I have been using the modified printer for a while now and I'd like to leave some comments on the actual usefulness of the project. PCB manufacture has been decreasing in its price, so I rarely use the “haxmark” anymore. It gets reactivated when I need a board fast and it does a very decent job.

The general idea

If you already know how the process works, feel free to skip to the next section. For those of you who don't want to go three links deep to the full text, here's a short overview of how a regular laser printer can be used for PCB manufacture:

The goal is to be able to etch the PCB straight from the printer. I use regular copper-clad PCB material without photo-resist, so ordinary toner will be the etch resist. Basically the goal is to let the printer print on the PCB just the way it would print on paper. Regular toner is really durable, so there won't be any surprises when the PCB is bathed in chemicals.

From a mechanical point of view there are quite some modifications to be made. Because PCB material is a lot thicker and more rigid than paper, the printer's feed mechanism won't work and large parts have to be removed. Instead, the PCB material is placed on a thin metal carrier, which is pulled in at the front and ejected through a newly cut slot on the back. The optics and drum are still in place and the toner is transferred onto the copper.

Because a lot of the mechanics are missing, a lot of sensors are, too. Normally, the printer's brains will get signals as the paper is passing through the internals and will stop everything when something unexpected happens. Thus, if the printer should actually print and not only produce paper jam errors, these signals need to be simulated. A small PCB with a microcontroller functions as a replacement for all the removed mechanics by feeding the signals to the printer's main PCB it expects.

However, the printing has to function without the fuser. It wouldn't provide enough heat and certainly have problems with the thickness of PCB and carrier. The PCB exits the printer with the toner particles stuck on solely by electrostatic forces. Those are stronger than you might think, but obviously not string enough for the etching process. A simple solution is to place the PCB in acetone vapour which doesn't degrade the printing quality at all but effects great durability.

And that's pretty much all it takes. Etching works as it always does, but the process before it is greatly simplified.

The process

I design my PCBs in KiCAD, but the CAD program of course doesn't really matter. These are my steps for etching a PCB from start to finish:

1. I export the copper layer as PostScript and run a badly hacked-together script to align the PCB in the upper left hand corner of a DIN A4 page (with a empirically determined margin). Otherwise, the position of the PCB in the CAD program's drawing area would matter and double-sided PCB would be impossible to get aligned.
2. I choose a piece of copper-clad PCB material and straighten it by bending it around.
3. The surface is cleaned with abrasive cloth. The copper dust and other residues get wiped away with some acetone. I also take care of any sharp edges.
4. I attach the PCB material to the carrier with a piece of copper tape along the top edge. I use 5 mm wide tape, but that doesn't really matter as long as it attaches the PCB well and doesn't get in the way. Strictly speaking, there needs to be a connection between carrier and copper surface so the electrostatics can work. Yet, I found regular copper tape with no conductive glue sufficient (it doesn't insulate that well).
5. Printing works as usual from the PC side. The carrier is placed where the rollers can grab it and with a push of the “OK” button the print is started. The carrier exits the printer and is inspected.
6. I place the PCB in a small closed container with a bit of acetone in it. The PCB is elevated, so that only acetone vapours can reach it. After a few minutes, the toner is hardened sufficiently.
7. Inevitable small inaccuracies are corrected with a permanent marker and the PCB is ready for etching!

For double-sided PCBs I just do the whole process twice and cover one of the copper surfaces with wide tape while etching. It would be possible to etch them in one go, but I didn't bother (yet) to build a jig for holding the PCB in the acetone vapour so the previously printed side won't be marred.

Pros and cons

This process isn't perfect. If you are thinking about converting a printer for this job, my experiences might help you in your decision.

What works well:

• The resolution is great. Structures as fine as 5 mil are no problem. Tolerances are no problem either.
• The toner is very durable. I thought about using it for printing on metal control panels.
• The alignment for double-sided PCBs isn't as hard as you might think. I added a small piece of sheet metal for alignment on my carrier which makes this as easy as it gets.
• You see clearly what's going to be the result after etching and you can fix things before it's too late. Either it's black, or it isn't. No in-betweens and no guessing as with UV exposure.
• And, of course, its fast.

Where's room for improvement:

• The printer's drum is delicate and you should watch out for sharp edges or you won't have fun with it for long.
• It is more sensitive than I thought to unevenness in the PCB material. This is the largest problem for me because even small differences lead to holes in the print and practically all PCB material I have is badly warped. When etching, I spend most of the time bending PCB material to get it straight.
• It needs a conductive surface. If you thought you could use this for silkscreen on a single-sided PCB, nope, sorry.

For my use case, the pros outweigh the cons. I still have all the stuff for UV exposure of photosensitive PCB material around but I didn't use it since I built the haxmark.