Static electricity
     and dust problem

     by Reinhold Rutks

In last year's edition of "Nordisk Emballage Marknad" was an introduction to static electricity in industry in the article "Static electricity - friend or foe?". Here is a continuation of the subject static electricity, where we focus on studying the problems of unwanted particles and health problems associated with static electricity.
It may sound a little strange that static electricity could have an impact on dust and health problems, or if you like, problems with unwanted particles. We all know about the problem, but perhaps it is not so many people that have understand the connection? Sure you have the recurrent dust on the TV screen at home!

We have previously found out that static electricity can cause problems where you process or handle at least one component which is an insulator. This is a question of definition - namely, one can produce dust and/ or health problems even with a conductive material, but here we are talking about unwanted particles that are acting as insulators.


Why stick to the dirt?
Two surfaces charged with different electrostatic charge, i.e. the have different electrical potential, exerts an electrostatic attraction on each other. The size of the attraction force is determined by the potential difference, and distance between the surfaces. This means that the attraction-force increases with increasing potential difference, and with shrinking distance between the surfaces.


Some practical examples when you want the particles to be trapped:
Firstly, in electrostatic powder painting it is important to verify that the object is grounded, and secondly the surface of the object is exposed of charged powder using a special designed gun.

The charged powder is attracted to the grounded object, with sufficient potential difference, so much that you for example can paint the back of a round table leg. Flue gas separation is often done with electrostatic filters to separate particles. Here one uses a technique in which the flue gases first pass a charging zone where the particles are being charged with one polarity, and then pass a so-called collector zone where usually a large number of collector plates are kept charged to an opposite potential. By using adapted gas velocity, physical dimensions and the potential of the collectors' one can reach a predetermined separation. If you know the current conditions it is possible to control the "dirt".

What we want to discuss?
Now we don't want to use unwanted particles, we want to get rid of them! It is therefore important to try to understand the presumptions by which one can avoid the product works "like a magnet" for dust and unwanted particles.

To mark off our discussion, we now mainly stick to the problems associated with packaging and handling, such as manufacturing, converting, printing and the packing element.

When a "dirty" article reach the customer, the cause, can of course to be found in a variety of locations in the production chain. Even if the packaging is clean and of adequate quality, the end result may be inferior if the packing is not tight enough to prevent the contamination of particles or the product itself is dirty. It can in many cases be necessary to discharge and air blow details with ionising air before packing, but even a pure packaging can not solve the problem to pack a dirty product.

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Packing of dirty products
As a matter of course, it is expected that packaged products should be free from dirt when they are unpacked. It can be finished products or components that come from a subcontractor. Problems with plastic details maintaining high surface finish of the end-product are common.
Some typical examples are interior and exterior car detailing, housings and display windows for mobile phones, syringes, valves, and packaging for healthcare and pharmaceutical industries, etc.


Some types of packing problems
When filling dry goods such as coffee, chocolate powder, sugar and other foods or chemicals and pharmaceuticals, etc., can the combination of packaging material and material to be packed cause severe dust problems.
In certain circumstances, one may be afflicted by a package that has been electrostatically charged with different polarities on the inside and outside of the package, which makes it difficult to fill the package with, for example, a powder of the same polarity as the inside of the package. This result in that some of the powder being filled will be repelled by the package inside and expelled from the package itself, after which the powder it is attracted by the outside of the package since they have opposite polarity. The same type of problem can occur even when filling tubes with creams, pastes and liquids, except for the fact that the package cannot be blown with ionising air or vacuumed clean as the surface become sticky / wet.
When filling beer or soft drinks in cans, statically charged particles on the inside of the can can cause problems.


Quality Problems in packaging production
Unwanted particles such as dust, fibers, splint, etc. are often the reason for complaints of packaging such as cartons, bottles, cans, foils, bags and more. "Dirt" can cause svere problems when forming such as impression marks on your product via molding tool, lamination and embossing rollers, etc.
In machining operations such as rolling cutting, sheeting, punching, etc, electrostatic charging and a lot of unwanted particles are generated.
The tools used need to be maintained, but even decent maintenance will generate particles. The quality of the printed surfaces can be very difficult to maintain. In screen printing, dust

may cause distinct marks in pressure which in turn results in clogging of the screen, and the print quality will gradually deteriorate and eventually lead to production stops, and cleaning. At the tampon printing, the problem is similar. An increasing problem today is the printing on cardboard.
With higher quality requirements and more and more complex printing, fibers, cutting dust and dirt will cause problems often so severe that one must stop for cleaning after the printing of as few as 25 sheets. It is quite clear that the costs are enormous and the planning of the production is very difficult.
Label printing is another area with increasing problems due to higher production speed, higher quality requirements and new label material which is more likely to build up electrostatic charges. It is not enough today to achieve sufficient contrast in the barcode, now, the label even have to provide added value and make the product more attractive and selling.


How can you tackle problems with unwanted particles?
As shown above, there is not a simple standard solution that solves all problems. As soon as one could formulate the problem it is important to systematically follow up all possible sources of the problem. Often there must be a combination of measures in order to achieve an adequate solution. The general level of hygiene within the facility are a key factor.
Do you drive diesel-powered trucks passing sensitive items? Do dirds flying in and out of the premises chasing flies? Does the staff follow defined cleaning routines, and if so, are they sufficient? To approach these problems, one should in an appropriate manner unravel the fundamental issues first, so a little common sense can not hurt. Thereafter, it will remain a number of problems that require a different approach angle.


Active problem solving?
In many cases, the problem can be solved through the use of antistatic equipment, in which the sensitive material is discharged so it will not be able to attract more dust and dirt than the current environment. Most problems such as wrapping of diry products can be avoided if one discharge details, e.g. injection moulding, assembly and so on. With a preserved charge the details will otherwise accumulate particles until they are packed.

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An interesting problem is the unpacking of dust-sensitive components, where the components have been packed as clean and dust free, but where they are charged up by vibrations or movements during the transport. When unpacking, the wrapping is often dirty and the packaging details that being unpacked have become statically charged, so they attract dirt from the wrapping and the new surroundings.
Thus, you can ask yourself whether it is absolutely essential that the production today is perfomed in cleanrooms, maybe it is quite enough to keep good hygiene standard and continuous discharging of the product? Ionisation, (i.e. antistatic equipment) has been shown to improve the purity of lower cleanroom classes so much without having the cleanrooms rebuilt when the requirements have been tightened. In a similar way, it is probably not necessary to carry out some procedures in cleanrooms. For those cases where one has to deal with a "dusty" product, it often pays if the product is actively discharged by blowing with ionised air or in many cases discharge and vacuum the product clean.

Some typical case studies
Here is a brief description of some common scenarios;

• In the manufacturing and printing of cardboard large amounts of dust is always formed that comes with the cardboard before it is printed. By discharging and vacuuming in the wellmachine, you get a better overall environment in the rest of the plant, but with high-quality prints the cardboard sheets have to be discharged and vacuumed cleaned. To achieve an acceptable result you need to integrate discharging, air blowing and vacuuming featuresinto one unit.

• Printing of plastic bottles and cans are sensitive to dust and dirt. With the help of an integrated discharging, blowing and vacuuming unit, you can achieve a better quality and fewer stops for cleaning. When filling spices in plastic bottles, spices stick to the outside of the can. Using blowing with ionised air (discharging) and dust-extraction techniques cans can be kept clean before the label is affixed - production disturbances will disappear and the quality increases.

Static electricity is not the only things that sparkle - it can cause serious problems that are easy to fix when you understand the relationship.

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