Rate Limitation Due to Solid Polymer Fragments in the Extrudate

2 Rate Limitation Due to Solid Polymer Fragments in the Extrudate 

Flight width and flight clearance were 13.5 and 0.12 mm, respectively, in all sections of the screw. The extruder had a smooth-bore feed section. Three rings of pin mixers were evenly positioned in the middle portion of the metering section of the screw. All pins in a single ring had the same axial position as the type shown by Fig. 8.25. The specific rotational flow rate for the metering section was calculated at 5.0 kg/(h rpm). The compression ratio was 2.7. 

Although the full history of this screw is unknown, the mixing pins appeared to have been added to the screw after the screw failed to mix and homogenize the extrudate. Pin mixers have been retrofitted to screws for many years as a way to improve the distributive mixing for an extruder. Here the pin mixers would distribute the solid polymer fragments evenly in the extrudate, hut the mixers lacked the ability to trap and disperse them. Pin mixers were clearly the wrong type of mixer to use for this application. Instead, a dispersive mixer such as a Maddock-style mixer would be preferred see Section 8.5.1. 

The Screw Simulator was also used to measure the melting flux of TPU resins at conditions similar to those expected in the melting section of the screw. At these conditions, the melting flux was measured at 0.2 kg/(m s). This melting flux is relatively low. For example, the melting fluxes [11] for linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) resins are 0.3 and 0.4 kg/(m2- s), respectively. The relatively low melting flux for TPU resins is a contributor to the solid polymer fragments in the extrudate. 

Maddock-style mixer 2 0.50 mm undercut on the mixing flight 

---