- December 07, 2012, Kelly Robinson
Static charge can cause a number of problems in web forming, coating, laminating, and converting operations, including shocking operators, igniting flammable solvent vapors, and attracting airborne contaminates to the web. Passive dissipaters, like static brushes and ionizing cords, or active dissipaters, such as AC static bars, are used to neutralize static. How far away can a static dissipater be mounted and still neutralize a web?
The static bar in Figure 1 is properly located to neutralize charge on the top surface of the web. The “common sense” answer is that the static bar is most effective when it is as close as possible to the web. However, the web flutters on spans, so the dissipater usually is located far enough away so that it will not touch the web.
I have 3 rules for deciding how far away to mount most static dissipaters.
- RULE #1: Follow vendor recommendations.
Ask your static equipment vendor to recommend the appropriate spacing for their equipment. My experience is that vendors know their equipment. They can offer informed and expert recommendations.
- RULE #2: The minimum spacing is twice DPOINT-TO-POINT.
Most static dissipaters generate ions using an array of sharp points. The static bar in Figure 2 has a row of pins that generate ions. Measure the point-to-point spacing DPOINT-TO-POINT. The minimum gap is twice the point-to-point spacing.
GAPMIN = 2×DPOINT-TO-POINT
For example, a static bar with a point-to-point spacing of 0.5 in. should be mounted no closer than 1.0 in. from the web. If the static bar is positioned to close to the web, there will be stripes of static on the web exiting the static bar because the static bar fails to neutralize charge in lanes on the web midway between pins.
RULE #2 applies to passive static dissipaters. A strand of tinsel has a sharp point at the end of each fiber. So, the minimum spacing for tinsel is twice the maximum distance between fiber tips. A typical strand of tinsel has fiber tips at least every 0.25 in., so the tinsel should be located at least 0.5 in. from the web.
Some static brushes have a full “nap” of conducting fibers similar to a paint brush. A static brush with a full nap will have good neutralization uniformity even when it is mounted so that it touches the web. Similarly, ionizing cords have ionizing points that are very close together. Ionizing cords also will have good neutralization uniformity when they are mounted to touch the web.
- RULE #3: The static dissipater must be the closest grounded object.
Use RULE #3 to find the maximum distance for most static dissipaters. There is a new type of static bar called a long range or a distance bar. These active dissipaters can neutralize distant webs. I’ll discuss this technology in a future column. RULE #3 applies to passive dissipaters and traditional AC static bars.
In your mind’s eye, draw a line across the web in Figure 3. This line is the center of a cylinder that just touches the closest pin on the static bar. Ions generated by the static bar will reach the line on the web when there are no conducting objects inside the cylinder.
We can use RULE #3 to maximize the performance of our static dissipater. The static bar in Figure 4 is as close as possible to the web (RULE #2: GAPMIN = 2×DPOINT-TO-POINT). With the static dissipater located close to the web, we can draw lines A and B almost half way to the nearest rollers where the static bar is the closest grounded object. Clearly, ions generated by the static dissipater can reach any point on the web between lines A and B. The performance of the static dissipater is higher when lines A and B are farther apart.
To maximize the performance, mount the static dissipater so that lines A and B are as far apart as possible. Static dissipaters should be located on web spans approximately midway between rollers. Look for locations that are far away from other metal objects such as machine guards and frame members.
When deciding how far to mount a static dissipater from the web, first ask your static equipment vendor for their recommendation. In general, the minimum distance is twice the point-to-point distance on the static dissipater. This ensures neutralization uniformity. And, the points on the static dissipater must be the closest conductor to the web. Note that a new generation of long range ionizers or distance bars can be located farther from the web. I’ll discuss this technology in a future column.
I invite you to ask questions about this column and to suggest future topics. My e-mail address is: Kelly.Robinson@ElectrostaticAnswers.com.
Static control expert Dr. Kelly Robinson, president of Electrostatic Answers, has 27+ years of experience in problem-solving and consulting. Contact him at 585-425-8158; firstname.lastname@example.org; www.electrostaticanswers.com.