Meaning
Elastic or plastic deformation occurring along the mating surfaces of two plastic components during assembly can compromise the structural integrity of the welded part. Controlling joint line deflection is a critical requirement in both ultrasonic and laser welding, where precise contact between the parts must be maintained. If the joint line deflects too much under the clamp force, it alters the focus of the laser or the energy concentration of the ultrasonic horn.
This deformation is set by the geometry of the parts and the stiffness of the polymer material.
Mechanical Load
Application of the clamp force must be balanced to prevent excessive squeeze-out of the molten polymer while ensuring full contact. Minimizing joint line deflection during this phase requires optimization of the fixture design and horn contact area. If the fixture does not provide adequate support directly beneath the weld area, the parts will bend, leading to uneven weld depth and localized flash.
Amorphous resins are more sensitive to this issue because they soften gradually over a wide temperature range.
Moulding Influence
Processing conditions during injection molding directly affect how much the assembly will flex under load. Residual molded-in stresses can cause the parts to warp prior to welding, which increases the initial joint line deflection before any clamp force is applied. Adjusting the pack pressure and cooling time in the molding cycle reduces this warpage, resulting in more flat and predictable mating surfaces.
Weld Failure
Excessive movement at the joint during the solidification phase leads to micro-cracks and weak bonds. High joint line deflection often results in a significant reduction in the pressure-holding capability of hermetically sealed assemblies.
