Flight helix angle

Figure 7.25 Optimum barrel groove helix angle versus the screw flight helix angle at various values of the barrel groove depth...

Lastly, it was assumed that the details of the screw geometry do not affect the heat transfer to the barrel this is not completely true. The number of flights, the flight clearance, the flight helix angle, and the flight width all affect the heat transfer from the polymer melt to the barrel. If we want to study the effect of these parameters in detail we have to use a more complicated, numerical analysis. 

When the flight helix angle cp, = 45°, the inequality simplifies to ... 

Minimum slot flank angle for flight helix angle of 45°... 

Increased flight helix angle in transition section... 

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... 

Most screws of SSEs are single flighted, with Ls = Ds, referred to as square-pitched screws. The radial distance between the root of the screw and the barrel surface is the channel depth, H. The main design variable of screws is the channel depth profile that is H(z), where z is the helical, down-channel direction, namely, the direction of net flow of the material. The angle formed between the flight and the plane normal to the axis is called the helix angle, 0, which, as is evident from Fig. 6.8, is related to lead and diameter... 

The value of the helix angle is therefore a function of the diameter. At the tip of the flight it is smaller than at the root of the screw. A square-pitched screw, neglecting the flight clearance, has a helix angle of 17.65° (tan 6 = 1/n) at the flight tip. 

We now take the drag and pressure flow terms in Eq. 9.2-5 and substitute the relevant numerical values. We assume a square pitched screw, neglecting the difference between mean and barrel surface helix angle, and neglecting shape factors and flight clearance. We further assume that flight width is 10% of the barrel diameter. We can make these simplifying assumptions because, at this point, we only wish to select the barrel diameter and the screw speed. The channel width can be expressed in terms of the screw diameter as follows ... 

There are many barrier-type screws that differ from each other by the channel depth profiles of the melt and solids channels, by the helix angles, profiles, and the number of flights. We briefly review here just a few if these types of screw. The first barrier-screw design is due to C. Maillefer14 and is shown in Fig. 9.40, in which the auxiliary channel follows roughly the solid-bed profile. Clearly, at certain conditions the auxiliary flight can restrict flow rate, but at all times it prevents solids from leaving the screw. 

Fig. 2 Geometrical diagram of an extruder screw 1) diameter of the barrel (Db) inside diameter of the barrel 2) channel depth (H) distance from screw roots to barrel inner surface 3) flight clearance (c5f) the distance in between the flight and the barrel inner surface 4) channel width (ff(r)) distance in between two neighboring flights and 5) helix angle (6r) angle formed in between the flight and the direction perpendicular to the screw axis.

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