Welcome to the SeeMeCNC Community

Just got my RostockMax up and running. But there are some problems I couldn't detect the issue.

When I tried to print the Hollow Cube, I couldn't get rid of the gap as shown on following picture. Also there are always some chunk in between two builds. Is it something about Retraction?
[img]https://lh5.googleusercontent.com/-lIGJ ... AG2075.jpg[/img]

Also, it's pretty obvious the shape was twisted, I tried to re-calibrate for many times but got same result:
[img]https://lh5.googleusercontent.com/-700J ... AG2076.jpg[/img]

Anyone experienced these issues?

You're gap is a sign you need to work on calibrating your
1) E-Steps per mm on the extruder
2) Extrusion Temperature
3) Flow rate (depending on what slicer you are using it's several settings that calculate this such as nozzle diameter, filament size setting, and the various terms each slicer uses for a general compensation constant)

As for the twist, make sure you check your towers again on at least 3 out of four of the sides of each tower. After that begin looking into the delta arm blues threads.

These are just likely problem areas for you to look into, it isn't a guarantee that is the source of your issue. Also, don't get too enthusiastic and try to fix everything at once (it's difficult...so so difficult...), be systematic so you know exactly what fixes what. It's just such a critical learning step in the early stages of setup that will help you later (not that you don't know that, but it is worth noting if it helps someone searching threads later).