Static Control Keeps the Lines Moving
If you’ll forgive the wordplay, static control technology in flexible packaging applications is anything but static. It’s moving fast, and two factors in particular are driving developments. First, with the economic climate uncertain and capital tight, converters need to squeeze as much productivity as possible from existing lines. And second, brand owners’ continuous search for added impact and performance from packaging means those lines are running an increasingly diverse range of materials.
Line speed and static go hand in hand, and when you add in the range of substrates now available you have a potent cocktail for potential production problems. Today’s lines are capable of running at upwards of 1200 feet/min, packaging is more complex, and lightweight substrates such as PET and PVC are especially vulnerable to the effects of static.
Static is InevitableStatic charges are an inevitable side effect of converting processes involving high-speed webs of material. They only become a problem, however, when they’re uncontrolled, leading to lost production. Static arises when an external force – generally friction, pressure or separation – causes an electrical charge to move from one material to another, leaving one positively charged and the other negatively charged. The size of charge depends on contributing factors such as the speed and force of the friction, pressure and separation: increased force or faster processes cause larger charges to be generated. If one of the materials is conductive, it won’t hold the charge, but if the material is non-conductive the charge is unable to move across the surface, creating a static “pool” of electrical charges.
Uncontrolled static causes a variety of problems. Products misbehave: Print can be out of register, tiny holes appear in extruded film, and static charges can cause blockages in the most modern machinery.
Dust attraction is a problem: Airborne particles are attracted to charged surfaces, leading to high reject rates and rework levels where the packaging calls for completely clean substrates – for example, in the pharmaceuticals industry.
And operators are endangered: The static voltage that causes an unpleasant shock when getting out of a car is around 15kV. Levels of 30-60kV are commonplace in converting, causing acute discomfort and, due to the recoil effect, increasing the risk of more serious accidents.
Charges are almost always generated as the substrate leaves the reel and travels through the first converting stage – often by a relatively simple procedure such as slitting it down to narrower widths. The real production hurdles occur after this point.
For example, if two webs are brought together in an automatic bag-making line, static can make it almost impossible to line up the two materials. During rewinding, static can affect the tension so much that contact, or even close proximity, with other surfaces results in dust and dirt contamination. It has been known for the web core to be crushed or fall out altogether because the tension of the web has become so extreme.
Passive and Active Static ControlsMethods of controlling static can be divided into “passive” and “active”. Passive solutions include induction techniques such as anti-static copper tinsel, one of the oldest and simplest methods. Tinsel is placed close to the surface of the web, and its points interact with the static field, creating a neutralizing field. However, induction methods cannot eliminate all static – they typically reduce it to a threshold level that remains high. Ensuring that equipment is adequately grounded is a further passive solution.
Active static control is more effective and utilizes ionization technology, employing high voltage AC or “Pulsed” DC to produce ionized air to neutralize surface charges. In both, the voltage is fed to an array of titanium emitter pins mounted on an ionizing bar, creating a high-energy “ion cloud” of positive and negative ions. In its AC guise, as the AC cycle changes, positive or negative ions are produced in approximately equal quantities, and a statically-charged surface of either polarity passing close to the cloud is quickly neutralized.
An AC system can only generate ions in accordance with the AC frequency. Pulsed DC ionization allows control of both frequency and the relative balance between positive and negative ions. Lower frequencies allow ionization over longer distances, and the balance control allows output to be adjusted to suit the charge polarity on the target. Pulsed DC thus offers optimum solutions for specific materials and more demanding applications.
Given that the foundations of modern converting technology were laid around a century ago, static is a relatively recent phenomenon, appearing on the scene after World War II, when machinery got faster and synthetic substrates proliferated.
Static’s here to stay now, but, thankfully, so are the solutions to control it.