(1) I've heard the origin of such a problem could be the under-use of one of the Black channels - say MK in this case. If one goes for long periods of time without printing matte is it possible that there could be some critical arteriosclerosis in the transportation route from cartridge to printhead? In particular the ink switch mechanism?
As I think most large format print makers know, manufacturers all indicate that 6 months the guideline time in which to use an ink cart and 12 months pretty much the maximum suggested time to let a cart be open and plugged in. This is to maintain quality and ensure sufficient ink flow through the lines, regardless of manufacturer. Some people regularly go beyond this without issue, but those are the guides. If you go a long way past this, though, then there is a risk that the ink deteriorates and can lead to blockages or air bubbles and so on. It's not particularly an issue that I've seen in the switching mechanism for printers with them, but that is likely going to be a place where little blockages or air bubbles might get caught and cause a blockage because, well, physics :-)
(2) Although not stated in the manual for either the 4900 or the 5000, would owners be advised to do a periodic ink switch and print run just to keep both lines "juiced"? If so about how often would be safe in a dryish environment?
I think it depends on what the users are doing. If you never intend to swap, I wouldn't bother. If you swap because of need, then you are almost certainly swapping enough to prevent any issue. I don't think you need regular attention to this UNLESS you never swap but think you might want to years later or that you may want to sell second-hand to someone and ensure they have the option to swap. Basically, those who are affected by this are a very, very small percentage because most people fall into one of the base categories of "never" or "from time to time" and neither of those will encounter an actual problem.
If you really just wanted to protect the lines for possible future use, I'd go with a monthly swap and nozzle check, but that's not based on any real experience doing it because, really, almost no one would never need to do this (and certainly I never have in my usage).
(3) Would a succession of powerful cleans of that channel pair, interspersed with purge prints, even over a period of several days usually suffice to safely restore ink flow, rather than prematurely calling in a technician (which tends to be costly in labour charges and wasted ink)?
Maybe. But if you realise you're in this possible situation (and any machine with the original cart in it, unused, is a candidate) and you're wanting to protect the head, calling in a tech to do it properly (and actually have a look at the lines, maybe even replace them or flush them to prevent any rubbish getting near the head) is going to be a lot cheaper than replacing the head or the printer.
If the printer isn't showing ink flow after two or three heavy cleans on that line, I'd be concerned and get a tech. And, as I said, if I knew the situation with the printer, I'd maybe even call one before even switching.
Mark
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