A plastic 28mm ROPP (Roll-On Pilfer-Proof) cap is a type of closure commonly used for sealing bottles, especially in the beverage and pharmaceutical industries. These caps are often made of plastic and feature threads on the inside that match the threads on the bottleneck. When the cap is twisted onto the bottle, it forms a secure seal, and the pilfer-proof feature ensures that the contents remain safe from tampering until the cap is broken or removed. The 28mm refers to the diameter of the cap, which is a standard size used for many bottles.