Take back control2 April 2019
The esteemed Kelly Robinson speaks with Converting Today with regards to the best practice in the management of static control. Through a series of provided figures, solutions are provided to combat a plethora of issues usually encountered within the process.
We have been trying to control static in the printing and converting industry for more than 100 years. Even with our diligent efforts and modern static dissipators, static sparks continue to ignite flammable solvent vapors, shock operators, and damage sensitive products such as release liners. And, even when these efforts succeed and sparks are suppressed, static charges on our products attract airborne contaminants and cause sheets and labels to stick and jam.
By no fault of our own
We can reliably solve these many static problems using a ‘fault tolerant’ static control system. The fault tolerant static control system presented in the first figure is designed to maintain satisfactory static control, even when any single system component fails. The overarching strategy is that, at the end of the day, everyone goes home safe and sound. When a failure occurs, static control may diminish. However, we design the system to maintain sufficient static control to prevent disaster.
The first step is to identify areas in our operation where static is a risk. For example, one risk area is a solvent coater where flammable solvent vapors may be present. A second risk area is a winding roll where operators must handle finished rolls. Static that accumulates on the winding roll may cause severely shock nearby operators.
The fault tolerant static control system in figure one has two layers of protection. The first failsafe layer protects risk areas from static. For example, for a solvent coater where flammable solvent vapors may be present, we protect this area with a powered static bar located on the web span entering the solvent area to ensure that no static enters. This common and best practice is effective as long as the powered static bar functions normally. However, should this powered static bar fail, the solvent area is exposed to the threat of static on the incoming web represented by the ‘fault’ in the aforementioned figure.
The second layer of protection maintains satisfactory static control, even if the powered static bar fails. For the second layer of protection, we find where static is first deposited onto the web using a static survey, and dissipate static on the web exiting the source. When static is controlled at the source, a charge-free web enters the solvent area. The first layer of protection is redundant. Of course, if our static control system has a failure in the source control layer, our failsafe layer prevents ignitions.
Static threats penetrate into risk areas only when there are two, simultaneous failures. Our verification procedures must detect a failure and our maintenance procedures must fix the failure before a second failure occurs. General machine verification procedures are commonly performed just prior to running a job during set-up. Common set-up tasks include mounting the rolls of raw material on the unwinds, properly threading the web, checking machine set-points are checked, installing machine guards, and many other things. Include in our job set-up a visual ‘fit for use’ inspection of all static dissipators. Use a checklist to verify that each static dissipator on the line is present, in good working order, properly spaced, and clean. This visual inspection would prevent many static problems.
Look closer for comfort
Let’s take a closer look at the static control system for the solvent coater in the second figure. Solvent coaters are a high-risk area of our operation because flammable solvent vapors may be present. Our static control system must suppress all sparks because sparks that are 10 times smaller than I can feel can ignite common solvents. The minimum ignition energies or MIE’s of common solvents are in the range 0.1mJ to 3mJ. The smallest spark I can feel is 6mm long and drawn from my finger to a door knob
after I walk across carpet. The energy of this spark is ~30 mJ.
We can suppress sparks in the solvent coater in figure two with a static control system having four elements:
- In-feed powered static bar
- Out-feed powered static bar
- Well-designed out-feed area of the coater
- Static dissipative pressure roller.
First, the in-feed powered static bar is our first-layer, failsafe protection against static on the incoming web. While I recommend using a powered static bar, a passive dissipator such as a tinsel strand or static brush may be used. The in-feed static dissipator is prone to failure from being splashed with coating solution. Many powered static bars provide a verification output or a ‘clean bar’ indicator when maintenance is needed. Passive static dissipators must be verified manually.
The in-feed static dissipator may be placed either above or below the web. I recommend installing it above the web so that the web protects the dissipator from being splashed with coating solution.
Second, the out-feed static bar protects against coater failures. In normal operation, the coated web exiting the coater and entering the dryer has no static. Most coating solutions have enough conductivity so that the coated layer allows static on the web to flow to ground through the coater. However, suppose that the pump that delivers coating solution fails so that the coating roller runs dry. The coating roller presses the web against the backing roll that can deposit a large amount of static on the web. The static on the out-feed web can cause a spark to the nearest grounded object that might be the shroud on the coater, a machine frame, or the first idler roller after exiting the coating nip. The powered, out-feed static bar dissipates static on the web exiting the coating nip before a spark occurs. The out-feed static dissipator is prone to failure from being splashed with coating solution. Many powered static bars provide a verification output or a “clean bar” indicator when maintenance is needed. Passive static dissipators must be verified manually.
Third, the out-feed area of the coater must be free of metal objects between the coating nip and the out-feed static bar. Imagine that you shrink yourself to a very small size and become a spot of static charge on the out-feed web. You will with throw a spark to the first metal object that you see. The first object that you see must be the out-feed static dissipator, which will smother you with dissipating ions before you can throw your spark. So, the out-feed area of the coater must be well designed.
Our fourth static control element for the solvent coater in the second figure is a static dissipative pressure roller. The web can deposit static charges on the pressure roller. In normal, steady-state operation, static charges accumulate on the pressure roller until sparking occurs. Static on pressure rollers in solvent coaters must be controlled to suppress sparking. I recommend using a static dissipative roller. Most roller vendors provide static dissipative rollers. However, we must specify the volumetric resistivity of the roller material.
Take it to the maximum
I implore you to use figure three to find the maximum volumetric resistivity for your pressure roller. To make the third figure, I require that static on the surface of the pressure roller dissipate within one revolution. Faster web speeds require lower resistivity rollers. And, larger rollers turn slower than smaller rollers. So, larger roller can have higher resistivities than smaller rollers. For example, with a web speed of 600 feet per minute, a 4-inch diameter roller must have a volumetric resistivity less than 1×10+9 W-m.
I have seen static dissipators used to dissipate static on some coating pressure rollers. While this works, I recommend instead using a static dissipative roller. If the static dissipator on the backing roller fails, sparking may occur. Our static control system is not a ‘fault tolerant’ system because a single failure results in sparking.
The corona treater in the fourth figure is an example of a source of static. Controlling static on the web exiting a corona treater is part of our second layer of ‘fault tolerant; static control in the first figure. A corona treater is commonly used to improve the wettability of a surface prior to coating. The treatment may also improve the adhesion of the coated layer to the web.
A corona treater is a small chemical reactor that oxidizes the web surface. This change in the surface chemistry may be detected using a dyne pen or by analytical methods such as x-ray photoelectron spectroscopy or XPS.
Corona treaters have at least two unwanted by-products: ozone and static charges on the web. Ozone is formed in the electrical, corona discharge in the treater. Ozone irritates the respiratory system and harms lung function so it is collected by an air handling system built into most corona treaters.
Static charges on the treated web surface are a second unwanted by-product of corona treatment. A common misconception is that the improved wettability from corona treatment is caused by this static charge. The benefits of corona treatment are caused by the chemical changes of the treated surface. Neutralising the static charges has no effect on the wettability of the treated surface or on the subsequent adhesion of the coated layer.
The best practice is to dissipate static on the web exiting a corona treater with a static dissipator facing the treated surface. You may use a passive dissipater such as a tinsel strand, an ionizing cord, or a needle bar. All passive dissipator leave a small amount of static on the web. To dissipate all of the static exiting a corona treater, use a powered static bar.
Static control in the printing and converting industries have challenged us for more than 100 years. Commercially available static dissipators are very effective when they are installed as part of a “fault tolerant” static control system that protects against static ignitions, operator shocks, product damage from static sparks, attraction of airborne contaminants, and static jams.