HP 7475A Plotter: Refilling the Pens

Plotters date back to the days before companies started using DRM to protect their monopoly positions, so refilling plotter pens requires little more than prying out the plug and squirting in more ink. Refilling the disposable liquid ink pens and the green ceramic pen suggested this would work.

I shaved down the side of a Genuine HP pen to find out why the plug didn’t pop out. It turns out the plug has a long and aggressively ribbed profile to ensure a gas-tight fit:

HP Plotter Pen - exposed plug
HP Plotter Pen – exposed plug

The easiest way to refill those is to drill an off-center 1/16 inch hole in the plug, then inject ink into the sponge with a syringe and blunt needle (and bulk ink!) from an inkjet cartridge refill kit. Angling the needle through the sponge close to the pen wall, then filling slowly, loads the sponge from the bottom up and expels the air along the way.

Inmac pens have a shallow plug, more of a flat cap, that pries out with zero drama:

Inmac Plotter Pen - removed plug
Inmac Plotter Pen – removed plug

Dripping the ink atop the sponge seems to work well, although that sponge is definitely over-filled.

Inmac caps push back in place with zero drama.

The pens have fiber nibs with vent channels along their sides that allow air into the reservoir, so overfilling the sponge nets you a mess when you take the cap off the nib: those same channels allow excess ink to run from the reservoir around the nib, without (much to my surprise) wetting the fiber tip.

About 0.2 ml of ink fills the reservoir to saturation, 0.1 ml leaves it wet, and 0.05 ml seems to work well. The 1.0 ml syringes I’m using require about 0.05 ml to fill the (blunt!) needle shaft & hub, plus the syringe tip below the 0.0 ml index, so measuring the ink by drops might make practical sense.

The old physician’s trick of expelling that air by inverting the syringe and pressing the plunger until liquid squirts from the needle is so not happening…

I’ve had zero success refilling fossilized pens, probably because the OEM ink slowly evaporating from the nib clogs all the gaps between the fibers with pigment or coagulated solvent. Preemptively refilling good pens when they first show signs of running dry generally works well.

Given the number of New Old Stock pens I have that are still in their original wrappers, this is more of a “Does it work?” exercise than a necessity.

But, y’know, maybe becoming the last plotter pen refiller on the planet will be my ticket to fame & fortune! For sure, we’ve all seen over-hyped Internet startups with worse business plans and (the admittedly few) typewriter repair shops occupy a stable niche.