By: Joe Fox, Director of Technical Services
To the beginner, dielectric sealing can be intimidating. The array of controls provided on modern heat sealers may seem bewildering. This need not be the case. Taking the time to understand how the RF heating process, materials and equipment controls interact will enable the user to utilize any RF Welding press to its full potential. This article will define the process variables and the potential problems they introduce.
Dielectric heat sealing, otherwise known as high frequency or RF welding, uses electricity to generate heat within a material instead of forcing heat into the material from the outside.
The amount of heat generated within the material is dependent upon the dielectric loss of the material. Materials with very low dielectric loss do not heat even though the RF Heat Sealer is turned on and is fully functional.
Materials with high dielectric loss, such as PVC or polyurethane, can be heated and sealed almost instantaneously. Polyethylene and polystyrene are examples of low dielectric loss materials that cannot be heated with RF Welding Equipment. Although this eliminates some materials from consideration for RF systems, heating selectivity through material choice can be a very useful phenomenon in application use.
When two (or more) layers of thermoplastic with sufficient dielectric loss are exposed to an RF field, they can be brought to a liquid or near liquid state very rapidly. These multiple material layers can then be blended into a single material layer stronger than the individual layers themselves. Proper adjustment of the hardware (controls) enables the user to manipulate the process variables and produce the desired results.
Most RF systems allow the user to adjust the following parameters:
1) Pre-seal time: (1 to 3 seconds typical)
The amount of time that elapses after the press closes and before the RF heating cycle begins.
The pre-seal time is used to allow the press enough pressure to stabilize and for the materials to come into firm and intimate contact.
If the press is equipped with a heated platen, pre-seal time allows the tooling to transfer heat to the material. Heated tooling is used to relax stiff material for better contact, maintain constant tool temperature for first / last part consistency or to increase dielectric losses in materials for easier sealing.
Effect of too little pre-seal time:
Arcing
Burning
Inconsistent seal thickness
Effect of too much pre-seal time:
Cold flow of material before reaching welding temperature, especially if a heated platen or tooling is used
Thin seals
Time wasted
2) Seal time: (2 to 6 seconds typical)
The amount of time the RF energy is applied to the materials in order to perform the intended work (welding).
Effect of too little seal time:
No or poor weld
Inconsistent weld thickness
De-lamination of materials
Rough or no tear when using tear seal dies
Effect of too much seal time:
Overwelding
Arcing
Thin seals
Excessive squeeze out
Bubbles
NOTE: Seal time interacts with RF power. Typically RF power is adjusted to bring the seal time within a 2 to 6 second window.
3) Cool time: (1 to 6 seconds typical)
The amount of time after the RF power shuts off that the press remains closed. This allows the molten material of the weld to solidify or quench before the press pressure is removed.
Effect of too little cool time:
Product may stretch or distort when removed from die.
Material may rebound causing poor definition (puffy appearance) this may be particularly troublesome in embossing operations.
Effect of too much cool time:
Time wasted
4) Pressure: (30 to 80 Psig typical for average size tooling in a “C” frame press)
NOTE: Pressure requirements vary widely based on application and materials involved.
Pressure is required to bring the materials to be welded into intimate contact and to force the air from between the layers.
Pressure moves the tooling once the material to be heated reaches the molten state. The moving tooling breaks the surface tension between the layers of molten material and mixes them, producing a weld.
Effect of too little pressure:
Arcing
Incomplete welds
Thick or variable weld thickness
Effect of too much pressure:
Thin welds
Excessive extrusion
Die wear
Cold welds (see below)
Cold welds:
When the pressure applied is excessive, tooling can be forced through the materials before they are hot enough to break the surface tension between layers. The result is that the tooling reaches the proper dimension for a completed seal however the materials remain two discrete layers. Tension applied to the interface will result in delaminating and failure. Tear seal tooling is particularly sensitive to pressure adjustments in order to prevent cold seal, dulling of cutting edges or cutting buffers. Excessive pressure actually makes the finished part difficult or impossible to separate or tear out cleanly.
5) RF Power: (Dependant on application, size and materials to be welded)
The RF Power level required for achieving optimum performance is that which melts the materials as quickly and evenly as possible without dielectric breakdown (arcing). RF power requirements and RF equipment capabilities are ideally matched by the manufacturer in order to provide good value with adequate reserve power for future applications.
