A Motor vehicle wheel arch has a body of thermoplastic material with an opening and a mat closing the opening and fixed to the body by strips of thermoplastic material. A method of making the arch includes injecting the strips onto the mat and allowing them to solidify in contact therewith. An edge is formed on a reduced thickness portion of the strip. In cross-section, the edge thickness relates to the edge width l by a mathematical function f such that: ##EQU1##

where h is the height of the edge at its junction with the strip.