(CH2)6N4, Hardening agent for novolaks (e.g. phenolic resins as binder for shell mold sands).

Novolaks would not harden without hexamethylenetetramine; it triggers duroplastic cross-linking. Under the influence of the heat from the molding tools, resin is transformed from the A resole state (meltable), via the B resitole state (plastic) to the C resite state (hardened). In this process, the hexamethylenetetramine breaks down to formaldehyde and ammonia.

Additional references:
No-bake process