Radiofrequency electromagnetic leakage fields from plastic welding machines. Measurements and reducing measures.

Operators of unshielded plastic welding machines are often exposed to radiofrequency (RF) electromagnetic leakage fields that substantially exceed all present occupational standards. Measurements show that the Swedish ceiling values (SE = SH = 250 W/m2) in many cases are exceeded at distances up to 1 meter from the electrode. To reduce the stray fields to an acceptable level at the location of the operator, RF field suppression devices should be fitted to the machine. We have studied the strength and the extent of the RF leakage field under various operating conditions and also investigated different methods for reducing the leakage field. The following measurements have been performed: E- and H-field strengths as a function of distance from the electrode, and as a function of load/tuning; the time dependence of [E]2 for various combinations of tuning and welding times producing a welding seam with the same strength; isopower density curves for SE and SH = 250 W/m2 with different types of RF emission control devices fitted to the machine; the RF voltage between the electrode and the welding table and the RF voltage on the machine casing. By decreasing the RF power and increasing the welding time the field strengths at the location of the operator can be reduced to levels below the ceiling values. The RF voltage between the electrode and the welding table ranged from 800 V up to 2100 V for the different plastic material that was welded. The RF voltage on certain parts on the chassis could be as high as 200 V. In order to reduce these voltages and the stray fields the machine should be equipped with a "large capacitive shield" in cases where this is possible.