Heated bed/Cooling fans

Hi all!
I am a 53 yr. old design engineer and have been using a Makerbot 2 for a few years. I am new to building my own printer but have decided
to put together a Delta from scratch. I am working on designing the print bed and from reading posts concerning print bed heating it seems
clear that to prevent curling I need to use a heated bed. What I don't quite get is why it seems that most printers have a fan blowing on the
part as it is being extruded to cool it. It seems counterintuitive to heat the bed and then blow on the part to chill it.
Anyone care to help me understand this?

I would really appreciate a little help with this concept. :-)

Kory

PLA needs to be solidified out of the nozzle otherwise it gets distorted easy, The heatbed will have less effect on higher layer's. too much heat from the bed can cause warping.

As he said.

Also, PLA usually has a heatbed temp of around 50C - 60C, which the bed heater will have no problem maintaining, even with a fan blowing. ABS needs a heatbed temp of around 100C that is harder to maintain, but rarely needs a print cooling fan so the issue doesn't arise.

Thank you both. It is a good bit clearer now.

KK

With PLA, the heatbed is used because the PLA sticks better to the bed with it. But the heatbed is not essential to print PLA: with some surface material (special build plate, hair spray, paper glue...) heat is not needed.
The heatbed is essential for ABS, to maintain the piece warm and avoid warping. And for ABS, you must not use the fan. The best is to have your printer in an enclosure.

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Prusa i3 - e3d v5 - Gnu/Linux - Pronterface - Slic3r - Octoprint - Rpi - French

The cooling fan is essential for PLA when you print small layers, i.e. short layer time, as the heat does not dissipate into the air quickly enough and the part can end up looking melted. I tend to keep the air flow small and very directional using a cowl with a nozzle pointing at the part this does not cool the bed too much. I remixed a couple of other parts to make a compact fan cowl for a Delta with e3d hotend here it works pretty well, with very low mass. [www.thingiverse.com]

With ABS it isn't as important to have a parts fan, but print very slowly when printing small layers and if you use a fan make sure its blowing from several, ideally opposite directions at the same time, or a circular fan cowl to prevent the part from bending toward the fan as it cools.

Edited 3 time(s). Last edit at 10/26/2015 02:23PM by DjDemonD.