ASSEMBLY END USER: Simply Structural

September 1, 2007
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Adhesives prove to be a cost-effective alternative to welding for marine and industrial applications.



4-Star Trailers incorporated structural adhesives into its production with the goal of improving the strength of its products. Their switch from rivets, adhesive tape and welding to adhesive bonding has resulted in a quieter trailer. (Images courtesy of 4-Star Trailers)

Proven in a variety of end-use markets - including more than 50 million cars, trucks, specialty vehicles and boats worldwide - structural-adhesive bonding is an established joining method. Because the adhesives are polymers, and this article discusses their use in making “structural” connections, the term “polymer welding” will be used to describe structural adhesive bonding. The benefits of polymer welding have been widely touted for decades, and are best summarized as building a better product, faster, cheaper, and with greater ease. Polymer-welded parts often command premium prices due to aesthetic appeal that can grow your top-line revenue. Couple this with lower production cost and greater reliability, and profits could be increased substantially.

When compared to mechanical fasteners and welding, structural adhesives widely distribute stresses, act as a sealant, reduce noise and vibration, offer excellent durability, help prevent corrosion, and maintain the original mill-finish quality of the exterior surface of thin panels. Cost savings can be significant: welding requires skilled operators, while bonding simply requires good operators. The ability to polymer-weld different materials allows for flexibility in choosing the right material for the task, not simply using what can be joined with a traditional fastening method.

Although the growing market acceptance of adhesives as a cost-effective alternative to welding is intriguing to many, companies with roots and expertise in welding are still reluctant to give “glue” a try. Reasons many say they continue to weld include the perception that welds look stronger, unfamiliarity with the adhesive-application process, misinformation about adhesives, and fear of change. However, those who have integrated adhesives into their business have benefited from immediate return on investment with decreased labor and material costs, improved production cycles, and better end product. But how does one get started? The key to success is embracing the conversion process, which includes understanding the chemistry behind adhesives, outlining a proper implementation plan and partnering for success with a reputable supplier.

Chemistry: Back to Basics

The first step to understanding adhesives lies with gathering information. Different adhesives perform differently, and finding the right one for your product is key to success. Since no one adhesive can meet all needs, it is important to understand each product’s merits.

Of all the polymers available for sealing and bonding, only five are regarded as structural, in that they provide strength and durability comparable to or greater than the materials they bond. These are: acrylics, cyanoacrylates (CAs, or sometimes superglues), epoxies, urethanes and anaerobics. CAs have limited structural applications, as do anaerobics, also known as threadlockers. For all intents and purposes, the list of chemistries can be narrowed down to acrylics, epoxies and urethanes.

Understanding the type of chemistry being used is important. Generally, acrylics excel at bonding unprepared metals, composites and thermoplastics. Epoxies provide the highest strengths and are great on prepared metals, composites, and natural substrates. Urethanes offer resiliency and flexibility, and are ideal for composites, thermoplastics, natural materials, and prepared metals. More detailed information on the unique abilities of these polymer-welding products is available from many sources.

It is important to remember that a structural adhesive used to polymer-weld two or more substrates into a single part is a thermoset polymer, which will not melt or change with environmental exposure, temperature, or time. It is good for the life of the part and can resist a wide operating temperature range. Acrylics and epoxies can withstand temperatures of -40-400°F; most urethanes are good up to 300°F or 350°F, with a low end slightly better than the others. Exposure to water, salt water, humidity, oil, gasoline, solvents and other factors or environments will not affect bond strength in a properly designed joint.

With a basic understanding of product characteristics and associated chemistry, the next step is to review what works best for specific materials, such as metal, thermoplastic, composite or natural products:

Metal Bonding. Without question, toughened acrylic adhesives are the finest unprepared metal-bonding adhesives available to polymer-weld similar and dissimilar metals. If you paint, prime or prepare the metals, urethanes or epoxies are useable. Epoxies will be the best choice for extremely high heat resistance or if exceptionally high shear strength is required. For impact resistance and toughness, choose a toughened acrylic.

Thermoplastic Bonding. Urethanes bond an array of plastics, including the hard-to-bond polyolefins (polyethylene and polypropylene) when combined with surface treatments. Epoxies and acrylics are good, but can be too stiff for these lower-modulus materials. Other chemistries work well as a “modulus bridge” between the softer plastic and stiffer substrate when cross-bonding plastics to metals or composites.

Composites (SMC, Fiberglass, FRP, Pultrusion, Carbon Fiber, etc.). Urethanes and acrylics are known for their composite-bonding abilities. These products need little or no surface prep, are easy to handle and have a quick room-temperature cure. For very high-strength and demanding performance applications, epoxies are widely used, but these unique products are very specialized, especially those for aerospace or aircraft uses. Toughened epoxies that can be heat-accelerated are often good candidates.

Natural Products (Wood, Stone, Plywood, Natural/Synthetic Rubber, Fabrics, etc.). Epoxies and urethanes are the best candidates, since acrylics often will not cure on these more porous surfaces. Urethanes work very well if the substrate has residual moisture that they can react with. Rubber-like materials will need a surface pretreatment to gain a good bond, while stone materials must be clean so that no dust or loose surface material interferes with a strong bond to the base substrate.

If you need to cross-bond one material class to another, look for a common product chemistry with the help of your adhesive manufacturer and then explore the unique qualities it brings to your situation. If your adhesive supplier has a range of chemistries and products to choose from, pair their product-selection expertise with your manufacturing knowledge to get a workable fit between the competing needs of your part, materials, process, and desired economics in order to improve your assembly method.

Implementation Procedure

Nothing is more vital to a successful project than a commitment to follow an implementation plan. Such a plan starts with a review of your goals and leads to a plant tour in order to audit your current process so that an estimate of potential savings can be made to justify your investment. If you agree with this scoping effort, then substrate testing to confirm bond performance is next, followed by plant trials or part redesign testing, leading to the preproduction test run for approval before production. It is likely that you’ll have trouble achieving the desired results if all parties are not committed to the applicable steps or if critical steps are omitted from the process.

Testing Success

After identifying the best adhesives for your project, the next step is auditing your facility in order to define the best applications for the adhesives. Audits normally require a tour of the manufacturing facility and discussions with designers, manufacturing engineers, and shop-floor personnel. Specific applications are selected and prioritized.

Bond trials are the most effective method to demonstrate the ability of a proposed adhesive process. Using the information gathered during the plant tour or audit, applications are chosen based on their ability to positively impact costs, process, aesthetics and/or throughput. Trials also answer process questions that may arise in this kind of change and help to fine-tune any unfamiliar manufacturing steps. It is important to note that decision makers capable of changing the process or design are needed at trials, as their approval is crucial to the overall switch.

Testing procedures vary, depending on the overall objective. In some cases, laboratory testing of the substrates is all that is required, though it is often necessary to actually attempt to break the bonded assemblies. This is sometimes done by brute force with a sledgehammer and chisel. However, it also can be quite a sophisticated process. Testing can range from in-house procedures - some facilities have specific test parameters - to field tests, which involve parts or assemblies being put into use for a period of time. Still other times, particularly in the transportation sectors, the testing is based on standard test specifications.

Simply put, the testing process allows everyone to feel comfortable that the application is sound and robust. The adhesive supplier often adds other tests to the process in order to verify the product’s performance. Such tests may include both physical and environmental testing in a laboratory setting. Long-term environmental testing is critical if the application is going to be exposed to outside conditions. Another factor to review is the track record of the particular adhesive in similar conditions. Accelerated aging tests will help uncover any deficiencies before they can cause trouble for your product.

A common mistake that is made is assuming that the failure of one type of adhesive eliminates all adhesives. The failure very well may be caused by the supplier not making a wise product selection, failure to understand the process ramifications, or choosing the wrong application for an adhesive at the outset by addressing emergency situations with a tool not suited to the problem.

Following is an example of recent success of adhesives in a new area. This example illustrates why polymer welding is a rapidly growing segment of the product-assembly market.

No Horsing Around

Users can credit adhesives in reducing their labor. As a leading manufacturer of custom aluminum horse trailers, 4-Star Trailers conducted extensive testing to develop fixtures and test the strength of their bonded designs. Kris Coleman of DTI, LORD Corp.’s distributor, provided several test samples. Two doors were created for the testing process: a traditional welded frame door, which used double-sided tape and rivets to hold the outer panels on (a common method of construction in the industry), and a door constructed entirely using adhesives. The doors were hung to a heavy steel frame and a hole was drilled in the lower corner of the door to simulate where a horse may kick and the panel would experience the most stress. Then, the chain from a ¾-ton winch was threaded through the hole. The welded door pulled off the frame at 40 lbs. of force. At 950 lbs. of force, the door pulled out 5.5 inches and broke apart. In contrast, the bonded door only pulled out a half-inch at 1,000 lbs. of force from the winch. The team was so amazed with the results that they bought a 3-ton winch, hooked it up, and performed the test again. Only when 2,500 lbs. of force was applied did the bonded door move one inch.

The same test was repeated at a later date. This time, the welded door was irreparably damaged at approximately 960 lbs. of force. The team never broke the adhesive door on the first test, so they set out to find the breaking point by adding 800 lbs. per stroke of the winch. Somewhere at 4,000-4,400 lbs., the bonded door popped loose, but when they released the winch, it went back to its original position and laid flat with easily repairable damage.

It was determined that the bonded doors were four times stiffer with double the impact resistance of 4-Star’s welded doors. This stiffness prevents the door from flexing easily and provides excellent security for cargo. Since integrating adhesives into their door assembly, Patchell said 4-Star has realized tremendous savings in labor and materials. Not only has the company been able to reduce labor by 50%, the switch from rivets, adhesive tape, and welding to adhesive bonding also resulted in material cost savings. In addition, the use of adhesives has resulted in a quieter trailer. Fewer squeaks and rattles result in less fatigue and stress - a better overall travel experience - for the animals.

The Right Partner

In addition to an innovative spirit and a commitment to the proper testing and procedures, finding the right partner is key to making the switch to adhesives. Select a product supplier with a long track record in your industry, as well as expertise in process support, application-engineering support (helping to redesign parts for bonding), technical capabilities (including both field and in-house testing), training capabilities, appropriate product delivery channels, and safety expertise. Not all adhesives are created equal, and neither are all adhesive suppliers. A supplier that offers multiple chemistries is better able to match an adhesive to your needs instead of the opposite. The right one will be able to apply the right adhesive for your application in order to reduce costs, improve quality and appearance, allow for greater flexibility in design, and increase throughput. In short, if you are still riveting, bolting, welding, taping, screwing or crimping a part, adhesives may offer a significant advantage in cost and/or performance.

For more information, contact Dan Bradshaw at dan.bradshaw@lord.com.

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