- November 04, 2013, Kelly Robinson
Static brushes and ionizing cords are passive dissipators used to neutralize static charge on webs, polymer covered conveyance rollers, and on unwinding rolls. Often, passive dissipators are mounted so that they touch webs or unwinding rolls. Is this necessary? If not, what are the advantages and disadvantages of having the dissipators touch?
Passive dissipators such as static brushes, strands of tinsel, or ionizing cords need not touch a web to dissipate static charge. The static brush in Figure 1 relies on corona ionization to produce ions to dissipate static. Both positive and negative corona ions are produced in ionization regions, which, if the room is very dark, are visible as faint, blue glows at the tips of the bristles.
The negative charges on the web in Figure 1 attract positive corona ions from the static brush. The corona ions move through the air across the gap. As the corona ions move across the gap, they spread out and deposit relatively uniformly on the web.
Of course, larger gaps mean that the corona ions must travel farther, so the neutralization performance of the static brush decreases. Typical gaps are in the range 0.25 to 1.0 in. The maximum gap for the static brush to effectively neutralize the web depends on the geometry.
Brushes, tinsel, ionizing cords, and rods that don’t touch the web have several advantages. The static dissipation performance is relatively uniform. Each individual static charge on the web is approximately equally distant from the static dissipator, so the flow of ion across the gap is nearly uniform. Also, the static dissipators last longer because there is little wear and tear when you don’t touch the moving web. Finally, the static dissipator causes little contamination since there is little wear and tear.
Draping the ionizing cord so that it touches the winding roll in Figure 2 offers an important advantage. The ionizing cord neutralizes the outside lap even as the winding roll builds. So, having the static dissipator touch the moving web works well in applications in which the web path changes, such as for winding rolls and unwinding rolls.
Brushes, tinsel, ionizing cords, and rods that touch the web have several disadvantages. The static dissipation performance is spotty. At a microscopic scale, the ionizing tips slide and bounce along the moving web. The deposition of corona ions from these tips illustrated in Figure 3 is very local because the gap is so small.
This non-uniform charge may have little impact on our end-use application. So, draping ionizing cords on unwinding rolls and on winding rolls is often practiced. However, this non-uniform charge pattern can contribute to sheet sticking and jamming in cut-sheet and label applications.
Another advantage of contacting static dissipators is that they stay cleaner in paper converting and other applications that have relatively high levels of dust and debris because the sliding and bouncing contact with the moving web shakes dust and debris from the dissipator. However, the working life of contacting static dissipators is shorter because of wear and tear from the moving web.Finally, contacting static dissipators can become a source of contamination because of the wear and tear from touching the moving web.
Passive static dissipators need not touch the moving web to work effectively. They work effectively at gaps in the range 0.25 to 1.0 in. from the web. They should touch the web only when there is a clear and necessary advantage from touching the web.
I invite you to ask questions about this column and to suggest future topics.
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; email@example.com; www.electrostaticanswers.com.