I got the new electronics and all four pumps running, did some testing with it and besides one thing everything seems to work ok.
The remaining problem is, that by mounting the nozzles in a horizontal arrangement, they often eject not only one droplet, but rather one lage and multiple small satellite droplets, which is a bad thing.
If the nozzles would be mounted in a vertical arrangement, all droplets and satellite droplets would hit the paper at roughly the same spot so that they would merge together, but in the horizontal arrangement they hit the paper at the entire flight path of the main droplet.
So mounting the nozzles in a vertical arrangement would be good, but by doing so there would be forming of ink drops on the nozzle again which would block the nozzle orifice until they fall down on the paper.
The satellite droplet appear more often when there is some air in the lines.
Another thing is using the 6mm OD / 4mm ID tube in the case that air can get inside the tube it can happen sometimes that the ink forms a channel inside the air bubble so that the ink can flow through the bubble instead of pushing it out of the line.
Maybe lines with a smaller inner diameter could help.
So, I have an idea how I could prevent drops from forming in the vertical orientation - will try it out later.
Here is another printhead prototype with a vacuum duct at the nozzle to prevent any drops from forming at the nozzle. This is the best drop preventing design that I built so far, because it does not just collect the drops, but can completely prevent them from forming at all.
I will design another one for all four colors for more testing. It looks like (I hope) I'm on a good way with this design :)
So, here is another printhead for all four colors.
It looks like I have finally build a working printhead, pump and driver setup. Even though there are some unwanted satellite droplets, which I think are caused by a partially clogged nozzle or a bit unprecise alingment while assembling the printhead, I'm really happy with the result because the machine is basically working as expected. The ink is drawn by the piezo pumps from ink containers through check valves to the printhead and can be ejected in a drop on demand method. That's what I wanted and now it works. In terms of reliability it works a lot better than the design from two years ago, that I could not get to work with four colors at all and with one color only if everything was 100% tuned right in terms of air in the lines and water level. The new design can work with some air in the lines and can also force larger bubbles out of the nozzle again in some cases.
Now I can write building instructions for it and upload all files and infos.
I also want to work on a software for it to do more testing in the future.