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Beyond a simple bond

Beyond a simple bond

Newton, MA--Often a replacement for mechanical fasteners, adhesives seem to have a simple task: bond materials together. But the role adhesives play in product development continues to expand. In many applications, adhesives can now deliver benefits ranging from resistance to impact and hostile environments to improved aesthetics and structural integrity to reduced costs and assembly time.

Take speaker assemblies for example. Thermal resistance has become increasingly important in speaker manufacturing. Over the years, the overall size of speakers has decreased, while power requirements have increased significantly.

These changes have increased operating temperatures and affected the requirements for speaker components, particularly the thermal resistance and heat-aging properties of the adhesives used. As a result, adhesives must now withstand higher temperatures for longer periods of time than ever before.

To address this problem, speaker manufacturer International Jensen Inc., Schiller Park, IL, recently tested Loctite's (Rocky Hill, CT) new PRISM 4204 instant adhesive on its speaker assemblies. The company has used instant adhesives to bond its speaker assemblies for years. However, as thermal requirements increased to 250F, standard adhesives could withstand temperatures of only 185F.

PRISM 4204, a rubber-toughened cyanoacrylate, can endure temperatures up to 250F. It also offers improved resistance to impact, vibration, humidity, and damp environments. And it achieves fixture strength within 20 to 30 seconds on most substrates, curing to a clear, durable bond line.

International Jensen used the adhesive to attach the speaker's flexible components. First, the adhesive was applied directly to the voice coil. To assist the fixturing process and speed the cure, an accelerator was sprayed on the adhesive joint. The spider was then attached to the voice coil. Next, the cone was attached to the voice coil in a similar fashion.

To affix the whizzer to the cone body, a bead of adhesive was applied on the inside diameter of the cone. The joint was treated with an accelerator to speed the fixturing process.

Results of the adhesive test have been excellent. "All of the company's adhesive concerns have been successfully addressed," says Haresh Kapadia, principal polymer engineer at International Jensen, "including workable viscosity and rapid room-temperature fixturing." Most important, the adhesive meets the upcoming 250F OEM requirement.

Broader benefits. Choosing the right adhesive doesn't just improve the product; it can significantly impact the manufacturing process as well. Chris Ervin, plant engineer at Drexel Heritage Furniture, Drexel, NC, knows this all too well.

With processes that can involve more than 100 people to build a single piece of furniture, this industry ranks among the highest for labor-intensive manufacturing. Controlling costs and quality remains a constant challenge.

One of Drexel's most time- and labor-consuming applications involved attaching a decorative, solid-wood panel to a headboard's front facing. The entire process took eight to ten people two days to complete. But by switching to Jet-Weld TE-100 thermoset adhesive from 3M's Industrial Tape and Specialties Div. (St. Paul, MN), the product can now be finished in one to one-and-a-half days--with just two people, says Ervin.

In the original manufacturing method, two people apply PVA adhesive to a poplar overlay panel. Two other individuals position the overlay on a headboard and pin-nail it in place. Next, two workers place the 20-lb headboard flat on an iron-rail track, and stack up to 20 pieces atop each other.

Workers then place a set of top rails, aligned with those below, over the headboards, and roll the unit onto the press. Up to 600 psi of force compresses the headboards. Workers then "bail" the compressed unit using two 150-lb headblocks (one on top, one on bottom) and eight 30-lb turn-buckle clamps (four per side) that extend between the eight 40-lb bottom and top rails. The boards are compressed for one day. This process prevents the poplar panel from bowing and permanently secures the panel to the headboard.

When curing is complete, four workers release the clamps, headblocks, and rails; remove the headboards; and return them to the assembly line where workers counter-sink nails, apply putty, and sand off excess putty. The entire process takes from 28 to 32 hours to complete, including two quality checks.

In addition to time and labor issues, the headboard manufacturing process presented adhesives problems. The conventional hot-melt adhesives the company used reheated in the finishing box and made assembly more difficult.

Making the move to 3M's Jet-Weld adhesive revolutionized the headboard process, claims Ervin. Jet-Weld, a one-part, moisture-curing urethane, has thin glue lines similar to those of PVA adhesives, long open times (up to 10 minutes), and fast set times (1,000 lbs of overlap shear in 10 minutes on maple).

Now, a worker applies the Jet-Weld adhesive to the panel with the squeeze of a trigger on a lightweight pneumatic applicator. Two others place the overlay panel on the headboard, engage four clamps for 30 seconds, and proceed to the next piece. The adhesive eliminates the nailing, the trip down to the press room, the cold pressing and bailing operations, the return trip to the work room, counter sinking, putty applications, and subsequent sanding.

Using Jet-Weld also did away with one quality inspection, two parts lifts, material handling, set-up time, and scheduling problems. The result: reduced production time per part from 28 plus hours to several minutes.

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