RF Heat Sealers

Broadcast antenna tower workers and dielectric RF heat sealer operators are the two highest exposed occupational groups in the United States. Richard Tell Associates has performed numerous exposure assessments of RF heat sealers, performing measurements of electric and magnetic fields, induced body currents and contact currents. The photo to the left is of a long bar type sealer used for fabrication of tents.






Heat sealers have the potential of creating extremely strong RF fields because of their design. A high power RF generator applies voltage to a sealing electrode that is brought into contact with the material being sealed, typically some form of vinyl, such that the dielectric losses in the material lead to rapid heating and subsequent melting of the material. The sealer on the right has a short sealing electrode and spotting lights to help operators align the material for precise seals. The timers control the length of the sealing cycle.

Some sealing machines use long bar type electrodes for creating extended seams, such as in the manufacturing of tents and other large objects. In other cases, smaller electrodes are used to create a customized pattern of sealing such as in producing vinyl protective covers, notebooks, etc. The electric fields are fringing fields that curl around the edges of the sealing electrodes. Hence, operator exposure occurs since these fringing fields emerge outward, away from the electrodes. Generally, the electric fields are predominantly vertically polarized. Strong magnetic fields can exist because of the RF current flow within the material being sealed between the electrodes.


The strong electric fields result in induced body currents that can be monitored at the foot by measuring the current flow between the operators feet and the floor as shown here. Because of the coupling of the operator's body to the near-field environment of the sealer, the magnitude of the induced body current can often substantially exceed that value that might be predicted on the basis of a uniform field exposure.


In some cases, electric fields can be significantly reduced in strength by fabrication of shields that are attached to the ground of the sealer system. The photo below on the left shows an example of a partial shield system while the photo on the right shows a completely enclosed shielded system sometimes referred to as a box shield. Application of these shielding methods is, however, limited by the production work being accomplished since the shields can interfere with movement and placement of the materials being processed.