Issue #0406/1 - As Canon launches the biggest desktop ink jet print heads in the world, we take a brief look at the effects of the print head deign and configuration on print speed.
Where inkjet printing is concerned, don’t let anyone tell you that size does not matter. They can tell you that small is beautiful – that is fine, as is the ‘need for speed’.
‘Size matters’ – the length of the print head is critical to print speed. Imagine cutting the grass of your personal football (soccer) pitch measuring 80yds x 120yds (75m x 110m). You would need to walk the width of the pitch 360 times to mow it cross-ways with a lawnmower that has a 12 inches (30cm) cutting width (not allowing for any overlap!).
In all, you would have walked 16 miles, 633 yards and 2 feet (26 Kilometres, 334 metres and 72 centimetres!) – more than a half marathon.
If, however, you were to tackle this football pitch with a tractor mower with a cutting width of 2 yards, it would take only 60 passes of the tractor, which would have travelled only 2.72 miles (4.39 km).
‘Need for speed’ - if the tractor mower in the example above were travelling at walking speed, the job would be accomplished in one-sixth of the time that it would take to mow the pitch with the small mower. However, a mower is capable of travelling much faster than walking speed – say 4 times faster. This would mean that the total mowing time could be reduced by 24 times – quite an improvement.
One other factor is involved here – one that makes a three-times difference to print speed. That is the configuration of the nozzles in the print head. The traditional print head in the accompanying images, still used in many of the Lexmark photo printers, shows how the three lines of nozzles (one for each colour) are all arranged in one single line. Please note that the positioning of the nozzles has been indicated with coloured lines because they were not sufficiently visible in the raw image.
So, although the print head measures approximately 11.25mm, each colour comprises only 3.75mm, meaning that a swath of ink measuring only 3.75mm can be laid down at one time. Furthermore, only one colour can be laid onto one point on the paper at one time. In order to construct the image, the 11.25mm head has to pass over the paper as if it were a 3.75mm head (one-third of the effective print speed).
If this is contrasted with all the other print heads shown, the biggest single difference is the fact that the lines of nozzles are arranged in a parallel formation instead of in-line. This allows all of the colours to be laid on the paper at the same time, meaning that the full width of the print head is utilised all the time and the maximum print speed is achieved for the size of head.
At draft print speed, the head lays down all the ink for the image in one pass of the print head. So, like our lawn mower, an A4 page can be covered in (roughly) just over 30 passes of the Hewlett-Packard print head, whereas it would require about 50 passes of the Canon head and 80 passes of the Lexmark head.
Looking at the image of the Epson print head, it is even possible to count the individual nozzles – all 48 nozzles in each 10mm (0.394 inch) line are clearly visible. This means that the real resolution of the Epson printer is about 120 dpi, which is a restriction of Micro Piezo technology. A resolution of only 120 dpi is barely adequate even for draft output. Realistically, it would probably require three passes of the head to produce reasonable draft quality.
Compare this to the Canon print heads, the natural resolution of which is 1,200 dpi. At 120 dpi, Epson’s print head has to pass across the same portion of the paper many times (10 times) just to build up an image equivalent to the 1,200 dpi of the Canon print head. In fact, even the high-density Canon print heads will use more than one pass to create a high quality photographic image, so whatever the Canon needs, the Epson theoretically needs 10x that number – hence the slower print speeds experienced with Epson printers.
Hewlett-Packard’s PhotoSmart 7960 is another fast printer. The cartridges used in that ink system has 300 nozzles per colour, giving a natural resolution of 900 dpi. Taking the nozzle density and head size together, the Canon configuration has the edge on Hewlett-Packard at full photo quality because although the print head is 35% smaller than the Hewlett-Packard head, the density of nozzles is 33% higher, meaning fewer passes across the same part of the paper. Canon’s print speed for a 6×4 inch photo on the Bubble Jet i905D is 33% faster than on the Hewlett-Packard PhotoSmart 7960.
‘Small is beautiful’ - overall, it is the combination of print head length, nozzle density and firing frequency that are the major contributors to print speed. However, drop size has an inverse effect on the equation – the smaller the nozzle size (hence the smaller the drop size) the finer and smoother the image that can be printed but the longer it takes to achieve. This element of printing relates to the grain size in tradition solver halide photography.
It is rather like trying to cover a snooker table with balls – to fill it with snooker balls would certainly need a lot of balls and would take quite a time to achieve … but … try and cover it with marbles and the job takes very much longer and uses a far higher number of marbles than snooker balls. The result of the increased numbers of drops required is that print speed can only increase as development of head length, nozzle density and firing frequency outpaces the decrease in drop size.
In order to produce a pleasing image, the grain must not be visible, meaning that the dot size must be small enough for it not to be discernable to the naked eye at normal viewing distance – even drops of 5pl can be seen with the naked eye when viewed very close, at normal viewing distance they are not discernable.
In practice there has always been more scope, relatively speaking, for increasing the firing frequencies, head sizes and nozzle densities than for reducing the drop size. Hence, inkjet development has always seen this balance work in favour of print speed, while also allowing print quality to improve as drop sizes have reduced.
~End~
