Label printers understand that a clean web is a productive web. With contamination being an inevitable by-product of high-speed webs, it is important to have an effective web cleaning system in place to increase productivity by reducing maintenance downtime and minimizing discarded product.
The types and levels of contamination differ in label printing, as do the reasons why web cleaning is necessary. Is it solely a maintenance issue or also a customer requirement? Given that any web cleaning system will have to fit into an existing production line, how does this influence the choice of system?
These are crucial factors that will contribute to the final choice of web cleaner, which is why it is fundamental to understand the basics of web contamination — why it happens, and how to tackle it.
What are the key causes of contamination?
Contamination takes on the form of dust, dirt and assorted airborne particles that deposit on the web’s surface during the printing process. There are two main reasons why a web attracts contamination: the boundary layer created by any moving web and the generation of static charges. An effective web cleaning system must be capable of tackling both.
The friction or separation between two objects can lead to the generation of static electricity, creating an electrical charge on the material surface that will attract contamination to the web.
The boundary layer is caused by the ambient air that the web drags along when in motion. This movement draws contamination to the substrate, trapping it either beneath the layer – and therefore directly on the web’s surface – or holding it within the layer. The slower the web is moving, the thinner the boundary layer, whereas the layer’s thickness will increase as the speed intensifies.
Contact or non-contact?
There are two basic web cleaning technologies for breaking the boundary layer on a label converting line: contact web cleaning and non-contact web cleaning.
Contact systems typically incorporate two types of cleaning methods: vacuum technology, or adhesive rollers. Several different designs of contact vacuum system are available. One employs a low-friction component of the cleaning manifold – a highly-polished steel faceplate, for example – to make contact with the web, breaking the boundary layer and releasing the contaminants, which are subsequently vacuumed away.
Other contact vacuum systems incorporate brushes to free debris before vacuuming. While this approach is effective when large particles need to be removed from heavily contaminated surfaces, it can be too harsh a cleaning method for use on sensitive materials (although some systems feature retractable brushes to avoid damaging abrasion). In addition, the necessity of removing contaminants from the brushes raises the possibility of cross-contamination. Furthermore, brush systems qualify as consumables and therefore require a significant capital investment.
Systems using adhesive rollers incorporate twin elastomer rollers, which are in contact with the full width of the web, breaking down the boundary layer and lifting contamination from the web’s surface. This is then transferred to a second roller with a high adhesive mass, which removes debris from the first roller, preventing recontamination. Adhesive-roller systems perform well on small particles (down to as low as 0.5 micron), but are not designed for more demanding applications where higher levels of contamination are involved, as the adhesive rolls can quickly become tainted. For this reason, adhesive roller systems also require closer monitoring by the operator than other web cleaning systems. Another limitation of tacky roller systems is the machine speed, normally limited to less than 250mpm, which can prove problematic for label applications, for which high production rates are a priority.
The choice of non-contact technologies is between ‘blow-and-vacuum’ and boundary layer systems. Blow-and-vacuum systems employ air knives on either side of the web to strip the boundary layer and all traces of contamination from the web’s surface. The presence of vacuum airflows ensure that turbulent air is captured and subsequently removed. It is an efficient method and the systems are typically compact, which makes integration into existing label converting lines more straightforward.
Boundary layer systems are a relatively recent development. High-speed, non-contact rollers rotate at a distance of between 0.5 and 1mm from the surface of the web and in the opposite direction. The roller design is optimized to generate its own high-speed boundary layer, the greater energy of which destroys the boundary layer carried by the incoming web. This exposes even the smallest particles on the web to a very powerful cleaning force. The low-pressure, high-velocity region created by the roller lifts and removes the particles from the web and carries them to a vacuum extraction chamber.
A web cleaner alone is not enough to guarantee a spotless web, which is why it must be combined with an effective static control system to counter the charges on the web’s surface that also cause contamination.
A static charge is typically generated by the separation (such as when the web unwinds) or friction (as the web travels over non-conductive nip rollers, for example) between two objects. These charges are more likely to occur when processing film and, if not neutralized, pull contamination already on the web to the surface of the material. Charges can be positive or negative in polarity, with paper usually being positive and film negative.
Active static control provides the most effective means of neutralizing charges. The technology uses ionization. A voltage is fed to an array of emitter pins mounted on an ionizing bar, creating a high-energy ‘cloud’ of positive and negative ions. Any statically-charged surface, of either polarity, passing close to the cloud, is quickly neutralized.
We noted at the outset that a clean web is the key to productivity. Label printers must be able to deliver on this promise and do so profitably. Installing a web cleaning system can increase the productivity of the converting line and ensure products are processed to a consistently high quality. Arriving at the right choice of web cleaning system, however, means considering a wide variety of factors, from analyzing the source of contamination to understanding how your customers’ needs are likely to develop in the future. Only then can you make an informed decision on which of the different approaches to web cleaning is right for you.