Water-injection foaming devolatilizing method

A water-injection foaming devolatilizing method, comprising the steps of:

• melting and kneading a polymer in a water-injection dispersing zone of an extruder having a screw;
• injecting water into a polymer melt so as to be dispersed into the polymer melt which is being kneaded;
• and
• vaporizing volatile components contained in the polymer melt together with water in a devolatilizing zone having a vent port and located on a downstream side of said water-injection dispersing zone, so that the volatile components are removed and discharged through the vent port,
• wherein a pressure-reducing expansion zone is provided between said water-injection dispersing zone and said devolatilizing zone so as to be upstream of the vent port, and further comprising gradually reducing a pressure in said pressure-reducing expansion zone so that bubbles in the water which is dispersed into said polymer melt are expanded to substantially their maximum size through a lack of kneading of said polymer melt in said pressure-reducing expansion zone and said bubbles are broken down at a downstream end portion of said pressure-reducing expansion zone by passing said polymer melt through slits in said downstream end portion.