Method for producing expanded granules

A process for production of expanded pellets from a polymer melt comprising a blowing agent and a thermoplastic polyurethane, said process comprising the steps of:

• a) pressing the polymer melt through a perforated disk controlled to a temperature between 150° C. and 280° C. and into a pelletizing chamber,
• b) using a cutting device to comminute the polymer melt pressed through the perforated disk into individual expanding pellets,
• c) discharging the pellets from the pelletizing chamber using a liquid stream,
• wherein the blowing agent comprises CO2 or N2 or a combination of CO2 and N2,
• wherein the pelletizing chamber is traversed by a stream of liquid which is controlled to a temperature between 10° C. and 60° C. and the pressure of which is from 0.7 bar to 20 bar above ambient pressure,
• wherein the pressure and temperature for the liquid in the pelletizing chamber and also the temperature for the perforated disk are chosen such that the pellets are expanded in the pressurized liquid by the blowing agent they contain so as to produce expanded pellets having an uninterrupted skin,
• wherein the temperature of the liquid in the pelletizing chamber is lowered when the pellets undergo an uncontrolled expansion which does not produce an uninterrupted skin and is raised when there is no or insufficient expansion of the pellets and wherein the temperature of the perforated disk is lowered when the pellets undergo an uncontrolled expansion which does not produce an uninterrupted skin and is raised when there is no or insufficient expansion of the pellets.