A bag forming apparatus includes a pair of opposed sealing bars arranged on opposite sides of a forming plane, a pair of guide arms, each extending from a first arm end through the forming plane to a second arm end and carrying a respective one of the pair of opposed sealing bars, a mounting frame to which the pair of guide arms are movably mounted between the first and second arm ends, and a drive shaft connected to the second arm end of each of the pair of guide arms such that shaft rotation is operable to move the pair of opposed sealing bars into and out of engagement across a web of bag forming material traveling therebetween in the forming plane. The drive shaft can be translatable relative to the mounting frame to adjust a closure gap between the pair of opposed sealing bars.