Problems with HIPS and ULTRAT

Good morning,

I would like to start printing some pretty large objects (diameter around 160mm) and I've been told to use either ULTRAT or HIPS to prevent warping and defects on bottom layers. I've been running some test prints and I encounter two problems with these materials:

HIPS: The external layer does not mold with the bottom layers and its makes very fragile prints. Zcode:4238


ULTRAT: I'm experiencing pretty large amounts of stringing with this material only and I fear it may affect the precision of the prints. Zcode: 4239


The infill is set to medium on both prints, with minimal layer thickness. The fan speed is set to 100% for ULTRAT and auto for HIPS i think.

Here are pictures of the test prints:


Image uploaded from iOS.jpg


Image uploaded from iOS (1).jpg

Do you guys have any idea how to solve these problems? 

Thank you!