Cavity transfer mixing

Figure 3.32 Distributive mixing sections (a) Pineapple mixing section, (b) Cavity transfer mixing section.

Another distributive mixing section is the cavity transfer mixing (CTM) section shown in Fig. 8.100. 

The cavity transfer mixer (CTM) is a distributive mixing device used as an add-on unit to extruders the barrel and screw have hemispheres cut out of their surfaces to give an overlap of half a pitch. 

Fig. 23. Screw sections for enhancing distributive mixing in single-screw extruders (a) Dul-madge mixer, (b) Saxton mixer, (c) pin mixer, (d) pineapple mixer, (e) cavity transfer mixer, (f) slotted screw flight, (g) Kenics and Ross ISG static mixers [148]...

Add-on s — Barmag add-on torpedo, Staro-mix , Maddox, Cavity Transfer Mixer (CTM), Twente Mixing Ring (TMR), the Extensional Flow Mixer (EFM), Dynamic Melt Mixer (DMX), static or motionless mixers (SM) from Koch, Ross, or Kenics, etc. 

Figure 3.13 Mixing enhancers for single-screw extruders [20]. (a) Pineapple, (b) Dulmage, (c) Saxton, (d) pin, and (e) cavity transfer.

Cavity-transfer mixer n. A two-piece device installed at the end of an extruder screw to accomplish both distributive and dispersive mixing. The stator is a barrel extension into whose inside surface is machined an array of many hemispherical cavities. The rotor is a screw extension whose exterior is similarly contoured. The lands of rotor and stator have the usual close clearance of screw and barrel. As the melt steam passes through, it is smeared between the lands and is repeatedly cut into small globs and recombined, passing from rotor to stator, stator to rotor, until it emerges. 

See also Dulmadge mixing section, Maddock mixing section, and cavity-transfer mixer. 

A further alternative is the cavity transfer distributive mixer, which comprises a rotor with hemispherical depressions located within a closefitting stator, also configured with hemispheres. Polymer melt continuously transfers between rotor and stator cavities, and experiences substantially increased shear strain, resulting in increased mixing. 

Internal modifications. Here belong the screw modifications, as well as the devices that need to be attached to the screw, e.g., Barmag s torpedo, RAPRA s cavity transfer mixer (CTM), a multi-screw planetary unit. All three examples also require modification of the corresponding barrel section. By contrast, the developed in the University of Twente torpedo with perforated, freely rotating sleeve, can be used without affecting the barrel, e.g., to improve mixing of the injection molding extruders. 

Further improvements in the quality of mixing obtained can be brought about by modifying the screw or by the use of special mixing sections, these usually being located in the metering section of the screw. Typical examples of such sections are pins, barriers, Barr type mixing sections and cavity transfer mixers. 

Figure 2.24 Photomicrographs showing improving mix quality over five cavity rows of a cavity transfer mixer nsing HDPE. Adapted with permission from R.W. Shales, Mixing of Thermoplastics in Single Screw Rextruders, Department of Chemical Engineering, University of Bradford UK, 1989. [PhD thesis])...

Figure 2.28 Diagrammatic representation of mixing action. Reproduced with permission G.M. Gale, Development of the Cavity Transfer Mixer for Plastics Extrusion, Rapra Members Report No. 104, Rapra Technology, Shawbury, Shrewsbury, UK, 1984, Figure 8.8. 1984, Rapra Technology)...

Photomicrographs in Figure 7.19 of cross sections of strands extruded at 80 rpm show no significant improvement in mixing by using this particular barrier screw. The cavity transfer mixer is covered in Section 9.4. 

A technique which avoids this problem is to decouple mixing from extrusion. An arrangement used by the author is shown in Figure 8.17 [17]. The primary objective was to obtain data on the influence of cavity size of the cavity transfer mixer (CTM) (described in Section 9.3). At the same time a variety of mixers were evaluated in an attempt to compare this particular mixer with a range of known mixing devices. An equal length of screw and a plain annulus (like the Couette model in Chapter 2) were used as controls. In addition to mixing performance, data was required on power consumption, temperature rise, and pressure drop. 

Figure 8.23 shows that the two cavity transfer mixers had the best mixing performance with either no visible striations or virtually none at 20 rpm. The mixing performances of the small and large cavity versions were very similar. 

In order to improve the homogeneity of the compound, a device is sometimes attached to the screw head. Among these the Rapra cavity transfer mixer (CTM) is very effective in improving distributive mixing with least resistance to the flow of compound. Figure 12.7 [4]. The improvement of dispersion is not intended. 

RAPRA cavity transfer mixer F>S- 4.4 Typical designs of mixing zones... 

Electrochemical template-controlled sjmthesis of metallic nanoparticles consists of two steps (i) preparation of template and (ii) electrochemical reduction of metals. The template is prepared as a nano structured insulating mono-layer with homogeneously distributed planar molecules. This is a crucial step in the whole technology. The insulating monolayer has to possess perfect insulating properties while the template has to provide electron transfer between electrode and solution. Probably, the mixed nano-structured monolayer consisting of alkylthiol with cavities which are stabilized by the spreader-bar approach [19] is the only known system which meets these requirements. 

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