On a roll25 November 2019
Web cleaners are a common feature on printing lines, and remove dust and debris from paper and other materials using a variety of methods. The choice of which unit is most appropriate for one’s line, therefore, is an important one. Michael Shaw talks to Sheila Hamilton, technical director at Teknek, about the types of web cleaner on the market and the future of his essential technology.
The common assumption that household dust is mostly dead skin is only partially true. In fact, this grey particulate matter is a mixture of carpet fibres, soil and airborne particles that either drifted into the home through open windows and doors or clung to residents’ clothes and shoes, unwanted fragments of a world we’ve purposely tried to shield ourselves against with bricks and mortar.
Appearing like so many smashed owl pellets under the microscope, this dust does little more than slowly grey our mantelpieces, windowsills and kitchen surfaces. Its influence on industrial applications, however, is less benign. Air filters, for example, cease to function if weighed down by particulate matter. The converting process in packaging, too, can be held up by the presence of this grey menace.
“Dust or particle contamination is one of the biggest causes of barrier contamination in printing,” explains Sheila Hamilton, the technical director of packaging manufacturer Teknek. Leading causes include sluicing dust as well as any particulate matter clinging to staff members’ clothing. The speed of the web material – usually foil, film or paper – as it passes through the print rollers is also a catalyst for the accumulation of debris, as it can “generate a lot of static, which attracts any dust in the atmosphere around the production process”.
If such detritus is deposited on the web, a variety of defects can occur depending on the size of the particle. If they are large, then pits, dents and other repeating defects on the label are commonplace. Smaller contaminants, meanwhile, can cause voids in high-value labels and lead to the rejection of an entire print run. Additionally, “sub-micron-sized particles can cause significant levels of defect in the display and flexible electronics industry”, says Hamilton.
Fortunately, packaging and label manufacturers can rely on a piece of equipment that minimises this risk: the web cleaner. These machines “are designed to clean the packaging line, to ensure that there are no repeating defects in your print”, says Hamilton. “As you’re printing thousands of metres of good packaging, you want to make sure that the print quality is the same throughout.”
Method and mode
To ensure the quality of the print remains consistently high, web cleaners safely remove dust particles from the web material, thereby preserving the integrity of the entire production line. As such, they are a common sight in most label manufacturing facilities.
However, there is no standard type of web cleaner, with most being designed to remove only certain sizes of particle. The choice of product, therefore, is crucial. “Both the type of material that causes the contamination and the size of the particles that cause defects should be considered,” says Hamilton, in addition to the possibility that certain units – such as those with brushes – might also damage the web. Electricity usage is also an issue.
“The high power required by vacuums and ultrasonics results in substantial amounts of noise, which may be a health and safety issue for the operators,” explains Hamilton. The space taken up by the unit and “ease of set-up for different production runs and ease of use is also an important factor”.
Fortunately, there are many different types of web-cleaning systems that can cater to a wide variety of needs. One is vacuum technology, where the particles are literally sucked away from the web. This avoids contact with the material, and reduces any chance of extra detritus being deposited onto the web during the cleaning process. However, it’s not useful in every situation.
“The challenge with air technology is the size of the particles that you can remove,” says Hamilton. This is because webs travelling through the print rollers at more than 150 feet per minute (fpm) develop a boundary layer of air just above their surface, which attracts particles that cannot easily be removed with non-contact web cleaners. “Vacuum nozzles can also cause scratching through fluttering of thin films with the air pressure [used], especially if combined with ultrasonics,” Hamilton adds.
Whether or not this is actually a problem for a packaging manufacturer depends on the quality control of their printing line. Simply put, there is no scale of web contamination currently in place across the packaging industry. And some clients demand higher web quality than others.
“This is particularly true for the wine label market,” says Hamilton. “People expect the print to be perfect because, after all if there are flaws [on the label], that will become unacceptable to the consumer. And so, cleaning the film before it’s printed is essential to remove defects like that.”
The removal of large particles, meanwhile, demands web cleaning methods that make contact with the surface of the web. Many of these are very effective, if rudimentary. Use of a tack cloth equipped with an adhesive is a low-cost option best suited for narrow, ‘stop and go’ processes with less stringent quality standards. Even so, it is difficult for cloths to sweep up all the tiny particles that collect on the web, and it is up to the operator to judge whether or not it has become clogged up with dust during the cleaning process.
Moving brush systems also make contact with the web, collecting dust using a spinning brush before collecting them with a vacuum and a flicker blade. Again, while this method is useful in picking up larger pieces of debris and is effective on irregular surfaces, it is ineffective in removing tiny dust particles and cannot be used on sensitive substrates.
Additionally, “one of the problems with the brush and vacuum is that the brush can cause scratching on scratch-sensitive plastic films”, says Hamilton. “High-gloss plastic films and metal foils are particularly susceptible to damage through scratching of the surface.” Ensuring the brushes themselves are clean can also result in cross-contamination and an unwanted rise in operational costs.
Contact vacuum systems, meanwhile, promise the best of both worlds. A combination of static elimination systems, brushes and air knives that prize dust motes away from the substrate before a vacuum sucks them all up, it can – in theory – remove particles as small as 25μ from the web. Tape systems also offer the kind of contact necessary to pluck out some of the smaller detritus from the web during operation, albeit with a greater need for periodic manual checks. Using a specialised polymer roll, this type of web cleaner penetrates the barrier layer of air and, with each rotation, collects dust and transfers it to a nearby adhesive roll.
Unfortunately, such methods generate static charges of their own during operation, complicating cleaning even further. Water wash web cleaners generally avoid this, although they represent the highest investment for any packaging manufacturer. Again utilising polymer rolls, these units are maintained by robotic wash heads and ideal for substrates running at high speeds with greater vulnerability to contamination. In this respect, the choice of materials water wash web cleaners can be used upon is confined to recycled CRB, virgin-coated board stocks or URB.
The variety of web cleaner types has pushed their manufacturers into exploring how the differences between units can be bridged. A great deal of attention has been paid towards reducing the amount of static electricity generated during the cleaning process. “Controlling static charge on high-speed webs during processing is an issue which can also affect the efficiency of web cleaning,” explains Hamilton. “Traditionally, web cleaners use static bars to reduce static, but these bars can build up contamination on the individual emitter pins.”
To combat this problem, manufacturers like Teknek have invested heavily in static dissipative cleaning rollers, which use adhesive rolls that minimise static generation. Other new-found efficiencies include the use of ultrasonic signals to dissipate dust on the web to enable easier cleaning. However, this method, while efficient, demands a higher initial outlay than other web-cleaning systems.
Even if it isn’t possible to know quite yet what innovation will disrupt web cleaning in the years to come, one thing remains certain – these machines, with their ability to clean substrates at speed and, in so doing, minimise downtime, play an essential role in modern printing and packaging. Further developments in this area are bound to enhance what is already a remarkably streamlined process.
Low-static cleaning: the future of defect reduction
Among recent releases by Teknek is its latest in next-generation contact-cleaning technology: NT cleaning rollers. These latest innovations allow Teknek to solve even more contamination issues.
NT cleaning rollers are unique: they provide the cleaning power of Teknek rollers with the added advantage of being static dissipating. With a good surface resistance, the machine’s static levels remain well below 100V compared with over 3,000V or more in other systems. Not only does this allow for the cleaning of products that are static-sensitive – for example printed circuit boards – materials that are prone to becoming static-charged can also be easily cleaned.
To complement the NT cleaning rollers, Teknek has also launched its new range of adhesive rolls. GAR (Gold Adhesive Roll) provides the same high cleaning performance as other Teknek adhesives but, like NT, it is static dissipative and a key component of the low-static cleaning system.
For the first time ever, boards used in the surface-mount technology (SMT) process can be cleaned in a safe static environment of less than 50V. Damage due to electro static discharge and electrical overstress (ESD and EOS) are eliminated. Such is the effectiveness of the system that for most SMT applications Ionisers are not required after cleaning.
The ability to control the static level with the cleaning process allows high-speed cleaning of thin films such as those used in display and flexible printed circuit board (FPCB). These films are easily charged while moving through the production process. Once highly charged, the films have a high tendency to wrap around rollers used in the process, including the cleaning rollers.
The recent introduction of NT and GAR within the high-volume display sector saw reduction in static-related quality issues fall from 4% to 0%. This not only allowed payback within weeks but increased output.
Moreover, thin materials that have been difficult or impossible to clean with traditional cleaning rollers are easily cleaned with this new technology. Polyimide widely used in the FPCB market is very thin and is easily tribo-charged. Having struggled for many years to process sheets of this material at speed, NT has recently been used to process such at up to 40m/min.