Abstract: A process for producing bottle preforms from the melt of polyethylene terephthalate and/or its copolyesters, includes selectively introducing an inert gas into the continuous flow or partial flow of the polyester melt from a poly-condensation having an intrinsic viscosity between 0.5 and 0.75 dl/g, subsequently bringing the melt to an acetaldehyde content below 10 ppm in a melt after-condensation reactor and to an intrinsic viscosity of 0.75 to 0.95 dl/g and thereafter guiding the melt into an injection molding tool and processing same.

Abstract: A process for preparing linear copolyesters which contain omega-hydrocarboxylic acid units, from polyesters that contain at least 90 mol % polyethylene terephthalate units, as well as the usual amount of diethylene glycol units and possibly the known catalysts and stabilizers, includes adding to the already prepared polyester melt at least one compound of the group of lactones, optionally along with further additives but without additional catalysts in a system sealed off from gas exchange and at a higher pressure than the lactone vapor pressure in a proportion of up to 10 mol %, referred to the final melt, and mixed intensively with the melt. The resultant copolyesters, after a total dwell time of less than 30 minutes and a temperature in the range of 265.degree. to 310.degree. C.