You set print speed to 300 mm/s in the slicer and nothing changes. It happens to everyone, and the reason is that your speed is not set by the number you type. It is set by how much plastic the hotend can melt per second, which is volumetric flow, measured in mm³/s.
Once you are against the flow limit, raising the speed setting does nothing; the printer is already at the end of what it can do.
The calculation
The plastic leaving the nozzle each second comes from three numbers:
flow (mm³/s) = layer height × line width × speed
0.2 mm layer, 0.42 mm line width, 150 mm/s:
0.2 × 0.42 × 150 = 12.6 mm³/s
Now compare that against what the hotend can actually deliver. Roughly:
| Hotend | Typical limit |
|---|---|
| Standard 0.4 mm, PLA | 8-15 mm³/s |
| High-flow hotend, PLA | 20-30 mm³/s |
| 0.6 mm nozzle | Higher, more heated surface |
| PETG | Below PLA |
The 12.6 mm³/s above is at the edge for a standard hotend. Go to 200 mm/s and you are asking for 16.8 mm³/s, which it cannot supply: under-extrusion, gappy walls, weak layer bonding.
The filament has a limit too
The flow ceiling does not come from the hotend alone. The filament itself stops melting all the way through above a certain rate.
Some manufacturers give a maximum volumetric speed on the spool or datasheet. If not, Orca Slicer's Max volumetric speed test measures it: it prints a wall at increasing flow and you look for where it breaks down.
Enter that value in the slicer and the printer will not exceed it, whatever your speed settings say; it throttles itself. That is the right way round: leave speed free and cap the flow.
The number the test gives is the limit where extrusion breaks down; how far below that limit to stay on a load-bearing part is in volumetric speed and part strength.
How nozzle diameter changes speed
Going from 0.4 to 0.6 mm widens the line, so more plastic flows at the same speed. But the real gain is elsewhere: fewer lines, fewer layers.
With a 0.6 nozzle you can print 0.3 mm layers. The same part finishes in a third fewer layers at 0.3 than at 0.2. That alone cuts real time off the print, with the speed setting untouched.
The cost is detail. Fine text, sharp corners and small features disappear at 0.6. A good trade on a functional part, not on a miniature.
When walls are thin
A second forum thread runs into a related problem: thin walls on the model do not print at all, or come out gappy.
The cause is that the wall is thinner than the nozzle's line width. The slicer cannot fit a line in there, so it skips it.
In Bambu Studio and Orca the fix is under Quality:
| Setting | What it does |
|---|---|
| Wall generator: Arachne | Produces variable-width lines that adapt to thin areas |
| Minimum wall thickness | Above 85%: thickens walls that are too thin |
| Minimum feature size | Around 5%: forces small features to print |
Switching to Arachne alone solves most thin-wall problems. The classic slicer uses a fixed line width; Arachne thins and thickens the line as needed.
A model that looks complete in Prepare but comes out with pieces missing in Preview has the same cause; the forum case solved by lowering line width is in the skipped thin parts article.
Practical order
- Find or measure your filament's maximum volumetric speed.
- Enter it in the slicer.
- Leave the speed settings alone; the flow cap will do the braking.
- Want it faster? Raise layer height, not the speed number.
- Still not enough? Go up in nozzle diameter.
Fiddling with the speed number comes last on the list, because that is where it makes the least difference. Where to change speed is covered separately.
