No More Mess: Bonding Plastic to Metal without Adhesives
By James Cruz, EWI
My first job out of college at Honda of America Manufacturing was a dream come true – the chance to play with robots and build cars! They handed me several crisp, white uniforms on day 1. Within that first week on the production line, however, I had soiled my pretty white pants with a huge, black, gooey streak of gunk. I had formally been introduced to one of the adhesive sealants used in automobile assembly.
Over my decade at Honda, I grew to understand what a pain adhesives can be in production, seeming so often to end up everywhere except where they’re wanted! They’re expensive and have limited shelf lives. Year-in-year-out, they were one of the top reasons for downtime in my department because they coated proximity switches or clamps and caused equipment issues.
Now, I fully appreciate the value of adhesives and understand that they – especially when combined with welding – can significantly improve joint performance. But what if you had a way to direct-bond a polymer to a metal without all the side effects of adhesives?
How to join dissimilar materials is one of the most common questions we get at EWI. In the aerospace industry, that might be Ti-6242 to Ti-5111. Among medical device makers, maybe Nitinol to stainless steel. But in both of those industries as well as others, clients ask us about polymer-to-metal bonding. Because at EWI, we have demonstrated the process of direct joining of polymers to metals with incredible results.
Recently, EWI’s Senior Technology Leader for Polymers Jeff Ellis wrote a paper about using a polymer lid on an EV battery box. While a gasket / adhesive + mechanical fastener approach would work, wouldn’t it be great if we could direct bond the lid? So, Jeff came up with two novel processes, both of which require little cycle time:
- The first process is to laser etch the surface of the metal. EWI has optimized the process to melt microscale valleys. The melt from the valley is pushed up the edge of the valley, creating a protrusion with ends that overhang the valley. This enables a surface that is clean and has a mechanically functional topography. We use a Laser Marking Technologies 100W 1064 nm pulsed fiber laser for the etching.
- The second process is to melt the polymer into the functionalized metal surface. This requires both force and heat, which cause flow into the microfeatures. We have used two different methods for heating with similar results. The first is through transmission laser heater. A 1µm continuous laser is shined through the polymer to the metal surface to heat it. The second method uses an induction coil to heat the metal and thus also the polymer at the interface. Both methods employ compression to encourage the viscous melt to fill the metallic microfeatures.
Resulting parts have shown shear strengths of more than 1000 psi. Also, initial testing has produced leak-free parts when tested using an air decay method. Finally, when cycled from 0 to 65°C, they and have shown no difference in mechanical strength despite the difference in coefficient of thermal expansion between the materials.
While there is still much to investigate with this application, but it has great possibilities for manufacturers. We look forward with working with you to explore applications of this exciting new technique. Want to learn more? Contact me directly at [email protected].