The venting performance of a vented extruder screw is primarily determined by the parameters of its venting section. Many factors influence the venting effectiveness. Research using the simulation device shown in Figure 14-2 to study the venting performance of vented extruder screws has shown that the venting section length L, the residence time of the melt in the venting section t, the shear rate γ to which the melt is subjected in the venting section, and finally the degree to which the melt fills the screw channel in the venting section (expressed by the fill factor F) are all directly related to venting effectiveness.
To ensure good venting performance, the screw channel in the venting section should not be fully filled with melt. To quantify the degree of filling, the fill factor F is introduced. It is the ratio of the cross-sectional area of the melt to the cross-sectional area of the screw channel. For ease of calculation, it can also be regarded as the ratio of the channel depth of the first metering section H₁ to the channel depth of the venting section H₂, as illustrated in Figure 11-13.
Experiments and analysis of existing machines indicate that for general venting screws, under the conditions of venting section length L ≥ 3D and fill factor F ≤ 0.5, a relatively good venting effect can be obtained if a shear intensity K greater than 100 can be ensured.
For venting screws with an L/D ratio between 24 and 30, the venting section length L ranges from 2D to 6D, with 4D being the most common. The channel depth of the venting section is approximately M times the channel depth of the first metering section H₁ (M = 2.5–6), which can be selected according to the statistical curve in Figure 11-14. The fill factor F and the shear intensity K are then verified using appropriate formulas, and finally the screw strength in the venting section must be checked.
