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Bonding Plastics 101
January 22, 2001
7 Min Read
Don't feel too bad if you don't know much about bonding plastics. You are not alone. "Many engineers don't know how to use adhesives properly," laments Dr. Barry Kostyk, who conducts bonding seminars for practicing engineers and students when he's not busy overseeing some of the largest adhesive accounts for 3M (St. Paul, MN). As he sees it, the brunt of the blame lies with engineering curriculums that favor the certainties of screws over the comparatively unknown territory of "glues." "Even as adhesives grow in popularity, too many mechanical engineering programs still focus on mechanical fasteners," Kostyk argues.
But forget about what you didn't learn in school. A better reason to keep up on adhesives has to do with their constant state of change-especially as it relates to plastics bonding. Adhesives developments over the past few years have rendered more kinds of plastics "sticky" than ever before, expanding the universe of adhesive-substrate combinations. "Even many knowledgeable users are unaware of all the possibilities today," says Pat Courtney, who has done more than his share of educational outreach, first as an application engineer and now as engineering project manager for Loctite Corp. (Rocky Hill, CT).
Most engineers do know that failed joints can turn a plastic assembly into a pile of parts. So it makes sense that adhesive selection often starts with bond strength and moves quickly to questions about substrate compatibility, thermal resistance, and environmental conditions. A good starting point in designing for adhesives, these "basics" can usually be gleaned from the bond strength tables put out by adhesives suppliers. Yet because adhesives exert a strong influence on overall materials selection, part design, and manufacturing costs, Courtney and Kostyk both advise going beyond the basics. It's the things that the bond-strength tables don't tell you-things like the six design principles that follow-that can really boost your success with adhesives.
1. In the beginning, there were adhesives.
Well, not quite. In fact, adhesives selection often takes place late, or even last, in the design process. "That's a big mistake," Courtney stresses. To keep costs low and bond strengths high, plastics and adhesives should be selected at the same time. Courtney has worked on projects in which engineers picked low-cost polypropylene over pricier polycarbonate only to find that their polypropylene parts bonded poorly without expensive plasma treatments or costly specialty adhesives. "Oftentimes your overall costs can be higher with a less expensive resin," Courtney says. And the seeming advantage of a stronger plastic can disappear if it results in flimsier bonds and failed assemblies.
2. Remember, Grasshopper, the weaker bond can create the stronger joint. "Understand that there are different types of strength," Kostyk says, drawing an analogy with the way oak trees face a windstorm with brute strength while willow trees tend to bend. "Joints also behave differently in terms of how they withstand loading and then recover," he says. Take that behavior into account, and you'll find that adhesives with a seemingly "weaker" bond strength can actually create stronger joints. As an example of this principle, Kostyk cites a non-plastics application in which stainless steel scuff guards had to be attached to the aluminum wing of an Airbus plane. The first choice, rivets, would have created a stronger bond than the adhesive tape that won the job. But the tape did a better job of handling a potentially joint-ripping mismatch between the two materials' coefficients of thermal expansion. The same principle holds true in plastics where Kostyk has seen tape beat out stronger adhesives in joints that had to exhibit a hinge-like flexibility. "Strongest isn't always best," Kostyk concludes.
3. Design a forceful joint. The right adhesive and the wrong joint design go together about as well as the Titanic and an iceberg. "Joint design is as important as selecting the right adhesive," Courtney says. To maximize bond strength, he suggests avoiding joint designs that are prone to peel and cleavage forces. "These are the most difficult forces for an adhesive to hold up against," Courtney explains, noting that adhesives fare better against shear and tensile forces. To get the right forces in the joint, try tongue-and-groove designs in place of flat-to-flat junctions, Courtney recommends. To enhance strength even more, increase a joint's width rather than the amount of overlap. "The leading edge of the joint is what bears the load, so it's better to increase joint width," Courtney says. Because the flexing of plastics under load can create the undesirable peel and cleavage forces, stiffening joints wherever practical can help too.
4. Tally total costs. Design requirements will certainly tell you how strong the adhesive has to be, but they won't always tell you how much it should cost. "You should be cognizant of over-engineering," says Kostyk, who has seen many cases of adhesives overkill.
And remember the adhesive's real cost won't appear on some price list. Its cost contributions emerge only when you evaluate it in the context of the overall design and the manufacturing process. Consider the influence of gap size on adhesive costs, for example. Courtney explains that designs with tight-tolerance gaps can reduce adhesive costs. Sure, the tight gaps could add cost to molding and assembly operations, but often not as much as "filling a large gap with an adhesive that costs ten or twenty dollars per pound," he says. Small gaps also tend to cure faster, further reducing assembly costs.
5. Cure time counts. Speaking of cure time, it too should inform the adhesives selection game. Courtney says that long-cure adhesives, while sometimes offering a price or strength advantage, often require assemblies to go through an expensive fixturing step that could offset any advantages. At the same time, he adds, long-cure adhesives can be a good choice for very large parts that need the strongest possible bond, or for low-volume jobs in which fixturing won't create a production bottleneck. Courtney's bottom line about cure time: "The decision to accept a long cure time should be a conscious decision, not an afterthought."
6. If in doubt, test-and always be in doubt. Assembly testing addresses an important distinction between adhesives and mechanical fasteners. As Kostyk explains, mechanical fastening's long engineering history has produced tables filled with reliable design values. "You can go to a table and pick out a bolt," he says. "And you'll probably get the right bolt." Adhesives are less straightforward because additives, fillers, and differences in polymer chemistry create bonding differences-even between grades within the same resin family. All polypropylenes, for example, are not created equal. "An adhesive may get a good bond to grade X but not to Grade Y," says Kostyk. "So it's wrong to think generically."
This variability means that bond-strength and substrate values from tables should be treated as a starting point, not gospel. "Use the tables to narrow down the choices to few likely candidate," Kostyk counsels. "Then test your assemblies."
For more fundamentals on adhesive bonding of plastics, see the fastening and joining channel at www.designnews.com.
|Relative surface energies of common materials
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