Mixing, pins

Although the full history of this screw is unknown, the mixing pins appeared to have been added to the screw after the screw failed to mix and homogenize the extrudate. Pin mixers have been retrofitted to screws for many years as a way to improve the distributive mixing for an extruder. Here the pin mixers would distribute the solid polymer fragments evenly in the extrudate, hut the mixers lacked the ability to trap and disperse them. Pin mixers were clearly the wrong type of mixer to use for this application. Instead, a dispersive mixer such as a Maddock-style mixer would be preferred see Section 8.5.1. 

Figure 3.3 View of the screw in the metering section of a 24 1 nonvented extruder. The hot melt flows around the downstream side of the mixing pins, where a sharp drop of pressure takes place. The area between the mixing pins and the flight is corroded as a result of steam explosion and acetic acid formation from hemicellulosics of the material.

As shown in Fig. 5. mixing pins are rows of metal pins inserted into the root diameter of a screw, while slotted flights are slots that are cut... 

Mixing pins and slots in the screw flights will improve axial mixing because they achieve a short-term splitting and reorientation of the fluid. The effect of mixing pins on backmixing is illustrated in Fig. 7.160. 

Particle tracking results in a mixing section with elongaticnal mixing pins... 

These results were obtained using a three-dimensional BEM flow analysis. It is clear that one row of mixing pins has limited effect on axial mixing. Backmixing can be improved by varying the spacing between the pins, thus intentionally creating streams of different axial velocities. 

Distributive Peiiet residues melted > t mixing (pins) (shear element) 5 Laminar shear mixing (rapid but slowing)... 

Figure 8.14 Mixing pins arranged radially. Reproduced with permission from D. Boes, Kunststoffe, 1974, 64, 11, 641. 1974, Hanser Publishers)...

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