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eldrick

Why is hotend fan wasting half of its output ?

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I don't think the hot end is wasting nearly half of the fan output even in the worst case, as the fan is anyway cooling down the aluminum structure. As aluminum conducts heat pretty well, the lone extruder benefits from being the only heat-producing element in the print head.

However, I see your point and I'm quite interested in the topic especially due to the closed chamber I have built for my UM2. Because of the closed chamber, I'm now a bit less worried about additional fan noise in the print head, as high-frequency noises get fairly well eliminated by the dome I have built. For this reason I have decided to try one of these:

http://www.yeahracing.com/catalog/yeah-racing-hacktronic-tornado-high-speed-ball-bearing-25mm-htn304-p-3471.html

I haven't received mine yet, but I'll let you know about my experience once I have incorporated one to my UM2 print head.

 

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Many bad prints in years past were because the fan blew on the nozzle tip instead of the part. The nozzle tip always gets some wind but if it gets too much you get underextrusion issues because it is colder than the heat chamber.

This was particularly a problem when Ultimaker was testing their new UM2 design (before it came out) and Cura changed the way the fan is turned on quite a bit - it now comes on slowly so that the PID temperature controller doesn't allow the head to cool too much when they first come on. Also there was an issue related to air bouncing off the glass and hitting the nozzle.

So the cooling of the white teflon part is instead cooled by the 3rd fan on the UM2.

 

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Still thinking about this...

"So the cooling of the white teflon part is instead cooled by the 3rd fan on the UM2."

So if it gets more cooling, no harm, right? I'm already using one of the alternate fan housings from Youmagine, which does a better job of avoiding cooling the tip, and actually seems to let me print with lower fan speed.

With a deflector to put more of the fan output onto the teflon piece, I'd also block the second extruder hole, which could be contributing to cooling the nozzle if the stock metal shroud was used.

Hmmm - perhaps I should use a thick piece of alu as a deflector, to let it help cool the alu bottom plate...

 

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it occurs to me that a partial fan duct for the 25mm fan could perhaps draw more air from above, rather than sucking in air from directly above the heated bed and hot-end.

 

If you blow through a straw the air coming out the far end will go very straight at first and you get a kind of rocket propulsion like effect and you can feel the wind 5 feet away.

If you suck through a straw there is no directionality at the far end of the straw - air come in equall from all directions. All 360 degrees. Even back along the straw!

So trying to get the fan to suck cooler air might not help as much as you think.

Anyway the hot end can easily be at 260C (for printing nylon). Although the teflon can handle 260C it is more likely to deform under pressure when at temperatures at 250C or higher. The heated bed is typically 50-75C for PLA and 90-110C for ABS. Even 110C is so much cooler than 260C that I don't think it matters too much if you use 110C air versus 20C air as long as you have good air movement. I could be wrong.

I think that the angled thing that aims the air at the teflon shouldn't hurt much and I agree, the left side of the head is the hotter side. But it's possible you are interfering a bit with the airflow and reducing the effectiveness of the 3rd fan but I doubt it - I bet its fine. Many people print without realizing their 3rd fan never worked and discover it when they complain that the PLA is getting soft high up in the print head and getting stuck up there (especially with lots of retractions).

So maybe that deflector will help keep you from getting jams higher up in the bowden. It's never been a problem for me on UM Original nor UM2.

 

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