How do HP Printheads work?
The HP Printhead is essentially at the very heart of your printing system. it is the part responsible for firing the ink drops onto the page to create text and images. Recently, technological advances with printheads have seen significant changes including increased numbers of nozzles to provide more precise output e.g:
Designjet 510 - HP 11 printhead - 600 nozzles per inch
Designjet T520 - HP 711 printhead - 1200 nozzles per inch
What the inside of one of HP's printheads looks like
HP Printheads use of Thermal Technology
The earliest form of commercial inkjet printing was continuous inkjet (which as the name suggests, deflects ink drops from a continuous flow of ink to form images on the page). By contrast, HP Designjets are 'drop on demand inkjets' and the printheads fire the ink drops as needed to form the image. Unlike Piezo Electrical Inkjet technology (which pushes the ink out by using mechanical force) the HP Designjet Printheads use thermal technology to push the ink out, together with an integrated circuit which routes signals to the hundreds of ink nozzles providing precision and predictable output.
HP Thermal Technology - what Coffee and Thermal Technology have in common!
Interestingly, the concept around thermal inkjet technology first started with a coffee pot. An engineer in the 1970s was watching his coffee brew and noticed that his perculator had no moving parts - just a heater element in the bottom that forced the water up and over the coffee grounds. He wondered if that same thermal energy principle could be used to eject ink. In 1979 HP proved that this could be done.
How the printhead works
The heating element (comprising of tiny resistors) rapidly heat a thin layer of liquid ink. The heated ink causes a bubble to form, expelling the ink through a nozzle. The heat also forces air out of the ink molecules.
The ink is then fired out at a velocity of up to 20 metres per second. This super heated vapour explosion lasts just 2 millionths of a second, and is repeated thousands of times every second for each of the hundreds of microscopic nozzles on the Printhead. As the ink is rapidly expelled out of each chamber nozzle, this in turn creates suction, allowing further ink to be pulled back through into the chamber nozzle.
Depending upon the printer model between 4,000-15,000 ink nozzles are used to fire ink drops down to the paper surface,
with up to half a million ink drops per second being released during a typical printing job.
The printhead skims about 400ths of an inch above the paper surface, and as the paper transfers through the printer, the printer control board orchestrates the ink shooting sequence so that millions of individual droplets are precisely and accurately dropped in the correct place, in order to form the text and images on the paper.
The ink is release in a CMYK sequence (first Cyan, then Magenta, next Yellow and then the 'Key' colour Black). Drop accuracy is critical and the control board uses complex print masking algorithms to blend multiple colours together on the page.
Two other useful blogs covering a similar subject matter include:
How does a 4 colour CMYK HP printer manage to create different colours
What does optimized dpi mean?