The inks that you mention Sailor Doyou and Noodler's Red Rattler Eel have not been used in this pen, the only two inks have been Parker washable blue and a Diamine, the Parker caused no problems.
I didn't mean using them stains the pen - I mean using them removes stains - that's right, some other ink stains the pen, but using these inks removes that stain. There are threads about this - I have not experienced it personally, but AmberleaDavis has posted before and after pictures - but her pictures may have been a third "cleaning" ink, which is why I recommended searching before buying.
I think that the most likely cause of the cracking is due to the insertion of the nib and feed, I cannot imagine anyone being capable of putting so much pressure on the nib that it causes plastic to crack can you, really?
I'm strictly thinking of directional forces:
1) Removing / inserting the nib and feed when done correctly (straight out, with no lateral force (no "rocking")) would only stress the plastic by causing expansion-style pressure pushing out against the section all around. But it's designed to have those things inside, so that pressure alone, even repeatedly (within reason, of course), seems unlikely to cause cracking.
2) Applying any pressure to the nib while writing applies lateral force against a specific portion of the section - the only relief from that pressure is cracking.
Personally, I don't think either of these is likely - I just think #2 is at least as likely as #1 to cause problems. And yes, ideally, one is applying almost no pressure at all on the nib when writing. I wasn't suggesting either of these were or weren't the cause so much as exploring the comparative forces involved in each.
Speaking with my brother when this happened to my Ecos, he said it was a known manufacturing problem in all plastics dealing with the cooling process (if I recall correctly - but definitely a discrete portion of the process) and was probably caused by them not wanting to slow the process sufficient to reduce the risk - in other words, they chose a middle ground between speed and quality, called it good enough, and accepted that some barrels will crack and that it was cheaper to replace these than to alter the manufacturing process. Since he knows a lot more about these things than I do, and since others have posted information here about how to see the internal stresses happening in plastic that's just sitting there doing nothing (using a polarizing filter, as I recall), I tend to think this is the real cause, and those whose barrels crack just got unlucky.
Post #63, by "Drone" posted 13 November 2016 - 20:53 on this page: http://www.fountainp...en-magic/page-4