The CRD Mixer

As discussed in Section 7.7.3, dispersive mixing in shear flow is substantially less efficient than in elongational flow. Important requirements for dispersive mixing elements were formulated by Rauwendaal [66] they are  

The mixing section should have a high stress region, HSR, where the material is subjected to high, preferably elongational, stresses to break down agglomerates and droplets. 

The HSR should be designed such that the exposure to high shear stresses occurs only for a short time to avoid excessive power consumption and melt temperature 

All fluid elements should experience the same high stress level multiple times to achieve uniform and efficient mixing. 

If we analyze current dispersive mixers based on these requirements, we find that most current dispersive mixers only meet these requirements partially. 

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As expected, some particles flow over the modified flight while others flow through the groove. Again, this effect will increase the distributive and dispersive mixing capability of the mixer. Simulation of the CRD mixer is discussed further in Section 12.4.3.6 (see Figs. 12.48 to 12.50). 

The second application of the CRD mixer was in the production of color concentrates (CC) on a single screw compounding extruder. The machine was a two-stage extruder and two CRD mixers were used at the end of the first and second stages. The CC quality was improved to the point that dispersing agents could be eliminated from the compounds run on this extruder. CRD mixers are used in single screw extrud-... 

The CRD mixer described above is normally designed to break up agglomerates into much smaller aggregates or even individual particles the final size is typically at the micron or submicron level. However, the same principle can be applied to break up large clusters of unmelted polymer particles into smaller clusters or individual particles (usually pellets) at the millimeter level. A number of variations of the CRD mixer have been developed with the specific objective to break up clusters of unmelted particles-these mixers are called the Cluster Buster or CB mixer [89]. An example of the CB mixer is shown in Fig. 8.94. 

Computed particle tracking in the CRD mixer with a high Ap simulated across the mixing section... 

The CRD Mixer. Current dispersive mixers have two important drawbacks. One, they rely mostly on shear stresses to disperse materials rather than elon-gational stresses dispersion is more effective in elongational flow than in shear flow. Two, the material passes over the high stress region only once. Advantages of elongational flow over shear flow are... 

The CRD mixer can disperse gels shear-based mixers cannot disperse gels. 

In a wide range of single screw mixers reviewed by Schnt [49], the designs and applications were very diverse. The potential for a reduction in gels in extruded films was of specific interest. The results reported for the CRD mixer were varied, but this may have been dne to the type of gels present (see Section 14.11). 

A relatively recent development in static mixers is the Dispersive/Distributive Static Mixer (DDSM) [308]. This mixer is specifically designed to generate strong elonga-tional flow for improved dispersive mixing. The DDSM is an extension of the CRD... 

The MW results for the Maddock screw, general-purpose screw, and barrier screw do not differ markedly for each barrel temperature profile, the CRD screw achieved the best MW results, resulting in the highest MW values. In all trials with the CRD screw the MW values were above the specification limit. As a result, this screw was selected for the production process. Figure 8.115 shows a photograph of a slotted CRD mixer. 

Most extrusion dispersive mixers are ineffective because shear is the main mode of deformation and the plastic melt is exposed to a high stress region only once. A mixer developed to overcome these shortcomings, is the Chris Rauwendaal Dispersive (CRD) mixer [100-104] shown in Fig. 12.48, see also Section 8.7.1.1. 

The last column in Table 3 is the most important when it comes to distributive mixing effectiveness. The Dulmage, Saxton, CTM, TMR, and CRD all do very well in this category. The Saxton and CRD mixers combine good mixing with low cost. 

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