Fully wiped screw

The clearances 6 and S are not required for the fully-wiping screw (see Section 2.5.2). This leaves four independent factors. Other six-fold combinations of independent geometrical values can also be used. 

Figures 5.8 and 5.9 and Table 5.1 provide an overview of the geometric variables in a screw. In Figs. 5.8 and 5.9, diameter DE of the fully wiped screw geometry is larger than the barrel diameter D because of the clearances required in this special case.

From the specified geometry data DG, A, Z, s and 8 of the extruder according to Table 5.1 we obtain the values DE, R, DI, RI, KWO and KBO of the fully wiped screw profile, (x, y) are the coordinates of the fully wiped screw and xa> ya are the coordinates of the profile to be produced. The planar profile of the fully wiped screw is thus... 

The tip width of the screw to be produced, KW1, is not the same as the tip angle of the fully wiped screw. KW1 is determined by finding the intersection with the tip of the flank curve of the screw to be produced. The solution is determined mathematically using the following equation... 

Fig. 6.42 Schematic cross sections of co-rotating fully intermeshing twin screws for single-, double-, triple-, and quadrupled-flighted screws. [Reprinted by permission from M. L. Booy, Geometry of Fully Wiped Twin-Screw Equipment Polym. Eng. Sci., 18, 973 (1978).]...

M. L. Booy, Geometry of Fully Wiped Twin-Screw Equipment, Polym. Eng. Sci., 18, 973 (1978). 

Since the - mathematically precise - system is intended to be fully wiping, the central shaft can be a wax blank that is shaped to its corresponding contour by the metal moved screw. The moved screw (Fig. 2.1) with its metal tip x then forms the flank arc y (bold) in the fixed wax shaft. As all mass points of the moved screw describe circles with a radius equal to the centerline distance, including the tip x, the flank arc y of the wax screw must also be an arc with a radius equal to the centerline distance of the two screw shafts an astonishingly simple solution. 

Figure 5.8 Geometric variables of a double-flighted screw profile, cross-section Green fully wiped contour. Red actual contour. Blue barrel wall...

If the ratio of centerline distance to diameter is too small, the screw cannot intermesh properly with the external wall. This occurs when the tip angle is negative. The fully wiped geometry can be calculated according to Table 5.1 for the boundary condition KW0 = 0. The gives the following condition... 

In the case of the planar offset we take the planar section of the fully wiped profile and move this inwards by half the screw-screw clearance. To calculate a screw profile according to the planar offset, we proceed as follows ... 

Booey, M.L Geometry of fully wiped Twin-Screw equipment. Polymer Engineering and Science, 1978, Vol. 18, no. 12... 

The reactive extrusion was performed in an APV Baker MPF50 fully wiped corotating twin-screw extruder. The maximum screw length used was 14 D. The extruder utilized five heating zones and the barrel wall was kept at a uniform temperature of 130°C in the last four zones. The first zone, closest to the feed port, was kept at a temperature of 120°C. The amount of monomer-initiator mixture fed to the extruder ranged from 1.5 to 4.5kg/h. The pressure in the die varied between 0 and 13 bar. After every adjustment of the extrusion parameters samples were collected when the extruder had reached steady state. In order to stop the reaction immediately the samples were directly frozen in liquid nitrogen at the outlet of the extruder. 

Limited number of screw geometries allow fully self-wiping action... 

Twin-screw extruders have an important role in the compounding industry. There are various possible geometries for twin-screw extruders. These include directions of screw rotation and level of intermesh. Two screws can rotate in the same direction (co-rotating) or in opposite directions (counter-rotating). The screws may be separated, tangential, intermeshed and fully intermeshed and self-wiping. 

---