Apparatus for forming a plastic attachment on an article includes clamping mechanisms which position the article and form a mold surrounding the appropriate section of the article. During the process of molding the plastic attachment, the article may be moved through the mold to facilitate forming the attachment along a length of the article. The apparatus is illustrated by an annular mold for forming mating plastic couplings around the ends of tubes such as corrugated and smooth-walled metal pipes.