Pin mixing section

Figure 3.29 Pin mixing section on the screw of a single screw extruder.

Pin Mixers. Pin mixers have been in use for many years and come in many shapes and sizes. Often the pins are circular, but other shapes can be used as well, eg, square, rectangular, and diamond-shaped. Two examples of pin mixing sections are shown in Figure 53. 

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

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. 

Erwin [13] commented, These mixing sections are most remarkable for their variety. Vanes, pins, strange ducts of unusually shaped pieces of metal are incorporated in the melt channel to improve mixing. ... 

Similarly with pins, after the streams are split, they re-unite behind the pins so that, longitudinal mixing is only slightly improved compared to a screw devoid of a mixing section (Figure 8.15(a)) [16]. 

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. 

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]...

For a variety of reasons the mixing in the extruder may not be uniform and the melt inhomogeneous as a result. With a view to preventing this the root of the screw may be fitted with pins that protrude into the material to help break-up laminar flow or, more usually, the tip of the screw may be modified with mixing elements or sections (in effect the screw is lengthened, so that a section of rings, cams or kneading discs can be fitted to it). 

In a raised bridge device, the explosive or initiating mix completely surrounds the bridge wire. Most raised bridges are made of fine, round cross-section wires, but some are in the form of ribbons. The bridges in these devices are either soldered or welded to the conductor pins. 

Miiser [25] examined yield of much larger tips modeled as incommensurate Lennard-Jones solids. The tips deformed elastically until the normal stress became comparable to the ideal yield stress and then deformed plastically. No static friction was observed between elastically deformed surfaces, while plastic deformation always led to pinning. Sliding led to mixing of the two materials like that found in larger two-dimensional simulations of copper discussed in Section IV.E. 

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