by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : March 2011
Technologies & Techniques tion for UV inks. A thicker ink film may not be thoroughly cured at the same speeds. • Use a calendared paper sheet. It will be less porous be- cause of the compression of the surface paper fiber. • Apply a “sizing” primer on press to take into account extreme, uncontrollable substrate variations. Paper coatings. These are used to fill the voids between the paper fibers and level the surface of the substrate. Coat- ings can also provide a white or glossy surface to enhance brightness and improve graphics. Generally, the paper coat- ing provides a good surface for adhesion. But, occasionally we see paper coating that is poorly adhered to the fiber and can flake or dust off during printing; or subsequent convert- ing. Sometimes it is difficult to determine the cause of the adhesion failure; and a sophisticated analysis is required to determine the actual point of failure. (See Figures 3a and 3b.) Paper coatings can also show variations in hydrophilicity across the web. Ink absorption tests can identify inconsistent, water-loving surfaces that affect drying speeds. Paperboards with uniform levels of reception allow water-based inks to dry acceptably, but and areas of increased lipophilicity will not permit adequate penetration at elevated press speeds. The Solutions • Although a penetrating primer offers a quick fix, the best course of action is to involve the paper supplier. • The only way to get around variable ink penetration in the short term is to slow the press down to increase the dwell time. This provides more time for liquid inks to penetrate before being set by the dryers and ensures adequate dryer capacity. The paperboard supplier will need to ad- dress long-term needs. Paper dust. Paper dust presents a huge problem for paper and paperboard printers. It gets into the sumps, absorbs ink, sticks together and forms lumps that get under the doctor blade and create streaks. Lumps can prematurely wear doc- tor blades, anilox rolls and cylinders, as well. Paper dust can build up on plates and cylinders and clog anilox roll or gravure cells. (See Figure 4.) The resulting defects include missed areas, dark spots and intermittent loss of color strength. UV inks are typically more susceptible to paper dust contamination because of their higher viscosities. The Solutions • Brush and blow the cut edges of a paper roll outside of the press- room. • Keep sumps closed and use inline filtering systems. • Filter work-off ink through cheese cloth or a similar medium to remove particulate contamination. • Improve pressroom housekeeping. • Use vacuums, adhesive rolls or other web cleaning devices. • Employ static control techniques. • Partner with your paper supplier to reduce paper dust. Low surface energy. Inks and substrates have surface en- ergy (dyne level). When the substrate’s surface energy is low and the ink’s is high, the ink will tend to bead up on the sub- strate’s surface instead of wetting out the surface and laying smoothly. This issue primarily affects film substrates. Some films may have lower surface energy than the ink, particularly if the ink is water-based. (See Figure 5.) If the substrate has low surface energy, inks may adhere poorly, as evident by doing a tape or crinkle test. The print defects and adhesion defects can occur at different times. When there is poor adhesion, it is necessary either to raise the substrate’s surface energy, or decrease the ink’s surface energy. Otherwise, poor wetting of substrate may result in mottling, an orange peel effect or pinholing up to complete reticulation of the ink on the film surface. The Solutions • Select a film with a more appropriate surface energy. • Decrease the surface energy level of the water-based ink by adding solvents or surfactants. • Corona, or flame treat, the substrate. This is best done in-line with the printing operation to ensure maximum treatment. Note: It is not always possible to treat previ- ously untreated film in-line adequately, particularly LDPE. In-line treating is often only viable if the film was already treated at the time of extrusion. Figure 3a. example of adhesion failure of paper coating after printing. Figure 3b. example of paper coating failure before printing. Figure 4. example of Paper dust contamination. 20 FLeXO march 2011 www.flexography.org Surface Energy Non-Wetting Liquid Wetting Liquid A liquid on a relatively low energy surface will not spread out and wet the substrate surface; while a higher energy surface allows the liquid to spread and better wet the surface providing more intimate contact between the liquid and the substrate. Figure 5. surface energy. FLX_March11.indd 20 3/18/11 1:32 PM