Protection from the charge20 February 2017
Protection from the charge
Protection from the charge
Uncontrolled static can cause products to misbehave – print can be out of register, tiny holes appear in extruded film, and static charges can cause blockages in the most modern machinery – but dust attraction is arguably the biggest issue in a sector concerned with quality control, as airborne particles are attracted to charged surfaces, leading to high reject rates. In the narrow-web industry, with its strict quality requirements, this is a serious issue.
Static electricity results when two or more surfaces are in intimate contact with each other and are then separated. In the label-converting workflow, with rolls of stock becoming unwound and processed through a web, two surfaces are perpetually contacting and separating, hence, static is a constant – and something that must be kept in check.
Explaining the science behind static electricity, Matt Fyffe, vice-president and general manager for Meech USA, says, “Static arises when an external force causes an electrical charge to move from one material to another, leaving one positively charged and the other negatively charged. Speed and force of the friction, pressure and separation are all factors that contribute to the size of the charge, with increased force or faster processes leading to larger charges being generated. If one of the materials is conductive, it won’t hold the charge, but if the material is non-conductive, the charge is unable to move across the surface, creating a static ‘pool’ of electrical charges.”
Brian Farno, application engineer manager at Exair Corporation, says static electricity occurs most often on the surface of non-conductive materials, but can also occur on ungrounded conductive surfaces. “A static (non-moving) electric charge is created whenever two surfaces come into contact and separate, or when friction occurs between them. When two materials are in contact, electrons may move from one material to the other. Material atoms with weakly bound electrons tend to lose them, while material atoms with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect. When the materials come in contact, are separated or have friction between them, they retain this charge imbalance.”
Static electricity presents multiple challenges to label printers and converters, as electrostatic charges develop on the surfaces of webs while they are transported through various converting processes such as printing, rewinding, slitting, coating and laminating.
Kim West, marketing director at Simco-Ion, stresses that in addition to static causing detrimental effects to a print run, the phenomenon also presents a safety issue: “The charges often cause contamination to the web material and defects in printing and coating uniformity, resulting in quality problems and customer dissatisfaction. Significant charges can also cause electrostatic shock to personnel or could even ignite vapours in solvent-based inks.”
There are a number of ways static can impact a converter’s process. West points out that static charges are created and present during both the printing and finishing processes, and as little as 1,000–1,200 volts can attract contamination. “The substrate path on web presses, at each print station, can pass very close to the floor and this is where debris could be picked up and cause print defects. Incorporating static bars at each print station will help control static and lower the chance of contamination.
“Depending on the substrate, a static charge may start at the unwind and will continue all the way through the press to the winder many times, accumulating static charges unless neutralised. High static charges attract particulates and debris from several feet away to the web, resulting in poor print quality and other issues,” West explains.
“In many cases a static charge will come in on the label or web stock and present issues such as misfeeds or shocking the operator as they are loading and unloading the material,” states Farno. “During the printing process, static electricity can cause spider webbing in the ink or repel the ink and keep it from reaching the label surface. When static causes freshly printed material to cling to another surface, it could smudge the image.”
Converting labels to a different length or width can also be problematic. “Slitting and cutting operations can generate a static charge, causing dirt to cling on labels, improper feeds and jamming, or labels and webs not being cut to correct lengths because they hang up in the cutting area,” Farno says.
High speed, high charge
Static and high-speed webs go hand in hand. “Today’s lines are capable of running at upwards of 1,200fpm, and we’ve noticed within the digital print sector that inkjet is evolving at a remarkable pace; its potential for increased speed and ever higher print quality is almost certain to continue,” Fyffe notes. “However, with greater speeds come more static charges on filmic materials, which in turn will attract a higher degree of contamination to the web’s surface. Digital printers are especially susceptible to static, as it directly affects the print quality. A high charge on the substrate can repel or prematurely attract the ink being laid down by the printhead. This means that print quality and product cleanliness will become an even bigger concern and thorough antistatic and web-cleaning solutions will be a necessity.”
Exair’s Farno agrees that digital printing has presented some hurdles due to the increased speeds of printing, as well as the ink being cured by UV lights. “If there is a large static charge on a surface, it will take a fraction of a second longer to dissipate the charge,” he explains. “If the machine is running at full speed, then the fraction of a second may mean the entire charge isn’t eliminated before the ink is applied. This can cause the spider web effect on the ink before it is cured. The other static hurdle created is due to the UV lights that are used for curing. This will decrease the relative humidity in the material as well as the area, which, in turn, increases the potential for static charge. Both of these hurdles create a need for a static eliminator that is not only fast but can eliminate static from a distance.”
Label material, substrates, inks, constructions, and even types of printers and converters play a factor in the level of static charge that might be generated.
Of all of the variables at play, according to Farno, the label material and substrate play a particularly critical role in the process. “Some labels don’t want to separate from the substrate due to a large static charge, while other labels repel from the product they are being applied to because both the label and the product have a large static charge of the same polarity. No matter what the material, ink type or construction is, static can virtually always be eliminated at the point it causes an issue with the use of an active static eliminator.”
Simco-Ion’s Kim West emphasises how today many industrial materials are using insulators, such as plastics. “Their desirable qualities make them essential to use, however, they are likely to become electrostatically charged during processing,” she says, adding, “one of the largest and most consistent trends in the labelling industry is the utilisation of flexible film-based materials. Additionally, label and packaging converters are looking for ways to increase line speeds. The use of plastics coupled with increased speeds escalates the chances of contamination being drawn to the production process. Unmanaged static electricity during the process can lead to a variety of issues. For many, understanding where static is generated is the key to success. Therefore, the need for monitoring the static environment is becoming more and more critical. Many industries and applications are requiring a cleaner and high-quality finished product; thus, a newer static neutralisation system is a necessity to remove contaminants and understand your specific needs.”
Simco-Ion’s IQ Power Static Neutralizing System is a multipatented lineup engineered to include sensor technology with a user-friendly interface.
“IQ Power is ideal for label converters who need smarter, more integrated, yet easy-to-use solutions to controlling static issues. There is no complicated software to install or maintain,” West explains. “Our Sensor Bar is an active multipoint sensor bar that offers standalone monitoring or closed-loop feedback functionality that works in conjunction with our static-neutralising bars. It adjusts as needed in real time to ensure you are receiving the lowest possible residual charge. The distinctive design allows for various configurations, allowing the ability to position the sensors across the target to receive a complete understanding of any electrostatic charge issues unique to the application. And with our ‘smart addressing’ technology, new sensor bars can be added in no time.”
However the charge is dealt with, it is important to make sure everything is considered, from the speed to the material used, because a little foreplanning will save a lot of headache later. Fortunately, with such experts available, companies will hopefully no longer have to fear fire or rejected materials from charge damage.