How to Combat Static When Converting Pressure-Sensitive Films

Static electricity may seem benign. However, the same force that allows you to rub a balloon on your head and stick it to the ceiling can produce converting nightmares. That’s because static electricity can lead to dust and debris being attracted to an on-press web or lead to misfeeds when sheets of material cling together. Static can also cause tracking miscues, create star or fork-shaped defects, and even ignite solvent-laden vapors on a machine. This can lead to product delays, higher return rates, and dissatisfied customers.

The problem is often a seasonal issue, with the worst time occurring in the fall and winter when dry air facilitates the conditions that can lead to static electricity. The composition of pressure-sensitive film lends itself to generating static and results in a number of converting challenges. For example, you could start with a roll that has no electrical charge whatsoever and create 20,000 volts simply by unwinding the roll or separating the sheets. Fortunately, there are several steps you can take to eliminate the problems associated with static electricity.

One way to combat static electricity involves the use of an ionized air blower, which sends ions at the web that help reduce static.

Identifying Causes and Locations of Static

Static is caused by an imbalance in a material’s atoms. When two surfaces come in contact, the atoms pass electrons back and forth. The imbalance is caused when the surfaces are separated, leaving some electrons stranded in an atom from the other material. The atom containing the extra electrons will have a negative charge, and the atom that lost the electron will have a positive charge. This separation results in static and explains why it is generated when one sheet glides over another as it feeds into the press, or when rolls unwind, pass over an idler roll, or rewind.

Factors that Increase Static Electricity

There are three specific factors that are likely to increase the generation of static electricity:

1. The faster the two surfaces are separated, the more electrons will be stranded
2. The closer two surfaces are to one another, the higher the number of electrons that will be exchanged (this is why nipped rubber rolls often generate a lot of static)
3. The composition of the material determines the level of electron activity. A conductive material, such as metal, can transport electrons more freely to dissipate the charge than a non-conductive material, such as plastic. This is why plastic is used to cover metal wires; it serves as an insulator and prevents someone handling it from being electrocuted. Pressure-sensitive films also tend to act as insulators. 

Signs of Static

There are a couple of simple signs that can alert you if the material is likely to generate static as you run it. For example, if the material gives you a shock during handling, or if the hair on your arm stands up when you place it near the film, static is probably present.

A more technical approach to determine if static electricity is present is to test your material with a field meter. Field meters are tremendously helpful in helping you understand and control static. Make a map of the web path of your press and take static readings along the path. The results will help you identify where the static control device should be installed.

Flexcon engineers use a hand-held field meter to quantify the charge on a Statistical Process Control technician's audit and document the unwind voltage on all of our PS coating machines. The field meter measures the volts per inch of field generated by a charged web. As little as 500 volts per inch can attract dust and dirt. If the charge is 12,500 volts per inch, it can generate a one-inch spark. For safety reasons, Flexcon initiates static control measures for any area generating more than 5,000 volts per inch. You can adopt a similar approach with your presses and converting equipment.

Controlling Static

There are basically two approaches when it comes to controlling static. The first is to make the material more conductive, decreasing the tendency to generate static, by adding an anti-static component, if possible. You can ask your supplier to determine if such components are a part of your product, or if they can be added. Some films have anti-static coatings available, and some topcoats can have an anti-static chemical added to their formulation.

The second approach is to upgrade your processes. The first priority is to make sure that your machine is grounded. There should be a heavy, bare copper wire, perhaps with green shielding, attached between the frame of the machine and a ground point, such as a water pipe or a spike driven into the ground. You can also consider applying conductive grease to the idlers. Both of these techniques provide a path for charges to dissipate.

You could also consider the use of humidifiers. These can reduce the relative dryness in your facility and decrease the likelihood of static electricity. Elevated humidity levels, however, can lead to their own set of challenges, and are therefore not used as often as some other techniques.

Types of Static Control Devices

Given that static is generated when two surfaces separate, it can be created at a number of points during the manufacturing or converting process. Even if you correct the cause of static being generated at one point, it can be generated at the next point where two surfaces separate. That is why if the material cannot be made conductive enough through the use of an anti-static ingredient, you must control static at any point where it is being generated. The field meter will help you identify these points so you can install a control device.

When it comes to control devices, there are two basic types: passive and active. Passive devices include; tinsel and conductive brushes, which are relatively inexpensive. Active devices include electronically powered bars and blowers, which cost more. Flexcon uses a combination of these approaches to reduce the likelihood of static.

In terms of passive devices, the material generates a charge that ionizes air, making a conductive pathway to the ground. As the device drains the charge, the static is dissipated. When the film’s charge is no longer strong enough to ionize the air, the device is no longer effective. This is the main limitation of passive devices: it will reduce the charge, but it will not eliminate it.

Passive devices work best when they do not touch the web. If they do touch the web, static dissipation is limited to the contact point. The tinsel or brushes should be placed between a ¼ inch to a ½ inch from the film’s surface, and about three to six inches from the separation point. They should be on the same side as the separation to dissipate the surface charge rather than directing it through the web.

Most active devices are powered by electricity. They have a series of charged points surrounded by a grounded grid. The voltage between the points and the grid is enough to break down the air into both positive and negative ions. These ions float to the web and combine with the positive and negative charges on the surface, canceling out the surface charge. These devices can reduce the surface charge to zero. More expensive models can be made explosion proof as well.

Power bars are placed between one inch and three inches from an unsupported web. Blowers use air flow to extend the range of ions between two feet and four feet. These are particularly effective with unwinds and reels where the web changes position as time passes. The closer the blowers can be placed to the web, the more efficient they are. That’s because the ions will recombine and cancel each other’s charge as they travel toward the web.

Static Within Your Process

There are many suppliers of static control equipment that will evaluate your processes and recommend a static control device. After installation is complete, evaluate the device's effectiveness by conducting another audit with your field meter and quantifying the change in the amount of static generated.

If problems persist, talk with your Flexcon Sales or Technical Service Representative. They may be able to recommend a product construction that could minimize static within your process.

While it can be challenging to identify the root cause or causes of static and develop a system for controlling it, implementing a workable solution that uses some of the ideas mentioned above will help you reduce remakes and downtime. You should see immediate, measurable improvement, improving efficiency and creating a safer work environment.