It has been tested time and time again. Bonded inserts are the best way to attach a syntactic foam plug to a base structure.
Syntactic foam plug assist materials typically fall in to one of three broad categories: thermoset epoxy, copolymer epoxy or thermoplastic.
Thermoset epoxy syntactic foams (HYTAC® W, WF, WFT and other standard 350° or 450° epoxy syntactics) generally have poor tensile and shear strength, making direct threading inadvisable. Failure due to over-tightening, improper screw/bolt length, or a cantilevered wobble of the plug is typically catastrophic and results in cracking of the entire plug or pull-out of sections too large to repair.
Copolymer syntactic foams (HYTAC® FLX, FLXT and C1R) are more durable to impact and production abuse, yet they are still subject to the same limitations when attaching to a base.
Thermoplastic syntactic foams (HYTAC® XTL and B1X) have a toughness that prevents propagation of a failure throughout the entire plug. A threaded piece that pulls free generally damages the threads only. Thermoplastic syntactic foam is easy to drill and allows you to retap the plug, install a slightly larger thread and get the plug back on the machine in a matter of minutes.
At CMT, we compared a threaded insert to a scored insert bonded into place using standard quick-cure adhesive. While our test yielded similar tensile pull-out values, it also showed very different types of failure. Threaded inserts failed catastrophically with the entire plug being destroyed. The bonded insert simply pulled out in a failure of the adhesive, not the entire plug. The advantage of the bonded insert is that it can easily be readhered inside the plug.
Typical threaded insert
Helicoil inserts are NOT recommended for use with thermoset epoxy or copolymer syntactic foam and will typically cause plug breakage.
HYTAC® Slotted Inserts
Scored insert available from CMT. Suitable for use with any syntactic foam. Available with ¼-20 or 6 mm internal thread.
Click here for a PDF installation guide detailing the process and adhesive to use CMT inserts for HYTAC® materials.