Is it safe to refill ink cartridges before they are completely empty?
No, refilling cartridges prior to depletion might cause your printer to fail. The printer is designed for ink cartridges to be used until they are empty. If a failure occurs from refilling, you should insert a new cartridge — genuine HP or compatible — to continue printing. (Page 7)
From the official HP Designjet T120 Printer manual, page 7
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