Testing Tips for Transportation Adhesive Applications

Your homework is done, you have studied every aspect of your design, consulted with sales reps, and selected several adhesives for evaluation. You have a design and the makings of a process, including a thorough review of the adhesive manufacturers’ Technical Data Sheets (TDS).

About that TDS…it contains only one universally true statement:

The user should test the product in its intended application to determine its suitability

Or words to that effect, usually found in fine print.

Why is this important to my adhesive bonding application, you ask?

Because while every TDS is based on a laboratory testing protocol, it is fundamentally a marketing tool that has undergone a legal review prior to becoming available in the public domain. Values represented in the TDS are not defined as “average”, “minimum”, “maximum” or “best case” and thus only peripherally apply to your application. Regardless, it is unlikely the bonding conditions and surfaces match your application. It is the user’s responsibility with assistance from their adhesives supplier to validate TDS values for a specific application.

As an experienced materials engineer retired from a vehicle OEM, I strongly recommend developing an individualized test plan for your joint design and process predicated on Failure Mode and Effect Analysis (FMEA) (see Figures 1 & 2 for an ASQ sample FMEA template for design and process, respectively). This is a common standard requirement for new designs in vehicle applications but is often not used effectively. It is an ideal starting point to any test plan as it communicates specific critical factors that require your team’s attention, and thus guides the test plan.

Figure 1: Failure and Effects Analysis (Sample Template) modified for design.

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Source: American Society for Quality (ASQ)

Figure 2: Process FMEA Example

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Source: American Society for Quality (ASQ)

Overall, your test plan is being developed to validate that the material will perform as intended in the vehicle design and assembly process. Your material selection is wholly dependent on its performance in the real world. Given that every material has its inherent strengths and weaknesses, there is no perfect model for material selection; therefore, application-specific trade-offs must be carefully assessed.

My advice is:

1 – Do not re-prove gravity
Your process shouldn’t be driven by fear of the unknown, which can unwittingly lead a test plan down one rabbit hole after another. Regardless of whether the program is 500,000 cars or 50 specialty vehicles, it is your job to guard against unnecessary test expenses. Instead, leverage both internal and external experts to help prioritize and direct any troubleshooting. Where possible, use surrogate data – data that has been developed in other, similar applications or projects.

2 – Know your data
Measuring and documenting the critical data from your product and process is critical to defining the test plan. Unfortunately, this is not always easy or readily measurable. The process side should be well under your control and ability to measure, with the correct measurement benchmark: For example, a paint oven air temperature is often irrelevant, instead measure the actual substrate temperatures and conditions.

Actionable field data is ideal, but field data often arrives too late in the process to be useful. This usually means you must use experience or simulated data – analytical or test. This is important to document and review when better data comes in as it can have a significant impact.

3 – Show your data
This is not just good engineering practice, it makes life easier in the long run as memories are seldom perfect and good records are essential to correcting errors. After a measurement is made and an adjustment is required to account for varying conditions (Ex: running a vehicle temperature measurement when the ambient temperature is only 35°C and the requirement is for an ambient at 50°C) document the basis for that adjustment. This could save you time later when you have a test failure or need to reconsider your material selection. We cannot have perfect data but we can document what we do and why.

Finally:

4 – Execute your plan
You identified the critical factors in your FMEA and you now have a plan. The key to success is to work it, even if it initially produces a failure or otherwise does not meet your expectations. Bonding failures happen for a variety of reasons ranging from inadequate material to a poorly prepared surface. The cause of failure could be related to equipment (i.e., how well the material pumped or mixed), to how well it was cured, to an unexpected surface condition as Jim Swope recently discussed.

Myriad issues can occur when introducing a new adhesive. An adhesive’s odor once caused a work stoppage on a heavy truck assembly line; the material had been cleared through our internal Health and Safety department but a key CAW (Canadian Auto Workers) plant representative was not informed. Regardless of the suspected culprit, work the problem, involve your supplier early, and take note of your findings as you investigate and document in your FMEA.

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