Clean sweep8 August 2016
Much has been written in previous editions about the money and resources that can be saved by maintaining web tension and reducing static or other imperfections in converting operations. However, the most fundamental element, web cleaning, needs more attention devoted to it. Converting Today speaks with Joe Rodibaugh, technical coordinator at Polymag Tek, about what converters should do to get the best performance and longest wear from their operations.
The most common issue that customers face when looking for web cleaning solutions is that they feel there is parity between options. However, according to Joe Rodibaugh, technical coordinator at Polymag Tek, the best method to clean a company’s web will be based on a number of diverse factors. He recommends a four-step process to get the best results.
“You need to know your process contamination standards,” Rodibaugh says. “Then you can identify the variables and operating parameters that must be considered and communicated. After this step, you need to consider operational principles, identify the advantages and disadvantages before finally analysing the total cost of ownership.”
According to Rodibaugh, understanding process contamination standards is the first step. “Across the industry, there is little or no standard for web contamination. The general consensus is that the web must be 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. At this step, you need to ask ‘why web cleaning?’.”
A company should identify if web cleaning is necessary for its product lines or whether it is something that customers specify. It should find out if web cleaning is necessary 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.
These questions inform the company about the necessary standards to conform to and lead to the second step: identifying what exactly needs to be cleaned and the relevant operating parameters that should be met, such as how comprehensive the cleaning needs to be and which part of the process will be its responsibility.
There are many sources within a contamination process, including air (atmospheric), substrates, equipment and people. Different processes also create contamination. Processes such as slitting, shearing, die cutting, and stamping, along with general debris created from conveyance, can all create contamination.
“Before selecting a web cleaner, you have 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, cleanroom, solvent and aqueous environment,” Rodibaugh says.
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.
Once variables and operating parameters have been defined, it is necessary to take the third step: balancing the pros and cons of each type of cleaning method to weigh up how it adheres to operational principles and meets the requirements of the complete project.
This is a rudimentary web cleaning method where a cloth with adhesive contacts is applied to the surface of the web. Tack cloth is cost effective when there is a narrow web stop-and-go process that does not require strict quality standards. This is generally only used for single-side cleaning. Disadvantages are that the cloth cannot capture small particles and may break apart, adding more contamination to the process. The process relies on operators to identify when the cloth is saturated and change it when necessary. Tack cloth should not be used on thin or sensitive substrates.
Passive static reduction
This reduces static charge to make it easier to clean contamination from the web’s surface. The advantage is the low cost. The disadvantage is that this method reduces but does not eliminate the static charge on the web.
Active static elimination
This neutralises the static charge on the web, assisting in cleaning and decreasing the attraction of contaminants downstream. The disadvantages include higher costs than passive products and the maintenance required when cleaning emitter pins. This method still does not eliminate existing contaminates from the web nor does it impact the boundary layer of air.
These are used with non-contact vacuum systems. Compressed air, which blows in the opposite direction of the web, breaks through the boundary layer of air and lifts contaminants off the web’s surface. The advantage of this method is the ability to remove dust and dirt from irregular shaped objects, which works well at slow speeds. The disadvantage is air can blow contaminants into the local atmosphere, allowing them to deposit back on the web.
Moving brush systems
This method uses a spinning brush to remove contaminants and then collects them by using a flicker blade and vacuum. It is effective when removing large particles from highly contaminated substrates and can be used on irregular surfaces. Disadvantages, however, include potential cross-contamination when removing contaminants from the brushes, high equipment and operational costs, inefficiency on sensitive substrates and poor performance on small particles.
Non-contact vacuum systems
This method can be used for large particle contamination and at slow speeds. A disadvantage of this method is its inability to break through the boundary layer of air. Some systems claim to break through by using high-speed rotating brushes close to the web surface, but stop before making contact with it. In theory this may work, but the positioning requires little to no web flutter.
Contact vacuum systems
This makes use of brushes, static elimination and air knives to release particles from the surface of the substrate and then suck them up through a vacuum. It is effective on particles that are greater than 25µm with speeds of up to 800fpm. The brushes are retractable, so they can be used on sensitive substrates. Contact vacuums incur only moderate investment costs, but have the disadvantage of cross-contamination when cleaning the brushes.
A specially formulated polymer roll (contact cleaning roll) is used to nip the substrate between another contact cleaning roll or idler. When the contact cleaning roll nips the surface of the substrate, it breaks through the boundary layer of air. Single nip configurations remove 96.9% of contaminants from the web, down to 1µm. The adhesive tape needs to be checked and maintained manually, and with high contamination levels, may make costs prohibitive. Contact cleaning also creates static.
This method uses a specially formulated polymer roll (contact cleaning roll) to nip the substrate between another contact cleaning roll or an idler roll. Water wash systems are generally used for substrates with high levels of contamination and that run at fast speeds. Single nip cleaning is 96.9% effective at removing contaminants. This is 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 this method faces is the high investment price, although ROI is typically less than a year.
The final step
Having analysed the customers’ needs and determined the most appropriate cleaning method, the final step in the web-cleaning process can be addresses: the total cost of ownership.
For example, if choosing between the vacuum method and the tape-contact method, the initial investment prices are roughly equal, but the vacuum system uses electricity, which may increase costs, whereas the cost for tape-contact systems is fixed. In addition, vacuum systems are less efficient than tape contact cleaning systems. The point is that price will vary depending on specific cleaning requirements.
Converters hoping to make the best decisions and save the most money on long-running systems will consider all aspects of the process before deciding their cleaning method.