An unmixed plastic sheet, which is made of a thermoplastic material and which is composed of a number of body segments that are adjacent to one another in a plane, interconnected and made of a closed-cell foam material. The body segments are thermoplastically welded to one another at their abutting lateral faces while forming flat weld seams. The flat weld seams form a plastic intermediate layer, which has few pores, and which is provided in the form of a web structure that, when viewed from above, is reticular and reinforces the plastic sheet. The inventive plastic sheet is preferably used as a core layer in sandwich composites.