Plastic Assembly News

Criss-Cross Energy Director Design For Ultrasonic Welding




The use of the criss-cross energy director design has proven to be beneficial for many ultrasonic plastic welding applications. This weld joint has been used in the medical, electronic and cc-energy-directorautomotive industries and has been a good joint design for achieving strong bonds with hermetic seals.  Essentially, the criss-cross energy director design utilizes the standard energy director shape where a triangular shaped bead of material is molded into the plastic wall. This molded-in triangular ridge of plastic is very effective at reducing the cycle time to achieve a weld and in compensating for non-uniform wall surfaces.  Depending upon the wall thickness and the application, the energy director typically varies in height in a range from .010 to .035 of an inch.  The peak of the energy director should be sharp with a triangular shape formed from a 60º or 90º included angle.  The energy director design has been used for years as a means of focusing the energy to improve weld strength and reduce cycle time.  The energy director has typically been placed only on one half of the part and runs along the surface to be welded. Without this energy director the weld quality would be suspect for many applications. The criss-cross design adds additional energy directors to the mating part, which increases the amount of material interaction.  On the mating surface opposite the perimeter energy director, a series of perpendicular energy directors are molded-in to mate with the perimeter energy director.  When a hermetic seal is desired these additional energy directors should take on a saw tooth pattern with each energy director repeating from the base of the proceeding energy director.  Because there are energy directors on both mating surfaces, the energy director height on each half should be reduced to prevent excessive material flash during welding.  Typically, it is recommended that the criss-cross energy director height be approximately 60% of the standard design. 

The joint is particularly effective when used in combination with a tongue and groove joint design.  The tongue and groove design provides the alignment necessary for a good ultrasonic weld joint and the groove serves as an excellent reservoir for the melted material. This pooling of plastic material helps contain the material and reduces the likelihood of a leak path.

tongue-and-grooveBecause of the increased material flow with the criss-cross energy director design, it is recommended that a tongue and groove joint be used to capture the additional plastic and contain the flash.

This criss-cross design certainly increases the mold cost, but we’ve seen applications where the weld strength has far exceeded expectations and hermetic bonds have been achieved that might not have been achieved using the standard energy director on one mating surface.   As always, each application is unique and should be evaluated thoroughly before implementing a joint design.       

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