Screw helix angle

An extruder is coupled to a die, the output of which is given by (KP/ij) where P is the pressure drop across the die, i] is the visco.sity of the plastic and is a constant. What are the optimum values of screw helix angle and channel depth to give maximum output from the extruder. 

The same treatment applies to the regenerated nip surface film, dc. The velocity gradient would be zero at the surface, except perhaps when elastic forces predominate. The surface exposure time may be approximated by 0 = distance dc/irDN sin(f)y where cross-sectional plane. 

A polymer with viscosity 2S0 Pas is to be extruded using a vented screw extruder, with details as follows. Diameter of bturel D = 75 mm, screw speed Af= 40 rev min S screw helix angle < -17.7 , and screw flight width e = 7.5 mm. 

Take the screw helix angle to be 17.7°, the screw speed to be 80 revmin and the temperatures at the first and second stages to be 200°C and 230°C, respectively. Viscosity data for polystyrene are given in the graph above. Take the density of polystyrene within the screw to be approximate constant at 981 lig m . 

Figure 7.25 shows cpb as a function of the screw helix angle when 4 = 4 = 0.2, fj = 0.6, and Hj = 15.24 mm (0.6 in). The optimum barrel helix angle increases with the screw helix angle and with reducing barrel groove depth. The optimum barrel angle is relatively insensitive to the internal coefficient of friction as shown in Fig. 7.26. 

The constraint that the tip of one screw element wipe the flank of its mate in self-wiping, corotating twin-screw extruders leads to a unique relationship for the shape of the screw channel (Booy, 1978,1980). Figure 13 is an isometric view of this channel and Fig. 14 is a cross section of the channel in a plane that is perpendicular to the plane which defines the helix angle. Figure 14 shows the actual shape of the channel, which is described by the following expressions ... 

The helix angle and the channel widths at the screw core or root are designated with a subscript c, and they are calculated as follows ... 

Thus the screw has a narrower normal distance between flights at the screw root because the helix angle is larger and because the lead remains the same. 

A mechanical clearance between the top of the screw flight and the barrel wall helix angle at the barrel 6c helix angle at the screw core 6 r) helix angle at radial position r... 

Figure 7.13 Comparison of literature drag and screw rotation for deep channels [45], The experimental data for screw rotation and barrel rotation and the theory lines were for screws with a 7° helix angle...

In polymer extrnsion, show that a square-pitched screw has a helix angle of 17.65°. 

In its simplest form, the ratio of screw diameter is the basis for scaling. The ratio of the large diameter D2 of the large scale unit to the small diameter Z>i of the lab unit is represented by the lower case d as shown in Table 4. The primary scaling variables are channel depth H, screw length L, helix angle cj), and screw speed N. The ratio of the primary variables of the two scales is then expressed as a power of the screw diameter ratio, d. 

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