Minimum Layer Time and Cooling Speed Limit

When a printer deposits a layer faster than the plastic can solidify, the still-molten material from that layer gets pushed around by the nozzle on the next pass. On small features — tower tops, slim columns, fine details — this causes drooping, stringing, and loss of dimensional accuracy.

Slicers address this with a minimum layer time setting: the printer will slow down or pause until at least X seconds have elapsed on a given layer. The resulting speed cap is:

max_speed (mm/s) = perimeter_length (mm) / min_layer_time (s)

Typical minimum layer times:

• PLA with good cooling: 5-8 seconds
• PETG (slower cooling, stickier): 10-15 seconds
• ABS/ASA in an enclosure (no part fan): 15-25 seconds
• Flexible TPU: 10-20 seconds

What counts as perimeter length. The slicer sums all travel in the layer: perimeters, infill lines, support. For a simple square tower at 20 x 20 mm with 3 perimeters and 0.45 mm line width, the perimeter alone is about 4 x (20 + 3 x 0.45 x 2) x 3 ≈ 270 mm. At a 10-second minimum, that is 27 mm/s — below typical print speeds and so the speed cap will trigger.

Why not just set minimum layer time to 1 second. Insufficient cooling means the layer below is still molten when the next layer lands. The compressive force from extrusion pushes the soft plastic outward, creating bulging sides and rough surface finish. Longer minimum times give cleaner small features.

Fan speed interaction. Higher fan speeds accelerate cooling significantly. With a strong 5015 blower at 100%, PLA can tolerate minimum layer times as short as 3-4 seconds for typical features. With a weak 4010 fan at 50%, 10+ seconds is safer.