PrusaSlicer 1.39.2-beta

Slic3r Prusa Edition Slic3r Prusa Edition 1.39.2-beta

Summary:

The Slic3r 1.39.2-beta improves the cooling slow down logic (the "Slow down if layer print time is below certain time" feature). We have found out, that the cooling slow down logic in Slic3r 1.39.1 does not serve our Prusa i3 MK3 printer well, leading to uneven extrusion of external perimeters (see the issues github.com/slic3r/Slic3r/issues/4070, ↗ #536, #806, #841, prusa3d/Prusa-Firmware#602).

An FDM printer lays down new extrusions over another already extruded and solidified layer. When printing small objects, a single layer is extruded too quickly to cool down and solidify, which Slic3r compensates by slowing down the print to give each layer more time to cool down. Without the cooling slow down, the print surface quality and dimensional accuracy would likely suffer, especially on overhangs.

The initial cooling logic before Slic3r Prusa Edition 1.34.0 slowed down all extrusion moves proportionally until the minimum layer time was achieved. This is actually the strategy commonly used by other slicers. In Slic3r PE 1.34.0, we adopted the cooling logic improvements by @alexrj. This new strategy strived for a constant external perimeter speed while slowing down the other extrusion moves proportionally as before. Only when the minimum layer time was not reached, the external perimeters were slowed down as well.

While this new strategy sounds logical, it lead to wild changes in extrusion velocity rate for our Prusa i3 MK3 printers, as we extrude the infills much quicker than the perimeters.

The new strategy slowed down proportionally the infill and inner perimeters first, slowing down the very fast infill to a quite fast infill, quite slow inner perimeters to extremely slow inner perimeters, and leaving the external perimeters intact. When the extremely slow inner perimeter extrusion was finished, the nozzle pressure was very low, leading to uneven extrusion and gaps at the start of the much faster external perimeter.

This leads to an extruder pressure deficit, which shows up as gaps at the seams, and inconsistent extrusion starting with the seam.

We have reworked the cooling slow down logic to fit our printers better. The reworked cooling logic does not slow down the extrusion moves proportionally, but it slows down the fastest extrusions first (usually the infill) until it matches the second fastest extrusion (usually the inner perimeters), then it slows down the infill and inner perimeters in tandem, until they reach the extrusion speed of the outer perimeter, and all the extrusions move with the same speed. From that point all the extrusions are further slowed down simultaneously.

This reworked logic leads to lower changes of nozzle pressure, therefore the print surface quality improves.