Method for joining two parts and a unit of joined parts joined by the same

A unit of joined parts comprising:

• a first part and a second part, said first part having a first flange portion extending along an entire periphery thereof, and said second part having a second flange portion extending along an entire periphery thereof and a margin portion extending along an outer edge of said second flange portion;
• and
• a preformed linear ridge formed on at least one of said first flange portion, said second flange portion and said margin portion, said preformed linear ridge extending along an entire periphery of the at least one first flange portion, second flange portion and margin portion and extending toward an opposed one of said first flange portion, said second flange portion or said margin portion in an overfolded state;
• said unit being formed by matingly contacting said first flange portion with said second flange portion and overfolding said margin portion into contact with at least said first flange portion and plastically deforming said preformed linear ridge by compression so as to form an air-tight seal between the joined parts.