**2.5 Die**

268 Thermoplastic Elastomers

Some kneading screws have interrupted flights to improve dispersive mixing, increase backflow, or increase mechanical energy dissipation into the extruder (Huber, 2001). Main characteristics of screw design include: (i) screw length; (ii) screw diameter; (iii) screw channel depth; (iv) screw channel width; (v) axial flight land width; (vi) clearance between screw and barrel; (vii) screw helix angle; (viii) leading flank angle; (ix) trailing flank angle; (x) screw pitch; (xi) direction of drag flow; (xii) direction of pressure flow and (xiii) direction of leakage flow (Figure 2) (El-Dash, 1981). The screw pitch (t) is the distance between corresponding points on adjacent thread profiles, and the number of parallel screw channels or leads (n) is defined as the number of screw pitches in the axis distance that the helix

advances in one turn.

Fig. 2. Main characteristics of screw design

mechanisms as well (El-Dash, 1981).

reduce leakage flow (El-Dash, 1981).

The barrel is divided into feeding, kneading and high pressure zones (Figure 1).

The sleeves surrounding the screw can be solid, but they are often jacketed to permit circulating of steam or superheated oil for heating or water or air for cooling, thus enabling the precise adjustment of the temperature in the various zones of the extruder. And most sleeves are equipped with pressure and temperature sensing and temperature control

In twin-screw extruders, the sleeves are usually smooth but can be constructed with longitudinal or helical grooves (Huber, 2000). In single-screw extruders, the sleeves are usually fluted on the inside, with either straight or spiral grooves. Parallel grooves are often cut or more often cast into the barrel. Spiral grooves provide high forward flow, while straight grooves hinder it. The latter thus result in a lower flow rate, but more mechanical shear. The clearance between the screw and its sleeve is usually kept to a minimum to

**2.4 Barrel or sleeves** 

The die presents two main functions: give shape to the final product and promote resistance to material flow within the extruder permitting an increase in internal pressure. The die can present various designs and number of orifices (El-Dash, 1981). Dies may be designed to be highly restrictive, giving increased barrel fill, residence time and energy input. Die design and its effects on functional properties and quality of a final product are many times overlooked.
