When converting materials, static charge is unavoidable. And although a carefully tuned charge can be helpful in some processes, you want to avoid it in most situations. What causes static electricity during converting? And how can you control the adverse effects and improve your product’s quality?
Causes of static charge during converting
All materials consist of atoms. An atom has a nucleus containing positively charged protons and electrically neutral neutrons, surrounded by negatively charged electrons. The number of protons determines the element. An atom is electrically neutral, and therefore not statically charged, when the number of protons and electrons is equal.
But as soon as materials touch each other, they can exchange electrons. When that happens, the number of protons and electrons is no longer equal, and the material becomes statically charged. This is apparent through an attraction effect, a kind of invisible adhesion.
Why is this? Simply put, the charged material tries to restore the lost balance. Its electric field attracts opposite charges and can also attract neutral particles by redistributing the charge within them.
Factors that determine the degree of static charge
What determines the degree of exchange and the resulting charge? Among other things, the characteristics of the material. The Triboelectric series indicates how easily a material gains or loses electrons. The further apart two materials are in the triboelectric series, the greater the charge they will typically generate when they contact and separate, and the stronger the resulting electrostatic attraction may be.
The surface properties of a material (surface area, roughness, coating) also affects the degree of exchange, because they influence the area and nature of the contact.
Environmental factors such as humidity also play a role. Higher humidity increases the conductivity of many surfaces and materials, allowing excess charge to dissipate more easily. This explains why there is much more static electricity when the air is dry during frost.
Above all, the nature of the contact is important: higher contact pressure and faster separation can increase the amount of static charge generated. In converting processes, repeated contact and separation between the substrate and rollers, winders, guides or belts can therefore generate significant static charge, especially at high production speeds.
What happens once the material is statically charged?
Once a material has become statically charged, it tries to restore the electrical balance. In conductive materials like steel, electrons can move freely to neutralize a charge.
But in poorly-conductive materials such as paper, plastic, or wood, the opposite happens. Charge cannot move freely in these materials, so the material holds the static charge until it slowly leaks away, is neutralised by ions, or finds a sufficiently conductive path. You can see this clearly after the winding process in paper production, where static charge can be present in stored paper rolls after a long time.
What does static charge mean for your converting process and product quality?
As long as the substrate is statically charged, it exerts its characteristic attraction force. In some processes, this attraction comes in handy, for example, in lamination or in stacking.
But when further processing materials — paper, cardboard, plastic — the static charge and resulting attraction cause problems such as:
- jamming or misfeeding of the substrate
- ignition or explosion hazard in the presence of combustible dust or flammable vapours
- attraction and retention of dust and contaminants from the environment
In the event of a jam or misfeed, it becomes immediately apparent that the substrate is statically charged, so action is usually taken right away. And in an environment where there is a risk of explosion, there are strict safety precautions.
But number three, the attraction of dust and particles from the environment, may not be detected in time, and results in:
- poor printability (hickeys)
- unhealthy work environment
- poorly adhering labels or tape
- contaminated end products
As you can read in our article about the adverse results of dust in converting processes, it is clear that you need to control static charge and its consequences. So, which measures are available?
How can you control static electricity during converting and reduce the resulting damage?
How can you control static charge during your converting process and limit negative impact? One step is to ground conductive materials and equipment, because it gives charge on conductive parts a controlled path to dissipate. Also, ensure proper humidity levels and keep the production environment clean.
Operations such as slitting and rewinding continuously generate dust and fibres from the material itself. Repeated contact and separation with rollers, guides and belts can also continue to generate static charge throughout the process. The charged substrate can then attract and retain newly generated or airborne particles.
To deliver a clean product, this dust must be removed before the substrate is wound into the roll or the particles become trapped between stacked sheets. To achieve a thorough dedusting, Weducon developed the HE web cleaning system, a well-thought-out system consisting of ionization, suction, and filtration. (Read a detailed explanation of the Weducon cleaning principle here.)
Ionization produces both negative and positive ions, which neutralize excess positive or negative charge on the substrate surface. This reduces the electrostatic forces holding particles to the material, allowing them to be removed by the controlled airflow and extraction. The removed dust does not enter the production environment but is collected in the self-cleaning filter.
Static electricity — will you be in charge?
It is clear that every converting process results in static charge. But taking the right measures prevents static charge from damaging product quality and process performance. Contact us to discuss your production process; we will gladly help you find the right solution for a clean, unhindered production process.
