Eliminate Printing Defects Caused by Static Electricity
I think there's a lot of opportunity to improve sales performance by improving sales compensation strategy. Let's start with the basics.
By William J. Larkin, Founder, Alpha Innovation, Inc.
For the digital and screen printing industry, the most common problems associated with static are dust attraction, printing defects and shocks to operators. Learn how to identify and mitigate these real-world challenges.
Dust attraction is a continuing problem, not only because it is impossible to keep a shop free from air born dust and lint, but also due to the increased use of treated plastic substrates with high static charge. The high static field actively attracts particles to their surface. When a particle of dust gets on a screen, it can create a repeatable defect. A piece of lint in the wrong place can ruin a wide-format print, and everyone has experienced the frustration of finding a particle defect within a completed laminate.
Surface static charge also is the cause of many common printing defects. The root cause is that the static field on a surface will push and pull the ink particles away from their intended trajectories causing printing defects including ghost tracks, overspray and fogging. Not only can these particles land in the wrong places, but they can also be knocked back toward the printheads, which can cause clogging of tiny ejector holes.
Shocks to Operators
One of the most common shock problems to people is really the opposite. The person is not getting shocked but actually discharging static to metal. The operator gets charged as he/she handles plastic sheets from a stack. The lifting of the sheet transfers a static charge to the skin, which is conductive. This stored charge then discharges to any metal near the operator's skin. The person lifts a sheet from a stack, and then feeds it onto a machine. When the operator touches metal, his/her body will discharge to the metal.
At one company, for example, an operator was receiving painful shocks whenever he attempted to lift a screen printed UV dried substrate from the bed of the press. The plastic sheets were so highly charged, that he became charged instantly as he lifted the edge of the sheet. He would then discharge back from his skin to the metal bed. This type of shock also is common with many types of laminators.
The solution to these problems is not conventional because in some cases, it is necessary to simultaneously ground the operator and eliminate the static on the substrate. By providing the operator with a grounded, ionizing wand we were able to simultaneously ground him and ionize the charged substrate. Now, as he lifts the sheet, he slides the wand under the media to remove the trapped charge, and thus avoid the static shock.
Many plastic substrates - including styrene, polyvinyl chloride (PVC), polycarbonate and various types of foam boards - carry a very high level of static charge. This is because most of these materials are treated with high-voltage corona to improve their printability. This treatment not only adds a high initial charge to the substrates, it also increases their propensity to carry an even greater charge, such as lifting, sliding, rubbing, etc.
Some methods that have been attempted in the past to eliminate static include, antistatic sprays, direct grounding and adjusting the level of humidity.
Antistatic sprays must leave a residue to be effective, and this can affect the surface printing. Grounding is not effective because, by definition, an insulator cannot be grounded-and it is the insulative surface that will hold the static charge. If simple grounding worked, there would be no need for static eliminators. The truth is, static charge on a plastic surface must be ionized away. Humidity is generally not effective because many substrates are not hygroscopic, meaning they can't absorb moisture.
Certain static problems have been difficult to solve with conventional static eliminators because it is impossible to remove static from a sheet when it is lying flat, while in a stack, or while on the bed of a machine. The static charge is hiding between the sheets in the stack, or between the sheet and the bed, and cannot be removed with conventional static eliminators. The sheet must be lifted into space exposing the field, or we must slide the static eliminator between the sheet and the machine to ionize the static field.
Conventional static eliminators are difficult to design and use in this manner. We have been designing useful ways to remove the charge from under the sheet, and have had success with a PSA ionizing tape that can be placed on the platen of a press. This will ionize the troublesome voltage from the substrate as it passes over.
In roll-to-roll printing, when a substrate like styrene or PVC is unwound, there are two common static problems:
- Dust is attracted from the floor and the air. In this case the elastic cord from Alpha Innovation is used to eliminate the static. The operator simply places one length of the cord across the unwinding roll and contacts metal using an attached magnet.
- Static on the substrate going into the printer also can cause printing defects. The operator prevents this by placing a length of the elastic cord across the sheet, before it contacts the printer. He/she will ground the cord using the magnets.
Regeneration of Static
In some cases, static can regenerate on the substrate due to contact with the printer's pull rollers on the platen bed. There are two new solutions to this problem. One is a static dissipative coating for the rollers to stop the generation of static, and the other is the PSA tape, which is placed on the platen after the rollers.
Wide-format hybrid printers accept both flexible and rigid substrates. These types of printers have had success using the elastic cords across the substrate, as it enters the printer. Many of these printers also have pull rollers, which also benefit from the static dissipative coating.
As in the earlier example of the screen printing company, an operator picks up a foam board to move it onto the flatbed. Just before bringing the substrate over to the registration points, he passes the ion wand under it, or on both sides using the double wand. It is also good practice for the operator to wear ionizing gloves to prevent fingerprints and static generation on the surface. The substrate is ready to be printed without defects. There is no need for rubbing or cleaning the substrate, as this will cause more static generation.
With new static problems continually arising with the implementation of new printers and media, we are constantly developing innovative solutions to solve static problems.
William Larkin is the founder and technical director of Staticure® - a division of Alpha Innovation, Inc. The company develops static eliminating solutions for the digital printing, screen printing, label and converting industries. The company has been providing solutions for static problems for nearly 30 years. email@example.com
This article appeared in the SGIA Journal, July/August 2011 Issue and is reprinted with permission. Copyright 2011 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.