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Article about static electricity

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Article about static electricity

Static electricity
     - friend or foe?

by Reinhold Rutks,

Static electricity - friend or foe?

In this article we will try to give you some background information and tell a little about problems which are occuring in industry and point out a few suitable solutions. But, perhaps it isn’t correct to single mindedly talk about problems - there is an ever growing discipline with good reason may be called "utility use". Could you imagine not having a copying machine? paragraph below gives some examples on “utility use” applications.

Static electricity is an often overlooked factor in industry. This is true in spite of us all having many possibilities to relate to the phenomenon by our own experiences such as the sparks that we get when getting out of the car, touching a door handle after having walked across the fitted carpet of your hotel or the sparks from your synthetic sweater when you pull it off. you remember the physics lesson when the teacher rubbed an ebonite rod with a cat’s fur or charging objects or perhaps himself with a van de Graff generator?? Well, then you have “seen” the problem demonstrated to you.

In every business which processes or handles at least one component which is an insulator there might be a reason to bring static electricity to the agenda. Insulators that we all are familiar with from everyday life are e.g. paper, plastic, board, textiles, glass, rubber etc.

How can one recognize and identify problems with static electricity?
It is quite obvious that the sparks you get when handling an insulator are caused by static electri-city, but it is usually not possible to identify problem spots by “feeling your way” by the touch of your finger. Static electricity usually manifests

itself as a loss of production or a quality problem where the underlying cause remains concealed until active measures are taken to study the problem.

There are three main criteria after which it may be practical to divide the problems;

  • Runability problems - machines or processes give poor performance often with reduced production capacity as a result

  • Quality problems - hygiene problems, dust, dirt and particles stick to the product, poor tolerances, lost specifications.

  • Work environment problems - discomfort or danger to personnel

Runability problems are probably constituting the largest part of the problems, and are at the same time the most difficult to identify. We are talking of electrostatic attractive forces which influence how a material is acting in a machine or process. The detail or material is stuck on a surface such as a machine frame, a roller, work bench etc., and only occasionally is it possible to “see” the problem. Many times the force isn’t stronger than causing a disturbance which by reducing production speed will go away and leave an adequate product. This is often interpreted as being a mechanical problem and it is with indignation concluded that the machine supplier has been “cheating”, since the machine cannot run at the specified speed. Runability problems are very common in packaging machines, printing presses, converting machines, conveyors, vibrating feeders, filling machines and many many others.

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Quality problems may of course be a direct result of runability problems. An ever more important and more common problem is dust, dirt fibres, chips etc. which stick to the product. You have most likely noticed how quickly dust is attracted to your TV screen at home, it doesn’t matter how often it is cleaned, the dust immediately returns. This is a very plain example on how hygiene problems and static electricity are interrelated.

Work environment problems are often apparent and usually constitutes an operator getting sparks due to materials or working areas being electrostatically charged with the subsequent discharges as a result. It is also relatively common that personnel wearing clothes made from the wrong material are being charged with annoying sparks as a consequence. An extreme case of work environment problem is what you could call work within explosion hazard areas, where a discharge could lead to explosion or fire and which in unfortunate circumstances has killed people.

What is static electricity?
Somewhat humouristically one might say that static electricity is the worlds oldest electricity, or "already the ancient greeks"..and so on.
Static electricity is an imbalance in the charging state of a material, which means that the material has had an excess of electrons and become negatively charged or a deficit of electrons and become positively charged. Just like with ordinary electricity one can talk about transportation of electrons, but with one important distinction - the static electricity, or more correctly charge, is static or with only very limited movability. That is the reason why it isn’t possible to produce electricity by rubbing a cats fur against an ebonite rod, although the charge buildup may be impressive.

A static charge can be generated mainly by three different mechanisms;

  • rubbing material surfaces against each other,
  • dividing of material surfaces (e.g a web leaving a roller)
  • influence - an object present in an electrostatic field will orient its charges against the outer field.

It is obvious that one cannot avoid all situations when static electricity may occur, hence we have to accept the presence of static electricity.

Can I choose antistatic materials?
Naturally there are many situations when one can choose materials which are not insulators, e.g conductors or antistatic materials.

Antistatic materials do not conduct current but are dissipating well enough to prevent that charges are accumulated on the surface. These types of materials are used e.g in electronics packaging, work bench and floor coverings and in clothes and shoes for work in esd sensitive areas.
There are a number of limitations with antistatic materials; they are generally speaking more expensive, have other material properties such as colour, workability, permanence etc. When used in the food and pharmaceutical industries yet other complications occur.

Can you measure static electricity?
Electrostatic fields can be measured by means of an electrostatic fieldmeter. The field is measured contactlessly since the measurement cannot consume any current.
It is relatively well known practice to measure a materials surface and volume resistivity in order to find out whether or not its ability to dissipate static charges or to be insulating enough is adequate for its purpose. A more relevant measure of dissipation is relaxation time, or the time it takes until a controlled charge that has been deposited onto a surface will dissipate enough to reach e.g its half time value.

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The measurement is a problem to the technician in general because of the extreme values that cannot be measured with ordinary instruments;

- the current is too small
- the voltage is too high
- the resistance is too high

The ordinary multimeter is useless. But with the above mentioned measuring instruments, the measurements get both possible and understandable with some practice.

Can you use air humidifiers?

An excellent way of discharging a surface is to pour water onto it, in most cases this is not possible. It is common knowledge that static problems get worse during winter as indoor air gets dry due to heating. By controlling the relative humidity it is possible to reduce some problems with static electricity mainly when handling paper, wood, textiles and similar materials which can absorb humidity from air. There is a number of practical limitations with moistening, such as corrosion, discomfort to personnel, growth of bacteria, materials sensitive to moisture etc. which means that it cannot be used as a standard cure. Many materials also lack the ability to absorb moisture from air.

Can one discharge static electricity?
Equipment to eliminate static electricity, so called antistatic equipment, is being used in almost every branch of industry. Equipment can be subdivided into active and passive eliminators, where passive eliminators are e.g. carbon fibre brushes (often to be found in office machines), copper tinsel or oldfashioned christmas tree glitter. The passive ionizer finds its driving potential in the charged surface and is able to discharge as long as the potential is high enough. Active eliminators, or ionizers, are producing ionized air, which constitutes the carrier of charge from the ionizer to the charged surface. Active ionizers are powered either by an electrical voltage source or by a radioactive source and are in a modern design touch proof.

The active ionizer produces both positive and negative ions since the electrostatically charged surfaces adopt both positive and negative charge in an uncontrolled manner.

A charged surface attracts ions of the opposite polarity but repels ions with the same polarity. For this reason one can regard the whole thing as beeing a transport problem - it is all a matter of keeping a high enough ion concentration close to the actual surface for a time long enough so that the surface may be discharged.

Is there charging technology?
The use of charging with static electricity in copying machines (xerox machines) and in electrostatic painting is not altogether unknown. This area which we may call utility use is an area in strong development and many applications are not yet commercially exploited.
The technology assumes that at least one of the components present is an insulator i.e. possible to charge and that the charge does not dissipate (flow away) as long as the desired effect is supposed to remain. Here one would use high voltage of one polarity (plus or minus) for charging against earth or against the opposite polarity (i.e.both plus and minus). Some typical examples may be to electrostatically charge protective paper on thin rolled plate, to electrostatically pin together the pages in a book prior to transporting to the glue binding machine, in-mould labelling, i.e. sticking a label into the mould of an injection moulding machine for producing plastic pieces.

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Are there any examples on problem solving with ionising technique?
The list of problems which can be cured with ionising equipment can be made almost infinitely long.

Let us just list a few typical problems related to the packaging area;

  • Shrink film is stuck to idler rollers and is torn off.

  • Small objects being placed in blister pack”jumps” out of position before the package is sealed.

  • The welding seal of vacuum pouches for coffee is failing due to coffee stuck in the seal.

  • When filling pharmaceuticals in plastic containers part of the substance ends up on the outside of the container.

  • In labelling of shampoo bottles labels are wrinkled although the dispenser in set correctly.

  • As confectionary is filled into plastic boxes the charge build-up knocks out the electronics.

Dust problems are usually related to static electricity;

  • The white plastic components are grey after only two days in the intermediary storage.

  • A lot of missing dots in the print of display packaging is the result of dust and dirt deposited onto the print cylinder.

  • Packaging film attracts dust and wraps up unwanted particles with the product.

Static electricity is not hocus pocus, it can be measured, removed or used according to what is needed. You just have to try, it isn’t that difficult.

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