a method for controlling the viscosity of molten polymers , such as polycarbonate , takes place prior to a molding operation such as injection molding , extrusion , thermoforming or blow molding . when a significant reduction of viscosity is desirable , the plastic melt is submitted , at constant temperature , to the action of a vigorous mechanical extensional shear vibration , with minimum or no external pressure , at a constant amplitude and frequency , causing the melt to become highly elastic , and simultaneously it is fatigued , for a certain time at that temperature , maintaining this high elastic state , until the macromolecules have partially or totally disentangled , in a controllable manner , at which stage the melt is ready for a molding operation such as a simple quenching operation or an extrusion process followed by quenching to produce pellets or compounds with a better mix or a lower viscosity when remelted , or an injection molding operation where the melt viscosity has been greatly reduced allowing a better processability of the injected part , for instance allowing the use of a lower temperature of injection , a lower pressure of injection or both .