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FLEXO Magazine : June2010
Technologies & Techniques www.flexography.org june 2010 FLeXO 53 Despite their name, halos in a printed image are anything but a God-send. The halo effect can be defined as a printed image surrounded with an unde- sired line encircling the dot. Refer to the picture for a visual reference of a halo problem that a press opera- tor may be experiencing. Like many troubleshooting prob- lems, there are multiple causes of the halo effect. The causes can range from very simple adjustments to more involved ink formulation issues. Causes of the halo effect could include the following: • Disproportionate ratio between the plate cylinder diameter to the actual gear. • Ink volume transferred is too high. • Too much pressure between the plate cylinder and the substrate. • Doctor blade pressure is too low. • Excessive pressure exists be- tween the anilox roller and the plate cylinder. In order for an operator to deter- mine the true origin of one of the above mentioned causes, it is necessary to check the outer diameter (OD) of the cylinders and gears. Also, the deck and impression settings would need to be checked and/or the deck reset. This resetting of the deck will rule out, or identify, impression settings as the cause. Once the causal analysis has been completed and the root cause has been determined, the following corrective action steps could be implemented dependent upon the pre-deter- mined root cause: • If there is a disproportionate ratio between the plate cylinder diameter and the actual gear, the press operator should check the thickness of each element on the plate cylinder, including the mounting tape, plate, cylinder, etc. in order to determine the appropriate ratio. The ratio between these two points is highly important in order to achieve the desired clean print. • If the ink volume being transferred is too great, the press operator must consider changing the anilox roller to one of lower volume and higher line. • If the pressure has been determined to be too great between the plate cylinder and the substrate, the press operator should reduce the pressure thus eliminating the halo effect. • If the doctor blade pressure is found to be too low, the press operator should adjust the doctor blade pressure to the appropriate level. • Should excessive pressure exists between the anilox roller and the plate cylinder, the press operator should reduce the pressure to the appropriate level thus alleviating the issue. If you are still experiencing halo issues, the operator should contact the ink vendor for further causal analysis to be performed. As with any process printing, operators must be given the tools to provide a repeatable, quality finished product to the customer. With troubleshooting issues like halos, prepress plays a large roll in predicting success. Prepress part- ners should ensure plate cylinders or sleeves have a runout or total indicated runout (TIR) of less than .0005in. The printer should also en- sure the correct cushion back and plate material is used for optimum transfer and minimum dot repro- duction. Prepress also assumes responsibility for ensuring that all bearings and/or bushings are in proper working condition so that bounce or slop is not a factor. Producing a repeatable press characterization and implementing processes will allow operators to follow what was learned and measure all components such as density and dot gain. n ABOUT THE AUTHORS: Patrick Mollman is a technical lab manager in the Product Stewardship Division of the lab at Siegwerk. He attended Creighton University majoring in Chemistry. In 1995, he joined Color Converting Industries (now Siegwerk) as a lab assistant. As a lab technician, he spent most time improving ink manufacturing and disper- sion. He also spent several years in Print Services developing process printing inks and testing on press. Roger Hall is a national account service representative at Siegwerk. He began in the printing ink industry in 1980 at Inmont. In 1987, he joined Color Converting Industries (now Siegwerk) in the R&D lab. Hall has held many positions in- cluding customer in-house technicians, technical positions in R&D and his recent position. He has spent a great deal of his time with Siegwerk working to develop and improve Sieg- werk’s flexographic and gravure technologies. Troubleshooting Guide: Halos By Patrick Mollman and Roger Hall Art courtesy siegwerk north America FLX_June2010_mech.indd 53 6/10/10 9:39 AM