The principle of Ink Jet Printers
As is perhaps obvious by their generic name, Ink Jet Printers use a non-impact method of image transfer by squirting ink through nozzles with microscopic holes.
Whatever technology is used in your printer hardware, the final product consists of ink on "media" (paper usually), so these two elements are vitally important when it comes to producing quality results. The quality of output from inkjet printers ranges from poor, with dull colours and visible "banding", to excellent, near-photographic quality.
Two entirely different types of ink are used in inkjet printers: one is slow and penetrating and takes about ten seconds to dry, and the other is fast-drying ink which dries about 100 times faster. The former is generally better suited to straight-forward monochrome printing, while the latter is typically used for colour printing. Since different inks are mixed to create colours, they need to dry as quickly as possible to avoid blurring. If slow-drying ink is used for colour printing, the colours tend to bleed into one another before they've dried.
The ink used in inkjet technology is basically water-based (although EPSON® tend to use a solvent based ink to suit their piezo-electric technology) and this caused the results from some of the earlier printer models to be prone to smudging and running. Oil-based ink is not really a solution for this problem because it would impose a far higher maintenance cost on the hardware. Printer manufacturers are making continual progress in the development of water-resistant inks, but the output from inkjet printers is still generally poorer than from laser printing.
One of the major goals of inkjet manufacturers is to develop the ability to print on almost any media. The secret to this is ink chemistry and most ink manufacturers will jealously protect their own formulas. Printer manufacturers and ink manufacturers invest large sums of money in research to make continual advancements in ink pigments, qualities of light fastness and water fastness, and suitability for printing on a wide variety of media.
Today's inkjets use dyes, based on small molecules (less than 50 nanometres) for the cyan, magenta and yellow inks. These have high brilliance and wide colour gamut, but are neither light or water-fast enough. Pigments, based on larger (50 to 100 nanometres) molecules, are more waterproof and fade-resistant, but they aren't transparent and cannot yet deliver the range of colours available from dye-based inks. This means that pigments are currently only used for the black ink. Future developments will likely concentrate on creating water-fast and light-fast CMY inks based on smaller pigment-type molecules.
In continuous tone printing, there's an unlimited palette of solid colours. In practice, 'unlimited' means
16.7 million colours, which is about 17 times more than our eyes can distinguish. To achieve this, the printer must be able to create and overlay 256 shades per dot per colour, which obviously requires precise control over dot creation and placement. Continuous tone printing is largely the province of
dye sublimation printers. However, all of the mainstream printing technologies can produce multiple shades (usually between 4 and 16) per dot, allowing them to deliver a richer palette of solid colours and smoother halftones.
Six-colour inkjet printers have now appeared on the market, specifically geared for delivering
photographic-quality output. These devices add two further inks -
light cyan and
light magenta - to make up for current inkjet technology's inability to create very tiny (and therefore light) dots. Six-colour inkjets produce more subtle flesh tones and finer colour graduations than standard CMYK devices, but are likely to become unnecessary in the future when ink drop volumes are expected to shrink to around 2 to 4 picolitres. Smaller drop sizes will also reduce the amount of half toning required, as a wider range of tiny drops can be combined to create a broader palette of solid colours.
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Print Quality
Good print quality 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.
Ink spreads and penetrates. Here is illustrated (not to scale) the effects of printing on ideal (coated papers) and on other papers. Spread is more accentuated when using 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.
Looking at the print with a magnifying glass, you would see a continuous line rather than a string of minute dots.
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The Ink Challenge
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 colour tone, feathering, inter-colour 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 (and usually longer than the life of your printer).
In practice, most printers make a trade-off between opting for higher resolution and providing more levels per dot. This is often determined by the printer's intended use. Graphic arts professionals, for example, are interested in maximizing the number of levels per dot to deliver higher 'photographic' image quality, while general users will require reasonably high resolution so as to achieve good text quality and reasonable image quality.
The simplest type of colour printer is a binary device in which the cyan, magenta, yellow and black dots are either "on" (printed) or "off" (not printed), with no intermediate levels possible. If ink dots can be mixed together to make intermediate colours, then a binary CMYK printer can print only eight 'solid' colours (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't sufficient to deliver quality colour printing, which is why there are half tones.
Below, listed under four separate headings: Ink, Material Compatibility, Drop Ejection and Print, are the parameters that must be considered and weighed when formulating a jet ink.
Jet Ink |
Material Compatibility |
Drop Ejection |
Print |
| Good Stability |
Non-Corrosive |
Inform Drop Size |
High Optical Density |
| Low Viscosity |
Plastics Compatible |
High Drop Velocity |
Colour Quality |
| High Surface Tension |
Adhesives Compatible |
High Drop Frequency |
Fading Resistance |
| Conductivity |
No Particulate Formation |
No Orifice Wetting |
Lightfastness |
| Long Shelf Life |
|
Non-Crusting |
Waterfastness |
| Non-Flammable |
|
Non-Clogging |
Solvent Resistance |
| Non-Toxic |
|
|
Smear Resistance |
| No Biological Growth |
|
|
Off-Setting |
| Dye Solubility |
|
|
Crack Resistance |
| |
|
|
Media Sensitivity |
| |
|
|
Spreading |
| |
|
|
Feathering |
| |
|
|
Dry Time |
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Pigment Ink & Dye Ink
The features of Pigment Ink
Does not dissolve in water, pigment is colloid type.
Strong Water Fastness, not easily de-colourised, more permanent than Dye.
Depth of colour is thicker.
The features of Dye Ink
The differences between Pigment and Dye |
|
Pigment |
Dye |
| Good Stability |
Non-Corrosive |
High Optical Density |
| Solubility in Water |
Undissolved |
Dissolved |
| The formulation method |
Dispersion (Colloid) |
Dissolved |
| Hydrophile property radical |
No |
Yes |
| Light Fast |
Excellent |
Inferior |
| The size of particle |
50nm ~ 200nm |
Less than 10nm |
| Application |
Organic solvent (paint) or outdoor |
Textiles |
The differences between Pigment Ink and Dye Ink |
|
Pigment Ink |
Dye Ink |
| Light Fastness |
Excellent |
Inferiority |
| Chromaticity |
Narrow Colour Gamut |
Wide Colour Gamut |
| Waterfast |
Excellent |
Inferior |
| Colour Vision |
Relatively Dull |
Relatively Bright / Vivid |
| Stability of Head |
Relatively Inferior |
Relatively Excellent |
The world-wide tendency in inkjet ink development is heading toward Pigment ink with high waterfast, lightfast and permanancy properties.
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Any Trademarks or Registered Trademarks referred to remain the property of their respective owners and are used solely for product identification. The information contained herein has been accumulated by Ink Sales Plus over a lengthy period from our own knowledge or long-lost sources and much, much more is available on hundreds of sites on the internet. For your convenience, we have paraphrased, assembled and collated some of the more interesting and relevant aspects here. It is given as a matter of general interest and Ink Sales Plus makes no representation as to its accuracy. Some sections have been reproduced in part or in whole with permission from InkTec & Image Specialists Inc.
