Ontario Specialty Coatings Corporation
THE LEADER IN DIGITAL IMAGING COATING TECHNOLOGY
The best evaluation of a printed ink jet coating is to simply look at it and compare it to competitive products. Whether your finished product is paper, film, tyvek, non-woven, or textile, a truly good print result will stand out. Not only will you notice it, but your customers and their customers will notice. Print quality, in a phrase, is what everyone is looking and paying for.
However, to be able to compare print results across different coatings and different media substrates, its helpful to have objective measurements of print quality. Several years ago Hewlett Packard Corporation (HP) put together several different test procedures in an attempt to establish some minimum quality standards for ink jet paper printability. These tests are applicable to most media besides paper and have endured as measurements of printing quality. HP had originally established acceptance criteria for each test but with the rapid advances in hardware, software, and media, some of these criteria have fallen into disuse or are no longer applicable. Manufacturers now use their own established standards that they have defined to compare the print quality of different coatings and media. There are usually one or two very high quality products in each category of ink jet media which can serve as a standard.
Optical density is a measurement of the reflected image density. A standardized test pattern, (which is usually only standardized for your operation), is printed on the media. The optical densities of the individual pigment black, cyan, magenta, yellow, and composite black are measured using a densitometer (example: Macbeth RD918). The measurements are then compared to a standard which could be your present formulation or a competitive product. The higher the optical density reading, the better the coating is maintaining the dyes and/or pigments in the top of the coating surface where they produce the best looking print result.
Spreading and Feathering (Bleed):
When two different colored inks are printed next to each other, it is prefereable that the border between the two colors be as distinct, clean, and free from invasion of one color into the other as possible. When one of the colors intrudes into the other the boundary between the two colors becomes tattered and uneven. This is known as spreading and feathering (bleeding). It is due to the random uncontrolled movement of ink jet ink upon or within the coating just after the ink is applied to the media. Spreading is an overall expansion of the printed area, while feathering is usually defined as the wicking of the ink along a narrow band or line away from the main image area. Both can be characterized as "bleeding". A certain amount of spreading and feathering is necessary to correctly translate the desired image to the media, but too much will cause the image to appear distorted or out of focus. Spreading and feathering can be quantified by a digital imaging system which looks at a specific portion of the standard test pattern (usually a small, distinct object like the dot on an "i"), measures the area of the image, and compares that to a standard. With practice, this test can identify very small differences in spreading and feathering tendencies between different coatings.
Halo can be characterized as the whitish area which forms around the edge of area printed with pigmented inks caused by the printing of dye based inks in area directly adjacent to the pigmented ink print. The dye based ink intrudes upon the pigmented ink and forms a whitish edge around the pigmented ink.
Ink Drying Time:
Ink drying time measures the rate of ink absorption into the surface coating of the media. Usually, the faster the drying time the better. Again, a standard test pattern is printed in composite black ink. The image is then blotted and the optical density of the transferred black is measured. These measurements are then applied to a mathematical model developed by HP to determine the drying time. Composite black is used because it is the combination of cyan, magenta, and yellow and is therefore the greatest volume of ink that can be applied. Assuming that the composite black ink dry time is adequate, one can assume that the individual cyan, magenta, and yellow ink dry times will be at least as good or better. Drying times between developmental coatings and the standard are then compared.
The waterfastness test provides an objective measurement of the amount of ink transferred from a printed area to a non-printed area when water runs over the sheet. A standard test pattern is printed onto the sheet and 250 microliters of deionized, distilled water is applied to the sheet and allowed to run from the printed area to an adjacent unprinted area. After allowing the sheet to dry, an optical density measurement is then taken on the printed, wetted area and compared to the optical density on a printed, dry area. The closer the optical density of the wetted area is to the dry area, the better the waterfastness of the printed coating. This test is run on a standard and developmental ink jet coating and the results are then compared.