Process for stabilizing oxymethylene copolymers

In a process for producing a stabilized oxymethylene copolymer which comprises heat-melting an oxymethylene copolymer having unstable, unblocked terminals obtained by the copolymerization of trioxane with a cyclic acetal and/or a cyclic ether to heat decompose it in the presence of available heat stabilizers;

• the improvement which comprises heating said oxymethylene copolymer having an unblocked terminals in a mixer at a temperature between the melting point of said oxymethylene copolymer and a point 100.degree. C. higher than said melting point under a pressure of 0.1 mmHg to atmospheric pressure for a mean residence time of 5 to 60 minutes, said mixer comprising a casing, a jacket for a heating medium fitted over its external wall, and at least two stirring shafts equipped with a plurality of scraping blades disposed in said casing, said stirring shafts being capable of rotating in the same or different directions, wherein (a) the scraping blades of one stirring shaft and those of another stirring shaft are deviated from each other in the direction of the stirring shafts so as to avoid collision and the tips of the scraping blades of one stirring shaft are capable of rotating while keeping a slight clearance from the inside surface of the casing and the surface of the other stirring shaft, or (b) the scraping blades of one stirring shaft and those of another stirring shaft are located in matched positions in the direction of the stirring shafts and the tips of the scraping blades of one stirring shaft can rotate while keeping a slight clearance from the inside surface of the casing and the stirring blades of the other stirring shaft;
• and in the meantime, rotating said stirring shafts of said mixer to heat decompose said oxymethylene copolymer while renewing the surface of the molten oxymethylene copolymer.