Low power microscopy around the home

Inkjet ingenuity - a tour around an inkjet cartridge

by Dave Walker

The inkjet printer cartridge is a miniature engineering and electronic marvel. How does it scan so fast yet precisely place millions of tiny ink spots on the paper to build up crisp text and images? With just a 10X hand lens, low power microscope or macro attachments on an SLR camera some of the fascinating features that make it work are revealed ...

Note added 2002: This macro/micro tour is now rather dated as it describes a disposable 'thermal' inkjet cartridge that was current at the time of writing (Jan. 99) so the features described don't necessarily apply to modern cartridges. Some also use piezoelectric technology for creating the inkjets rather than the thermal method used in the cartridge described below.
The 'tour' is also not possible for many newer inkjet cartridges. For example, many modern inkjet printers have the print head as an integral part of the printer and separate from the ink reservoir.

There are plenty of common objects around the home worth studying using low to mid powers of a compound or stereo microscope with top lighting. One miniature marvel worth looking at is the discarded ink cartridge of an inkjet printer (see header and footnote).

The images right and below show the electronic circuit tracks on the plastic film that wraps around one side of the cartridge and the base (labelled '2' in top image). The pins on the printer body contact the circular gold contacts on the cartridge, which via the tracks connect to the inkjet circuitry in the base.

The image left is a shot of the plastic film at a different lighting angle. I include it as I like the pattern the tracks form!

You don't need a microscope to see these sort of images. They were taken with a 50 mm SLR lens and camera extension tubes (attached to a video camera for image capture but an SLR camera will give good prints). They can also be admired by eye just with a 10X hand lens, a much overlooked but cheap and useful item.

Now to the 'business' end of the inkjet cartridge - the print head plate (ca. 7 x 4.25 mm) on the cartridge base (labelled '1' in image at page top). The image right shows a close up of the arrangement of the tiny holes for each inkjet on a Hewlett Packard 550 colour cartridge. (Also captured with the SLR lens and extension tubes).

The pins along the top and bottom connect to the film tracks shown above. There are three labelled pairs of jets, one set for each of the three colours. The black and white cartridge just has a longer single pair of jets along the centre of the plate. (Note the arrangement of the three sets of jets as they align with the circuitry behind this plate shown later).

With all three SLR extension tubes and 50 mm SLR lens it is possible to see the tiny jet holes in close up without a microscope and using equipment which many photographers will own. Just one of the three sets of jets are shown. This set occupies an area ca. 1.25 x 0.7mm. For each colour there are two banks of eight jets i.e. 48 holes in total on the plate, modern inkjets have more. The holes are slightly proud of the surface, each on a circular base. Note the jets are slightly staggered in each row, possibly for greater control over the ink distribution.

It's evident at this magnification and with the low angle lighting, to see why too many recycles of the cartridge isn't recommended as the plate gets progressively scratched by the continuous movement over the paper. In time the jets will wear and print performance is degraded.

To see what the inkjet circuitry behind the plate looks like, with care and a sharp knife the plate can be severed round its edges and gently prised off. This also allows the tiny print head plate to be put under a compound microscope and examined in more detail.

Part of the inkjet plate is shown left above viewed under the compound microscope with a 9X objective in transmitted light with a little top light as well. The stagger of the holes is more clearly seen and the holes were measured at ca. 0.046 mm or 46 µm in diameter. I suspect newer printers and cartridges with much higher resolutions than the HP550 series will have smaller holes. Notice how large dust is in comparison (the white specks on the left side of the image above). The jets must be kept free of debris in the ink so each of the three colour ink reservoirs has a filter stub at the base to remove any tiny debris before the ink is fed to the print head. (Filter shown right above, apertures ca. 10 µm).

The image right shows the electronic circuitry (ca. 8 x 5 mm) behind the removed inkjet plate. This is the part of the cartridge that is never seen but is where the tiny jets of ink are created and forced through the holes in the plate.

The areas labelled '1' in the image right are holes in the circuit board and align with the three labelled rows of jets shown earlier on the inkjet plate. Behind the board the holes also align with ducts from the main inkjet reservoirs to allow the filtered ink for each of the three colours to reach the print head. The gold coloured tracks labelled '2' carry an electrical signal under software control to the perimeter of each aperture.

The HP 550 series is a thermal inkjet printer, some more modern types exploit the piezoelectric effect. It's not possible to deduce much more about the workings of the inkjet cartridge from visual inspection. But there are many resources on the web e.g. the PC Technology Guide that describe inkjet technology.

So if you had no previous interest in this type of macro studies and low power microscopy, I hope I've passed on some of my own enthusiasm for man's miniature engineering and electronic marvels. There are plenty of discarded items around the home worth studying in this way.

Comments to the author Comments to the author sent via our contacts page quoting page url plus : ('dwalker','')">Dave Walker are welcomed.

Please note. The author is an amateur photographer with an enthusiasm for macroscopy and microscopy. He therefore can't help with any printer or cartridge problems. Printer manufacturers' websites should have FAQ's and troubleshooting sections.

Further reading. There are a number of sites on the web who describe various aspects of inkjet printers and how developing technologies are constantly improving their performance. The keyword 'inkjet technology' in a search engine will find a number of them.

Web resources: A terrific web site describing how many parts of a PC and related technology work, with links to further resources is the PC Technology Guide.

The article 'Inkjet printing: joining the jet set' by Peter Gregory is available on the Royal Microscopical Society's web site (follow their 'Publications' and 'Proceedings' links). The article gives a fascinating overview of inkjet technology and inkjet composition.

Footnote: As remarked in the 'Note added 2002' in title, the tour only applies to the older style disposable thermal inkjet cartridge. Even these are worth recycling with inkjet refills if possible, but if the cartridge is really redundant, give it a really good soak outside in water and then dry before dismantling. They can be very messy otherwise, even when empty, and will readily stain work surfaces, carpets, hands etc. The cartridge disassembled was a colour one but the same can be carried out with a black and white cartridge. Return to article.

Disclaimer: No responsibility is accepted by the author or other contributors to Micscape, Microscopy UK or Onview.net Ltd for damage to property or injury caused by disassembling a cartridge. Or for dismantling a printer/cartridge that is not suitable (as remarked in header) for this sort of tour. It is up to the reader to take appropriate care. Youngsters should not disassemble one without adult supervision as it needs a sharp knife and possibly a hacksaw to disassemble.

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Published in January 1999 Micscape Magazine. Revised August 2002.

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