Web contamination: keep it clean

The most common issue that customers face when researching cleaning solutions is that they can often feel an equivalence between options. Joe Rodibaugh, technical co-ordinator at Polymag Tek, knows the best method will rest on diverse factors and recommends a four-step process for best results.

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 product lines or whether it is something that customers specify. It should find out if web cleaning is necessary to guard the equipment from 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 of the standards to conform to. The second step is to identify what exactly needs to be cleaned and the relevant operating parameters that should be met.

There are many sources within a contamination process – air, substrates, equipment and people. Processes such as slitting, shearing, die-cutting and stamping, along with debris created from conveyance, can all create contamination.

“Before selecting a web cleaner, you have to consider all the variables. This includes the type of converting process, the sources of cross-contamination and their relative location in the process, substrate composition, 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. 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.

Tack cloth

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. 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. However, this method reduces but does not eliminate the static charge on the web.

Active static elimination

Neutralising the static charge on the web, assisting in cleaning and subsequently decreasing the attraction of contaminants downstream. The disadvantages include higher costs than passive products and the maintenance required when cleaning emitter pins. This does not eliminate contaminates from the web nor impact the boundary.

Air knives

These are used with non-contact vacuum systems. Compressed air, which blows in the opposite direction of the web, breaks through the 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.

 

Quite often, tooling can be the majority of the cost for smaller projects and can sometimes stop a project in its tracks.

 

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.

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 contamination when cleaning the brushes.

Tape cleaning

A specially formulated polymer roll is used to nip the substrate between another contact cleaning roll or idler. When the roll nips the surface of the substrate, it breaks through the boundary layer. Single nip configurations remove 96.9% of contaminants, down to 1μm. The adhesive tape needs to be checked and maintained manually.

Water wash

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 subsequently determined the most appropriate cleaning method, the final step in the web cleaning process can be addressed. The last stage is surmising the cost of ownership.

For example, if choosing between the vacuum or tape-contact method, the initial investment prices are roughly equal.

However, 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.

As Rodibaugh concludes, “When analysing 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 immediate costs with the level of cleaning efficiency. For instance, when comparing vacuum systems and tape contact cleaning systems, it is always best to consider that the initial investment prices for vacuum systems and tape contact cleaning systems are comparable. 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. Finally, vacuum systems have a lower cleaning efficiency and particle size effectiveness than standard tape contact cleaning systems.”