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If I have a free-standing printing head including ink tanks, what would I have to do to just make it spray some ink? Are there some standardized signal encodings or something?

I'm not talking about generating an "output" with a moving head. Just a free head that should spray some droplets of ink.

And where would be a good starting point to find out more about such kind of knowledge?

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How are you going to remove the ink if it does not transfer to paper and ends up clogging the jets? –  Tony Stewart Nov 16 '12 at 18:58
    
Sorry, I don't understand that question. Remove the ink from where? I want to be able to spray a short "ink pulse" from the head (which is not connected to any printer anymore). Or do you mean what I'll do if the head is "polluted" by ink making it stop working? I'll just clean it, I think. Or is there a some reason speaking against that? –  Foo Bar Nov 17 '12 at 8:34

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A lot of that is proprietary but I've spent a lot of time reverse engineering print heads and building printers :) Usually the firing voltage is high in the 24/30 V range. If you just want it to spit you can usually drive some current through it at the appropriate voltage and get some ink to come out (I assume you have a resistive head and not a piezo most heads now a days are resistive). I'd start by trying to just pulse some through it as you don't want to hook it up continuously and burn out the head.

Your just looking to get a short burst, heat up the tiny element behind the nozzle and get the ink to fire out.

Whose head are you using?

Actually Printing

To actually print something takes more effort, we developed dedicated silicon just to do that. But basically you convert the pixels or dots you want to print into high voltage firing pulses that activate each nozzle. So usually the first step after spitting a blog of ink onto a page is to print out some test pattern by firing multiple nozzles. You'll notice that there are a lot more nozzles than there are pins though :) So to figure out the right sequence you'll need a scope or better yet a logic analyzer.

Printing and moving across a page or object? That's a whole different story that will involve some encoders and control loops :)

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I should add that you can usually get a head to work just using a bench top power supply and some wire. Just make sure you're making momentary contact and not leaving the power on! –  Some Hardware Guy Nov 16 '12 at 16:23
    
Thanks. I'm not having a specific head now. I'm just doing some brainstorming and information gathering. When I think I got enough informations I'll just get some old printer somewhere and try it out. Do you recommend a specific type of head that does best results? –  Foo Bar Nov 16 '12 at 16:26
    
Yes get the cheapest low end, older printer you can find from HP like this: hp.com/hpinfo/newsroom/recalls/fax1010.html The lower the resolution and the older the head technology the easier it will be for you to get working. –  Some Hardware Guy Nov 16 '12 at 16:30
    
Another tip it's best to check out the firing pulses first on a scope. If you turn the head element on too long you just evaporate the ink, if don't turn it on enough then nothing comes out. If you leave it on really long you'll burn it out. It's pretty satisfying to get that ink to finally spit out onto some paper though so don't give up. –  Some Hardware Guy Nov 16 '12 at 16:34
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+1 for "Whose head are you using?" - That deserves to go on a T-shirt :-) Very useful insight into the arcane arts of picoliter printing, thanks! –  Anindo Ghosh Nov 16 '12 at 16:48

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