Is a fan needed?

How important is a fan on a Mendel? I gather that this is used to cool the extruded plastic faster so that it's hard enough before the next layer is extruded. And how do you mount a fan on the Mendel? I found one way in Thingiverse but there are no comments.

In Skeinforge, I can only see an option to turn the fan on at the start and off at the end. Is that at the start of the entire build or on layers where it's needed, for instance, on narrow towers?

I don't use a fan since I moved to heated beds. A fan makes the the object detach. Instead I slow down if the layer is less than 20s to make it last 20s and I make multiple objects if they are very small.

I have seen somebody on Thingiverse use a directed blower to just cool the object close to the nozzle. Ideally it would blow air at the same temperature as the bed so as not to cool it.

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Thanks. I've noticed that the extruded filament size on my parts get narrower when I build bigger parts compared to small parts when I have cooling turned on with the slow down option in skeinforge. I adjusted the flow rate on the hexagon example in replicatorg but when I printed a larger part, the perimeter didn't fuse to the inner loops and I measured the width to be only 0.5-0.6 instead of 0.72 mm. I thought that this might be due to the change in feed rate/flow rate. So, I thought that using a fan with the orbit option might work better. Does this make sense? Maybe it will work even without a fan.

I just saw that ducted fan in Thingiverse. It looks like a good design. The air would pass the hot end so perhaps the air flow can be adjusted to get the right temperature.

The fan shouldn't affect how wide your extrusions are. The problem with orbit is that your nozzle will be dribbling the whole time it's moving around, and when you go to start the next layer the first bit of extrusion will have very little plastic. As long as your stepper extruder is slowing down properly with your feed rate, the extrusion size should always be constant.