Web Cleaning: Fact, Fiction and Method Selection
Customers looking for web cleaning options often compare all methods of web cleaning as if they are apples to apples. However, this is not the case. The most appropriate solution for your web cleaning application is dependent on a variety of factors. I recommend a four-step approach to determining the best web cleaner for your process.
- Know your process contamination standards.
- Identify variables and operating parameters that must be considered and communicated.
- Play fact or fiction concerning the operational principles, identify advantages and disadvantages.
- Analyze the total cost of ownership.
Know your Process Contamination Standards
Throughout much of the industry, there is little or no standard for web contamination. The general consensus is that the web must be clean enough to not adversely effect any follow-up operations or final usage. Certain industries tend to have more stringent quality requirements with quantifiable standards (medical, food, pharmaceutical, electronics, aerospace). Ask yourself, “Why
- Is web cleaning necessary on your product lines?
- Does the customer or product specification require web cleaning?
- Is it required to keep equipment clean from substrate contamination to minimize downtime, waste, maintenance, and operational cost?
- What contamination needs to be removed? What is the particle size?
Identify Variables and Operating Parameters to Consider
There are many sources of contamination within a process, including air (atmospheric), substrates, equipment, and people. Different processes also create contamination. Processes like slitting, shearing, die cutting, and stamping, along with general debris created from conveyance, can all create contamination. Before selecting a web cleaner, consider the following variables:
- Type of converting process
- Sources of contamination and their relative location in the process
- Substrate composition, sensitivity, thickness, and width
- Web speed and tension
- Is the process continual with flying splices or intermittent?
- Dimensions of idlers and machine frame (space available)
- Cleanroom, solvent or aqueous environment
Compare Available Web Cleaning Methods
Once you have determined the quality standards, the source of the contamination, and the process parameters, you can compare web cleaning methods to find the right fit for your process. When comparing methods, it is important to consider what holds dirt and contamination on a moving web.
The Laminar Boundary Layer of Air: Substrates traveling more than 150 fpm 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.
Static Charge: The conveyance of substrates, especially films, creates a static charge on the surface of the substrate that can hold particles to the web. It can also attract atmospheric contaminants to the web’s surface. Many web cleaners offer static elimination as a standard option with the equipment.
Now, let’s explore some available web cleaning methods:
Tack Cloth: 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. 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 if needed. Tack cloth should not be used on thin substrates or substrates sensitive to contact.
Passive Static Reduction: This is not a web cleaning method, but rather a way to reduce the static charge on the web so that when used with true web cleaning methods, the contamination is more easily removed from the web’s surface. The advantage is the low cost and the ability to reduce the static charge on the web prior to cleaning. The disadvantage is that this method only reduces and does not eliminate the static charge on the web. The charge can quickly build again, and this does not impact the boundary layer of air.
Active Static Elimination: This is not a web cleaning method, but it neutralizes the static change on the web, assisting in web cleaning and decreasing the attraction of contaminants downstream. The disadvantage is the higher cost than passive products and some limited maintenance is required when cleaning emitter pins. This method still does not eliminate existing contaminates from the web, and does not impact the boundary layer of air.
Air Knives: Not a web cleaning method, but air knives are often used in concert with non-contact vacuum systems. The theory is that compressed air blowing opposite 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 of air. Additionally, the air can blow contaminants into the local atmosphere, allowing them to deposit back on the web.
Moving Brush Systems: This is a web cleaning method that utilizes a spinning brush to remove contaminants, then collect them by using a flicker blade and vacuum. This method can be effective in removing large particles from highly contaminated substrates. This method can also be used on irregular surfaces. Disadvantages include potential issues of removing contaminants from the brushes possibly causing cross contamination, high equipment and operational cost, it’s not for use on sensitive substrates, and it does not perform well on small particles.
Non-Contact Vacuum Systems: An effective non-contact web cleaning method best utilized on large particle contamination at slow speeds. The disadvantage to non-contact vacuums is the 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: A web cleaning method that utilizes 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 >25 microns with speeds up to 800 fpm. Brushes are often retractable for use with sensitive substrates. Contact vacuums represent a moderate investment price. A disadvantage is determining how to clean the brushes to avoid cross contamination. Also, there are no industry reports to verify effectiveness claims.
Contact Cleaning (Tape Systems): A contact web cleaning method that uses a specially formulated polymer roll (contact cleaning roll) to nip the substrate between another contact cleaning roll or idler. Since the contact cleaning roll nips the surface of the substrate, it breaks through the boundary layer of air. The contact cleaning roll collects the contaminants from the web and transfers them to an adhesive tape roll with every revolution, continually cleaning the roll. Single nip configurations remove 96.9 percent of contaminants from the web, down to 1 micron. Contact web cleaners carry a moderate investment price. 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 neutralize the charge created by contact cleaning.
Contact Cleaning (Water Wash): A contact web cleaning method that uses a specially formulated polymer roll (contact cleaning roll) to nip the substrate between another contact cleaning roll or an idler roll. The contact cleaning rolls are continually cleaned by robotic wash heads. Water wash systems are generally used for substrates with high levels of contamination and fast speeds. Single nip cleaning is 96.9 percent 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.
Beware of the Hidden Costs
When analyzing the best option for your process, consider not only the initial equipment investment, but also the cost of consumables, maintenance and man hours. Additionally, contrast these costs with the level of cleaning efficiency.
For instance, when comparing vacuum systems vs. tape contact cleaning systems consider:
- Initial investment prices for vacuum systems and tape contact cleaning systems are comparable.
- Electricity is a vacuum system’s consumable. Electricity consumed by the vacuum pump and compressed air on vacuum systems can equal or exceed the consumable costs of a tape contact cleaning system.
- Vacuum systems have a lower cleaning efficiency and particle size effectiveness than tape contact cleaning systems.