I do that regularly.. key is to not transfer air and to measure the cartridges.. I weigh the cartridges.. full and empty. I chart them. when the new one gets low enough I draw out the remaining ink from the old one using a blunt 16 gauge leurlock tip on a 50cc syringe. Do not use a sharp tip, it will destroy the seal.. I pump ink into the cartridge only when it has enough room to accommodate the difference by weight.. After filling I draw out any air by inverting the cartridge.. with the port vertical. Sometimes takes multiple passes to fill and once air is evacuated you've got a functional cartridge.
I've seen Roland 440 ml cartridges have as much as 100ml of additional ink in them despite showing empty.. Seiko color printer routinely has 95 ml of additional ink when empty.
I bought that ink! I wanna use it..
Btw 1 ml is 1 cc..
I've never had an issue with color, quality or operation by refilling cartridges with the same ink brand, or by topping them off this way in 8 years, and I use a thorough digital quality control process for my color management.
But if your machine reads chips, then you have a more complex issue that goes beyond mere plumbing and juice.
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95 ML is about $30 Canadian. I guess per ink cart thats a lot... if you do a lot of printing. I just had to replace a head on our Color painter, cost $4000 though. I wouldn't risk messing with the ink/mixing old and new to save $30. I guess to some it's worth it, but there are risks in it. I'd feel safer going to a cheap, OEM brand ink, than re-filling carts that way.
You may want to check your seiko, btw. our h2-74s pretty much will suck the ink until its nearly bone dry. I never measured how much ink is left in it afterwards, but it feels close to air tight, and I cant feel any ink in it. Next empty bag, I'm going to cut up just for fun and see.
[Edit] I also wouldnt recommend this with UV ink. At least with our flatbed (Uses 4 liter bags) By the time one is reporting at 0, the ink is like a sludge. I wouldn't mix it with the new stuff... Theres always quite a bit left too. We just write it off as cost of doing business.