Static can cause problems in medical device manufacturing processes, but active air ionization is an effective and practical fix.
Matt Fyffe (VP/General Manager – Meech USA)
In 600 B.C., the philosopher and mathematician Thales of Miletus reported that after rubbing a piece of amber on the fur of a cat the amber attracted and held feathers. This was the first account of static electricity (literally, electricity at rest).
What Thales observed was what we now know as triboelectric charging, where certain materials become electrically charged after contact with a different material through friction. While generating a controlled static charge has positive applications in some manufacturing scenarios—allowing the temporary adhesion between two or more surfaces of opposite polarity, for example—in many operations across a multitude of industries uncontrolled static electricity causes serious production problems. These range from downtime due to machinery jams due to product contamination to product loss in industries such as electronics, where even a low static voltage can destroy sensitive components. People can be damaged, too, with employees suffering electric shocks. Where flammable materials are used, there is also the real possibility of fires and explosions, as the tragic death of a Pennsylvania man in 2010 while filling his car with gasoline graphically demonstrated.
Uncontrolled static attraction is a particular problem for plastics industries and consequently for medical device manufacturers. Among the processes where it can be a problem are injection molding, blow molding, thermoforming, parts conveying and collection, and assembly processes. Even in the most stringent cleanrooms, static charge attracts particulates from people, processes, and equipment, so it is important to take appropriate measures to ensure static is kept to a minimum, if not completely eliminated.
This article considers the common production challenges arising from static, explains what static is, and describes the principal techniques for neutralizing static. In particular, it explains why active air ionization is an especially effective and practical solution.
The Damage Static Can Do
The primary problems resulting from electrostatic charges are electrostatic attraction, material misbehavior, and operator shocks.
Electrostatic Attraction (ESA). Not only are airborne contaminating particles attracted to charged surfaces; charged airborne particles can also be attracted to surfaces free of any charge. This problem affects most plastic-based industries in one form or another, but static in medical device manufacture it is the most common cause of rejected products. The problem affects a range of devices, including catheters, syringes, replacement joints, pacemakers, and stents.
Material Misbehavior. Uncontrolled ESA gives rise to problems besides product contamination. It can disrupt automated processes by misrouting, repelling, or causing parts to stick to each other or to equipment. This imposes significant cost penalties because it forces manufacturers to run their machines at much slower speeds than might otherwise be necessary.
Operator Shocks. Operator shocks are typically the result of an accumulated charge, or battery effect, occurring during the collection of parts in a bin or assembly area. While they can be painful, in most cases the effects are nonlife-threatening and short-lived. However, there are also cost implications in the recoil reaction associated with the initial shock, after which there can be a moment of disorientation, bringing with it subsequent hazards such as collision with other operators or machinery. More stringent health and safety standards place an increasing burden of responsibility on manufacturers to protect staff from static discharge.
Understanding Static When a material or object holds a net electrical charge—positive or negative—it is said to have a static charge. The term static is relative, as in many cases static charges will slowly decrease over time. How long the charge takes to decrease depends on the resistance of the material. For practical purposes, the two extremes can be taken as plastics and metal. Plastics generally have very high resistances, so they can maintain static charges for long periods; metals have very low resistances, and an earthed metal object will hold its charge for an imperceptibly short time.
The voltage present on a material depends on the amount of charge on the material and the material’s capacitance. The simple relationship is Q=CV, where Q is the charge, V is the voltage, and C is the capacitance of the material. For a given charge on a material, the lower the capacitance, the higher the voltage and vice versa. Plastics generally have very low capacitive values and therefore a small charge can produce very high voltages. Problems with static are most noticeable when working with plastic because the voltage level causes the attraction of dust, operator shock, and misbehavior of materials.
Static electricity results from an imbalance in the molecular construction of the material. In a balanced atom the positive charges in the nucleus are equal to the negative charges of the electrons orbiting the nucleus, so the overall charge is zero. This balance can change, however. If electrons are removed, the result is a greater positive charge in the nucleus; if extra electrons are added, the overall charge becomes negative. In both cases, static electricity is the result.
There are three main causes of such imbalances: friction, separation, and induction.
Friction. As two materials are rubbed together, the electrons associated with the surface atoms on each material come into very close proximity with each other, and can move from one material to another. The direction in which the electrons travel—from material A to material B or vice versa—depends on the triboelectric series, which is based on the order of the polarity of charge separation when a material is touched by another. When touched to a material near the top of the series, a material toward the bottom of the series will attain a more negative charge and vice versa.
In addition, the harder they are pressed together, the greater the exchange of electrons and the higher the charge generated. A practical example is if a piece of polythene is rubbed on a nylon carpet with gentle force, a moderate negative charge will be generated on the polythene, whereas if the force is increased, a larger negative charge will be achieved. The speed of the rubbing action also affects the level of charge; the faster the rubbing, the higher the level of charge. This is because the surface electrons gain heat energy generated by the friction, and this extra energy allows them to break their atomic bonds and transfer to other atoms.
Separation. When materials are in contact, surface electrons are in close proximity, and when separated, they tend adhere to one material or the other, depending—again—on their positions on the triboelectric series. The faster the separation, the higher charge generated, and the slower the separation, the lower the charge. A common example is a PVC web moving over a Teflon-coated roller. As the two separate, the electrons tend to adhere to the Teflon, generating a net negative charge on the Teflon and a net positive charge on the PVC.
Induction. The surface of a material in close proximity to a high positive voltage will tend to become positively charged. This is caused by ionization of the air between the surface of the material and the voltage source, which carries surface electrons away from the material to the source. This may occur when an operator is working near charged materials and becomes charged himself. On touching an earthed object, he will discharge to it and get an electric shock.
Neutralizing Static through Active Air Ionization
The same fundamental principle governs every technique for neutralizing static: Where a material has a positive surface charge, electrons must be added to the surface to rebalance the charge; where the surface charge is negative, the excess electrons must be removed.
The two basic techniques for rebalancing charge are conductivity and replacement. The former involves making an insulator conductive and then grounding it. Ways of achieving this include humidification and applying anti-static chemicals (either as coatings or adding them to plastics during manufacture). Carbon can similarly be added during manufacture to make plastic conductive.
When it comes to tackling static during the production process, the replacement technique using active air ionization is more practical. Active air ionization employs high-voltage ac or pulsed dc to produce ionized air to neutralize surface charges. The voltage is fed to an array of titanium emitter pins mounted on an ionizing bar. This creates a high-energy ion cloud made up of a high number of positive and negative ions, which are attracted to particles or surfaces carrying an opposite charge, thus rapidly neutralizing the surface.
The choice of ac or dc is determined by the application. 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, offering optimum solutions for specific materials and more demanding applications. For example, lower frequencies allow ionization over longer distances, and the balance control allows output to be adjusted to suit the charge polarity on the target.
Low-frequency operation lends pulsed dc eliminators to long-range neutralization. The relatively long duration of each half of the cycle causes large clouds of ions of alternating polarity to be emitted from the bar. This distance between the positive and negative ions close to the bar greatly reduces the rate of recombination (positive and negative ions coming together and cancelling each other out). Note that at long distances from the bar, fewer ions are deliverable to a statically charged surface, so the speed of neutralization is reduced. Therefore, when using pulsed dc equipment pay attention to the distance at which the bar will be mounted from the target surface.
An additional feature of pulsed dc systems is that the output waveform can be altered and the duration of the negative and positive sections can be increased or decreased. For instance, if the charge to be neutralized is known to be positive, the duration of the negative section can be increased and the positive part of the waveform reduced. This will increase the production of negative ions and decrease the production of positive ions, making the system more efficient at neutralizing the positive charge. Similarly, for a known negative charge the output can be biased toward positive-ion production.
As awareness of the problems uncontrolled static can cause grows, more medical device manufacturers are installing static-neutralization equipment. For example, one manufacturer specializing in the development and manufacture of mold tools, plastic injection-molded components, and the assembly of complex devices for the pharmaceutical, drug-delivery, medical, and healthcare industries uses static-control equipment for the assembly of injection-molded drug-delivery devices and throughout the injection-mold process, right up to hand assembly.
During assembly of the plastic components, ionizing blowers and nozzles neutralize the parts and remove the excess plastic-flash and statically attracted airborne contaminates. This is carried out within a Class 7 cleanroom. During the operation of the bench-mounted ionizing nozzle, the removed particulate is directed toward a tack-mat area, where it is captured to avoid future recontamination of product. Once clean, the drug-delivery device is manually inspected under an illuminated magnifying glass for cleanness.