How is Static Generated?
The main causes of static electricity are:
Contact & Separation between two materials – including friction, travelling over rollers etc.
Rapid heat change e.g. material going through an oven, or polymers during injection moulding.
High energy radiation, UV, X-ray, intense electric fields.
Cutting action e.g on slitter-rewinders, sheet cutters, machining of plastics.
Induction – standing in the electric field generated by a static charge or handling charged products and materials.
Contact and separation is probably the most common cause of static in industry where film and sheet are being processed. It happens when material unwinds or passes over a roller.
When the material touches the roller a small charge flows from the material to the roller causing an imbalance. As the material leaves the roller the voltage is magnified like the separating plates of a capacitor. The size of the resultant voltage is limited in practice by the breakdown strength of the surrounding materials, surface conduction etc. You often hear small cracks, or static discharges, as the material leaves the roller. This is where the static has reached the breakdown strength of the surrounding air.
The plastic film is neutral before the roller, but as the film separates from the roller electrons flow onto the film giving it a negative charge. The remaining positive charge on the roller will disappear quickly if it is an earthed metal roller.