HP 7475A Plotter: Refilling Disposable Liquid Ink Pens

A while back, Keith Ward sent a Big Box o’ Plotter Pens that:

• Should suffice for the rest of the plotter’s life… if not mine
• Obliterate any need for my Sakura pen adapters

After a bit of sorting, I had a quartet of “disposable” liquid ink pens with contents ranging from desiccated to gummy. With nothing to lose (and having already cut a clearance slot in the plotter case), I drilled a small hole in the top of each reservoir, squirted some inkjet printer ink into the void, and taped the hole closed.

Surprisingly, a little liquid love restored all but the black pen to working condition, if not perfect heath:

I think the blurred white disk floating in the reservoir sealed the end where you jam the tip in place to activate the pen. The blob of dark gunk shows the reservoir didn’t start with yellow ink, but I had nothing to lose.

The top pen in this picture is another style / brand with a smaller reservoir:

The white pen in the foreground has a 0.3 mm ceramic tip, contains its original green ink, and works as well as it ever did; it might be refillable, too.

The liquid-ink pens have a serpentine vent in the tip. This is a Genuine New-Old-Stock pen in a four-pen case labeled HP 5061-7566:

The serpentine path connects the exterior vent opening (facing you) to a tiny hole (on the other side of the blue shaft) into the ink chamber. As it turns out, a new hole drilled in the reservoir admits enough air to drain the (freshly refilled) liquid ink through the serpentine path all over the workbench. Having some experience with refilling inkjet cartridges, I deployed a towel decorated with colorful splotches in anticipation of such an unexpected event, although my fingers looked considerably more cheerful than usual for a few days.

The black pen never worked quite right, but the other three did fine. The ceramic pen is at the top:

Protip: the blown contrast and rear-surface bleedthrough behind the yellow ink should tell you it isn’t visible in normal room light. I must mix yellow with another color if I ever refill that pen that again.

KiCad uses only one pen for the entire schematic, even when you select “plot in color”, suggesting nobody has sent the “plotter” output stream to an actual plotter in a long, long time.

Despite the charm of watching the plotter crank out an entire schematic page, it’s not a compelling enough user experience to replace an inkjet printer. For an art project, one might be seeking an entirely different user experience and the answer might be different, too.

Selah.