, the upper ones the top. The infill sits in between.
In the slicer, the settings are named slightly differently depending on the program, for example “Walls”, “Perimeters”, “Wall loops” or “Top/Bottom shell layers”. The same thing is meant everywhere.
Typical values
The manufacturers’ default profiles are a good starting point. The Bambu Lab profiles for 0.2 mm layer height, for example, use 2 walls, 5 top layers and 3 bottom layers. Prusa generally uses at least 2 perimeters in its profiles.
| Use | Walls | Top layers | Bottom layers |
|---|---|---|---|
| Decoration, figures | 2 | 4 to 5 | 3 |
| Everyday parts, brackets | 2 to 3 | 5 | 3 to 4 |
| Load-bearing functional parts | 3 to 5 | 5 to 6 | 4 to 5 |
| Watertight parts | 3 to 4 | 5 to 6 | 5 |
The number of layers depends on the layer height. What really matters is the thickness of the top and bottom shell. At 0.2 mm layer height, 5 layers give 1.0 mm. If you print at 0.1 mm, you need twice as many layers for the same thickness. Many slicers therefore also have a minimum thickness for the top and bottom shells (for example “Top shell thickness”), which automatically creates more layers with thin layers. More on layer height in Choosing the right layer height.
Rules of thumb
- At least 0.8 to 1 mm thick on top. Less quickly leads to holes or infill showing through on the top surface.
- 0.6 to 0.8 mm is usually enough on the bottom. The base sits on the bed and does not need to bridge over the infill.
- More strength comes from walls. One extra wall usually brings more than 10 or 20 % more infill.
- Small parts benefit especially. Narrow ribs or clips often consist almost entirely of walls.
- Threads and screw holes: 3 to 4 walls, so that there is enough material for cutting a thread or driving in a screw.
Walls and infill belong together
The load in a component runs mainly through the outer skin. That is why walls are often more important than high infill. How much filling you need for which purpose and which pattern is suitable is explained in detail in Setting up infill.
For the top surface: little infill and few top layers are a bad combination. The top layers have to span the gaps in the infill. At 10 % infill or less, it is better to plan one more top layer. If the top surface still has gaps, Holes in the top surface will help.
Wall thickness as a multiple of the line width
A wall is exactly one line wide. The total thickness of the wall is therefore the number of walls times the line width. With a 0.4 mm nozzle, the line width is usually around 0.42 to 0.45 mm.
| Walls | Wall thickness at approx. 0.45 mm line width |
|---|---|
| 2 | approx. 0.9 mm |
| 3 | approx. 1.35 mm |
| 4 | approx. 1.8 mm |
This matters if you design parts yourself. A wall in the CAD model that is, say, 1.0 mm thick does not fit 2 lines cleanly. Older slicers fill the rest with a thin gap fill or leave it empty. Modern wall generators such as Arachne (available in PrusaSlicer, OrcaSlicer, Bambu Studio and Cura) adjust the line width flexibly and cope better with such dimensions. Even so, wall thicknesses that are a multiple of the line width print the cleanest and the strongest.
Wall order: inside to outside or outside to inside?
Most slicers print the inner walls first by default and then the outer wall. There are three common variants:
| Order | Advantage | Disadvantage |
|---|---|---|
| Inner, then outer | best overhangs, because the outer wall finds support on the inner one | outer wall is pushed slightly outwards, dimensions marginally larger |
| Outer, then inner | more accurate dimensions, cleaner outer surface | weaker overhangs, seam can become more noticeable |
| Inner, outer, inner | good surface with decent overhangs | needs at least 3 walls |
Rule of thumb: for parts with overhangs, stay with inner to outer. For fitting parts where tenths of a millimetre matter, try outer to inner or the inner, outer, inner variant if your slicer offers it (for example OrcaSlicer).
Other useful settings
- Only one wall on top surfaces: useful for small lettering or narrow areas that would otherwise consist only of wall lines.
- Reinforcing walls in certain areas: With modifiers or region settings, only certain spots get more walls, for example around a bore.
- Ironing: passes the hot nozzle over the top layer once more and smooths it. It costs time but is worthwhile for visible, flat top surfaces.
Printing watertight
A printed part is not automatically watertight. Tiny gaps remain between the round lines, and every layer starts anew at the Z seam. Snapmaker names the seam as the most common way for water to escape. With a few settings you can still get cups, vases or housings watertight.
| Setting | Recommendation | Source |
|---|---|---|
| Walls | at least 3 to 4, 5 to 6 are rarely needed | Prusa |
| Bottom layers | 4 to 5 | Snapmaker |
| Nozzle temperature | 5 to 10 °C above the profile or at the upper end of the spool specification | Prusa |
| Flow rate | 5 to 10 % more than in the profile, alternatively increase the line width by 5 to 10 % | Prusa |
| Layer height | no more than 60 to 65 % of the nozzle diameter, 0.2 mm is enough with a 0.4 mm nozzle | Prusa |
| Overlap of infill and wall | if the base leaks, raise from 10 % to 25 to 35 % (PrusaSlicer) | Prusa |
A few notes on this:
- Walls instead of infill: Prusa advises against thick walls with infill in between. Water can run into the cavity between the inner and outer wall, and dirt can collect there. Even, solid walls made of several wall lines are better.
- Uniform shape: According to Prusa, protrusions or small details on the wall are typical leak points.
- Vase mode: Vase mode prints a continuous spiral without a Z seam. Snapmaker recommends it only for simple vessels that are open at the top, not for parts with a lid, a flat top or mechanical details.
- Material: Snapmaker names PETG and PP as well suited. According to Snapmaker, PLA absorbs moisture over time and is better suited to short-term uses.
- Coating: If a part must never leak, Snapmaker recommends an inner coating of epoxy resin. For non-critical parts such as flower pots, Snapmaker says 2 to 3 coats of clear spray varnish are enough.
- Testing: Stand the part on a dry paper towel, fill it with water and leave it for 2 to 3 hours. If the paper stays dry, the part is watertight.
Watertight does not mean food-safe. Prusa expressly points this out.
Where the seam is least disruptive is explained in Hiding the Z seam. More on flow rate in Calibration.
Common mistakes
- Too few top layers with low infill: The top surface gets holes or grooves.
- More infill instead of more walls: Costs more time and brings less strength.
- Too many walls on small parts: The part becomes almost solid, prints slowly and, with shrinking materials, is more likely to warp.
Continue with: Setting up infill and Hiding the Z seam.
Sources
- Watertight prints: walls, temperature, flow rate, layer height, overlap, geometry, note on food safety: Prusa Knowledge Base: Watertight prints
- Z seam, bottom layers, vase mode, material, coating, paper towel test: Snapmaker: How to Make 3D Prints Watertight