There are several web cleaning technologies available for various converting applications, including static elimination, simple vacuum, multimodal, contact roll, and hybrid systems. Dust and debris control at slitter/rewinders, sheeters, coaters, or laminators rarely is addressed to satisfaction without one or another of these technologies.
DETECTING DEFECTS
To properly identify defect sources and the optimal positioning of cleaning systems, detailed surface inspection is required. Converters with the best quality control programs use surface inspection sampling as part of their production processes.
The quality and intensity of this inspection lighting distinguishes better surface inspection systems from others. Focused intense light is able to show surface debris in much finer detail than unfocused light sources.
Systems that use focused light beams that shine parallel to the web surface, brilliantly highlighting the side facing the light and casting a deep shadow on the other side, are considered the best. This combination of highlight and shadow gives the impression of magnification.
The first step in choosing a cleaning system is to determine which technology is best matched to the application.
STATIC ELIMINATION
One approach to web cleaning is static elimination bars that prevent the attraction of dust to the surface of the material being converted. By taking away the static charge on the surface, dust and debris can fall from the web. Often such bars are combined with air to blow the dust and debris away from the surface.
While this proactive approach to dust control does help, many companies that use static elimination to control dust eventually end up investing in add-on cleaning technologies with greater effectiveness.
CONTACT ROLL CLEANING
A contact roll cleaner uses a particle transfer roll (PTR) or tacky roll that lifts dust and debris as it moves across the surface of the substrate being converted. It then transfers that debris to a (tackier) collection roll, typically an adhesive roll where the debris builds until purged.
It can be a very effective cleaning technology for many converting applications if used on substrates with low levels of dust, such as films, foils, etc., and at lower operating speeds. However, on dustier materials such as paper-based substrates, contact roll cleaners can be expensive and impractical, given the fast rate with which dust and debris accumulate on the adhesive layer and the need to purge rolls frequently. The drag on production rates from maintaining collection rolls needs to be considered. While the upfront costs of contact roll cleaners are not exorbitant, the longer-term operational costs can be high. This technology is especially effective on highly scratch-sensitive substrates.
SIMPLE VACUUM SYSTEMS
A vacuum-type system often is considered an essential part of converting quality control. However, there is a range in quality in vacuum systems for the converting market, with vacuum power varying by as much as 10:1, and in some cases, differences are not strictly correlated with price. Maximizing the vacuum directly affects the cleaning performance.
It is also important to ensure the power of the vacuum applied by the cleaning system is applied evenly across the entire web width. If the seal making the connection between the web and the vacuum isn't secure, the power of the vacuum, as well as the uniformity, can be compromised significantly. For this reason, off-the-shelf-design vacuum systems may not perform as well as custom designs that take into account the width, speed, substrate, and proper positioning within the configuration of the converting equipment.
These systems offer moderate overall improvement in the cleanliness of the web as long as the amount of debris is limited.
MULTIMODAL SYSTEMS (MULTIPLE CLEANING ACTIONS)
For many converting applications, it is advisable to augment static elimination bars and vacuum actions with direct mechanical contacts on the web surface that are able to agitate the debris and free it up for elimination by the vacuum system. Heavy-duty brushes, and especially those located close to the vacuum orifice, are used to scrub the web surface aggressively to break loose dust, dirt, felt hairs, and fiber that are partially bonded to the web. Then, a high-velocity, multi-directional air flow across the surface helps direct the loosened dust and debris to a vacuum system.
This multimodal approach works best for many applications, including some of the coated substrates that many mistakenly think cannot be exposed to mechanical brushes. If scratch-sensitive substrates are involved, special attention to proper brush selection can be all it takes to use this high-efficiency method successfully.
HYBRID SYSTEMS
A new approach to the most demanding applications is a combination of multimodal cleaning systems and contact roll cleaners. These hybrid systems usually apply static elimination followed by multimodal cleaning systems to eliminate the lion's share of the dirt and debris. The contact roll cleaners then are used on the relatively clean web surface; they are relied on as a final touch and an especially helpful tool to eliminate the smaller dust particles (to 0.5 microns). Because there are prior cleaning steps, the contact roll needs to be changed less often and, therefore, costs less than when used as the single cleaning mode.
Overall, hybrid systems often are more expensive options (as much as twice the cost of a standard web cleaner) but can offer greater cleaning efficiency. One of the other advantages to a hybrid system is that the multimodal and the contact roll cleaners can be used independently if chosen. In some cases of converting materials with high dust levels, the multimodal method alone will suffice. If a substrate is sensitive to scratching, the hybrid system operator can use only the contact roll cleaner. This is not possible when using one or the other technology solely.
OPTIMAL POSITIONING
The rule of thumb is that the closer the cleaning system is to the source of the defects, the more effective it will be. By not locating the system just prior to where the defect is created, recontamination may occur. Custom design of cleaning systems typically enables optimized placement at or near the source of dust or debris for cleaning effectiveness.
COMPETITIVE TESTING
When comparing one cleaning system to another, it is beneficial to test several technologies and quantify their effectiveness in decreasing defect rates. Experienced cleaning consultants will be able to suggest the short list of cleaning systems that are used by those doing similar converting operations with similar types of converting equipment. They will know, for example, if a particular coated film or paper can be subjected to mechanical brush actions and which ones require the combined hybrid systems for effective cleaning.
Reputable web cleaning system manufacturers will be able to provide comparative test systems. Real-world testing, to the greatest extent possible, should be done in the actual plant environment at normal operating speeds, with the systems positioned where they will be in the permanent installation. If that is not possible, consider a pilot line or an off-line operation.
At the conclusion of testing, comparative results should be determined to find best-in-class technology for an application. Detailed return-on-investment projections then can be calculated before purchasing a particular cleaning system. When testing is impossible, plant management should ask for contact references with similar applications who can report on their experiences.
Making an apples-to-apples comparison of various web cleaning technologies and their relative appropriateness for specific types of defect costs is achievable by partnering with quality systems engineers with proven expertise in diverse converting applications and plant designs. Such consultations will clarify if more expensive hybrid cleaning technology is needed. Other types of cleaning technology may suffice in less demanding applications.
Mark Gibis is VP of Doyle Systems, Barberton, OH, a supplier and consultant to the converting industry for more than 85 years. Gibis can be reached at 800/445-3856; sales@doylesystems.com.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).
COST OF DEFECTS
Throughout the converting industry, it is customers' concerns with the costs of defects that propel most converters to invest in web cleaning technology. The problem with such a reactive approach is that when quality issues already have escalated to the complaint level, it may be too late to salvage a customer's business. In fact, demonstration of higher quality control using web cleaning technology offers a competitive advantage in the marketplace and a way in which one converter wrests customers away from another converter.
One trend compounding this problem is the growing number of customers that are becoming de facto competitors by investing in web cleaning systems. For example, more printers are moving slitting/sheeting operations in-house in order to increase their ability to address dust and debris issues that compromise printing quality and add to waste. In a related trend, sheeter converters are buying overruns and secondary quality products from mills, using web cleaning technology to improve stock quality, and then selling it to printers as the highest quality products.
When you consider the relatively modest costs of typical web cleaners (about $14,000-$25,000, depending on width) compared to the potential loss of business from inadequate quality control, the value of investment in web cleaning technology is apparent.
SUPPLIER INFO
Doyle Systems-PFFC-ASAP 301. www.doylesystems.com
Article Link: http://pffc-online.com/mag/paper_spot_less/
