How does an inkjet printer work?

How do inkjet printers work?

Inkjet printers are a familiar sight in homes and offices, used to print homework, newsletters and photos for the family or quotes, invoices, forms and colour business documents for small businesses. But have you ever stopped to think about how they work? Inkjet printers have functioned in roughly the same way since HP launched its original ThinkJet printer in 1984, but this is changing – and these changes are revolutionising printing.

How do traditional inkjets work?

In a traditional thermal inkjet printer, ink is fed in to thousands of tiny reservoirs in the printhead from the cartridge, then heated rapidly by a tiny resistor, which causes the ink to form a bubble. This bubble then propels the miniscule droplets through a nozzle on to the page, where each forms an equally miniscule dot. These dots form the lines, characters and subtle gradations of colour that we see in a finished printout, whether that’s a simple letter, a 20-page report packed with charts and graphs or a family photo.

hp_dropsIn black-and-white prints, dots of black ink, placed precisely in microscopically controlled quantities, create crisp, black text and smooth lines. In full-colour prints, coloured dots of up to eight different coloured inks are layered accurately in patterns or directly on top of one another, creating the impression that there are millions of different colours on the page.

It’s clever stuff, but the traditional inkjet has its limitations. The printhead is very small and can only deposit ink on a tiny section of the paper at a time, which means that the printhead has to travel right to left then left to right across the sheet, printing one line of dots at a time. When the line is finished, the paper transport mechanism – the mechanism that pulls paper in to the printer, under the printhead and in to the output tray – moves the paper in to place for the next line.

This limits print speeds, since the printer has to wait for the printhead to make its way across the page before the paper transport mechanism can do its stuff. It can also impact long-term reliability, as the printhead’s horizontal movement means another mechanism that’s subject to wear and tear. It can even affect quality, as the motion of the printhead makes it harder to lay down all those dots with such precision.

Inkjet printers have improved dramatically over the last 30 years. They’ve gone from speeds of 2 pages per minute (ppm) to more than 30ppm, while resolutions have leapt from 300 dots per inch (dpi) to 2,400dpi. However, the limitations of the printhead are now holding the inkjet back. They prevent it from printing faster and handling the kind of workloads you’d expect from a laser printer.

A better way

hp_prox_02HP’s PageWide technology removes these limitations by replacing the traditional single, moving printhead with an array of printheads that span the width of the page. In a PageWide printhead, each printhead has 1,056 nozzles for each of the four main inks – cyan, magenta, yellow and black – working out at 4,224 nozzles per printhead and 42,240 nozzles in the whole array. These nozzles enable a PageWide printer to print each line in a single burst as the paper transport moves the sheet through underneath.

The resulting printhead does something incredible: it puts thousands of droplets of uniform weight and size on the page at amazing speed and with equally impressive accuracy. This enables PageWide printers, such as those in the HP OfficeJet Pro X range, to print full-colour pages at speeds of up to 70ppm. That’s not just fast for an inkjet – it’s fast for any kind of printer.

What’s more, by eliminating the need for the printhead to travel across the page, HP has made the PageWide printhead more robust and reliable, not to mention better equipped to handle higher monthly workloads. By thinking outside the conventional inkjet box, HP has transformed the way that inkjet printers operate and the tasks they can take on.

Ensuring Great Prints

HP’s first PageWide printers – those in the OfficeJet Pro X line – have been designed to compete directly with laser printers and handle workloads of up to 6,000 pages per month. Yet where a conventional printer has the print head integrated into the ink cartridge, meaning both are replaced periodically, the OfficeJet Pro X has a separate printhead designed to last the entire lifespan of the product.

It achieves this using an optical tracking system that scans both the paper and the drops of ink in flight, looking for variations in alignment and ensuring that each nozzle is depositing the correct amount of ink at the correct time. If not, the printer can intelligently substitute working neighbour nozzles for nozzles that are jammed or misaligned. Meanwhile, an integrated service cassette cleans and conditions the nozzles and refreshes the ink in each reservoir to maintain great performance. Put it all together and you have a printer that keeps pushing out great prints, year after year.

Advanced inkjet technology

The PageWide printhead is a marvel of technology. The print heads are manufactured using the same photolithographic processes used to manufacture microprocessors in a plate thinner than a single human hair, and each print head in the array overlaps the others by 30 nozzles on each side to avoid any artefacts appearing where the printheads join.

It combines with a new precision paper-transport mechanism that maintains a constant speed for the paper underneath the print head and dampens any lateral movement, while using over 300 star wheels – thin metal gears that only touch the paper with their points – to move the paper through the printer without leaving tracks in the ink. Finally, a specially formulated pigment ink improves colour saturation, clarity and definition.

Together, these enhancements are paving the way for a brighter future for inkjet technology, where printers are  faster, more versatile and more capable than ever before.

There’s a lot more to HP’s PageWide technology than fast print speeds. Read our whitepaper on how they can reduce running costs and waiting times.

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