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Abstract
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  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  for various
combinations of tuning and welding times producing a welding seam with the
same strength; isopower density curves for  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.
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