Case Study 1: Can your vendor really do the job?  How will you know?

Our client had a design that involved joining two plastic parts using ultrasonic welding.  The integrity of the joint was critical to the function of the product.  An injection molding vendor was selected based on his familiarity with ultrasonic welding.  Prototype samples appeared to meet the design expectations.  However, soon after launch, joint failures were discovered by customers.  The product launch was abruptly cancelled to stem the tide of customer returns and to allow a remedy to be put in place.

How would the materials engineering perspective prevent such a failure?

The main problem here was that the weld joint was not seen as being a critical aspect of the design.  Consequently, proper attention was not given to specifying and evaluating the joint strength.  Both of these are critical to selecting optimum materials and controlling the variation of their properties.  In this case, the material that made up the weld joint was of concern.

The joint strength could be estimated using a simple mechanical analysis involving the shapes of the components and the properties of the materials.  Knowledge of the expected joint strength provided the basis for conducting tests to evaluate the joint strength.

Before selecting a supplier, samples of one of its products should be evaluated.  In this case, the evaluations would include cross-sectioning to evaluate the appearance of the joints and mechanical testing using standard tensile testing equipment to measure joint strength.  

Once the bad joints surfaced process development experiments were conducted.  Variances found in the welding processes were addressed with revisions to welding procedures and fixtures.  Ultimately the weld joint met the revised expectations for strength and production yield.  

Busted Myths

Remediation of the problem did not require re-tooling, changing materials, or any expensive and extensive analysis.  This issue would have been exposed early on by conducting a materials risk review.  The actual cost associated with identifying the root cause of the joint failure and implementing a remedy was trivial compared to the lost revenue and tarnished reputation.