Author: alistair

LIVING WITH STATIC

All of us involved with handling or processing plastic films will be aware of static electricity. But what is it, how is it generated, and how can we deal with it?

Static occurs when the surface of a material has either a surplus or a deficit of electrons. This can be caused by friction, where electrons are transferred directly from one surface to another. A source of static often encountered in the plastics world is the separation of two surfaces in intimate contact with each other, for example, removing the masking film from gloss materials. Heating and cooling, especially repeatedly, as in drying ovens will also create a static charge, as will placing the material in a strong electric field.

Static build-up in materials can be affected by humidity, with a more humid atmosphere aiding dissipation of the charge. In the past print shops have used atomising water sprays and even the odd bucket of water thrown across the floor! As static is cumulative repeated handling or heating can increase the charge. A particular issue for printers is the additive nature of static. A stack of charged sheets will have a much higher charge than the individual sheets. You may be surprised to know that static charges can be many tens of thousands of volts. This is not dangerous but can be very painful!

These charges are not permanent and will dissipate naturally, but this is a slow process and cannot be relied upon to solve the problem for us.

So what can you do to help safeguard yourself against static? Prevention, we are told, is better than a cure. The fairly obvious step of using substrates which are conductive or dissipative has the marked disadvantage of adversely affecting the appearance of the finished job, to say nothing of adding to the cost. Anti-static coatings may help. There are two types: hydrophilic coatings which absorb atmospheric moisture allowing the charge to be conducted away. Other types of coatings inhibit the transfer of electrons between surfaces preventing the build-up of static. Coated films, where they exist, are only a partial solution and are unavailable on many substrates. Inevitably we are faced with the necessity of removing the charge.

The simplest but still effective method of elimination is to conduct the charge away to earth. This can be achieved by bringing a copper tinsel into contact with the charged surface. To be effective the tinsel must be earthed and must contact all the substrate surface as it only removes the charge from areas contacted.

The natural dissipation of the charge can be accelerated by increasing the humidity.

More sophisticated measures involve ionisation by one means or another. Passive ionisation passing the charged surface very close to, but not touching, the bristles of an earthed conductive brush, usually copper or carbon. This will reduce but not fully eliminate the charge.

Radioactive ionisation, although effective, is no longer considered safe for use in our industry

High voltage ionisers create large numbers of positive and negative ions in the surrounding air. There are two types: AC and DC. AC ionisers produce equal quantities of both positive and negative ions. They are effective over short ranges (25-100mm) but this can be increased by the use of air blowers.

Pulsed DC ionisers have a greater effective range (up to 600mm). The polarity can be controlled to give more ions of either positive or negative charge. This can increase effectiveness if the polarity of the static charge is known.

It is important to place static eliminators correctly. They should be at the last possible place prior to the charge being a problem. If it is not a problem do not try to eliminate it