Optimize (PLA) print

Update Slic3r
The first improvement in print quality was achieved by upgrading Slic3r from 0.7.1 to 0.9.8. As described in my post of last January 17.

Airflow
My first print was a glass from thingiverse. The foot printed fine. The stem was troublesome. I had to blow air (with my mouth) to get a reasonable quality.

Left blown by mouth, middle with small 40 mm fan, on the right with big fan mounted in the printer frame

When printing the stem, the nozzle keeps adding hot PLA to the same small spot without waiting for it to cool down and harden. I added a small 40 mm fan to automate my mouth blowing. When I tried the glass again with this fan in place, it turned out that the airflow of the small fan wasn't enough. See the glass in the middle on the image above. In this interview with Alessandro Ranellucci, the developer of Slic3r, I noticed that he had a fan mounted in the back of his printer frame. I took a fan scavenged from an old computer power supply and mounted it on the frame using some tie-wraps.

A fan is mounted on the printer frame using tie-wraps

I noticed that when  printing big objects (objects touch a lot of the printer bed surface), the corners will bend more when the fan is on. So for some objects you want the fan on and for others you want it off. I installed a switch to easily switch the fan on and off, depending on the object to be printed. I assume a heated bed will take care of the bending in case of printing objects that touch a lot of the printer bed surface, but for now switching off the fan works okay for me.

Finer steps
My printer has a 0.35 mm nozzle and I originally had the layer height set to 0.25 mm. You can set this value in Slic3r. I tried a value of 0.1 mm. Slic3r will do all calculations necessary to deliver the right amount of filament for the height and speed setting. The result was very smooth. Downside is the much longer print time.