Canadian Plastics

Sticking Together

Last month, I introduced an old concept that, in my opinion, is due for a major revival: gluing simple subassemblies as an alternative to large, complex moldings. As the traditional "run-what-they-bru...

October 1, 2007   By Jim Anderton, technical editor



Last month, I introduced an old concept that, in my opinion, is due for a major revival: gluing simple subassemblies as an alternative to large, complex moldings. As the traditional “run-what-they-brung” job shops disappear, molders today are increasingly expected to take responsibility for design and engineering of the parts they mold. That gives the molder some latitude to find solutions that suit the customer and the processor.

What am I getting at? I once participated in an OEM job for a small insert molded part with several different end-use applications. The core of the part was the same for each vehicle, but the outer housing was designed to be model-specific.

To keep costs down, big multi-cavity tools were cut, one for each part number. But imagine another approach: What if the core of the part was common, with an outer molding welded or glued to the core to make a basic design fit several applications?

With a two-part design, the big tools can mold the common cores, while smaller, more flexible presses make the application-specific portion. Granted, a second operation is now involved, but robotic application of modern high-tech adhesives can make that process relatively cheap. The result is functional parts with less big press time and greater flexibility for just-in-time production.

The same logic can be applied to robotic interference fit assembly or welding. Most processors are allergic to second op processes as profit killers, but if you’re using pick-and-place robotics press-side, is it really slower to drop the parts into a fixture for post-mold assembly? It’s common these days to use robotics to orient parts into tray-like carriers for shipping, for example, and I’m convinced that a number of post-mold assembly processes could be integrated here.

The ultimate would be an in-mold assembly process that uses a high tack adhesive on a pre-loaded part to allow an end effector to withdraw the molding and build the assembly at the same time. It would be a sort of in-mold second op, kind of like a kid retrieving a quarter from behind the refrigerator with some bubble gum on a broom handle.

The catch? It’s probably not the cost of the process itself, but the surface preparation needed to get a good bond. Clean is king and if you need to use degreasers or surfactants, everything slows down until your parts are out of the rinse cycle.

Can it be done? Yes. Should it be done? Probably more often than it is. Will it be done? How deep are your pockets?


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