Bubbling on flat parts on prints

[n108bg 0]

I have been operating an Ultimaker 2 for a few months now, and within the last two weeks, i have been noticing issues with my prints capping off. I will get prints where the top is bubbly, bulging up between the infill lines, and sometimes, there will be no material on top of these bubbles. I'm having a hard time figuring out the source of the issues, has anyone had a similar issue?

 

[IRobertI 521]

I'm guessing you're experiencing what we call "pillowing". In short, cooling, lots of it:

http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#pillowing

 

[valcrow 145]

Try increasing infil to 25%. This will make the infil lines double as close together compared to default 20% and pillowing will be greatly reduced/eliminated.