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Since any gap between the spider and the bobbin hasbeen sealed, there is less risk of cone adhesive migratingdown into the gap during final assembly.Although some recently developed epoxies offergood bonds, they are often too slow for most high speedproduction environments. In production environmentswhere speed is not such a critical factor, such as in batchbuilding, some Loctite ® DURABOND epoxies workextremely well. DURABOND epoxies E-20HP, E-60HP orE-120HP are recommended for the most demanding andhighly stressed voice coil bonding applications. Sometake an hour or so at room temperature to achieveenough handling strength to allow meaningful end-of-linetesting. This is usually unacceptable. Epoxy cure timescan generally be reduced by using hot-plug inserts orheat tunnels, making DURABOND E-120HP useful forslow assembly lines and batch production methods.Unfortunately, the chemistry that produces a fast curetypically also seriously compromises other useful qualitiesof the epoxy. Often the fastest epoxies display lowpeel and sheer strengths, brittleness, and a catastrophictendency to crystallize under repeated thermal cycling.Some single component, frozen epoxies have beenused for the cone-coil-spider bond, but they are morecostly and require special storage in freezers. On theassembly line, the problems of thawing these adhesivesfrom -40°C just before use, contending with their shortpot lifes, and dispensing them from small disposablesyringes, make them impractical.For small drivers, and those where the stresses aremore modest, cyanoacrylates have been used with greatsuccess. In high-frequency and midrange drivers, andmoderate power low-frequency drivers, (typically withcoils under 50mm), cyanoacrylates are ideal. They addminimal mass, create a strong bond quickly, and haveexcellent environmental tolerance. Cyanoacrylate"instant" adhesives work best when the joint involvestwo close-fitting, reasonably parallel surfaces. On lowfrequency drivers, this can be obtained by formingcones with short neck sections that closely parallel thevoice coil, ideally with less than 0.25mm of radialclearance. Gel type cyanoacrylates are available forbonding those speaker parts which have larger gaps orpoorer tolerances. Redesign of the part may be morecost efficient however, because when large gaps must befilled, adhesive consumption can be quite high.Cyanoacrylates can be made to form uniform filletsor wedges, if laid as a heavy bead, and then oversprayedwith accelerator, creating a tidy, quick bond. The lowviscosity versions produce a uniform, cosmeticallyattractive bondline. The rapid cure of these adhesives inthe last few steps on the production line allows driversto be tested within minutes of assembly. Also, when driversare assembled with cyanoacrylates, the frequencyresponse and impedance curves taken at the end of theline are essentially the same as those taken after theadhesive is fully cured. The elimination of off-linestacking reduces space requirements and labor costs,and the improved QA feedback from the test stationallows almost immediate correction of problems.If low viscosity cyanoacrylates are applied and thenoversprayed with an accelerator, the resultant curereaction may be too rapid, which can seriously compromisethe bond strength and create a "popcorn" likeappearance to the adhesive. Typically, low viscositycyanoacrylates are already fast curing and do notrequire acceleration. An over-application of acceleratorcan have the same effect, also creating the "popcorn"like appearance.Assembling Speakers with Loctite ® AdhesivesRecommended products for the Voice Coil to Spider to Cone Assembly:Loctite ® Cyanoacrylate Adhesives 380, 410, 411, 422, 498, 4204, 4210, or 4211, orLoctite ® Epoxy Adhesives E-20HP, E-60HP, or E-120HP.See detailed Product Information on Cyanoacrylates on pages 22 and 23; andEpoxies on pages 28 and 29.13