Hey there,

I had a lot of fun testing out my polargraph machine so far, now I thought about getting more serious. One step to take was the pen issue: simple give-away ball tip pens do a good job, but their color vibrance is very limited (usually black and blue) and due to their small line width (0,10 mm or so) it takes ages to draw larger projects.
I had the (to my taste) best results with pens like those shown in the attached picture. The problem is that they tend to run out of ink in the middle of the drawing process, especially if it includes many dark pixels.

So while refilling my ink jet cartridges I had a simple but kind of brilliant idea: just refill the pens! This ink is dirt cheap, available in colors and easy to use.

Step by Step:
1. Get ink and something called "inkjet cartrige cleaner". It is a liquid that has almost the same ingredients as ink, but with cleaners and without color.
2. Get an empty pen. It makes sense to use a brighter color, so that Disassemble the pen: usually it is easy to pull our the back end of those pens with a pair of pliers. Your goal is to get to the felt-like tube-thing inside.
3. Squeeze out all the ink that is left in between the felt. Pour the inkjet cleaner to one side of it and squeeze it trough the felt tube. Do this a couple of times.
4. Pour the desired ink to the felt. It works best if you do this in steps, with some time in between. If there is too much ink in it the pen will bleed badly!
5. Do this with 2-3 pens per color. During the drawing process simply stop the machine and change pens. Its that easy.
6. Get creative! It may be possible to render Pictures in classic cyan, magenta, yellow and black, with the same color mixing effects your printer would use. It may be interesting to draw with a mixed color pen: one end of the felt filled with yellow, the other one with blue...

I meant to reply to the OP, but I guess I never did - Good idea about using the inkjet cart cleaner to flush the pen. I think those staedtler pens are great.

I dabbled in continuous flow mechanisms for the spectrum arts window drawing project, but never got anything working beyond making a big mess. My target was a pen barrel and tip, with a small piece of felt in it, with an IV line stuck in it, and ink in the IV bag. The bit I was missing was the drip chamber - where the flow is regulated, so it just dribbled out continuously. I had hoped that the felt and tip would stop that, but there was just too much weight behind the ink.

Once you get the continuous ink supply working, you've got to deal with the tip wearing down. All these factors add up and make for a significant engineering task, so I opted out! The commercial versions of this kind of thing (ie plan plotters), tend to use disposable pen cartridges - ball point or fibre tip.

My best results have been from kuretake zig millennium pens. I did a big, A1 skull, a dark one, over two days with 0.05mm millennium, and it was still going by the end.

sn