Abstract: Generally rectangular paperboard frame blanks are fed separately into a horizontal position from a stack at one end of a first leg portion of an L-shaped assembly line, and air filter mats are fed respectively to the frame blanks. Each edge portion of a frame blank is scored to provide three spaced parallel fold lines. As each frame blank with an air filter mat is conveyed along the assembly line, glue is applied along two spaced parallel glue lines on each of a first pair of opposite edge portions and the edge portions are doubly folded. Then the blank and mat are shifted at right angles and conveyed along the second leg portion of the L-shaped assembly line where the gluing and folding operations are repeated for the second pair of opposite edge portions.