An Experimental Method to Study the Sensitivity of Transmission Laser Welding of Plastic Parts to Interfacial Gaps 2009-01-1298

Hollow polymer-based automotive components cannot, in general, be directly injection molded because they cannot be ejected from the mold. The common practice is to injection mold two or more parts, and then join these together with a welding process. Of the many joining process available, laser welding has an advantage in geometric design freedom. The laser weld joints are also generally stronger than those of vibration welds because the weld joints are located in the walls rather than on external flanges. Eliminating the external flanges also makes the part more compact.

In transmission laser welding processes, the laser beam passes through a transparent part to its interface with an opaque part. The beam energy is absorbed near the interface in the opaque part, and heat flows back across to the transparent half to make the weld pool. So successful laser welds are possible only when there is a continuous interfacial fit between the parts.

The new method measures what sort of interfacial non-contact areas can be tolerated, without degrading the integrity of the welded part. The test samples comprise two “hemispheres” joined together on mating flanges. Within the flange region of the mold are ten sub-inserts that create 66 different molded-in interfacial gaps on each flange. These are then welded at various laser welding parameters, and for 2 levels of carbon black in the opaque materials. Void healing is evaluated by inspection of transverse polished sections and fracture surfaces, as well as by hydraulic burst testing. An ancillary result is the burst strength tolerance of the welds to the level and size of the porosity created when the joints are overheated.