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Nov 1st, 2007 Print this article Design Focus: The ‘perfect weld’By Tony Deligio That’s the goal, however elusive, suppliers of plastics-joining equipment strive toward, continually expanding their technology repertoire in an attempt to answer changing questions from new materials and compounds.
Sensitive electronics components, which can’t suffer loose particulates and are unable to withstand the jostling associated with ultrasonic or vibration welding, are increasingly being joined via laser welding from companies like LPKF.
Pictured are some basic joint designs suggested by Sonitek, which suggest a minimum height of .005 inch for the energy director. For heights greater than .020 inch, two or more directors should be designed in, with the sum of the heights equaling the formula dimensions, according to Sonitek.
Pairing a laser welder with a pair of halogen lamps, LPKF is putting forth “hybrid” welding as a solution for multimaterial, complex-shaped parts like automotive headlamps.
Infrared welding systems, like this unit from Heraeus, have joined the growing ranks of plastics-joining technologies, offering a noncontact, highly focused tool for welding. |
Sylvio Mainolfi, director of marketing at joining-equipment supplier Branson Ultrasonics (Danbury, CT), has watched his own company make the same progression, initially offering ultrasonic welding in the 1960s, adding vibration welding in the 1970s, hot plate in the late 1980s, spin welding in the 1990s, and laser welding in 2000. At present, the company is in the latter developmental stages of infrared welding technology, with a product launch possible in 2008.
“You might ask, ’Why do you need all these processes?’” Mainolfi says. “The answer is, [welding] is very application specific—you have to not only look at the material that’s being welded, but the size of the part and the geometry.”
Given the array of technology choices and variable parameters, Mike Johnston, national sales and marketing manager at equipment supplier Dukane (St. Charles, IL), has one request of processors considering plastics joining.
“If you’re planning on putting together a plastic part,” Johnston says, “before you even start cutting any metal, 100%, make sure you speak with a viable vendor of the plastics assembly equipment.” Dukane, which itself offers six joining processes—laser, ultrasonic, vibration, hot plate, thermal, and heat staking—sees a lot of joining projects, but not always at the development stage at which it would like.
“It’s so often, and it really makes the customer’s life and our life difficult,” Johnston says, “that customers already have the parts designed, CAD models finished, and they’re cutting steel, but haven’t really done a good job of figuring out what the joint design needs to look like.”
Mike Brunetti, VP of sales and marketing at Sonitek (Milford, CT), which specializes in the design and manufacture of ultrasonic horns and fixtures, says the biggest impact can come in the joints. “In many cases, we are still contacted after the joint has been designed and/or parts have been molded,” Brunetti says. “At this point, more often than not, we must do something creative with the tooling to overcome an improper design that generally increases the costs of the tooling or equipment. When possible, in very bad joint-design situations, we do recommend the customer re-do the joint.”
For customers that do come to Branson before steel has been cut, Mainolfi says processors should consider the part volumes, with some processes better suited for higher throughputs and faster speeds. Other considerations customers should be aware of include: whether or not they want to automate the process; the cosmetic standards that must be met; and the additive packages, especially if flame retardants are in play.
“People put a flame retardant into a material so it won’t melt when it’s exposed to a flame,” Mainolfi explains. “So if you look at it as a thermal process, and you’re trying to elevate the temperature at the joint area, it makes welding more difficult. Once you start to modify a plastic, it might push you into a different process.”
Designing in end-of-life
Of the methods to join plastics, broadly described as chemical (adhesive/solvent), mechanical (screw, fasteners), and capital equipment process (welding units), Mainolfi estimates that welding comes in second behind chemical, but recycling concerns and economics are favoring welding more and more. “Every time you use an adhesive, you have the cost of the adhesive and the labor to apply it,” Mainolfi explains. “If there’s a trend, people want to get away from that extra cost of an adhesive or solvent or screws and gaskets.”
In addition, end-of-life (EOL), which is gaining increasing importance in markets like automotive and electronics, favors welding since no foreign or dissimilar materials are introduced to the product, and items like screws don’t need to be manually removed.
“Automotive manufactures are being pushed, big time, by this recycling thing,” Dukane’s Johnston says. “It’s the metal fasteners they’re trying to move away from, so we’re addressing larger assemblies—like complete taillights or headlights—that in the past might have been screwed together.”
For Sonitek’s Brunetti, the impact of EOL has been limited. “Even though end- of-life reclaim is a common consideration in the upfront design of most products today,” Brunetti explains, “we continue to see just about the same volume/ratio of mechanical fasteners and adhesives today as we did 20 years ago.”
Sustainability as a trend, and its requirements for recycled content, also presents the potential challenge of reground content in parts. Mainolfi says regrind is workable, but it’s important that processors track the material’s heat history, with third- and fourth-generation resin affecting weld strength. Johnston says the consistency of regrind percentage is also important, with variation negatively affecting the process.
In spite of shifting challenges like regrind, Mainolfi says the desired result remains static. “Our serious customers are always looking for what we call the perfect weld,” Mainolfi says. “That was the force behind our coming out with laser welding, and that’s sort of what’s behind [infrared] coming out.”
Six easy questions
Branson Ultrasonics (Danbury, CT) poses these basic questions to customers interested in joining to help them narrow in on the appropriate process.
1. Which polymer are you using? Some are suitable for some assembly processes and not for others. Additionally, additives and regrind can affect the final result.
2. Are you using fillers or a mold release agent? Mold release can leave a residue that might affect weld strength, and talc or glass can change the required process.
3. Does your part have internal walls or ribs? Are the parts small or large? Does the part have a complex geometry? Are you welding one part at a time, or several? All these variables inform the process choice.
4. How strong does your final bond need to be? If your product is a tool or a piece of lawn furniture, your weld will need to be stronger than for a disposable package, for example.
5. Is a hermetic seal required?
Hermetic seals, common in pumps and filters, need to be factored into the design and knowing burst-pressure requirements is a must.
6. Does the assembly need to be flash-free and particulate-free? A weld on the perimeter of a display would need to appear cleaner than welds on part interiors. Welds on pumps, for example shouldn’t have flash, which can break off and contaminate the contents.
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