Posted on Wed, Aug 12, 2015
Ideally it’s in testing that engineers discover all the ways an assembly tape fastener or bond line can fail. As long as these failures happen in the lab prior to production, it’s all good. Knowing what doesn’t work is half the journey to finding what does when it comes to adhesive testing.
Testing and evaluations become more critical as the complexity and volumes of final assembly increase. That’s why at Gleicher, we go the extra mile to provide samples of adhesives and substrates to help engineers find or create the right solutions.
Narrowing Your Test Subjects
Fortunately the better OEM’s supply detailed product data sheets and material selection guides. Those are helpful to narrow the options to just those families of materials that meet your broad criteria. From there, it’s down to the details – what stresses, what exposures, and what assembly constraints.
When to Run an Evaluation
Because no one can anticipate all of the different possible surfaces and contaminants that may exist, it is imperative that the user conduct an evaluation to determine the suitability of 3M™ Tapes, surface preparations procedures, and any other processes that may have an influence on the tape or the bonded parts.
Likewise, where there are any changes in plastic or paint formulation, or suppliers of these materials, it is advisable to run evaluations to ensure that the change has not influenced the compatibility of the surface.
Follow normal R&D protocols to document all variables and conditions for each test.
Factors in Bond Strength
• Surface Contact: Bond strength is dependent upon the amount of adhesive-to-surface contact developed. Firm application pressure develops better adhesive contact and helps improve bond strength. Generally, this means that the tape should experience at least 15 psi (100 kPa) in roll down or platen pressure.
• Dwell time: Dwell time is how long the adhesive is left before beginning the testing. The length of time needed is based on testing room temps and application pressure. After application, the bond strength will increase as the adhesive flows onto the surface. At room temperature, approximately 50% of the ultimate strength will be achieved after 20 minutes, 90% after 24 hours and 100% after 72 hours.
• Temperature: It’s easy to overlook this powerful variable in the equation. While it’s always best to read the label, for most testing “room temperature” is a rule of thumb. If exposures introduce extreme temperatures in either direction you’ll want to look more carefully at the chemistry.
Pro Tip: In some cases, bond strength can be increased and ultimate bond strength can be achieved more quickly by exposure of the bond to elevated temperatures (e.g. 150°F [66°C] for 1 hour). Check your product label and talk with your Converter about your specific scenario.
• Surface Preparation: It can be easy to disqualify a good alternative because of missed steps in surface alignment, cleaning and preparation. To obtain optimum adhesion, the bonding surfaces must be well unified, clean and dry. Typical surface cleaning solvents are IPA/water mixture (rubbing alcohol) or heptane.
Rough Sample or Custom Prototype “piece of tape”?
For the best possible testing conditions we suggest using prototypes and not rough cut pieces. The closer you can get to the finished geometry for your “piece of tape” the closer you are to simulating the mass-production fastener that will be on the assembly line. Trying to hand-cut pieces to fit precisely into your product prototype can be more than a hassle – it can be the cause of test failure.
We hope this article was helpful. How do you test? Let us know in the comments below.
Contact Gleicher for Samples and Custom “pieces of tape” for your next project.