Static Beat | Static Control for Nip Rollers - Paper, Film & Foil Converter

Static Beat | Static Control for Nip Rollers

Published: December 04, 2018, By Christine Pietryla Wetzler

By Kelly Robinson

Nip rollers are common because they provide high traction and they isolate web tension zones. For example, a tension control nip near a winder can maintain constant tension upstream and allow the winding tension to taper improving wound roll quality.

While nip rollers are very important, they can deposit large amounts of static on webs. The best practice it is to dissipate static on webs exiting nip rollers. Let’s look at exactly where to install a static dissipator exiting a nip.

Figure 1: Measurements E_IN and V_{IN BOT} confirm that the incoming web is charge-free.

The polymer nip roller in Figure 1 ensures good traction between the web and the driven metal roller. High traction is important because this enables the tension of the outgoing web to be different from the tension of the incoming web. This tension variation is controlled by the torque applied to the driven roller.

While nip rollers are important for controlling web tension, they have an unwanted by-product; static on the web. Dissipate this static on the web exiting the nip using a powered static bar (see Static Beat | Choosing Passive or Active Ionizers) or a passive static dissipator (see Static Beat | How Do Passive Dissipaters Neutralize Static?). However, just where shall we install the dissipator? Would Static Dissipator #1 in Figure 1 perform better than Static Dissipator #2? Let’s take some static measurements to decide which location is better.

Table 2: Static “Stop Light” Levels

Let’s assume that we’ve done a good job with static control upstream of the nip roller so that the incoming web in Figure 1 is charge-free. Better yet, let’s verify our assumption. Use an electrostatic fieldmeter to measure E_IN in Figure 1 (see Static Beat | ‘GeeZE’ Rules for Static Readings).

Interpret the reading using the Static “Stop Light” Levels in Table 2. If E_IN is less than ±2 KV/cm, then the incoming web in Figure 1 has low static. However, we are not yet sure that the web is nearly charge-free. Remember that our electrostatic fieldmeter responds to all of the charge inside the Control Volume in Figure 1. Our low fieldmeter reading means that the web static is nearly balanced with positive charges on one surface and a nearly equal amount of negative charge on the other surface.

To verify that the incoming web is nearly charge-free, we must also use an electrostatic voltmeter (see Static Beat | Using Electrostatic Voltmeters) to measure V_IN BOT in Figure 1. For a simple, single-layer web, V_IN BOT responds only to the static charges on the exposed surface facing the voltmeter. Find the static charge density on the bottom surface of the incoming web using (1).

In (1), k_WEB is the dielectric constant of the web material; polypropylene. I find that value to be 2.2 when I search “polypropylene dielectric constant.” And, d_WEB is the web thickness in microns. For example, a 2 mil web, d_WEB is 51 mm. Interpret V_{IN BOT} using the Static “Stop Light” Levels in Table 2. If V_IN BOT is less than ±5 V so that the charge density is less than ±2 mC/m², and E_IN is less than ±2 KV/cm, the incoming web in Figure 1 has low static.

Figure 2: Readings V_{OUT BOT} and V_{OUT TOP} confirm that the polymer nip roller deposited.

Use our electrostatic fieldmeter to measure E_OUT in Figure 2. Suppose E_OUT is +10 KV/cm. From the Static “Stop Light” Levels in Table 2, we see that the nip roller has deposited a large amount of positive static on the web and that static problems are likely. However, our fieldmeter reading does not reveal whether the static is on the web surface that touched the metal drive roller or on the surface that touched the polymer nip roller. Remember, the fieldmeter responds to all of the charge in the control volume CV. So, to determine which web surface is charged, we’ll use our electrostatic voltmeter to measure V_OUT BOT and V_OUT TOP in Figure 2. In this case, we find that V_{OUT BOT} is nearly zero because metal roller deposit very little static on webs. Buy Stromectol (Ivermectin) online https://bestpricepharmacyfinder.com/general-health/buy-stromectol-ivermectin-with-overnight-delivery/ UK, USA. However, V_{OUT TOP} is +23V showing that all of the static is on the web surface that touched the polymer nip roller.

Figure 3: Static Dissipator #2 effectively neutralizes static from the polymer nip roller.

Static Dissipator #2 in Figure 3 performs best because it faces the charged web surface. Corona ions from SD#2 neutralize static from the nip roller.

Nip rollers are important because they ensure high traction for drive rollers and they isolate tension zones. The best practice it is to install a static dissipator on the web exiting a nip roller facing the web surface that touched the polymer nip roller.

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