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Static Causes and Cures

Static Electricity Causes and Cures

Contents

What Is Static Electricity? 4 How Is Static Created? 5 What Factors Affect Static Electricity? 6 Methods Of Elimination 7 Modes Of Operation of Static Eliminators 8 Addressing Common Electrostatic Problems 14 Overview Of Solutions 15 Glossary, Tribolelectric Series 18 About Meech 19 Static Electricity - Causes and Cures

About Meech:

Static electricity causes productivity, Our mission is to develop effective and quality and safety problems throughout durable products that are relevant to industry. In the form of electrostatic our customers’ needs; to maintain the attraction or repulsion it generates highest levels of response and service dust contamination and product and to develop the best possible misbehaviour. As an electrostatic support system for our clients in our discharge it gives shocks to operators, field of . starts fires and destroys electronic components by the million. As the first specialist manufacturer of static elimination equipment worldwide Meech has specialised in industrial to be approved as an ISO 9002 electrostatics for over 40 years. This registered company, we take our experience enables us to develop commitment to quality seriously. products which match the changing needs of our customers. The Meech The international quality approvals range encompasses four technologies covering some, or all of our products and includes over 40 products. include: Static Electricity - Causes and Cures Introduction

Quality products must be supported The purpose of this booklet is to look by good service. Meech Distributors at what static is, how it occurs, the throughout the world share our methods available to counteract it and commitment to quality and participate how Meech uses these methods in a in our continuous training programme. variety of industrial applications. It is This keeps them informed about not intended to be an exacting, developments in applications and scientific reference manual; instead its technology so they can offer the aim is to answer the most commonly highest level of customer service and asked questions about static electricity support. posed by Distributors and Customers alike.

To understand this booklet it is helpful to familiarise yourself with the various terms which can be found in the glossary at the end of this booklet. Static Electricity - Causes and Cures What Is Static Electricity ?

When a material or object holds a material, the lower the net electrical charge, either positive the higher the and vice versa. or negative, it is said to have a static Plastics generally have very low charge. The term static is a relative capacitive values and hence a small one as in many cases static charges charge can produce very high will slowly decrease over a period of . Conversely metals tend time. The length of time that this takes to have high capacitive values and is dependent on the resistance of the therefore a relatively high charge will material. For practical purposes the produce low voltages. This is why, in two extremes can be taken as practice, problems with static electricity plastics and metal. Plastics generally are most noticeable when working with have very high resistivities. This plastic, as it is the voltage level which allows them to maintain static charges causes the attraction of dust, operator for long periods of time; on the other shock and misbehaviour of materials. hand metals have very low resistances and an earthed metal object will hold There are two main types of static its charge for an imperceptibly small electricity, volumetric and surface. period of time. Volumetric static charges are charge Static electricity is usually measured imbalances within the body of a in . Whilst mains voltages of 220 material whereas surface static volts AC are considered dangerous, electricity is only present on the very levels of static electricity of 100 kV are outer surface of a material. common. The voltage present on a mate- In practice nearly all the static rial is dependent on two factors; the electricity problems found in industry amount of charge on the material and relate to surface charges. Whilst there the capacitance of the material. The is no way of neutralising volumetric simple relationship is Q=CV where static charges they rarely cause a Q is the charge, V the voltage and C problem and their effects are normally the capacitance of the material. It can minimal when compared to surface be seen that for a given charge on a static charges. Static Electricity - Causes and Cures How Is Static Created ?

There are three main causes of static the two materials are pressed together electricity; friction, separation and the greater the exchange of induction. and hence a higher charge is generated. Friction As two materials are rubbed together A practical example is if a piece of the electrons associated with the polythene is rubbed on a nylon carpet surface atoms on each material come with gentle force a moderate into very close proximity with each negative charge will be generated on other. These surface electrons can be the polythene, whereas if the force is moved from one material to another. increased a larger negative charge will The direction in which the electrons be achieved. The speed of the rubbing travel either from Material A to action also has that effect on the level Material B or vice versa depends on of charge, the faster the rubbing the the Triboelectric Series (See page 18). higher the level of charge. This is due Materials on the positive side of the to the surface electrons gaining heat Triboelectric Series will tend to give generated by the friction. This up their surface electrons and become extra energy allows them to break their positively charged whilst conversely atomic bonds and transfer to other materials on the negative side of the atoms. series tend to gain electrons and hence a negative charge. The harder

Contact + Friction + Separation Static Electricity - Causes and Cures Separation Induction The method of charging by separation Whilst of interest technically, induction is similar to that of friction. When two does not play a significant role in our materials are in contact the surface field. Static charges can be generated electrons are in close proximity to when materials are in the presence of each other and upon separation have a strong . The surface of a tendency to adhere to one material a material in close proximity to a high or the other dependent upon their positive voltage will tend to become relative positions on the Triboelectric positively charged. The method of Series. charging is caused by ionisation of the air between the surface of the material The faster the separation of the and the voltage source which carries materials, the higher charge generated surface electrons away from the and conversely, the slower the material to the source, (ionisation is separation the lower the charge. A explained in methods of elimination). common example is of a PVC web An example of induction is operators moving over a Teflon coated roller, as working near charged materials. The the materials separate the electrons operator will himself become charged will tend to adhere to the Teflon, and on touching an earthed object will generating a net negative charge on discharge to it, giving the operator an the Teflon and a net positive charge on electric shock - often mistaken for a the PVC. shock from the mains supply.

Contact + Pressure + Separation Static Electricity - Causes and Cures What Factors Affect Static Electricity

Among the many factors that affect electricity than most plastics. the generation and maintenance of Atmospheric deposits small a static charge are humidity, the type quantities of water on all surfaces in of material, repetition and change in their environment and hence surface temperature. static charges on materials have a tendency to dissipate to earth by Type Of Material current flow through the surface Some materials are more readily moisture. charged than others. For example materials such as acetate will gain a Repetition charge very readily whilst glass will Repeated actions such as friction gain a charge less readily. Also the or separation will increase the level relative position of materials on the of charge found on a material. For Triboelectric Series will determine example a plastic web moving over a whether a material charges positively series of Teflon rollers will increase its or negatively dependent on the other surface charge after every roller. material with which it has come into contact. For example hard rubber, Battery Effect when rubbed against nylon, will The combination of many charged become negatively charged whilst items can lead to extremely high when it is rubbed against polythene charges. For instance individual sheets will become positively charged. of plastic with relatively low surface charges when stacked together can Humidity generate extremely high voltages. Generally speaking the dryer the environment, the higher the level Change In Temperature of static charge and conversely the As a material cools down it has a higher the humidity, the lower the static tendency to generate charge. The charge. In relative terms water is a action of the cooling is to leave a net significantly better conductor of charge on the material throughout its Static Electricity - Causes and Cures entire volume. If the material is a very 1. Movement of electrons through the good the internal (volumetric) material itself. static charge can be maintained for extremely long periods of time. 2. Movement of electrons through However over time this charge another material in contact with normally migrates to the surface at the surface. which point it becomes a surface static charge. An example of this is an 3. Movement of electrons through injection moulding which is seemingly ionisation of the surrounding air. neutral when hot but can subsequently be found to have a large surface There is in actual fact a fourth method, charge once cool. sparking, which occurs when the surface voltage is sufficiently high to Methods of Elimination cause the air to become conductive. However the occurrence of sparking The fundamental principle for is normally due to the lack of neutralisation of static charges is the application of other methods! same whatever the technique used. Where a material has a positive Humidity surface charge electrons must be As previously noted moisture on (or delivered to the surface to bring the within) a material will tend to leach charge back into balance. Where the away static charges down to earth. surface charge is negative the excess For example paper generally has a electrons must be removed from the relatively high moisture content and surface to neutralise the charge. The does not maintain particularly high delivery or removal of electrons can levels of static. However if the paper be done by one of the three following is particularly dry static can become a methods, either severe problem.

Static Electricity - Causes and Cures Passive Ionisation A drawback of radioactive eliminators The close proximity of a conductor to a is the fact that they are only available charged object will tend to discharge it. on annual leases. The radioactive For example Meech Model 974 source loses its effectiveness over Carbon Fibre Brushes will reduce time and requires replacement on an static charges in materials passed in annual basis. close proximity to the brush. Active Electrical Ionisation Radioactive Ionisation By using high voltage AC or DC, Radioactive sources such as polonium ionised air can be produced which cause ionisation of the surrounding can then be used to neutralise surface air which will neutralise surface static charges. The use of AC or DC systems charges. Meech do not supply is application dependent. Further Radioactive Eliminators. information on this is available in the applications section later in the booklet, from the Meech Manual or from our technical sales staff.

Brush Casing Connected Passive Eliminator Brush to earth

Ionisation at Negative Ions brush tips

Surface with high Surface with small positive charge residual positive charge

Direction of material

Diagram 1 Static Electricity - Causes and Cures Modes Of Operation Of Static Eliminators

Passive Eliminators surface of the material. When this A charged object will generate an process has occurred in sufficient electric field between itself and any quantity the positive charge on the surrounding earthed object (or indeed surface will be reduced. In this any object of differing voltage). In the process there will come a time when case of the Meech 974 passive the electric field generated between eliminator the field is between the the surface and the tip of the brush is surface and the tips of the Carbon no longer sufficient to cause ionisation Fibre or Stainless Steel earthed brush. of the air and no further neutralisation See Diagram 1.The fine point at the takes place. In the instance of a end of the individual bristles causes negatively charged surface the the electric field to be highly opposite procedure takes place. concentrated at this point. When the strength of this electric field reaches Passive eliminators are thus useful for a sufficient value, ionisation of the air reducing high levels of static charge, molecules surrounding the tip occurs. tens of kV’s down to levels of a few kV’s. However by their very nature In the example in Diagram 1 the they are not able to completely positive charge on the surface of the neutralise the surface charge. material will cause electrons from the tip of the brush to jump to Radioactive Eliminators surrounding air molecules which will Radioactive eliminators employ then have a net negative charge and polonium 210 or other low level are thus negative ions. radioactive source. In the process of radioactive decay alpha particles are From the simple rule that opposite emitted to the surrounding charges attract, the negative ion will atmosphere. These high speed be drawn to the positively charged particles collide with the air molecules Surface of the material. When the ion and in doing so cause the air to reaches the surface the extra become ionised. This ionised air then the molecule is carrying is drawn from neutralises nearby surfaces in similar the molecule and delivered to the fashion to the passive eliminators. Static Electricity - Causes and Cures AC Eliminators are then repelled from the pin due to AC Eliminators operate at supply their like charge. frequency. The mains voltage, 110 or 240 etc. is greatly increased using a On the negative half of the cycle the ferro-resonating , Meech opposite occurs and negative ions are Model 904, to generate voltages of created at the pin point. These again between 4.5 and 7 kV. This high are repelled from the pin due voltage is fed to the ionising pins, to their like voltage. whilst the casing of the bar is Thus around the ionising pin a cloud of connected to earth. See Diagram 2. positive and negative ions is produced. If we look at the positive cycle of the In the absence of outside influences input waveform we will see that the the positive and negative ions are electrode pin will be at a positive attracted to each other or to a nearby voltage compared to the casing. This earth, such as the casing of the bar or generates a strong electric field nozzle, and would either neutralise between the two which is highly each other or be dissipated to earth. concentrated at the sharp point of the However with the presence of a electrode pin. This in similar fashion nearby static charge an ion will be to the passive bar generates positive attracted to on opposite charge on the ions at the pin point. These molecules surface of the material. At the surface

Bar Casing Connected Ionising Bar to earth

HT AC Supply to Electrode Pin

Ionisation at Electrode Point

Surface with high Surface with no positive charge residual charge

Direction of material

Diagram 2 Static Electricity - Causes and Cures of the material the electrons will be the bar is made of plastic and hence exchanged and the surface will be there is no proximity earth. The output neutralised. from the power supply is effectively a square wave switching from negative As the ionisation at the bar is not to positive at the chosen frequency. dependent upon the surface charge Looking at the positive half of the wave and ions are produced regardless of form the controller switches on the the proximity of a surface charge, high output voltage connected to the complete neutralisation of a surface positive emitters. This then sets up can be achieved. This is a significant an electric field between the emitter advantage over the passive and the surrounding earthed objects. eliminators. The speed at which At the sharp point of the emitter this charges can be neutralised is field is extremely strong, and in similar dependent upon the rate of ion fashion to the AC eliminators, positive production and speed of repulsion of ions are produced. The similar charge the ions from the emitter pins, which of the ion and the emitter drives the in turn is dependent on the voltage at ions away from the bar. the pin. The Model 915 Bar running at 7.0kV gives greater performance than On the negative half of the cycle the when running at 5kV. power supply delivers a high negative voltage to the alternate set of Pulsed DC Eliminators emitters. Again in similar fashion to Pulsed DC Eliminators like their AC the AC eliminators negative ions are counterparts produce ionised air by produced at the emitter point. using high voltage. Whereas the AC A statically charged object in the units operate at supply frequency vicinity of the ionising bar will attract or the Pulsed DC units operate at lower repel the ions, dependent upon their frequencies, in the case of the Meech relative polarities. When the ions reach Model 977v3 between 0.5-20 Hz. The the statically charged surface, 977v3 also features a variable output neutralisation takes place in a similar voltage from 6 - 14 kV peak to peak. manner as described in the AC The ionising bar consists of a series of Eliminators section. emitters connected alternately to the The low frequency of operation lends negative and positive outputs of the Pulsed DC equipment to long range 977v3. See diagram 3. The casing of neutralisation. The relatively long Static Electricity - Causes and Cures duration of each half of the cycle An additional feature of the Pulsed cause large “clouds” of ions of DC system is that the output wave alternating polarity to be emitted from form can be altered and the duration the bar. This distance between the of the negative and positive section positive and negative ions close to the of the wave form can be increased or bar greatly reduces the rate of decreased. For instance if the charge re-combination, (positive and negative to be neutralised is known to be ions coming together and cancelling positive the duration of the negative each other out). part of the output can be increased At long distances from the bar, less and conversely the positive part of the ions are deliverable to a statically wave form reduced. charged surface and hence the speed This will increase the production of of neutralisation is reduced. Hence negative ions and decrease the when utilising Pulsed DC equipment production of positive ions, making the on dynamic applications, such as system more efficient at neutralising moving webs, thought must be given the positive charge. In similar fashion to distance at which the bar will be for a known negative charge the mounted from the target surface. output can be biased towards positive ion production.

Pulsed DC Ionising Bar Dedicated positive and negative emitter pins

Positive Ions not attracted Ionisation at to surface charge. Electrode Points Negative Ions attracted to positive charge over long distance

Surface with high Surface with no positive charge residual charge

Direction of material Diagram 3: Static Electricity - Causes and Cures Other Aspects

Measurement High Accuracy Balanced Neutralisation In order to select the correct In the majority of applications the equipment for a given application it is reduction of a static charge to a few important to know the magnitude, and hundred volts is normally sufficient to in some instances, the polarity of the eliminate dust attraction and charge to be neutralised. In cases misbehaviour of the product. However where AC eliminators are to be used it in the electronics industry static is important to know the magnitude of charges of a few hundred volts can the static charge so that the correct have disastrous consequences with eliminator bar can be selected. In modern micro-processor chips. In cases where Pulsed DC equipment is these instances it is important to to be used, both the magnitude and ensure that the ionisation delivered by the polarity of the static charge are the neutralisation system is balanced. important to allow the power and bias In such cases the Meech Series 200 of the output to be set correctly. Whilst range of products is recommended. it is quite difficult to measure the actual This has been specifically developed amount of surface charge the surface for use in the electronics industry. For voltage can be measured by looking at example, the Models 221 and 225 the electric field it generates. Ionised air blowers constantly monitor the ion ouput to ensure very The Meech Model 983v2 measures exact neutralisation. Products can the electric field generated by a stati- be neutralised to within a few volts of cally charged object, the unit of which earth potential. This greatly reduces is volts per metre. By measuring the the risk of damage to components by, field at a known distance the surface in terms of static charges, relatively voltage can then be deduced. In the low voltages. case of the Meech Model 983v2, the unit is calibrated for a distance of 150 mm from the surface of the charged object. The surface voltage and its po- larity is then displayed on the screen. Static Electricity - Causes and Cures Static Generation As well as neutralising static it is also possible to generate static charges in materials by the use of high voltages. By passing materials through intense electric fields it is possible to charge materials either positively or negatively. This charge can be of assistance in a manufacturing process. A common usage of static generation is in high frequency welding applications where induced static charges are used to temporarily bond one item to another to hold them in place during the welding process. For further information on static generation applications please consult either the Meech Manual or Meech Static Eliminators Head Office. Static Electricity - Causes and Cures Addressing Common Electrostatic Problems

Identification of the problem in the form of the product itself, There are four main troublesome usually webs, fibres or sheets, sticking difficulties arising in industry, deriving to themselves or equipment, from electrostatic charges, with a fifth misrouting or repelling. Automated affecting only the electronics industry processes are particularly prone to this but with very serious and costly problem. consequences. Operator/Personnel Shocks Electrostatic attraction (ESA) This is becoming increasingly Airborne particles are attracted to significant as companies look to charged surfaces or indeed charged improved safety standards. Whilst airborne particles are attracted to a shocks can be painful the effects are surface which could be totally free of usually quite safe and short lived. any charge. This problem effects most However, in extreme cases, the plastic based industries in one form or debilitating effects can cause another, spoiling finishes of painted personnel collision or entrapment with products and causing rejects in quality associated machinery or can even in the food, pharmaceutical and initiate a fire or an explosion in medical industries. hazardous areas.

In the printing industry dust (ESD) attraction damages print finishes or This problem is associated to indeed printing plates. The film electronics assembly, installation and industry also suffers with low quality field service and also electronic prints and poor resolution projections component manufacture. in the cinemas. Voltages as low as 5 volts which have The microscopic nature of no real meaning in other industries, semiconductor manufacture can be can cause catastrophic failure of affected by this problem. electronic components or much worse, latent damage which results in field Material Misbehaviour failure, by far the most costly in terms This is another form of ESA. However, of repair and manufacturers’ instead of the contamination of reputation. products, the problem manifests itself Static Electricity - Causes and Cures Overview Of Solutions

Electrostatic attraction (ESA) D. Silk Screen Printing - Long Range Long range Pulsed DC Bars can be A variety of equipment is available to used to eliminate charged screens resolve this problem in many different without causing drying. Ionising guns ways. Some examples follow : or air curtains can remove attracted particles from substrates pre printing. A. Injection Moulding - Neutralisation Bathe components in ionised air. Often E. Semi Conductors - Long Range using AC Ionised air blowers or Pulsed Pulsed DC can be used for this DC bars. situation - stand alone or using air assist from HEPA filters. This method will not remove attracted particles but will help prevent charging Material Misbehaviour and thus prevent attraction of dust. A. Webs B. Injection Moulding - Particle Most commonly solved using AC Removal Where particles have already ionising bars. The Model 915 bar been attracted they can be removed offers the power to cope with fast, effectively by the use of high velocity highly-charged webs. ionised air. This can be achieved for small components with Meech nozzles Bars can be positioned either side or guns, larger components by the use of a web to achieve the best results, of 957 air curtains and with extremely usually at the point where the problem large components the Meech occurs, or just prior to this position. JetStream system. Note: The use of An example would be where a web ordinary compressed air will often fail exits a roller or at the exit rollers of a to remove dust. It can also worsen the flat bag making machine. problem by generating further static charges on the material. B. Sheets Two major problems exist with sheets C. Webs - they can repel or stick together when Single or doubled sided web collating. Ionising bars or a 935 blower cleaners could be deployed in this can be placed above the process to situation or high velocity blow off eliminate the static charges on the systems, air curtains etc., sheets and allow the process to continue. Static Electricity - Causes and Cures On sheet feeders, static attraction 935 ionising blowers are generally causes multiple sheet pick-up rather used on transport systems. Long than the single sheets required. range Pulsed DC equipment is Meech 957 air curtains can be used exceptionally useful for controllinhg the in this situation to create a blade of charges on parisons. ionised air which penetrates the top layers of sheets allowing free Operator/Personnel Shocks separation. This problem can occur in two ways C. Component Sticking And Repelling and can be resolved in two ways. On conveyors or bowl feeders, small plastic components can stick to the 1. Personnel can receive shocks belt or inside the track of the bowl from extremely highly charged feeder: this leads to down time and material as it discharges through ineffective production flow. Where them to earth. components repel each other they can 2. Operators can themselves jump off or out of the delivery system. become charged by handling or being in close proximity to highly Deploying ionising bars , long range charged material. When the preferably (AC bars with air assist or operator is presented with a path Pulsed DC) can resolve this problem. to earth , usually metal objects or Meech 935 blowers may also be used a machine, they discharge in this situation. violently often believing they have had a mains shock. Where larger components are concerned, repelling can cause The problem can be addressed by components to fall over whilst in either neutralising the material with process transport e.g. blow moulded bars or blowers or by continuously bottles on conveyors. The parrisons neutralising the operator with long on blow moulding machines also can range Pulsed DC bars. In extreme repel each other causing quality cases both methods can be combined. problems.

Static Electricity - Causes and Cures Electrostatic Discharge (ESD) In all of these situations ionisation is recognised as a solution and The electronics industry predominantly sometimes as a supplement to addresses this problem by the use of traditional methods. conductive or static dissipative materials in the work place and in This is a very specialised area for packaging. 200 range of equipment. Series 200 which Meech has developed the series All of these materials are earth bonded products are cleanroom friendly and to the same electrical potential thus give highly accurate, fast eliminating the possibility of potential neutralisation. differences and subsequent discharges. Triboelectric Series

However there are many areas in The position in the series indicates the electronics where such materials relative polarity that is likely to occur cannot be used practically . when two of the materials are rubbed together but does not necessarily Clean Rooms The materials can cause dictate the likely magnitude of the contamination.Also the laminar air charges. flows can generate static.

Powering-Up Testing PCB’s and components may short out and present an electric shock hazard.

Auto Assembly The movement and separation within the equipment generates its own static charges.

Product Assembly Where PCB’s are assembled into equipment made from ordinary static generative materials Static Electricity - Causes and Cures Glossary

Capacitance: A measure of the ability Ion: A molecule or atom with an of a substance to hold charge. electrical imbalance (a negative ion has an excess of electrons, a positive : The unit of measure of ion has a deficit of electrons). electrical charge. Resistance: The higher the resistance Conductance: The higher the of a material the harder it is for charge conductance of a material the easier to flow through it. Insulators such as charge will flow through it. Metals are plastics are very resistive. very conductive.

Electron: A small atomic particle with a negative charge (1.6 x 1019 ).

Meech International 2 Network Point Range Road, Witney OX29 0YD, UK Tel: +44 (0)1993 706700 Fax: +44 (0)1993 776977 email: [email protected]

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