Monolithic vs. COEX
Monolithic sheet is a single homogeneous material through the cross-section. COEX combines layers, typically a structural substrate with a surface capstock, so the surface and the structure can be engineered independently.
When COEX earns its place
Spec COEX when the part needs UV stability, scratch or chemical resistance, or a specific color/texture on a surface, while a different, lower-cost resin does the structural work underneath.
Common COEX systems
ABS COEX and TPO COEX are the workhorses at heavy gauge, an economical substrate capped with a durable, UV-stable, or cosmetic surface layer.