Keep it clean7 August 2017
Keep it clean
Keep it clean
Converting is an exceptionally complicated series of steps and moving items that must work together flawlessly in order to provide the best possible finished product. Companies seem not to realise the stress on their production lines until something goes wrong, by which time a very serious issue has developed. Fortunately, steps can be taken through correct web cleaning to ensure that all the parts are moving with and against each other in harmony, with minimum static-charge build-up and removal of contaminants that could impede the precision engineering from the line.
Web cleaning systems fall under two categories – non-contact and contact. As a general rule, the cost of a non-contact system is greater.
Cost aside, if the production line is used for substrates with sensitive coatings or that are prone to marking, then a non-contact system will be the best option. This is also the situation with companies operating in fields in which it is desirable to have as few objects as possible touching the material, such as pharmaceutical packaging.
Contact web cleaners fall under these headings:
- Vacuum systems: typically, the web is in contact with a low-friction part of the web cleaning manifold, allowing the contaminants to be vacuumed from the surface. Systems commonly incorporate active static-control bars before and after the vacuum slot.
- Tacky (elastomer) rollers: these systems use a sticky roller driven by contact with the moving web to pull off debris and fibres. The roller is in contact with a second roller that employs more adhesive, which, in turn, takes up the contaminants. The systems are relatively inexpensive, easy to install and provide adequate results. The main complaints from users are the cost of buying the replacement sheets for the second roller, the time it takes to fit them, and the decrease in performance if the sheets are not replaced and the roller becomes more saturated with contaminates.
- Brush systems: this style of system is probably the least popular among users due to the common design, which typically involves the web being ‘scrubbed’ by a range of brushes. Even if the web is not susceptible to marking, the process can appear harsh.
Non-contact systems can be broadly grouped as follows:
- Boundary layer systems: these function by creating a powerful layer of air that breaks the web boundary layer, which is then ‘sheared’ into a vacuum chamber. Boundary layer systems have low running costs and no consumable costs. As with vacuum systems, a range of extraction units are available.
- Ultrasonic systems: typically the most expensive style of web cleaner, these systems work by blasting the web with ultrasonic pulses from either side of a vacuum slot.
- Blow and vacuum systems: these include a combination of blowing and vacuum airflows to strip contamination from the surface. The systems can be very effective but some reports show they can require a lot of ongoing adjustment of the airflows to achieve the desired cleaning results.
With such a variety of options, selecting the right tool for the best cleaning job is a vital step. Experts in web cleaning companies typically recommend a four-step process.
Understanding the standards is the first step, and one that is already complicated by the fact there is little or no standard for web contamination. Therefore, the general rule is to ensure the web is clean enough not to adversely affect any follow-up operations or final use. Certain industries tend to have more stringent quality requirements with quantifiable standards, such as in the medical, food, pharmaceutical, electronics and aerospace industries.
Secondly, a company should determine whether web cleaning is necessary for its product lines or something that customers specify. It should find out if web cleaning is required to guard the equipment against substrate contamination and will help to minimise waste, downtime or the need for additional resources. It should ask what contamination needs to be cleaned and what the particle sizes are.
Then it is time to identify operating tolerances. How comprehensive does the customer want the cleaning to be, what standards are they working towards and which part of the process will be the web cleaner’s responsibility?
Before selecting a web cleaner, a firm has to consider all the variables: the type of converting process, the sources of contamination and their relative location in the process, substrate composition, sensitivity, thickness, width, web speed, tension, the continuity of flying splices, dimensions of idlers and machine frames, and whether it is a cleanroom, solvent or aqueous environment.
Once the quality standards are identified, different cleaning methods can be compared to determine the best option. While making these comparisons, companies should consider what causes contamination on a moving web and decide the best way to treat any potential maintenance or cleaning issues.
For example, substrates travelling more than 150fpm exhibit a laminar boundary layer of air that traps particles on the surface of the web. Non-contact web cleaners tend to be ineffective against removing particles. Products moving on the web, such as films, will create a static charge on the substrate that hold particles to the web but can also attract contaminants, which can become a discharge or cause a fire if improperly treated.
Find the right tool for the job
The next step is to weigh the pros and cons of each type of cleaning method, and determine how it fits your operational principles and the requirements of the complete project.
A rudimentary web cleaning method where a cloth with an applied adhesive contacts the surface of the web, tack cloth is very cost-effective if you have a narrow web ‘stop-and-go’ process that does not have strict quality standards. The process relies on operators to identify when the cloth is saturated and to change if needed. Tack cloth should not be used on thin substrates or substrates sensitive to contact.
Active static elimination neutralises the static change on the web, assisting in web cleaning and decreasing the attraction of contaminants downstream. The disadvantage is the higher cost compared with passive products, and limited maintenance is required when cleaning emitter pins.
Air knives, while not a direct cleaning method, can be used in concert with non-contact vacuum systems. The theory is that compressed air blowing opposite to the direction of the web will break through the boundary layer of air and lift contaminants off the web’s surface. The advantage is the ability to remove dust and dirt from irregular-shaped objects, which works well at slow speeds. The disadvantage is that even with large volumes of compressed air, this method fails to break through the boundary layer.
Moving brush systems use a spinning brush to remove contaminants, and then collect them by using a flicker blade and vacuum. Disadvantages include potential issues with removing contaminants from the brushes, possibly causing cross-contamination; high equipment and operational cost; unsuitability for use on sensitive substrates; and poor performance on small particles.
Non-contact vacuum systems are an effective non-contact web cleaning method best used to tackle large-particle contamination at slow speeds. The disadvantage of non-contact vacuums is their inability to break through the boundary layer of air. Some systems claim to break through the boundary layer of air by using high-speed rotating brushes that come close to the web surface but do not contact it. In theory, this may work, but tight tolerances on position require little to no web flutter.
Contact vacuum systems use brushes, static elimination and/or air knives to release particles from the surface of the substrate and then vacuum them up. It offers non-substantiated claims of being highly effective on particles greater than 25μm, with speeds up to 800fpm. A disadvantage is determining how to clean the brushes to avoid cross-contamination. Also, there are no industry reports to verify effectiveness claims.
Tape systems use a specially formulated polymer roll, or contact cleaning roll, to nip the substrate between another contact cleaning roll or idler. Disadvantages are that the adhesive tape needs to be manually checked and maintained. High contamination levels may make tape consumable costs prohibitive. Contact cleaning also creates static. Active static elimination is often incorporated with this method to neutralise the charge created by contact cleaning.
Contact cleaning by water wash is a method that uses a specially formulated polymer roll to nip the substrate between another contact cleaning roll or an idler. Water wash systems are generally used for substrates with high levels of contamination and fast speeds. Single-nip cleaning is 96.9% effective at removing contaminants. Water wash systems are the only web cleaner capable of cost-effectively cleaning recycled CRB, URB or virgin-coated board stocks due to the low annual consumables cost. The disadvantage is the high investment price, although ROI is typically less than a year.
The final step is to consider the total cost of ownership. Firms must bear in mind that not only will equipment investment, cost of materials and labour will add to the cleaning bill but each of these can potentially influence the final outcome.
Those who stand to make the best decisions and save the most money on longest-running systems will consider all aspects before settling on the right method for them to keep the web clean, lean, green and operating at its full potential.