Ivor, I've built 2 Mendel 90s with my Community College class (near Boston) and

3D Printing
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1

Ivor,
I’ve built 2 Mendel 90s with my Community College class (near Boston) and we are troubleshooting some issues. You seem to have a lot of knowledge so hoping you can help. I recognize our questions may be old and basic, but this is a new challenge for us.
We have Melzi board with cooling fan and duct that directs air around hot end and at target printed object. We are using a 1.75mm/0.4mm hot end (we chose this since we use Makerbots in our Lab and have lots of the Makerbot material).
Hot end seems to work fine, gets to target temp quickly (e.g. 220 C). It even melted our first PLA cooling duct (we are printing a hi temp version of duct to fix this). However, when we turn on fan, we cannot get temp above 135 C!
We are using Digikey 259-1687-ND fan with Air Flow 25.2 CFM and RPM 5400. I suspect this fan is too strong.
We tried changing Marlin/Arduino code (0 to 255 setting) but no speed control and fan shuts off below 240 setting. I see some cooling settings in slice3r but I suspect they will not work for same reason as Arduino change does not work? To solve our problem, I think we need to be able to control the fan speed, then find the speed value that allows us to cool the part being printed without dragging down the hot end temp.

Questions:
What is correct fan (CFM/RPM) for this purpose?
Is there additional code (Python?) we can use for speed control?
Do we need hardware fix for speed control (Cap and diode)?
Is fan really needed for cooling hot end, or just for printing PLA?
I assume we should somehow insulate hot end?
Anything else we should be looking at to get around this issue?

Thanks in advance for any help.
Damian

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Your fan is probably fine but you need to protect your hot end from the turbulence, and probably retune the hot end PID with the fan on. I don’t know the Melzi board so I can’t tell you if the fan output lets you change the speeds.

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If you have followed nophead’s original design you will notice that he wrapped the J-head block in silicone tape.

Additionally, his duct directs a small amount of air onto the silicone tape to disrupt heat that is convected upwards from the nozzle.

I use nophead’s standard extruder and duct setup on my Mendel90 but I have changed the fan to a Gelid Silent 6 model. nophead mapped cooling airflow some time ago and compared his fan and the Gelid one with the Gelid giving a slightly better airflow coverage.

I run the latest Marlin RC on my Mendel90 and have the cooling fan configured to give an initial fast run and a minimum of 16 for speed. This allows me to drop the cooling fan speed down to a very low level without any hardware modifications.

The hotend, in nophead’s configuration, does not require cooling for printing ABS it is primarily provided for printing PLA.