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Adhesives: making it with metals.

Adhesives: Making it with metals

Structural adhesives, long-time challengers of metal-joining methods, continue to win battles and make in-roads. Sure, there are plenty of metal-joining tasks where welding, mechanical joints, and fasteners that permit disassembly will hang onto their turf for a long time to come, but, in many areas of assembly, the only thing holding adhesives back is either ignorance of what they can do or lack of opportunity to prove it.

Surprisingly, there are no simple matrixes or computerized expert systems to pick out your adhesive, says Philip Brown, applications engineer for Loctite Corp, Newington, CT. You still need an experienced human to weigh all the variables. "I stress the need to understand an adhesive's uncured, curing, and cured properties. You have to fully consider handling, storage, processing, and timing needs--including what that adhesive will do in its final component environment. When you have narrowed all the options to a few, the user then opts for the one most convenient to use in his process."

Application engineering is critical. For example, in truck-trailer building, Loctite supplies an adhesive with a set time long enough to permit the panels to be accurately positioned. Initially bonded with a strength of X, the adhesive is still pliable enough for movement during final assembly. When heated during the painting process, bond strength increases to 3X.

"Viscosities are also important," adds Loctite's James Murratti, director, product management. "You don't want the material to migrate out of the bond. It must be heavy enough to stay in place, yet thin enough to wick into nooks and crannies. That's why we need thousands of formulations--and why there's no quick answer to the question 'Which one's the best?'"

Basic choices

Structural adhesives generally fall into five generic categories:

Epoxies. The most widely used, they offer high strength, good solvent resistance, low shrinkage, and good high-temperature resistance. A wide range of formulations are available, and they are relatively low cost. However epoxies are brittle, and their peel strength and flexibility are low. A two-component system, they must be mixed in exact proportions.

Polyurethanes. These are tough, have excellent flexibility and impact resistance, high peel strength, and moderate cost. However, they require special mixing and dispensing equipment, are sensitive to moisture, have toxic components, and have poor shear strength at room temperature.

Acrylics. Offer good flexibility, peel and shear strengths, fast curing, good bonding to oily surfaces, good moisture resistance, and do not require careful proportioning. However, they have low strength at high temperature, and are toxic, flammable, and smell bad.

Cyanoacrylates. These are single-component, with high tensile strength, rapid cure, are easily dispensed, and have good metal adhesion. However, they are higher cost. Thick bond lines do not cure well, impact and peel resistance is poor, and temperature exposure is limited to 180-220 F.

Anaerobics. Offer rapid cure, no mixing, good solvent resistance, good high-temperature resistance (350 F), and moderate cost. The fact that fillets do not cure in air aids cleanup.

Adhesive advantages

The basic advantages of structural bonding over other joining methods are ease of assembly and reduced assembly costs. The more specific benefits include: uniform stress distribution, ability to bond dissimilar materials, fatigue resistance, sealing benefits, noise dampening, and eliminating of preliminary (hole punching) or secondary (weld clean-up) operations.

Changing over from a mechanical fastener or welded joint to adhesive bonding usually requires redesigning the joint to take advantage of adhesive strengths (shear in wide-area lap joints) and to compensate for its weaknesses (peel or cleavage situations). Quick-bonding adhesives, such as the cyanoacrylates, are sometimes used to temporarily tack parts together for bonding later by other means.

Strength phobias

Adhesives still have a strength-perception problem, admits Loctite's Mr Brown. "We worked on a drill-bit application where the shank of the bit and its mounting adapter were press-fit and then welded. The press fit required tight machining tolerances. The problems were damaged parts during press fitting and tempering of the part during welding. They were creating a lot of scrap. By switching to anaerobic adhesives, they have been able to open up their tolerances a little bit, reduce machining cost, and solve these problems.

"In this case, the drill-bit manufacturer came to us with the positive perception that an adhesive could do this job. Too often, we get people who haven't had the training to understand how a plastic bond can be as strong as a metal fastener, and therefore will not listen to us at all.

"So we try to overcome that hurdle by education. Even those who can accept the idea of replacing thousands of rivets in an airplane wing with adhesive to get better stress distribution, go right back to old phobias when we talk about using adhesives to mount a bearing," Mr Brown explains.

Dispensing equipment

The trend in dispensing equipment is to custom-designed solutions for accurate delivery and minimal package size, as opposed to standard packaged solutions that make compromises. Some degree of flexibility is important--the ability to adapt to minor changes in product and delivery location or adhesive characteristics. Major changes in adhesive systems usually demand major equipment changeover and process modifications.

Robotics are delivering on the flexibility requirement. Robots can trace three-dimensional surfaces, repeatably and precisely, and are easily reprogrammed to new part shapes. The adhesive-delivery payload is relatively light and easily adapted to standard robots.

Most dispensing systems are shop-air pressurized where flow is controlled by adjusting pressure, ON-time, orifice size, dispensing needle type, and the delivery motion bringing the needle to and from the workpiece. Suck-back features can automatically pull the last drop back inside the dispensing needle to avoid untimely drips. Positive displacement systems are gaining acceptance over air-delivery because they waste less, provide better control, and are environmentally cleaner.

On the low end, there remains a demand for manual delivery methods where high-tech automation isn't practical. The adhesive may be delivered into position manually, but still metered automatically or semiautomatically.

Application examples

One good example is the spot welding of metal skins on computer products where the final surface must be flawless. Switching to structural adhesives eliminates both the spot weld and the grinding process needed to restore the surface for painting. Another example is in auto-accessory motors, where ferrite motor magnets were held with mechanical clips that corroded, popped loose, or fatigued and produced noise. Now, an adhesive is applied to the back of the ferrite, an activator on the housing, and the two clamped together to achieve a permanent bond in three to five minutes. Says Loctite's Murratti, "With adhesives, you make up for the variance in air gap, eliminate the cost of the clip, eliminate buzz and vibration, and improve the quality and longevity of the motor. Bearings in the motor are also adhesive bonded."

Removing solvents

The solvents issue--eliminating volatile organic substances proscribed by EPA--is forcing some to choose water-based primers and accept a processing delay for the less volatile fluid to flash off. The tradeoff here can be a higher-cost system that is as fast as the previous solvent system versus a slower, less expensive one.

As solvent-based adhesives move to water-based systems, says James A Graham of Lord's Industrial Adhesives Div, Erie, PA, the biggest challenge is simply change, not the need to make major compromises. There are performance benefits of water over solvents, and no need for speed concessions, he feels. "For example, you can get the same rheology at much higher solids content in water than you can in solvent because it's a dispersion rather than a true solution."

This can mean laying down more solids per application, much higher polymer molecular weights (tougher adhesives), and acceptable processing characteristics. "People fail to recognize that when you're flashing water, you can turn up the temperatures because water is nonflammable and nonexplosive."

Solvent systems, he adds, were a less expensive delivery system at the time major commitments were made in the early '80s. Those systems (including expensive solvent recovery) still need to be amortized. "Water-based technology is newer, and not simply a matter of delivering a new fluid in the old system. Keeping polymers stable and compatible in dispersion particles, and getting them to coalesce or not coalesce at temperatures in transit, storage, and then in cure--all of this is very sophisticated, but not necessarily more difficult, just different."

Potential users also have trouble getting over the higher cost of water-based adhesives. "They can't seem to realize that you get twice as many miles per gallon because there's twice as much adhesive per gallon. Plus, they haven't considered how much energy cost they would save by not needing to heat large volumes of air in their solvent-evaporation stations.

"When you do a full, economic analysis, you'll find that water in most cases is much friedlier than solvents. By the year 2000, there will be water-based alternative methods covering 99% of our environmental problems."

Future directions

What new adhesive solutions lie ahead? "We have been doing research in one-component systems that will cure at a fast speed and be relatively stable for industries now using two-component systems. This should be ready in a few years and make delivery systems much simpler.

Automotive assembly means building a part a minute, "It's interesting that automotive people, who've heard all about adhesive uses in the aircraft industry, are amazed when we take them into operations making 800 to 900 parts/day with structurally-bonded assemblies. They can't believe that this is already being done."

"The industries we deal with, particularly automotive, are in a materials revolution. The issues are whether adhesives can bond galvanized steel, cold-rolled steel, SMC, and now once again, aluminum. We are on a 1995 program for the auto industry for the all-aluminum Sable Taurus. Aluminum is difficult and expensive to weld, and weld quality is a major issue. Aluminum bonding is also very different to deal with than other substrates. But that's coming along, and we will solve that, probably with an adhesive similar to what we're using on galvanized steel."

"In today's economy," he adds, "cost is probably the worst enemy we have. Nobody in the industries we deal with will buy anything that is more expensive than what they are presently using. There has to be a cost reduction, or they won't change." His advice: look at total cost of the finished part, not just glue cost.

Glue replaces spot welds in car doors

Lord's Graham is particularly proud of a metal-bonding breakthrough in automotive: the adhesive bonding of door seams previously spot welded. "We are supplying the adhesive that bonds door hems on the '92 Ford Econoline van, a 260,000-unit run. In conventional doors, the inner and outer stampings are edge wrapped, crimped, and then spot welded to stiffen the assembly.

In the new process done for Ford by Budd Stamping, the inner reinforcing "hat" section is first stamped, then the exterior flat sheet. A robot lays down a bead of adhesive to the perimeter where the hem will be, the two panels are put together, and run through a hemming operation. One stamping operation forms a 90-deg bend on the outer sheet, another bead is placed on top of that, and then the second stamping closes the hem.

The engineering benefits are significant: the adhesive bond has greater stiffness and uniformity, it seals (saving a secondary operation), and there are no spot-weld corrosion points created in the galvanized substrate. Thus, the ultimate durability and reliability of the door is enhanced. Also, during railcar shipping to the assembly plant, there is no longer any loss of dimensional stability from vibration.

But all of these points were secondary, Graham reports. The key reason for the conversion was the elimination of spot welding and very expensive robots. "The bottom line was that it was much cheaper than spot welding."

PHOTO : Loctite Zeta modular system for semiautomatically dispensing gasketing material on motor housings.

PHOTO : Adhesive-bead location and closeups of the Ford Econoline door.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related article on use of glue on car doors
Author:Sprow, Eugene
Publication:Tooling & Production
Date:Nov 1, 1991
Previous Article:Minster reorganizes.
Next Article:Clamping technology vital to productivity.

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