Dielectric heat sealing, otherwise known as high-frequency or radio-frequency heat sealing uses electricity to generate heat volumetrically within a material, instead of forcing heat conductively into the material from its surface.

Dielectric heat is a passive form of heat… in that the amount of heat generated within a material is fully dependent on the dielectric loss of the material itself.  Materials which embody little or no dielectric loss will not heat… even though the RF heat sealer is turned on and is fully functional.

Materials with significant dielectric loss such as PVC can be heated and sealed almost instantaneously.  Although this eliminates some materials from consideration for RF heat sealing, heating selectivity through material choice can also be a very useful phenomena in application use.

When two or more layers of similar plastic with sufficient dielectric loss are exposed to an RF field, they can be brought to a liquid or near liquid state rapidly.  Combining this with concurrent pressure, results in the blending of the  multiple material layers into a single material layer that can be engineered to become stronger than the individual layers themselves.

The term engineered refers to the art and science of manipulating the process variables of the hardware, to produce the desired application result.  Success for an RF heat sealing application is not simply the ability to produce RF energy.  Consistent success is drawn from the details of good application analysis and engineering.  Let’s review the process, along with the variables available to  and for our manipulation in the engineering of the end product.

First and foremost is the ability of the hardware to provide consistent and sufficient energy levels to the area where the material is being exposed.  The RF generator takes alternating line current at 50 or 60 cycles per second, and converts it to a typical radio frequency of 27 million cycles per second, at relatively  higher voltages.

The two factors that cause a dielectrically lossy material to heat rapidly are the frequency and the voltage that the material is exposed to.  These are the only two  conditions the RF generator can manipulate to cause a change in material  heating rate. Remember, the material’s properties are in control of the heating rate from here.

Further to the establishment of heating rate, the variance of exposure time must then be employed to produce the resultant heat volume necessary to achieve the  material’s melting point.  Once the generator is certified as performing its function consistently and on a 24/7 basis, we must then prevent the heat that has developed in the material from escaping until the melting point is achieved.

In RF heat sealing, many applications involve relatively thin web or film materials.  Pressure for bonding while the material is molten is provided by a press system, integrated electrically to the RF generator.  Contact with the material can be through the press platen or inserted bars, or an inserted and intricately designed die or mandrel depending on the needs of the end product.

All of these, when in contact with the material at room temperature, will cause heat developed by the RF to be drawn away, or heat sinked from the material.  The thinner the material, the more time or RF energy will be necessary to bring the  material to the temperature necessary for bonding.  Should the material thickness approach .004″ or less, thereby increasing the surface to mass ratio even further, the material may become very difficult or impossible to bring to temperature.

Here, prevention of this heat sink effect becomes mandatory.  This can be done through pre-heating of the surfaces in contact with the material.  It can also be done by separating the material from these surfaces where practical with insulating, non-loss, high impact materials known as buffers.

It should also be noted that heat loss can occur through convection.  Care must be taken to avoid exposure of the material or dies to any air flow which may adversely  affect the consistency of results.

Once RF heat is developed in the material, and the heat sink addressed, the material has now been assured thermal equilibrium for each subsequent cycle.  The required pressure for a successful bond must now be established.

The press platens, either alone or in combination with an inserted die, provide direct  pressure to the materials being sealed.  They also serve as the RF electrodes.  All things being equal, the lower the pressure the poorer the seal and the higher the pressure the better the seal.  Too little pressure can result in an unacceptable seal and too high a pressure can result in the undesired thinning of the material.

Excessive thinning of the material can produce an objectionable bead along the sides of the seal, while also reducing the material’s ability to electrically insulate the upper and lower platens which, as previously stated, are the RF electrodes.  Once the material loses the ability to stand off the voltage necessary to heat the product, electron flow between the plates is the result.  This is known as arcing. An un-extinguished arc can damage the material, the die, or the buffer materials.

To obtain high pressure when needed, and yet avoid material thinning and /or arcing, the press and /or die can be fitted with mechanical stops to prevent the press from descending beyond a fixed point once the material is molten.

To insure uniform seals, pressure must be consistent and uniform at all points on the  seal.  First, press platens must be ground flat to the tolerance dictated by the application.  Next, they must be perpendicular to the movement of the cylinder shaft and guide rods.  Accordingly, they will then be parallel to each other.

In addition, dies must be precision machined and carefully assembled to provide the same flat and parallel presentation to whatever the mating surface may be.  They must also be rigidly constructed to prevent warping under conditions of heat and pressure.

Dies, their design and their preparation, are extremely important to the end result.  Not only do they participate in the application of uniform pressure as indicated above, but they must also uniformly distribute RF energy, contribute  to the end product shape and form, control seal depth, influence bead size and formation, and directly provide for a successful application in the case of a tear-seal product.

Once the primary factors of heat equilibrium, pressure, and die preparation are resolved, the establishment of the process recipe begins.  These involve the variation of power, the time over which the power input rate is applied, the dwell time necessary for material solidification (cool time), the establishment of the appropriate pressure, the use of buffer materials and make ready of the dies, and the placement or the handling of the materials to be sealed.

It is not our purpose here to introduce case studies of recipe development.  However, we do wish to present a generic format for an initial approach.  Keep in mind that many variations of this theme are possible and have been utilized based upon the specific application and level of prior experience.  What we present next is somewhat simplified but representative.

Once the press and dies are level and producing uniform mechanical images, and thermal equilibrium has been established, the process should begin with minimum power, moderate time, and medium pressure.  If the seal is weak, increase the power slowly to soften the material sooner providing lower mechanical resistance during the pressing cycle.

If this does not work within a reasonable range of power increase, an attempt should be made to work with additional time and /or pressure to accomplish the objective.  Increasing power further represents an increase in voltage.  Higher voltage increases the chance of arcing.  Power should be kept as low as possible, within the mandates of good sealing.  This approach provides a cushion between benign and potentially problematic operating conditions.

Finally, whether using automated material handling or placing material by hand, lateral stress on the material should be monitored and minimized during the seal to assure that the material is not separated or thinned at the weld while the  material is molten.  This type stress can be induced simply in the closing of the die and must be considered during the dies’ design phase.

In summary, RF dielectric heat sealing has the ability to produce welds that are stronger than the respective material layers being joined.  It is a passive heating method, which is interactive with, and dependent on the properties of  the materials being exposed to the RF field.  Consistently reproducible and verifiable operating recipes are the trademark of an RF seal and are based in:

  • Optimizing RF Power Levels
  • Selection of Exposure Times
  • Establishment of Thermal Equilibrium
  • Precision Design, Manufacture, and Make Ready of Dies (Buffers)
  • Careful Selection of Press Platen Surface Tolerances
  • Proper Leveling and Operation of the Press
  • Appropriate Selection and Application of Uniform Pressure
  • Care in Material Handling

Once the  recipe for success of the end product is established, the system can be  designed to monitor itself and report back to the operator any product cycles that  do not conform to the recipe for success.  This allows immediate detection and  removal of any products that do not meet pre-established quality standards.

The system can also be designed to deliver information on its own operating status to the operator, and provide self protection in the event of arcs or other abnormal conditions.  Overload protection, surge suppression, and component failure detection are all present.

Since the mid 1940’s, RF heat sealing systems have provided the consistent, quality seals  manufacturers of all product types have come to rely on.  Those who make, and  more importantly those who wear, space suits in our shuttle program can attest to this.

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