Dual extrusion - Inactive nozzle crashes into part

[kanDO_Bryce]

I have a Rostock MAX printer, which I have modified to run two extruders, one for ABS and one for HIPS.
I've set up the nozzles at the same height, so that they can both print a first layer that sticks to the heated bed.

However, when printing, the inactive nozzle 'crashes into' the material that the active nozzle has printed. This can have various effects: making a small dent, pulling a perimeter / skirt off of the bed, making big dents/ gouges in the part, or in the worst case dislodging the print from the bed.

See the photo here to show what I mean by the gouges in the part.
http://postimg.org/image/ut7fyqebp/

Has anyone else experienced this? And if so, how have you solved this problem?
The obvious, but complex solution is to actuate the nozzles, lifting each one up and out of the way when inactive. However I am not keen to do this as it is a considerable investment into a budget machine.

Note that I cannot lift one of the nozzles up, because if I do that nozzle will not successfully print a first layer that sticks to the bed. Plus that would worsen the crashing when that nozzle was in use.

Thanks in advance, would be interested to hear your opinions!

[Jimustanguitar]

I haven't done any dual extrusion, but it sure sounds like there's still a leveling problem.

I know you said the "inactive" nozzle is the problem, but Is it always the same nozzle dragging, or does it switch when you switch materials?

[JFettig]

Its definitely an issue and I didn't realize how big of an issue it is until I experienced it. I haven't been running dual nozzles for a little while because of it.

When I was, I found the most success to have the support nozzle a couple thousandths above the main nozzle, just slightly, I would make sure to print a raft so that it would stick better as HIPS wouldn't stick as good as ABS. I usually just drew in a paper thin raft to place under the support.

Make sure your support prints directly on the material below rather than skipping a layer.

[Polygonhell]

That's the big issue with dual nozzles and why I keep telling people they don't want to do it and why solutions like the cyclops are so appealing. The issue is most peoples builds aren't perfect enough to ensure the platform doesn't tilt very slightly across the bed and even is the build is perfect in that regard, the platform itself isn't held rigidly enough to prevent slight rotational movement especially towards the edges of the bed.

That means you can be perfectly adjust both nozzles in the center of the bed and still end up with one higher than the other as you move to the outside, and often which is higher will change depending on the position of the nozzle.

I ran dual extrusion using a kraken for a while and ended up moving the second nozzle out of the way when I wasn't using it, which because of this issue was most of the time. I recently pulled the Kraken off the machine and replaced it with an E3DV5.