By Mary Schilling, Itinerant Inkjet Investigator
Paper is a huge factor when evaluating and integrating inkjet into your production workflow. Differences in ink chemistry and carrier combination make each inkjet device print differently (or not at all) on low porosity media. This is challenging for the paper industry when trying to create gloss coated papers which work well across a range of devices.
Different coated mill grades will perform differently. The same coated mill grade will also perform differently in different paper weights. Higher gsm coated media, such as cover stocks, will have less porosity than text of the same paper grade. The higher gsm has more coating applied to fill in the thicker base sheet. The image below shows how gloss coated media will dry slower and show more color bleed and coalescence than matte or satin coated. This happens because the matte and satin coated media has less coating and more porosity, which speeds up ink drying.
Coalescence is the merging of printed ink drops when they make the slightest contact on low porosity media. Drops jetting onto other wet drops, or ink and media surface tension difference, will pull drops together. Coalescing drops make areas darker and give a mottled appearance. Mottle is more pronounced in areas of solid color and less in images, as seen in the example below.
Inkjet manufacturers are changing ink chemistry, or adding pre-coating options to the print process, creating a more compatible media surface. Chemistry changes and jetted or anilox applied coatings do, in fact, help with the inks wet-out and slow the ink spread for inkjet’s wet-on-wet process. However, this doesn’t completely eliminate the visual appearance of coalescence on very low porosity coated stocks.
Coated papers being more expensive, we expect higher quality from the printed piece. Gloss coated, which holds the ink high on the surface, will show any defect from inconsistent pre-coating, print head jetting and even inconsistent paper coating. Note that, even though we may not see a visual difference in surface coating, offset coated papers currently used in inkjet can come with inconsistencies that originate at the paper mill. If coating is not evenly applied across the entire paper machine, the roll or ream you receive will print and dry differently than the one before.
The lower the porosity on mill coated paper, the higher the chance for coalescence.
You may also see a difference in drying and coalescence from one side of the sheet to the other. This may not be a coating issue or may happen because the ink had less time to dry one one side within the inkjet device. Printing higher coverage on the first impression will allow more dry time within the inkjet device.
Remember, not all coated stocks are the same, this is why you will get better print results from a text weight matte coated sheet than from a gloss coated cover stock.
Testing paper at different speed and drying settings as well as testing multiple reams or rolls will help your press team better understand the variance which coated paper can bring to your workflow.
Test and run coated media wisely my friends.