Ink Jet Testing Protocol

by Annette Groscup


Designing a quality ink jet cartridge involves more than simply selecting component manufacturers. The creation of a reliable printing system requires a general knowledge of ink and media interactions, a test program that approximates end use and a means of ensuring consistent product quality.
A wide variety of quality assurance checks can help one meet these objectives, but the single most important check is how the system comes together to deliver the final product: a printed image. Print testing should be conducted in a manner that closely replicates typical consumer usage patterns. In order to design the best print testing protocol, it is important to understand how the ink and media interact to affect the quality of the final image.
Ink
Many physical properties of ink can be tested, but surface tension, viscosity and ink purity most directly relate to image quality. Both the surface tension and viscosity of ink affects flow through a cartridge sponge or bladder. A flow that is too fast may cause flooding and dry-time issues, while a flow that is too slow may cause banding or laydown variations.
How well the ink drops remain intact during flight between the print head and the media is a function of both the ink and the print head design. Usually, the print head variables are not easily changed, so the focus falls to the ink. Drop shape, size and integrity are dependent on the ink chemistry as well as the two important parameters of viscosity and surface tension. Ink drops that break apart in flight can cause overspray and satellite drops to appear on the print. Text sharpness and intercolor bleed are just two of the many image properties affected by the surface tension of the ink.
Viscosity not only regulates ink flow, but also has an effect on laydown, dry time and ink spread. If the ink viscosity is not controlled, then drop sizes may vary and an uneven laydown in solid block printing will occur. If excessive amounts of ink are applied to the media, the image dry time will increase, and smearing may occur as subsequent pages are printed or as the print is subjected to normal handling.
Insufficient ink spread will result in gaps or mottling on the image. Conversely, ink that spreads too far may affect text sharpness and can cause inter-color bleed or migration of ink beyond the image boundary.
Note, however, that properties such as surface tension and viscosity cannot be used independently as predictors of the ink's performance. The ink's actual behavior results from a complex interaction of all the various measurable physical parameters. Simply altering these physical parameters will not correct all print issues.
As image resolution continues to increase, print head nozzles are getting smaller and smaller, and ink purity becomes more important. Impurities in the ink can cause the print nozzles to clog, which can then lead to a wide array of print test issues ranging from lining to total print failure.

Media
Selecting the appropriate media for print testing is important but the wide variety of available media can make this a daunting task. It is widely known that the quality of an ink jet print is greatly affected by the media that is used. Testing for photo-quality appearance on copier paper or printing large files of black text on photo glossy paper may not adequately replicate end use.
An estimated 80 to 90 percent of all end user printing is done on plain bond copier papers; therefore it is critical to ensure that print testing is performed on a number of different copier papers. A wide variation exists in papers. The range goes from papers containing all virgin fibers to those containing large quantities of post-consumer recycled fibers. Cotton, sugar cane and rice papers are common in some geographic locations.
Length and orientation of the paper fibers, and the size and distribution of surface voids will affect how the ink is absorbed by the paper. While it is impossible to test on every paper, a thorough knowledge and understanding of the markets in which your cartridges are used will enable you to choose the best array of papers for testing.
Premium coated papers and photo glossy papers are also key media for print testing. Premium papers are designed to enhance the appearance of the printed image, and are ideal for presentations, reports, brochures and other applications that mix text and graphics. The coatings used on these papers may cause color shifts with certain inks, so a variety of images representing the complete color gamut should be examined.
Photo glossy paper was designed for photographs and high-quality graphics. print testing should ensure that photo-quality images that exhibit realistic and natural color reproduction are produced. Dry time issues on glossy paper are not uncommon, and evaluation of dry time should be incorporated into the test program. Although they are valuable components of a thorough test print program, these papers should be used selectively, and the test images chosen should approximate typical use.
Specialty media, including transparencies, cards, labels, tattoos, stickers and fabric are gaining in popularity and may be a valuable addition to your print test program. It is important to consider the typical end user applications when determining what, if any, specialty papers are used for print testing. Each type of specialty media has its own print issues. For example, dry time on transparencies, feathering and edge acuity on fabric and brilliant color reproduction on stickers illustrate a few of the issues that can be assessed with print testing.
Test Images
Once the appropriate media has been selected, it is essential to choose test images that best mimic end user conditions. Almost any image can be used for testing, but it is important to ensure that a variety of image and text types are evaluated.
Text printing, especially on copier paper, accounts for the majority of all ink jet printing, and is therefore the most fundamental of print testing performed. Character sharpness is important in evaluating text printing. The characters, especially as font size decreases, should be sharp, well defined and easy to read. Another important consideration in text printing is how well the ink dries on the paper. If drying is too slow, the print will smear as subsequent pages emerge from the printer.
Printing single blocks of any color at 100 percent coverage is one of the most stressful print tests. When performed at varying print speeds and resolutions, print issues such as lining, banding and laydown variation easily can be discovered. These large blocks of color also provide an ideal sample page for testing color match, lightfastness and other properties.
Remember, however, that the typical consumer will not be printing large, single-color objects. So, although these prints are important, they are only a small part of a well-designed print testing program.
Inter-color bleed, especially on copier papers, is another common print issue. Black and yellow tend to have the most inter-color bleed problems, but other colors may also exhibit this phenomenon.
There are several different ways to test for this: printing text on solid blocks of color, printing color blocks with adjoining edges, or printing lines of one color through blocks of other colors are all ways to evaluate for inter-color bleed.
Inter-color bleed issues tend to be minimized — but often not eliminated — on premium coated and photo papers so testing on these media should also be performed.The use of graphics for presentations is a common application for ink jet printing. Graphics can include text, blocks, pictures or graphs, and a test print that incorporates all four components easily can be designed. Premium coated paper is the typical media used for presentations, and print testing should be run on this media. Copier paper should also be tested as draft copies of presentations are often printed for review or handout purposes.
With the increasing popularity of digital cameras and scanners, end users are doing more photo-quality printing, both in the business and home markets. The most critical aspect of photo printing involves tonal reproduction, especially with skin tones.
Accurate reproduction of grayscale, vivid colors and metallic objects is also important. Each printer has a list of recommended photo media, and it is important to check print quality on these media. In addition, common third-party brands can be tested if appropriate.

A thorough print test protocol is the best way to evaluate how the complex interplay of printer, ink and media results in high-quality images. A basic understanding of ink, media and typical print issues will enable you to design the most practical print test program for your intended market.
Although it is impossible to imitate every potential print application, a well-designed print test protocol will ensure that your product is flexible enough to succeed in today's rapidly evolving ink jet market.
Annette Groscup