parts of the print higher than others

Dear Community,

I'm quite new in the RepRap business. I built myself a Prusa i3 with a Hexagon Extruder (very happy with the Extruder so far). I have only been printing PLA so far and had different problems that I was able to solve over the time and now I got some nice prints. However today I tried to print this Juicer: [www.thingiverse.com] and got a weird result. The part in the middle was far too high and I don't understand why. just too much material?! On the sides it was fine! (see pictures) and everytime it went to the middle it was pushed upwards and the rods were bent. Thank god I was around and had a look at it. Originally I was planning on printing it throughout the night. Any Idea where this could come from?

Also I printed a 20mm calibration cube with height 0.2mm and was very happy with the result except for the one egde were it retracted. This was quite ugly. (see third picture) Is this something I just have to accept or is there a way to improve it?


Thanks in advance for any help!
Cheers!

I get that sort of warping with the height being uneven & that edge if I don't cool my PLA fast. Maybe play around with your extruder temp & fan set up?

I don't have a fan facing the Bed. is that recommendable? also for ABS? I only have a fan to cool the extruder...

Fan is pretty much needed for PLA but a definite No-No for ABS. Best is one that is attached so it's pointed right at the nozzle like this one [www.lulzbot.com]