The power required for a good seal is directly proportional to the seal area of the product. After the pre-seal cycle, the main seal power is applied to the dies. Nice controlled rise of power can bring consistency in the RF Welding process and prevent flashes or material burns.
If too much energy is applied, the material will burn. While if the energy is not enough, the material layers will not bond. Partially sealed areas are caused by low RF energy.
As the material temperature rises, a state of equilibrium is achieved between the generated heat and heat loss through the dies. Heat loss is greater with thinner materials. Ultra-thin materialsí heat loss is too much and buffer materials are used to reduce the heat loss through the die surface.
When setting up a new sealing or welding job, the first test should be with minimum power, moderate time and around 80 PSI pressure. If the seal is weak, power should be increased gradually.
The dies must be held parallel to produce even pressure around the seal area. If there is too much extrusion or if the seal is too thin, the press sealing stops need to be adjusted. To set the stops, adjust the press to half the total thickness of the material. Close the press and adjust the stops finger tight. Then, insert the full thickness of material in the press and make a seal. Check the results and lower or raise the stops as required. Lock the stops in place.
If the seal is weak at a particular area, the dies are not level. The leveling screws should be checked and adjusted. If these adjustments are still unsatisfactory, the use of a shim may be required or the dies must be surface ground.
After several RF welding cycles, the dies warm up and the system parameters (time and power) may require re-adjustment. To eliminate parameter adjustments, some dies are designed with heated upper platens that pre-heat them to the proper operating temperature. This process is used in tear seal applications.
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