Canadian Plastics

Adsorbing Reading

When it comes to making money in the resin processing business, I prefer to take as much of the production process in-house as possible. This means serious attention to "second op" processes, but pays dividends in quality and on the bottom...

May 1, 2009   By Jim Anderton, Technical Editor



When it comes to making money in the resin processing business, I prefer to take as much of the production process in-house as possible. This means serious attention to “second op” processes, but pays dividends in quality and on the bottom line.

Last month, I described the property of “surface energy”, and what it means to virgin and engineered materials. That’s great, but why do we care? Mainly because, since few high-value resin parts are big money makers as they come from the mold or die, adding them to assemblies boosts value. And making assemblies with resin parts cost-effectively frequently involves bonding.

I’ve talked in the past about welding, and mechanical fastening using inserts, but what about good old fashioned glue? Sticking parts together is cost-effective, controllable, and can be a design “ace in the hole” where single molded assemblies are prohibitively complex in a single shot.

This is where the surface energy comes in. Every surface of any material has an associated energy, and the amount or value of that energy drives the material to either minimize the surface area, like a spherical droplet of molten solder; or maximize it, like naphthalene (mothballs) that try to become all surface by sublimating directly into the air.

Again, so what? It turns out that controlling the surface energy of your resin part controls the ability of a liquid adhesive to wet the part’s contact surface, since the surface energy fights the liquid’s energy of cohesion (the force that tends to pull the liquid together like a water drop on a waxed car fender). Treating the surface can involve anything from gamma rays to a gas flame to acid dips, but the key is to avoid adsorption. Adsorption (not absorption, which requires a porous material) is a process where stuff sticks to the energetic part surface, reducing overall energy.

There’s a ton of physics here, but from a production standpoint, the important thing to remember is that any surface that has to be modified to get it to stick is vulnerable to contamination by the dirt floating in the air of almost every shop floor, and at a speed much faster than an untreated surface. If you’re using a treating technology, or even if you’re simply washing the parts to take off adsorbed contaminants, the clock is ticking and you need to stick your stuff together as soon as possible. If it’s a critical bond, it might be worthwhile to experiment with the time between treating/cleaning and assembly to see if there’s a strength compromise by delay.

Naturally, the adhesive matters, but if you find that surface contamination is killing your bond, don’t automatically assume that you need cleanroom conditions. I once encountered a problem with dust and dirt contamination that defied all attempts at housekeeping. It turns out that in ventilating the line properly to draw off the fumes from the adhesive, we were drawing dust-laden air from adjacent lines over the parts, then up the fume hoods. The solution was simply to add a “make-up” air duct channelling clean air into the “glue station” to pick up the fumes without sucking up dust.

Cleanliness may be next to godliness, but I’ll take positive pressure any day. CPL

JIM’S BUZZWORD OF THE MONTH:
Adsorption (adz-orp-shun)

The deposition of a gas or liquid onto a (usually solid) surface. Adsorption is almost impossible to eliminate from energetic surfaces, but can be minimized by quick assembly or coating of the surface after treating or cleaning. Adsorption isn’t necessarily a bad thing; it’s the basis of desiccant-type low-cost compressed air dryers.

Check out Jim Anderton on CanPlastics TV, at https://www.canplastics.com/video/episode22.asp


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