Box parts such as box bottoms and box lids are fabricated from notched blanks of plastic sheet material.Relatively large boxes can be fabricated and the blank feed mechanism is actuated by a crank mechanism that runs at twice the speed of a main, one revolution per cycle drive shaft. An extension blank pusher can be added to produce smaller boxes. The tab folders have solvent applicators formed as grooved metal bars associated with a centralized solvent delivery and recovery system. The transverse folding dies are heated with a flexible, rope-like heating element to facilitate changes in die length. The feeding apparatus includes cam operated, vacuum cup lifter arm assemblies which lift the top blank from a stack and present it to vacuum cups on the feed pushers. These assemblies are longitudinally adjustable. The anvils that cooperate with the tab folders have heated bodies, but their clamp surfaces are insulated from the bodies.