In the 1960s Piretti joined the Italian design company Castelli as an interior designer, creating furniture for the home and office. In his twelve years with Castelli, Piretti obtained numerous industrial patents for his designs, mainly for chairs, and his work rapidly advanced the company's international reputation in the furniture design sector. His best known design is the 'Plia' plastic folding chair (1968), of which more than seven million copies have been sold since 1969, and it is still in production. He won the BIO 5 award and the SMAU award at the Ljubljana Biennial in the 1971, the German Gute Form award in 1973, and the design can be found in the permanent collection of many museums around the world, such as in Austria, the Czech Republic, Slovenia and the United States. In 1969, the matching 'Plia' side table was equally appreciated in the world of design.