problems with printing quality - two cooling fans?

Hi,

I am quite new in the printing business and about to calibrate my Mendel Prusa.
I use a Wades Extruder 0.5mm nozzle with 3mm PLA filament, Slic3r for generating G-code.
So far I printed with layer heights of 0.4mm or 0.3mm.

I tried to follow the calibrations steps on:
[reprap.org]
but have troubles getting the things nicely printed.

The filled box became resonably nice, but the problems started with the thin wall object (see attached photo).
The problem seems to be that the fresh PLA layers are put down on other layers which are still too hot and soft.
Hence I installed a 40mm fan continuously cooling the nozzle, with the result that the wall facing towards the fan is nicely printed (therefore confirming my suspect that it had to do with the temperature). However, the other walls are still quite messy and warped. Also lowering the extruder temp doesn't help a lot. I am printing at 175°C at the moment (the bed is at 60°C) I don't dare going much below that.

Also, the 50mm-tower comes out not very nice (see pic), altough much better than without fan (then it was just a blob of plastic)

The hollow box (again see pic) as bridging test is typically a complete mess (the photo is my very best result :-) ), fiddling with the Slic3r settings for bridging didn't give any improvements. (Though I read another thread on the Forum saying that Slic3r is generally not quite good for bridgings, which might be another reason(?))

Finally, I attached another pic of a recent failure. There, the thin wall seperating a 8mm hole from the surface broke open. Again I suspect that it has to do with temperature problems.

I am a bit puzzled since other reprappers seem to get really nice results with quite similar set-up and settings. I am a bit clueless about which parameters are left to optimize now. Changing ESteps doesn't seem to improve things as well. Is it possible that my PLA has bad quality?
Especially for the 50mm tower the calibration instruction says that one should lower the extruder temp. until something nice comes out. I don't think this works for me, although I have the cooling fan installed.
I thought about installing a second cooling fan on the opposite side, but so far I haven't read about anyone else who found this to be necessary.

Any ideas are greatly appreciated.

How fast are you printing? Small objects like the 50mm tower can require speeds as low as 10mm/s to give the layers a chance to cool. Two fans can also help. Or try printing two objects at the same time, spaced a little ways apart.

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For pla, are you able to lower the temp after the first few layers? and also is 175 the lowest you can print reliably? different plastics of pla have slightly different temps.

also does the slicer you use have a feature to cool down layers that dont have enough time to cool?

here is a thread thatr troubleshooted the cooling feature:

[forums.reprap.org]

I'm new to all this so I'm just guessing based off my reading/research.

Maybe 60degC bed temperature too high?

I thought a lot of people were printing PLA without a hotbed at all.

60° is good for first layer. After on my prusa I use 50°C

I Have a config with 2 small fans btw, mounted at 90° from each other. But while one is oriented toward the noozle, the other's job is to blow into the vents of the JHead.

O.4 layer with a 0.5 noozle is the maximum you should use (0.8x), and if you use 100% infill, the 50 mm tower is also a measure of the accuracy of extruder steps, much more precise thant the 20mm solid cube.

Think I solved the problem (at least for most situations):
I have a second fan installed, but the biggest improvement was made by the newest version of Slic3r, which seems to be more clever than the old version I used before. It seems to drive more slowly when needed.

for PLA i have a 120mm fan blowing across the heated bed with some tape over the bottom of the 120mm fan so its not blowing on the bed directly.

http://pic.twitter.com/tWjIxi7uqE

I wonder if its a good idea to print out a set of fan barn doors..attach them to the front of a fan to help control direction of airflow