Thermal or Piezo: Its All in Your Head
By Art Entlich Copyright 2005
The differences in thermal and piezo printheads is of some importance in
terms of inks that can be used, and other issues, but the type of
printhead does not necessarily preclude that one will always provide a
better quality result when looking at the output.
This was not always the case. One of the reasons the piezo ink head
system caught on (and Epson inkjet printers have always been piezo head
design) is because they were lightyears ahead of thermal printing
designs when they were introduced when it come to output.
Epson, part of the Seiko group, had a fair interest in piezo technology.
Some of you might remember the Seiko Quartz watches with a vibrating
quartz crystal that kept the watch accurate. This was the same
principle that piezo crystals work on. While thermal ink heads use a
resistor that rapidly heads a tiny quantity of ink at the tip of the
outlet forming a vapor bubble or boiled ink and pushing the ink in front
of it forward and out, piezo heads use mechanical force (vibration) to
move the ink.
When thermal ink heads were the only inkjet technology available for
consumers, they were still fairly low resolution, about 150-300 dpi
maximum, and they tended to have a large ink droplet which wasn't very
accurate in size, placement or shape.
When Epson came on the scene with a 720 dpi piezo head (in black only,
just like the thermal companies started with) and smaller and more
accurate dots, the industry competition began "heating up" (excuse the
pun).
It wasn't until Epson came out with the very first Stylus Color printer
at 720 dpi that the industry really took notice. It put all the thermal
printer output to shame.
The reason the piezo technology could do this was because it was using a
much more controllable process to move the ink through the nozzles. The
ink didn't need to be heated, and could use different formulations,
since it didn't have to boil the ink. In fact, as long as the viscosity
was within a reasonable range and had the correct polarity to stick to
or penetrate the paper reasonably well, and was small enough, it could
be put through the heads.
The life span of a piezo head was considerably longer, often by orders
of magnitude, over that of a thermal head. Back then, thermal heads
were integrated into the cartridge and were expected to be replaced with
each ink change.
Fast forward to today. The technologies still basically work the same
way, but both have evolved. I would say thermal has evolved further
relative to its beginning, but it also had further to go. They have made
the heads more resilient, such as the Canon head, which lasts numerous
ink cartridge replacements. The number of jets or nozzles has multiplied
considerably making the printing process much faster. The dots now can
be as small as one picolitre, and are more accurate and uniform in size.
The piezo head has also improved. It can also go down to one picolitre
in size, has been speed up, has more nozzles, but the cost per nozzle
appears to be higher, and so they have not kept up with the speed of
some thermal inkjets. They have, however developed variable dot
technology which allows for different size dots to be produced from the
same nozzle by varying the vibration of the piezo element.
Today differences in output quality can be minimal. The main two areas
tend to be in variations of inks available for piezo heads versus
thermal. Piezo inks come in everything from dye, to pigment to
pigmented to dye sublimation type. Thermal heads have a somewhat harder
time with some of these inks, although some use pigment inks. The head
life is still, in general, longer for piezo heads than for thermal type.
Because the piezo head is mechanical, it doesn't heat the ink or the
head. In spite of what some people have reported, I don't believe they
burn out from lack of ink flow, where thermal head may, and probably
will. With some of the heavier bodied inks, piezo will tend to clog
more, and require a bit more maintenance. Those same inks may not flow
at all with thermal.
However, although the head is significant, you need to consider a lot of
things when deciding on a printer. Cost of the printer itself, life for
the build (head, gearing, paper transport, ink waste inkpads, etc),
warranty, cost of ink, other cost of upkeep (head replacement, etc) ease
of use of software, driver color management accuracy, paper profile
availability, speed, paper types available for the inks that can be
used, ink permanence, CD printing, ability to use bulk inking system,
maximum paper size, how close to the edge it prints, how noisy,
manufacturer's customer service support, cost of servicing, etc. In
other words, you need to look at output and ability to do what you need
it for.
Five years ago, head technology may have been a deciding factor in
selection of an inkjet printer, today it a much more complex puzzle.
Art
