Sticky Business

Regular readers of my little soapbox know that I think making money as a processor is a lot more complex than just shooting resin. Automotive OEMs started the trend a decade ago of using suppliers as vendors of complete systems.

Nowadays, even smaller players are being asked to bid on several parts as an assembly, not just bulk moldings by the truckload. This leads to true assembly lines, with presses or extruders at the front end of a process that may include different resin types, metals, ceramics and a means to hold it all together. I’ll deal with the numerous ways to make and lose money screwing, riveting and welding plastics together in a later column, but this time I’m thinking about glue.

Adhesives get a bad rap, but in my experience it’s almost always a result of the wrong product for the application, poor part design, or poor assembly technique. The application part is easy: ask an expert. Technical sales staff have a wealth of knowledge, and can get you moving in the right direction. Bring in several if possible, and get proposals in writing, or at least take notes during your meetings.

Roughly speaking, your adhesive choices will break down into three basic types: solvents that dissolve resin part surfaces and “weld” the parts together, glues that bond at the molecular level, and adhesives that form a mechanical lock between the two parts.

There are thousands of product choices. What’s best? The simple answer is the method that gives a reliable joint at the lowest possible overall cost.

From your customer’s perspective, the primary issue will likely be joint strength, which is relatively easy to test: pull it apart and see what happens. Inside your plant, however, it’s a little more complex.

I like solvent-type adhesives because I feel that a weld with no interlayer is the simplest strong joint between two resin parts. But where does the solvent go when the parts are bonded? Everyone who built a styrene model airplane kit as a kid knows: into the atmosphere. Do you have an environmental issue? What about health and safety? Will you be required to track, separate and specially store your adhesive? If a worker is contact-sensitive, do you have an insurance issue?

Adhesives that bond at the molecular level mean there’s a chemical reaction taking place between the glue and the part’s resin substrate. The chemistry can be complex, but for the processor the key is getting the stuff to diffuse into the surface layer of resin in a reasonable time. The rate of diffusion depends upon the properties of resin and adhesive, as well as temperature.

Warmer is generally better, within reason. The nice part of bonding this way is that you can use an extremely small amount of adhesive to get a good bond and it’s often possible to use products with very low residual toxicity.

The third type really isn’t a true adhesive in that the parts are in essence mechanically attached by forming a solid interlayer that grips the nooks and crannies of a roughened surface to get a “lock and key” bond.

In the real world, bonding is usually a combination of chemical and mechanical grip, which is fine as long as there’s enough excess bond strength to account for the inevitable variability in surface roughness or contamination. And that’s probably the biggest issue in making cheap, strong joints in resin parts. Stay tuned.