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Questions and Exchange

June 1, 2000
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Questions and Exchange

Question: We are a small manufacturer of Arts and Crafts items that we sell as kits to consumers via mail order, specialty retailers, and more recently, the Internet. One of the kits we want to offer makes use of the little plastic containers that photographic film comes in. However, we can’t get anything to stick to them. We need to be able to have our customers glue two of these little canisters together side by side, but everything we try to use just falls off. When examining what actually fails, we see that the adhesive literally falls off the plastic, leaving no residue at all. Any suggestions? Answer:

Unfortunately, you’ve chosen a very difficult material to use an adhesive on. Film canisters used for 35MM film these days are molded from a polyolefin material. This type of plastic is economical, molds well and most certainly offers a suitable barrier to contaminants for the film itself, but presents a real challenge if you want to stick anything to it.

Polyethylene and polypropylene are among the most common polyolefins used today. These versatile polymers find use in hundreds of applications, but they fall into a category of surfaces one would describe as non-polar, or low energy. Low energy refers to the surface energy of the material, and unfortunately, the lower the surface energy, the more difficult it is to find an adhesive that will stick to it.

I think your best solution is to recommend your customers use either a double-faced pressure sensitive adhesive, or if you need a more permanent bond, one of the many consumer hotmelt-adhesive guns on the market. In the case of the pressure sensitive tape, the highly tackified surface of most tapes will overcome the low surface energy encountered and provide at least some level of strength. A hot melt adhesive, generally made from a close cousin to the polyolefin resin used in the canister, will form a decent bond that should approach the strength of the substrate itself.

- BOB SMITH

Question:

For many years now, our firm has manufactured a tank housing for a pump using two ABS clam shells. We have discovered a large potential cost savings if we could utilize a much less-expensive resin based on filled polypropylene. However, when we prototyped parts, we found that the ultrasonic welding process we have used for years simply does not produce the quality of bond we require. What alternatives do we have?

Many of the new engineering grades of polypropylene have successfully replaced components previously molded from ABS. You are correct in thinking that making this resin change can result in significant material cost savings over ABS. However, you have also discovered that welding polypropylene offers new challenges you may not have encountered before.

In some instances, the ultrasonic welding process can be fine-tuned to provide an adequate bond at reasonable production speeds. It would be worth contacting your equipment vendor to see what they may be able to do. If that approach does not work, you might consider using one of the alternative assembly methods using a consumable adhesive. There are systems available that use low-frequency energy to electromagnetically excite a specially filled resin material that is placed between your parts. These systems do require new application equipment and the earlier-mentioned consumable adhesive, but in many instances, the cost savings of the resin change more than compensates for the capital and material expenses.

- BOB SMITH

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