LETTERS

FIT FOR ADHESIVES

I enjoyed the adhesives article in the Nov. 3 issue ("Advanced Adhesives Foster Hybrid Structures"). As a die manufacturer, I'd like to know how precise the fit between metal and plastic surfaces should be for proper bonding. I suspect the metal component would be highstrength

R. Distel

There is reasonable latitude for variation in dimensional tolerance. The recommended bond-line thickness forBetamate low-energysubstrate adhesive (LESA) is 0.030 in. (0.75 mm.) However, tests show that bond lines from 0.02 to 0.16 in. (0.5 to 4 mm) work well. Molding small ribs into the plastic part in the bond area can control the minimum gap size by preventing adhesive squeeze out. Adding properly sized glass beads in the adhesive can also control minimum gap size. Maximum gap size is controlled by keeping the steel and plastic parts within dimensional tolerances. Fixturing devices such as snap fits or pop rivets hold parts together during cure and can be used to pull together assemblies with gaps to minimize adhesive use. This must be done carefully to avoid creating a new gap in another area of the bond line. - Dave Recktenwald, Dow Automotive