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Sealing Properties

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This section provides an overview of membrane sealing methods and some points for device manufacturers to consider when designing a sealing process.

Heat Sealing

Heat is transferred through a die that is applied directly onto the materials being sealed. As the heat melts the substrate plastic, the pressure forces the softened plastic into the pore structure of the membrane and forms a bond between the materials. The sealing parameters of temperature, pressure, and dwell time must be optimized for each process and material combination.
Key Considerations:
  • Apply a low surface-energy coating to the heater head to minimize plastic build-up 
  • A transparent seal area generally indicates a complete seal 
  • Simple geometries such as a round seal area yield better results 
  • Seal membranes to substrate materials with similar or lower melting points 
  • A minimum seal width of 0.05 inches (1.25 mm) is recommended 
  • Seal integrity can be tested using low air or water pressure in the reverse flow direction

Ultrasonic Welding

Ultrasonic welding is the joining of thermoplastics through the use of heat generated from high frequency mechanical motion or vibrations. The vibrations are created in a vertical direction; the heat is generated from the repeated collision of the materials.
Key Considerations:
  • Use a welder with high frequency and low amplitude (40 kHz) to reduce damage to delicate materials such as membranes 
  • Avoid excess vibration 
  • Proper horn, nest, and part design are crucial to achieve a good seal 
  • Use energy directors to reduce the required weld energy 
  • Cutting and sealing can occur with one pass of the welder

RF Welding

Radio frequency (RF) welding uses electromagnetic waves to excite molecules and generate internal heat in plastics. The heat, combined with pressure, bonds materials together. Only certain materials, such as PVC and acrylic, have the correct dielectric properties to allow RF welding to work.

For best results, seal the membrane between two plastic housings.