3920.63 - Of unsaturated polyesters:

This HTS category, 3920.63, encompasses plates, sheets, film, foil, and strip made from unsaturated polyesters, excluding those that are cellular or have been combined with other materials through lamination, reinforcement, or support. These are non-composite plastic materials, distinguished from other polyesters by their specific chemical structure.

Crucially, this category differentiates itself from siblings such as 3920.61 (polycarbonates) and 3920.62 (poly(ethylene terephthalate)) by its focus on a broader range of unsaturated polyesters. While 3920.69.00.00 covers "other polyesters" without specifying unsaturation, 3920.63 specifically pertains to those with unsaturated polyester resins, which often possess unique properties related to curing and cross-linking.

Within this classification, further distinctions are made based on flexibility. The subcategory 3920.63.10.00 is designated for flexible forms, implying a degree of pliability and resistance to breaking upon bending. Conversely, 3920.63.20.00 covers all other forms that do not meet the criteria for flexibility, encompassing more rigid or semi-rigid materials within the unsaturated polyester family.