The purpose of an inkjet print head is to eject a small amount of liquid onto a substrate. Usually the liquid is ink and the substrate is paper, but inkjet technology can be used for much more than just printing words on a page or pictures of your next-door neighbour’s prosthetic leg on your mobile phone case.
The two most commonly used inkjet technologies are thermal inkjet (TIJ) and piezo ink jet.
In thermal inkjets, a small amount of ink is vaporised by a heater in a chamber. The chamber has a hole (nozzle) in the bottom of it and as liquid is vaporised the pressure in the chamber increases and liquid is sent on its way through the nozzle. After the ink is ejected the bubble of vapour collapses, sucking more ink into the chamber through a feed hole at the top of the chamber.
Piezo inkjet works a little differently. Instead of a heater to create a bubble, piezo inkjet printheads rely on the piezoelectric effect to create displacement. Confused? It’s actually pretty simple. It means one side of the chamber is made from a piezoelectric material (usually lead zirconium titanate) that bends the wall a little inwards when electricity is passed through it. When the wall bends, ink is displaced and ejected through the nozzle. This would be the same as having a piston at the top of the chamber which moves down, squirting some ink out.
There’s also continuous inkjet in which a continuous stream of ink is directed by an electro static field, but it’s not widely used.
At this point you might be saying, this is all well and good, but my water pistol works fairly well for making drops of liquid. And that might be true, but the amazing thing about thermal and piezo inkjet technology is that it works with chambers large and small. How small? Very small!
Modern inkjet cartridges have thousands of nozzles around 10 micrometers in diameter that can repeatably eject drops of liquid ranging from 10 – 100 piezo litres. If you’re wondering a pico litre is 1 x 10-12 litres. I have trouble wrapping my head around a number that small because people don’t use numbers nearly that small in daily life. Just for comparison rain drops range in size from 0.5mm to 3mm. I could talk about raindrops for a while, they’re really interesting but if you take an average rain drop with a diameter of 1.25mm that would be enough for 160,000 50pL inkjet drops! Why is having small drops important? The smaller the drop is the better your resolution and the finer you can print!
Which system is better – thermal of piezo? Well unfortunately like so many engineering problems there’s no “best”. When choosing a space launch system, or building a bridge it all depends on your objectives and budget. It’s no different in printing.
Piezo printheads are difficult to build and control so they’re more expensive but can eject a wide range of liquids, both in terms of (viscosity and composition). Thermal printheads on the other hand can only eject liquids containing a solvent (usually water) that will quickly form a stable bubble when heated. Thermal printheads also have trouble ejecting liquids that are too viscous. Thermal printheads have the advantage of being cheaper so most thermal printing systems rely on removable disposable printheads. This eliminates the complex systems piezo printers rely on to prime and clean nozzles.
I could go on about about the advantages and disadvantages about each system, but in the end, as with rockets and bridges you just need to pick a system and go with it. We chose thermal inkjet because we wanted to keep the design of our printer as simple as possible. Not having to worry about priming and ink feed systems reduced complexity as well as cost and meant we could get a product out much faster. Other factors were the low viscosity of our water based inks and the fact that they are relatively harsh to the printheads.
In the next post I’ll talk about how each of the thousand nozzles are controlled. This isn’t a trivial problem when you consider that each nozzle can be fired thousands or even tens of thousands of times per second! I’ll also talk about my favourite topic… Manufacturing!
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