SubscribeSimon Hatcher B-Eng, Technical Director, Meech Static Eliminators, shares some thoughts on this potentially shocking topic.
Static electricity is a phenomenon that has been recognised for a thousand years, is experienced by everyone, but understood by few.Today’s high-speed production processes, often involving synthetic materials, are more prone to static effects than was historically the case; an example of this is in labelling. Labels used to be printed on paper and applied slowly, they are now usually printed on plastic and applied at high speeds, causing static to build up, resulting in misapplications, lost production and wastage.
All too often the dangers of static discharge in the workplace are ignored, sometimes with serious consequences. There have been instances of employees being absent on sick leave for many weeks as a direct consequence of receiving static shocks. Although these are exceptional cases, many employees receive shocks that are severe enough to literally make them jump. That reflex action can have serious consequences for example, dropping a breakable or heavy object, coming into contact with working machinery.
Meech has specialised in providing industrial electrostatic solutions for over 40 years and was the first specialist manufacturer of static elimination equipment worldwide to be approved as an ISO 9002 registered company. The company offers an extensive range of static control products, which are currently in use in a wide range of industries ranging from packaging to contact lens manufacture.
What is Static?
Any material that holds a net electrical charge, either negative or positive, is said to be statically charged.To give a comparison, a mains voltage is commonly considered dangerous at 220V, whereas static charges commonly reach voltages of 100kV. In many cases static charge slowly decreases over time and the length of time involved depends on the resistance of the material.
The voltage present on a material is dependent on two factors, the amount of charge on the material and its capacitance, Q=CV where Q is the charge, V the voltage and C the capacitance of the material, therefore for a given charge on a material, the lower the capacitance the higher the voltage.Plastics generally have low capacitive values, so a small charge can produce a very high voltage, which is why problems with static electricity often occur when working with plastic.
There are two main types of static electricity:
- Volumetric static charges, which are charge imbalances within the body of a material.
- Surface static electricity, which is only present on the very outer surface of a material.
In practice, most static electricity problems relate to surface charges.
Causes of Static
Friction occurs in many manufacturing processes either directly during the production process or while items are being transported between processes.As two materials are rubbed together the electrons on the surface atoms of each material come into close proximity with each other and move from one material to another.The direction in which they travel depends on their position in the Triboelectric Series (a ranking of materials by relative polarity). Materials on the positive side of the Triboelectric Series will tend to give up their surface electrons and become positively charged.The harder the two materials are pressed together the greater the exchange of electrons and the higher the charge generated.
Separation occurs where one material is peeled rapidly from another.When two materials are in contact the surface electrons are in close proximity and upon separation have a tendency to adhere to one material or the other, depending on their relative positions in the Triboelectric series.The faster the separation, the higher the charge generated.
Induction created static charge does not play a significant role in industrial applications. Static charges can be generated when materials are in the presence of a strong electric field
Static - Factors to Consider
Material
Some materials are more readily charged than others and the relative position of materials on the Triboelectric Series will determine whether a particular material charges positively or negatively dependent on the other material against which it is rubbed.
Humidity
Generally, the dryer the environment the higher the level of static charge. Water is a better conductor than most plastics, so when atmospheric humidity deposits small quantities of water on surfaces the static charge tends to dissipate to earth by current flow through the surface moisture.
Repetition
Repeated actions such as friction or separation will increase the level of charge, for example a plastic web moving over a series of Teflon rollers will increase its surface charge after each roller.
Battery Effect
The proximity of many charged items can lead to extremely high charges, for example individual sheets of plastic with relatively low surface charges when stacked together can generate extremely high voltages
Changes in Temperature
As a material cools it has a tendency to generate charge. One example of this is in injection moulding which may seem neutral when hot but can have a high surface charge once cool.
Common Problems Caused by Static
Electrostatic attraction (ESA)
Airborne particles are attracted to charged surfaces. This causes problems in many plastic manufacturing processes, for example spoiling the finishes of painted products or causing quality failures in the food, pharmaceutical and medical industries.
Material Misbehaviour
A sub-set of ESA, material misbehaviour manifests itself when products, usually webs, fibres or sheets, stick to themselves or equipment - automated processes are particularly prone to this problem.
Operator/Personnel Shocks
Personnel can receive shocks from highly charged material as it discharges through them to earth and operators can themselves become charged by handling or being close to highly charged materials.
Electrostatic Discharge (ESD)
This problem is usually associated with electronics assembly and electronic component manufacture, where voltages as low as 5 volts, which are of no consequence in other industries, can cause catastrophic failure.
Getting to Grips with Static
To remove static charge electrons must be delivered to or removed from a surface to bring the charge back into balance.
This is usually achieved by:
- Movement of electrons through the material, for example through raising the humidity of the environment.
- Movement of electrons through an adjacent material, for example if the charged material is in close proximity to a conductor.
- Movement of electrons through ionisation of the surrounding air. By using high voltage AC or DC ionised air can be produced which can then neutralise surface charges, this is known as Active Electrical Ionisation.
AC Static Elimination
AC eliminators operate at supply frequency.The mains voltage is boosted using a transformer, for example Meech Model 915.This high voltage is fed to the ionising pins, while the casing of the bar is connected to earth.During the positive cycle the electrode pin has a positive voltage compared to the casing.This creates a strong electric field between the two, which is highly concentrated at the point of the electrode pin where positive ions are generated.These molecules are then repelled from the pin due to their like charge.
During the negative half of the cycle the reverse occurs and negative ions are created at the pinpoint.Thus, around the ionising pin a cloud of positive and negative ions is produced.In the presence of a nearby static charge an ion will be attracted to an opposite charge on the surface of the material, thus neutralising it. As the ionisation at the bar is not dependent upon the surface charge and ions are produced regardless of the proximity of a surface charge, complete neutralisation of a surface can be achieved.
Pulsed DC Eliminators
Pulsed DC units, for example the Meech Model 976, operate at lower frequencies, between 0.5 - 20 Hz as well as featuring a variable output voltage. The ionising bar consists of a series of emitters connected alternately to the negative and positive outputs and because the bar casing is made of plastic there is no proximity earth.During the positive half of the wave the controller switches on the high output voltage connected to the positive emitters. This sets up an electric field between the emitter and the surrounding earthed objects.At the sharp point of the emitter this field is extremely strong and positive ions are produced.
The similar charge of the ion and the emitter drives the ions away from the bar. During the negative half of the cycle the power supply delivers a high negative voltage to the alternate set of emitters and negative ions are produced at the emitter point.A statically charged object in the vicinity of the bar will attract or repel the ions, depending upon their relative polarities. When the ions reach the statically charged surface neutralisation takes place.The low frequency of operation makes pulsed dc equipment suited to long-range neutralisation
Gerber Foods “delighted”
Gerber Foods Soft Drinks Ltd. produces the popular children’s drink range, Sunny Delight, under contract from Proctor and Gamble.The containers are high-density polyethylene bottles, which are bought in to the factory prior to labelling, filling and topping.Occasionally the lines experienced labels curling back on themselves instead of adhering correctly to the bottles, resulting in loss of productivity.
Meech Technical Advisor, Mark Jones, investigated the situation and ascertained that it was due to a build-up of static on the labels, induced when two non-conductive materials: the labels and the backing sheet, were pulled apart at high speed, an effect which was exacerbated during periods of low humidity. His recommendation, that two Meech Static Eliminator Bars be installed on each labelling line, has been implemented and there have been no further occurrences of the mislabelling problem.
Parkland Boosts Machine Efficiency
Parkland International manufactures slitting and re-winding machines.With ever shorter runs and an increasing range of materials, it is essential for Parkland’s customers to be able to optimise the machine for each job and to increase the speed and ease of changeover from one job to another. Modern slitter re-winders run at high speeds and this, in combination with certain materials, can produce a large build up of static on the material, causing a range of quality problems that lower productivity.The Meech system allows bars to be fitted some distance from the reel, out of the operators’ way, and therefore enabling easy job changeover.
The ability to easily adjust the system’s anti-static levels is important to optimise productivity and Parkland advises clients to calibrate the system to suit the materials they use, and then simply re-set the system when jobs are changed. The incorporation of Meech Static Eliminator systems into Parkland’s range of slitter re-winders provides real productivity benefits to customers in many industry sectors including food, medical and computer manufacture.
IT’S A WRAP FOR GEEST WITH MEECH STATIC ELIMINATORS
Geest PLC is a leading fresh prepared foods and produce company which supplies High Street supermarkets with a wide range of products including fresh pizza. When the Gosberton production facility in Lincolnshire started to experience productivity issues with the wrapping of its large, rectangular pizzas, Engineering Manager Colin Stead suspected that static electricity on the wrapping film was the problem.
Colin called in Meech Static Eliminators’ application specialist Mark Jones, who confirmed that the rapid unwinding of the PVC film was generating static in the film, to voltages of 10kV.This was causing machine jams and film wrap-around on the flow wrapper forming neck.Mark proposed a simple solution based on the well-proven Model 915 shockless elimination system.
As soon as system was installed the static issue was solved and the wrapping line ran smoothly at its required rate of 30pizzas/minute. “Everyone was very impressed,” comments Colin, “the equipment was very effective and solved the problem right away.”