Designer's Productivity Kit: How to choose & use fasteners & adhesives

By: 
September 25, 1995

Admit it: When you think about design--whether it be an intricate
electronic component or a straight-forward enclosure--you probably don't think
about how you're going to hold it all together until the end of the design
cycle.

Innocuous as they may seem, fasteners and adhesives are critical to the success of any product. For optimum joint design, engineers must balance performance, cost, and manufacturability. Factors such as availability, joint life-expectancy, handling, safety, and design for serviceability also come into play.

With so much to consider and an endless variety of fastening options to chose from, it's not surprising that some designs don't take advantage of ideal fasteners or adhesives. To avoid being overwhelmed at the choices and to improve your design, look to the experts. What follows is their advice on how to avoid common mistakes, cut costs, save time, and get the most out of new fastening technologies.

Think of the end use

If you haven't already chosen between mechanical fasteners and adhesives, tapes, or welds, focus on the product's end-use. "Parts subject to wear will need access and service, and require mechanical fasteners," says Larry Kline, senior product engineer at SPS Technologies, Newtown, PA. Mechanical fasteners can also provide a reusable solution in some applications.

Adhesives offer other benefits: Where a permanent bond is desirable, they can lower manufacturing costs, add shock protection and sealing characteristics, or improve the aesthetics of an exterior surface.

"Obviously, in any application the engineer has to carefully consider parameters and physical requirements such as stress, shock, load, and temperature," says Randy Ladutko, General sales manager for Avdel Textron, Parsippany, NJ. But Design for Serviceability should also influence the decision, he says. Will the product require frequent maintenance or replacement parts? Clearly, permanent adhesive bonds aren't suitable for an access panel. "But don't forget that modular products and upgrade options also mean that the product may require disassembly down the road," adds Ladutko.

Remember to evaluate the surfaces you're intending to bond, cautions Jeff Travis, senior market development administrator at 3M, St. Paul, MN. Says Travis: "Are you working with two rigid substrates? One flexible and one rigid? What about material compatibility?" For example, vibration can spell disaster for a mechanical fastener, yet leave certain adhesive bonds intact. Likewise, different coefficients of thermal expansion can wreak havoc on joints that include dissimilar substrates.

Although engineers are typically well-informed about the adhesive bond performance requirements of their application, they tend to be unfamiliar with important manufacturing issues such as surface preparation and time-to-cure, adds Travis. Such elements can have a profound influence on installed cost, bond strengths, and production speed.

Get suppliers involved early

Manufacturers agree: No matter which fastening technology you choose, early supplier involvement is vital. "The fastener supplier is rarely called in at the 'paper napkin' stage," reflects Avdel's Randy Ladutko. "Fastening is probably the most critical area of design, but most engineers relegate it to something they'll find in the corner hardware store." Supplier involvement can help the designer take advantage of the wide variety of sophisticated fastening systems available.

For example, extensive handbooks, such as SPS Technologies' 76-page reference for aerospace self-locking nuts, provide specifics about swaging, plating, and fatigue properties of hundreds of products. Likewise, 3M's updated Designer's Reference Guide and Loctite's intensive Design Guide for Bonding Plastics walk you through the selection of suitable adhesive chemistry and application and cure methods, as well as highlighting advantages and disadvantages of dozens of products.

Even the most detailed guide is no substitute for supplier engineering support, however. "Excluding the supplier from the early stages of design diminishes the possibility of using a standard--and therefore less expensive--product," says Jim Grady, manager of corporate engineering for Southco, Concordville, PA. "When the designer contacts us at the end of the development cycle, there are already set criteria and we're limited in what we can provide," he observes. "Early in the cycle, it's easy to specify an off-the-shelf product. If necessary, we have the lead time to pursue custom development and tooling, and everything can happen on the same launch schedule. Because you don't want to be waiting on a $4 latch for a $100,000 computer system."

Analyze installed cost

When banished to the end of the design cycle, fastening systems often bear severe budget and design constraints. There's more to specifying fastening technology than screws and glues, warns Ladutko at Avdel. "If the design engineer takes a hands-off approach with the idea that a fastener is a low-cost item, that mind-set can turn out to be very expensive."

For example, specifying a less-costly substrate can actually add expense if it's incompatible with conventional bonding methods or not sufficiently strong for a threaded fastener. And while a custom mechanical fastener may be only slightly more expensive on a piece-part basis, single-side access for installation or elaborate custom tooling can cause installed cost to skyrocket.

Tim McGuire, Manager of Product Engineering at Camcar Textron, Rockford, IL, advises engineers to "look at the overall design. Are you designing the fastener in your component where there's room for tooling? It's easy to locate a fastener in an engineering drawing, but maybe a component will prevent the drive gun or a person's hand from getting in to install it." By including fasteners in conceptual drawings, engineers can avoid such a scenario, says McGuire.

Avoid overkill in your specifications

Frequently, hidden costs stem from design overkill. That's where a supplier's expertise can save money. Penn Engineering and Manufacturing, Danboro, PA, halved the cost of most of the fasteners in an automatic bank teller machine in one instance, says Application Engineering Manager Pat Kelly. "The engineering group hadn't contacted us, and they were specifying stainless-steel fasteners throughout the assembly," recalls Kelly. "In 80% of the fastener locations, they didn't need stainless steel. They were spending more than twice the cost of an equivalent non-stainless-steel part with no benefit."

Likewise, balance the cost and performance of plating or other finishes when addressing corrosive environments, says Kelly. "We often see applications where our customers have specified products that work, but they're over-designed." Thread size is especially prone to such excess, he adds. Unless a metric M6 or M8 fastener gives the benefit of having a single fastener size in your component, it may be too large a thread. "When engineers over-specify, they don't get additional benefit, just additional cost," reflects Kelly.

Choosing an over-qualified adhesive, over-specifying bond overlap, and over-spraying are also budget thieves. For example, the lower tensile strengths of plastics make it common to create bonded joints that are stronger than the plastic itself, explains Richard Thompson, Senior Product Engineer with Loctite Corp., Rocky Hill, CT. Engineers can achieve equal overall assembly strength with a narrower overlap and less adhesive. Many engineers aren't aware that shortening bond overlaps actually reduces the bend and differential shearing effects of lap joints, says Thompson.

Use specials to beat design limitations

When a custom fastener or adhesive formula is genuinely called for, the extra expense and engineering effort during design can prevent failures and costly delays down the line. In fact, a well-designed special fastener can compensate for limitations of a design. Says Camcar's Tim McGuire: "Custom fasteners can reduce overall fastening cost in some applications. The overall piece-price is higher, but the fastener may be faster to drive, such as the Torx Plus[super{TM}], so it reduces overall installed cost."

Soft joints also benefit from custom fasteners. Plastic components, thin cross-sections, and gasketed joints aren't well-suited to proper fastening techniques. But custom fasteners tailored to the correct seating torque can overcome the restraints of such designs and prevent fasteners from stripping out or cracking the substrate, says McGuire.

In critical applications such as automotive airbags and seatbelts, price and production volume support custom parts. The technology is available, says McGuire, and "the more dramatic the impact of the failure, the more technology you can afford to put into place."

With adhesives and tapes, getting what you want out of a custom formula can be as simple as knowing what you need, says 3M's Jeff Travis. "Customers come to us and say they need a longer open time, or higher viscosity, or faster set, and we tweak a formula to make it work."

Examine manufacturing issues

Of course, even the best prototype joint design doesn't always prove sound in production. With adhesives, engineers need to be aware of issues such as bond curing techniques, shelf-life limits, and handling requirements. For both adhesives and mechanical fasteners, assembly time and automation and tooling expenses are important elements of a bond design's feasibility.

For instance, some adhesives shrink during curing. Flexible adhesives are better able to adjust to shrinkage than rigid systems. In applications that don't tolerate shrinkage, engineers should select 100% solids adhesives such as one-component epoxies, says Kelvin Yee, market development manger at Grace Specialty Polymers, Woburn, MA. For example, Grace's AMICON(R) epoxies are designed specifically to bond surface-mount devices in electronics applications, where they protect components from contaminants and excess solder filling once cured.

To address safety standards and OSHA requirements, engineers should consider automatic mixing and dispensing equipment to minimize worker exposure, says Ken Cressy, Industry Manager for Ciba-Geigy's Aerospace group in Los Angeles. In some cases, liquid adhesives can be replaced with alternative products. "If heat cures are suitable, as is often the case with pre-preg composite structures, you can go to a film adhesive, which gives higher strength and better application control," he says.

Many designers don't take into account that some substrates require surface preparation such as plasma- or chemical-etch, says Len Rantz, Director of Polymer R&D at Grace Specialty Polymers. Engineers should be aware that design constraints such as available heat, time, and dispensing techniques limit automation choices, he adds. "We're seeing a lot of infrared curing, induction curing, and hot air blowing, because engineers are looking for shorter cure times. There are more demands on the chemistry, because in some applications, there's no time to wait around while a part goes through a tunnel oven."

Design for Assembly isn't simply a matter of inexpensive, easy-to-install fastening systems, says Avdel's Ladutko. Engineers should consider not only the cost of fastening materials, but also the quality and integrity of the final joint. In an automotive fastener, for example, an anti-rattle feature such as one designed by Avdel can improve the fastener's function and may reduce parts count without any assembly-speed penalty. Not surprisingly, many high-volume automotive applications demand compatibility with automated assembly equipment. Hence, fasteners from Penn Engineering are available with tapered or dog-point ends, or an orientation feature for rapid, high-volume track-feeding in automated assembly lines.

Explore new fastening technologies

Adhesive and fastener suppliers are always introducing new products to answer application needs. For instance, to join magnesium components, engineers needed a thread-forming fastener rather than a thread-cutting part that would create debris from the flaky substrate, says Camcar's Tim McGuire. So Camcar developed thread-forming TapTite(R) "Magscrews" especially for magnesium.

For applications where loose hardware is a problem, Southco engineers designed the "Captive Nut." The N7 fastener replaces wingnuts or mounting nuts in installations where lost hardware can't be retrieved or would cause equipment damage. The fastener is suitable for joining aluminum, low-carbon steels, and annealed stainless steel. To speed assembly, an internal thread and ferrule help position panels being fastened. A through-hole version accommodates frame and panel thickness variations and gasket compression set.

As application demands change, fastening systems must adapt, observes Larry Kline of SPS. "Car engines today run a lot hotter than they were running ten years ago, for fuel-efficiency reasons. That requires high-strength, high-temperature fasteners." Similarly, aerospace manufacturers seeking to boost engine thrust run aircraft engines hotter. To meet those requirements and accommodate existing designs, SPS offers fasteners made from multi-phase materials such as AEREX 350, an alloy suited to high-strength service to 1,300F. "New materials come out every six months, and a lot of them are directed at fastener applications," says Kline.

New technologies are constantly developing for adhesive uses, as well. For example, Ciba-Geigy's Urelane 5774 FST adhesive meets flame, smoke, and toxic-gas emission limits, which are required for automotive, aircraft, and other applications for safety. The adhesive is slow to burn and releases no toxic fumes, says Industry Manager Ken Cressy.

If you run into a snag when designing a joint, call on suppliers for technical expertise. Says Penn's Pat Kelly, "We don't expect our customers to be experts in the fastener field. We welcome their calls and questions because, ultimately, it's going to be better for everyone." Many adhesives suppliers, such as Grace and 3M, devote R&D resources and engineering experience to customer-support labs. Says Grace's Len Rantz, "That's why the lab is here. If it was easy, engineers wouldn't need us."



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