When a part is thin and the overall layer time decreases below a certain threshold, the plastic has no time to cool down enough to solidify and properly support the next layer, resulting in low print quality. When the layer printing time is higher the issue does not exist, plastic cools down by itself due to the cooler environment.
Slicers also have the option to slow down printing when the layer time decreases below a certain threshold, so that there is more time to cool the part, but this only down to a minimum printing speed.
Assuming a variable fan speed is used, the following parameters play a role:
- layer time threshold to turn on the fan at the minimum speed
- layer time threshold to set the fan to its maximum speed and to start slowing down print speed
- minimum printing speed to be used for layers smaller than the threshold number 2. (below this threshold 3. the layer time starts to decrease again)
Obviously these parameter depend on the enclosure temperature, fan power, print temperature, flow rate, and so on. I'm assuming they are constant.
Well, while we have very clear methods to calibrate most aspects of a printer (max extrusion rate, temperatures, E steps, ...) I haven't seen any procedure to calibrate all the cooling settings.
Often you see people (even experienced ones) use a temperature/fan tower to pick a fan speed, but this is not really correct: there's no point in using fan for big layers, while a single fan speed won't be enough for small layers.
Ideally, with optimal calibration, the part strength is not lowered by the use of a fan.
How to systematically calibrate the above-mentioned parameters used to set variable fan speed?
Note: this is NOT about calibrating fan speed for bridges, which requires a different procedure, since excessive fan speed may blow away the bridge itself.