in an apparatus for forming and separating the side edges of plastic bags of the type which have opposed interlocking closure strips , an intermittent transport device transports a half - tube of thermoplastic sheeting to a welding device . the sheeting is stopped while welding is performed . at a milling station upstream of the welding device , a milling apparatus is actuated while the half - tube is stopped to mill off a short length of each of the opposed closure strips at a position which coincides with the position at which welding will later be performed .