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 : June 2014
the footprint—the area of steel in contact with the anilox roll—will increase, thereby increasing the potential for ink to get past the doctor blade. Although not indicated in the chart, the material tested was a long life tool steel with a laser hardened working edge. The extra hardness provided by the laser further reduces blade wear and helps to prevent the working edge of the blade from over deflecting. The next element for comparison is the width of the blade. The blade width should be what the chamber OEM specifies, since running wider blades was shown to increase the likelihood of spitting issues. A wider blade will extend further in the chamber. It will also deflect more under load—due to more of the blade being unsupported—and contribute to the spitting problem. The data in the chart also suggests a beveled blade is less likely to contribute to spitting than a lamella blade. Of the beveled blades, the sharper 15 degree bevel was preferred, followed by the longer 4 degree bevel angle. The lamella edge, by nature, has more difficulty dealing with high viscosities and higher blade loading pressures. The 15 degree bevel angle will provide a more firmly supported working tip for the doctor blade than either the 4 degree bevel or the lamella edge. The extra support provided by the 15 degree bevel will help to resist blade tip deflection and fluttering due to the higher viscosity. CONTACT ANGLE & APPLICATION PRESSURE Generally accepted doctor blade contact angles for flexographic print- ing will fall into the range of 25 degrees to 42 degrees with a desirable angle being near 30 degrees. Testing has shown that for UV inks, the contact angles should be at least 30 degrees or more to minimize UV spitting issues. The sharper contact angles will allow the doctor blade to cut through the film of ink and clean up the surface of the anilox roll, leaving the desired matte anilox surface appearance. Contact angles less than 30 degrees will tend to leave some surface ink on the anilox and contribute to the spitting problem. In extreme cases, a harsh operator will create a severely flat contact angle, due to excessive application pressure that can cause the tip of the doctor blade to lift off the anilox roll. The lifted tip will provide an easy path for the UV ink to get past the doctor blade (See Image 3). In chamber and most single blade reverse angle applications, the blade angle is fixed by the geometry of the equipment. If a blade analysis indicates that your contact an- gles are less than desirable, they can generally be improved by ap- plying less blade loading pres- sure. Contrary to the practices of a harsh operator, always run with the least amount of application pressure possible while still obtaining a clean wipe, indicated by a matte appearance on the anilox roll. If you can’t lower your pressure, investigate and determine if something is preventing from operating at a lower pressure. Areas to look at would be: • Improper end seals for the application • Excessive ink flow rate • Chamber or blade holder actuating mechanisms that need maintenance 64 FLEXO | JUNE 2014 Image 1 Image 2