The idea of a jet printer has its roots
in the last century when the two British Lords, Kelvin and Raleigh
structured the mathematical equations describing the conditions under
which a stream of liquid can break into droplets.
However, most of the research that made
the current jet printers possible began in 1968 and the first working
bubble jet printer saw the light in 1977.
Jet printers can be classified
according to the way the ink is handled. There are three basic systems:
-
Continuous Ink
-
Electrostatic
-
Drop-on-Demand
Most printers on the market are of the
Drop-on-Demand type and these can be further classified into:
-
Thermal Bubble
-
Piezoelectric
"Drop-on-Demand" means
exactly what it says - on the printhead receiving a specific electrical
pulse the "demanded" ink drop is formed and delivered to
paper. Shown here is how a bubble is formed and ejected from a
cartridge. Ink is first heated by voltage pulses. As the ink begins to
vaporize, a bubble is formed and begins to grow. The pressure formed
from the expanding bubble forces the ink out of the nozzle. The bubble
then cools and contracts creating a vacuum. The vacuum, in turn, pulls
more ink into the nozzle and a new cycle is ready to start.
"Thermal Bubble"
Bubble Jets, Hewlett Packard Deskjets,
Thinkjet and Designjet printers belong to this category.
In piezoelectric Drop-on-Demand printers
droplets are produced by a transducer, shown here. The Epson Stylus Color
Printer is an example of this design concept.
Print Quality
Good print quality, as we can expect,
depends on two parameters: Ink and Paper. The higher the DPI (dots per
inch) of the printer, the more critical is the paper (to achieve the
number of dots per inch that the printer can deliver). The way paper is
actually made is also important - for example, the amount of fiber
clumping as observed by holding a piece of paper to a strong light. In
general, the coating and substrate of the paper must match the resolution
of the printer. A 600 dpi printer printing on plain paper will not yield a
600 dpi print, and a 720 dpi printer printing on 300 dpi paper will not
deliver 720 dpi.
 |
Difference in dot spreading
between coated paper (left)
and uncoated paper (right) |
Coated paper is expensive
and the better the coating the higher the cost. Fortunately, unless you
are dealing with special applications not yet routinely handled by jet
printers, such as photographic reproduction, etc. you can print on
standard paper and have quite good results. From on e point of view,
having paper with less resolution may actually be an advantage. For
example, the unmagnified side of the letter M in the sketch will actually
appear darker and have more eye impact than the left portion, composed of
discernible and independent dots.
Ink spreads and penetrated. Here is
illustrated (not to scale) the effects of printing on ideal (coated
papers) and on other papers. Spread is more accentuated when using our
water resistant inks, but this is quite acceptable. Looking at the print
with a magnifying glass, you would see a continuous line rather than a
string of minute dots.
The Ink Jet
Design Challenge
You are probably using (or know that
millions use) ink jet printers. The specifications of a jet ink appear
tough and almost forbidding. Below is listed under four separate headings
(Ink, Material Compatibility, Drop Ejection and Print) the parameters that
must be considered and weighed when formulating a jet ink.
|
Inkjet
Ink |
Material
Compatibility |
Drop
Ejection |
Print |
| Good
Stability |
Non-Corrosive |
Uniform
Drop Size |
High
Optical Density |
| Low
Viscosity |
Plastics
Compatible |
High
Drop Velocity |
Color
Quality |
| High
Surface Tension |
Adhesives
Compatible |
High
Drop Frequency |
Fade
Resistance |
| Conductivity |
No
Particulate Formation |
No
Orifice Wetting |
Light
(UV) Fastness |
| Long
Shelf Life |
|
Non-Crusting |
Water
Fastness |
| Non-Flammable |
|
Non-Clogging |
Solvent
Resistance |
| Non-Toxic |
|
|
Smudge
Resistance |
| No
Biological Growth |
|
|
Off-Setting |
| Dye
Solubility |
|
|
Crack
Resistance |
| Pigment
Size |
|
|
Media
Sensitivity |
| |
|
|
Spreading |
| |
|
|
Feathering |
| |
|
|
Dry
Time |
In general, the final product is, as
always, a compromise. The ink ingredients are such as to ensure the
optimal medium between print quality, dry time, and nozzle performance.
The better the print, the longer the drying time. The shorter the drying
time, the more likely the ink will dry on the nozzle when the printer is
not in operation.
Ink is first analyzed for color tone,
feathering, intercolor bleed, laydown, text sharpness, and performance on
plain paper, coated paper and transparencies. Then viscosity, surface
tension and pH are measured. The interaction of these factors determines
print quality and short and long term stability. Long term stability is
purely an experimental factor - we can now say that our inks will
be stable in a bottle for at least three years.
After the ingredients, the solvents, the
glycol content and the proportions have been established and tested, the
ink is filtrated through a long process down to 0.2 microns. After
filtering the ink is tested again to check that the properties have been
maintained. After three experimental batches have been successfully
prepared and tested for Consistency, the formulation and procedure are
formalized and the ink goes into production.
