Humidity
- Humidity affects static build up. “Relative humidity” is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. The reason static is more problematic in cold weather is because of low humidity of the air. If the relative humidity drops below 30%, the static conditions become noticeable. So introducing moisture into the environment, either by steam or vaporizer in the sheeting area can reduce static.
- Here’s an old timer’s trick when sheeting coated grades. Mount a full width, 12” (308 mm) long absorbent pad prior to the cutter infeed. Position the felt such that it comes in contact with the incoming web flow. The moist mat dampens the web, offsetting static buildup.
Locating Static Prone Areas
- Static build up can be neutralized by ionization devices such as static eliminating bars or air ionizers. The key is properly locating the units on a sheeter for maximum effectiveness. Static can be a problem when material slides over itself as when overlapping sheets in the delivery section or being jogged into place in the stacker.
- A more comprehensive approach is to use a static detection field tester. Map the flow of material through the sheeter, recording results along the course from roll stand to stacker. Use these results to identify where static systems should be located.
Ionization Devices
- In the unwind and web conditioning areas, (prior to the webs being converted into sheets), consider hanging metallic tinsel across the web paths after an outfeed roll. Keep the garland within ¼” (0.06 mm) from the moving surface. Insure that the tinsel is grounded to the metallic side frames of the machinery.
- Rubber or polyurethane covered rolls act as an accumulator of static charge. To offset static build up in the draw drum section of the cutter, mount a static bar about 1” (25 mm) above a cutter’ squeeze roll.
- Static eliminating bars mounted in the high speed tape section above and below the sheet path, reduce the sheet clip’s charge prior to the overlapping process.
- Mounting a second pair of static bars above and below the sheet path in the slow speed tape section will treat some of the induced static build up. Note that if a full width apron is used as the conveying design, rather that narrow width belts, static bars on the underside of the sheet flow will be ineffective.
- At the end of the delivery system as the sheets enter the stacker, use compressed air or blower motor air to break up the overlapped clips. Mount the nozzles or fingers facing the stacker at the end of the slow speed tape section. Position them in a manner that air is introduced in the tail end of the sheet. The volume of air will be dependent on the weight of the sheet(s) and the cut off length – typically more air is required on longer lengths.
- On particularly static prone material, the air at the pile may need to be ionized at the nozzles or additional static blowers maybe required over the stacker directed down at the pile to prohibit sheets ‘climbing up’ jogger blades.
