Conveying Elements

Fig. 10.8 Fully intermeshing, co-rotating elements in configurations matching square-pitch screws, (a) Forward screw conveying elements with different numbers of lobes (b) forward staggered kneading paddles (a.k.a., disk elements) representing leaky screw elements, and (c) neutral configuration. [Reprinted by permission from D. B. Todd, APV (Baker Perkins) Systems, in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

Figure 4.16 Energy input in the pressure build-up zone for different conveying elements...

Figure 8.2 Grid on the surface of a right-hand and a left-hand conveying element with two threads...

Figure 8.3 Grid in the cross-section of conveying elements with different numbers of threads...

Figure 8.12 Distribution of the shear rate within a conveying element bottom enlarged illustration around a screw tip. The color scale ranges from +22 s 1 (blue) to +687 s 1 (red)...

Figure 8.15 Temperature development of the polymer melt on the screw surface of the conveying element over two rotations. The color scale ranges from 300°C (blue) to 325 °C (red)...

Figure 8.16 Geometry of the second example series of conveying elements with different pitches and different rotational directions...

Figure8.20 The screw zone comprises a conveying element-spacer disc-mixing element-spacer disc and kneading element at a pressure difference of 0 bar and melt flow of 0.11 kg/s. The top figure shows the pressure on the screw surface, while the bottom figure shows the axial velocity in the melt channel at half the channel height...

The above equations represent a very simplified approach for simple conveying elements. For special mixing and dispersing elements such as kneading discs, eccentric discs or grooved elements, the approach must be expanded. A simple estimate is, however, possible. 

Figure 11.6 Plastification section without backward conveying element...

The devolatization sections and the discharge section up to the screw tip contain predominantly conveying elements that are partially filled in the devolatization area and completely filled in the pressure build-up area. A homogenous melt is generally assumed in these processing steps where viscosity is altered up to the tip of the screw, depending on the temperature evolution. 

The pitch direction, i. e., the direction of the angle of rotation of the thread. If the thread of the element runs counter to the direction of rotation of the screw shafts, one obtains a conveying element. If the thread runs synchronously with the screw shaft rotation direction, the screw element will have a backward-pumping effect (Fig. 12.4). 

Conveying elements take the product to be metered - pellets, powder, ground product, melt, etc. - into the extruder and transport it downstream to the first processing section where it may be compressed. Then, the product is transported through processing sections,... 

Conveying elements are supplied with different pitches and lengths. They almost always wipe the barrel with small clearances to prevent leakage flows over the tips. This means that only single- and double-flighted elements can be used in extruders with a D()/D, ratio of more than 1.36. 

Conveying elements are designed with identically shaped and located face sections. This makes it easier to fit the elements onto the screw shaft to form a continuous profile with no offset angle with respect to the previous element. Double-flighted conveying elements are normally used. Their length is usually equal to a whole pitch or half a pitch, i. e., a complete profile thread or half a profile thread (Fig. 12.11). 

Figure 12.11 Ratio of pitch to length for conveying elements...

Figure 12.12 Conveying elements with increased free volume...

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