Why you need a fan on your printer
The three things I've learned for printing is calipers, calibration, and cooling. Having the proper tools and taking the time to calibration a machine is a must. But no matter how well calibrated a printer is, its almost useless if the parts don't cool enough while printing.
Here's a simple single wall test cube. The print on the left failed due to insufficient cooling time. It can be fixed by either slowing down the print from 60mm/s to 10mm/s or by adding sufficient cooling. The problem happens when the layers do not cool before another layer is printed. The PLA is still soft and starts to warp as more heat is added into the print. The temperature differential between the layers causes warping. By adding cooling fans or giving more time to cool per layer, the PLA hardens enough to stop warping. Made with Slic3r, 0.5mm nozzle, 0.25mm layer height. The only difference between the two is cooling.
Here's a print I recently did. Its a model of a Kuka KR180 robot. It has a few nice features, such as hidden screws and every axis is movable. Its still a little delicate though. You can find the model here on thingiverse. http://www.thingiverse.com/thing:4575#