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Fig. 4 – Linearly increasing velocity profile from 0.25 to 0.4 mm/s. Melt penetration 690 μm. Average failure load 5223 N.

Stronger Welds. Microscopic investigation of welded parts using linearly increasing weld velocity provides insight into the physical characterization of the weld regions. The length and depth of the weld zone correlates closely with the tensile strength of the samples, with a larger zone producing higher tensile strength values. Samples that had full coverage of the contact area, as well as resultant deeper penetration, showed high strength. These larger, more uniform melt regions penetrated well into the surrounding material.

Less Residual Stress. Observation of the samples with a polarized light source shows an additional improvement in the welding process afforded by a servo-driven welder. Optimizing the weld speed throughout the cycle allows the molecules to become less oriented and retain more of the amorphous structure that yields higher strengths. A reduction in number of colors, as well as the number of fringes, is evidence that these samples have less residual stress resulting from the welding process (see Figure 5).

Fig. 5 – Melt zone image for sample welded with profiled weld velocity photographed in polarized light.

The lessening of residual stress levels will be a key factor for medical device assemblies. High levels of stress can accelerate failure of plastic parts when subjected to a wide variety of environmental factors, such as ultraviolet exposure, chemical attack, and sterilization processes, as well as normal wear. These factors all hasten the failure of a welded plastic assembly, and any process that can minimize residual stress levels caused during welding will help mitigate their impact. This stress reduction can be considered a safety improvement in many products.


In summary, associating a specific weld velocity profile with formation of a homogeneous melt layer in the interface of the assembly offers a key approach to selecting optimum welding parameters. The significantly enhanced capabilities of servo-driven welders in controlling material flow and the rate of material displacement during every stage of the welding cycle, their high repeatability and accuracy, and the optimal implementation of these tools and features enable users to develop a robust joining process with high strength, lower occurrence of welding flash, and lower residual stresses. This approach is beneficial to the welding of small parts typical in the medical device and electronics industry, where strict requirements for strength and dimensional consistency are critical factors.

This article was written by Jason Barton, National Sales and Marketing Manager for Dukane, St. Charles, IL. For more information, click here.

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