Method of making bottom seal thermoplastic bags

Claim 1

In a method making bottom weld bags from a supply of elongate thin, flexible multi-layer web of thermoplastic material, which comprises the steps of converting normally continuously rotary motion to oscillatory motion for providing an oscillating drive having a web feed stroke in one direction and a reverse stroke in the opposite direction with said strokes progressively increasing and then decreasing in velocity from zero velocity to a maximum velocity and back to zero velocity between reversals of stroke direction, gripping the web between rotatable draw rolls and maintaining the grip of said rolls on the web during both of said strokes, separating transversely disposed seal bars and clutching a draw roll to the oscillating drive during the latter's feed stroke for drawing a trailing length of the web from the supply while advancing a leading edge of the web for motion between and past the seal bars, declutching said draw roll from the drive and preventing web motion at the end of the feed stroke, severing the web downstream of the seal bars and bringing the seal bars together for making a weld at the leading edge of the severed web, separating the seal bars and feeding a new length of web between the separated seal bars; the improvement comprising the steps of freeing said web for motion and clutching the draw roll to the oscillating drive during the final portion of the drive reverse stroke for rotating the draw rolls in a reverse direction sufficiently to strip the weld from the seal bars and maintaining the clutching of the draw roll to the drive after termination of said final portion of the drive reverse stroke and throughout the succeeding drive feed stroke for restoring the weld to its previous position between the seal bars and feeding a new length of the web past the seal bars while the seal bars remain separated.