- January 17, 2012
“Hot rolls” cause many static complaints. Winding a polymer web with static creates a hot roll that can cause problems when the roll is unwound in the next operation. Neutralizing static when unwinding a hot roll is difficult and almost always ineffective. So, what can be done?
Solve the problem at its source. Eliminate hot rolls by neutralizing static on the web before winding the roll. To identify sources of static, use an electrostatic fieldmeter to take readings on spans midway between conveyance rollers after each major process.
For example, sources of static in a blown film extruder are identified in Figure 1 by taking eight fieldmeter readings. Note that all readings are taken before any static neutralizers are added to the line.
Blown Film Extruder Example
1. Extruder | Blown film lines have a circular extruder that creates a cylindrical film tube. Fieldmeter reading EEXT is taken on the cylindrical tube just after exiting the extruder: EEXT ~ 0 KV/cm. Molten polymers typically have a small amount of electrical conductivity that dissipates static from the mixing and extrusion process. So, extruders usually are not a significant source of static charge.
2. Haul-Off Nip | The haul-off nip compresses the cylindrical tube into a two-layer web. The nip is usually a movable polymer roller pressed against a fixed metal roller. The haul-off nip isolates the lower tension in the hot cylindrical tube exiting the extruder from the higher tension used to convey the cool compressed tube. Fieldmeter reading EHO NIP is taken on a span midway between conveyance roller just after the haul-off nip; EHO NIP ~ -4 KV/cm. The polymer nip roller deposits negative charge on the tube. An ionizing string or static bar should be positioned just after the haul-off nip to neutralize static on the surface that touched the polymer nip roller.
3. Corona Discharge Treatment | Corona discharge treatment (CDT) promotes wettability and adhesion. In this example, only one side of the compressed tube is treated. Fieldmeter reading ECDT is taken on a span midway between conveyance rollers just after the CDT unit; ECDT ~ +6 KV/cm. CDT units deposit high levels of static charge on webs. Here, the compressed tube enters the CDT with negative charge from the haul-off nip, so the CDT adds a high level of positive static. Locate an ionizing string or static bar just after the CDT unit to neutralize static on the treated surface. Static must be neutralized before the backside or untreated surface touches a roller.
4. Tension Isolation Nip | The tension isolation nip controls the web just before the edges of the tube are cut so that the tube may be split into two webs. The nip is usually a movable polymer roller pressed against a fixed metal roller. Fieldmeter reading ETI NIP is taken on a span midway between conveyance rollers just after the tension isolation nip: ETI NIP ~ +3 KV/cm. Here, the fieldmeter reading on the compressed tube exiting the CDT reads +6 KV/cm, so the polymer nip roller adds negative static. An ionizing string or static bar should be positioned just after the nip to neutralize static on the surface that touched the polymer nip roller.
5. Edge Trim and Web Split | In blown film lines, the edges of the compressed tube are trimmed so that the tube may be separated into two webs. Fieldmeter readings ETRIM1 and ETRIM2 are taken on spans midway between conveyance rollers just after the tube is split into two webs: ETRIM 1 ~ +10 KV/cm, ETRIM 2 ~ -7 KV/cm. Splitting the tube into two webs is not a significant source of static. Rather, splitting the tube reveals charge on the exterior surfaces of the tube. The high positive charge on web 1 came from the corona discharge treatment. The negative static on web 2 is from the nip rollers. An ionizing string or static bar should be positioned on each of the two webs just after the split to neutralize static on the formerly exterior surfaces of the tube.
6. Winder Nips | The winder nip isolates the winding tension from the web conveyance tension. The nip is usually a movable polymer roller pressed against a fixed metal roller. Fieldmeter readings EW NIP1 and EW NIP2 are taken on spans midway between conveyance rollers just after the winder nips: EW NIP1 ~ +8 KV/cm, EW NIP2 ~ -9 KV/cm. Ionizing strings or static bars should be positioned just after the nips to neutralize static on the surfaces that touched the polymer nip rollers.
7. Winders | Prior to installing static neutralizers, web 1 is wound with high positive static and web 2 is wound with high negative static. Hot rolls are wound on both winders. However, after installing neutralizers, both webs are nearly neutral when wound and no hot rolls are produced.
Hot rolls cause static problems when they are unwound. Neutralizing static when unwinding a hot roll is difficult. The best strategy is to neutralize static at the source of charging before winding the roll. Use an electrostatic fieldmeter to find sources of charging and neutralize the charge with an ionizing string or a static bar.
I invite you to ask questions about this column and to suggest topics for a future “Static Beat.” 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; email@example.com; www.electrostaticanswers.com.