flat bottom bags are formed from a supply of substantially endless plastic tubing , gusseted along two sides to form a flat reel . a sleeve of predetermined length is withdrawn from the reel and allowed to hang along a vertical plane between two foraminous plenum chambers which are then moved toward each other to squeeze the length therebetween . simultaneously a sealing and shearing mechanism forms a transverse seal and cuts the sleeve length from the reel , the gussets being at the same time sealed in the seam . the plenum chambers are then subject to vacuum and are moved apart , causing the sides of the now formed bag to separate opening the same . upon opening of the bag , a forming mandrel is inserted therein and moved through the length of the bag until it reaches the sealed end . simultaneously a presser member is moved across the outside surface of the bag folding over the seam , and pressing the gusset portions into flat flaps and in addition forming simultaneously the flat bottom to the bag . the mandrel or the presser plate is provided with heat sealing means sealing the flaps to the sleeve sides .