Wave screws

Table 13.8 Screw 1 Channel Dimensions for a 114.3 mm Diameter, 30 L/D Barrier Double Wave Screw for Running a Starch-Resin Formulation...

Pan, P., Vlachopoulos, ]., Smith, N., and Sheth, H., Computer Simulation of Melt Plow in Wave Screws, SPE ANTEC Tech. Papers, 44, 97 (1998)... 

The first wave-dispersion-type screw was developed and patented by Kruder in 1975 [18], and the device was trademarked as the Wave screw. Numerous other wave dispersion screws were developed later based on Kruder s design. The term wave dispersion screw refers to screws with metering sections that have two or more channels with a flight between them that is selectively undercut to allow the dispersion of solid polymer fragments and molten resin. Several commercially available screws utilize this type of technology and are discussed in this section. These screws include Double Wave screws, Energy Transfer screws. Variable Barrier Energy Transfer screws, DM2 screws, and Fusion screws. 

Figure 14.6 Typical channel depths for a 114.3 mm diameter Double Wave screw. The channel depths in the peaks of the Double Wave section and the undercut on the secondary flight were 5.0 mm...

Figure 14.7 Schematics of a Double Wave screw channel a) schematic of a Double Wave section. The unfilled flight is the secondary flight, and it is undercut with respect to the filled primary flight, b) and c) are cross-sectional views perpendicular to the flight. Section c) is about 1 diameter downstream from section b). The flow direction is out from the page surface...

The performance of Double Wave screws was provided earlier by Kruder and others [20-22]. For these trials, the extrusion process was capable of higher rates and lower discharge temperatures than well designed conventional single-flighted screws. Three-dimensional flow simulations for a Double Wave and a Triple Wave section were performed by Fan [23] and Perdikoulias [24], These simulations showed the complex nature of the flows, including the acceleration of the flow as it... 

The success and breakthrough innovation of Kruder s wave screw was further enhanced by coupling the wave technology with an upstream barrier section and a material reorientation section. The reorientation section was positioned between the barrier section and the wave section. This spin-off technology was patented by Womer, Buck, and Hudak [36] In 2004. Other improvements were patented later [37, 38]. 

Kruder, G.A. and Nichols, R.]., New Applications of Wave Screw Designs for Ultra High Output Extrusion, SPE ANTEC Tech. Papers, 31, 137 (1973)... 

Some of the more widely used high-performance screws include the Energy Transfer (ET) screw [19], the Variable Barrier Energy Transfer (VBET) screw [20], the Double Wave screw [21], Stratablend [22] and Stratablend II [23] screws, and the Unimix [24] screw. Other devices can, however, act as a trap for solid materials from the melting section. These devices include Maddock-style mixers [25], spiral dams, and blisters. They will be briefly discussed in this section as many of them have been the subject of many articles on secondary mixing processes. 

The double wave screw (Fig. 5.17 ) provides both distributive and dispersive mixing. This screw has a second flight in the metering zone. The depth of both channels alternately increases and decreases. When the channel depth is shallow, melt is sheared and can be transferred into the adjacent channel. 

This screw was developed by Kruder of HPM [39] and is basically an extension of the single channel wave screw [40]. Polymer is forced over the center barrier flight by a cyclic variation of the channel depth. When one channel is increasing in depth, the other is reducing. When the first channel reaches its maximum depth, the other channel reaches its minimum depth. Then the first channel starts to reduce in depth and the other channel starts to increase in depth. This process is repeated many times. This screw design improves mixing performance, but the screw is relatively expensive to manufacture. 

Other mixing screws have been developed in the past to disrupt the solid bed and mix unmelted with melted material. The double wave screw shown in Fig. 8.80 breaks up the solid bed and mixes the material by forcing a cross-channel flow by the cyclic variation in channel depth. The principle of the double wave screw was used by Barr in his energy transfer (ET) screw [90]. The ET section is basically a double wave section with occasional undercuts in both flights to force a cross-channel mixing between the two channels. Modeling of the ET mixer is discussed in Section 12.4.3.2 see also Figs. 12.23 to 12.25. 

In variable depth mixers, the channel depth of the mixer is varied to obtain improved mixing. An example of a variable depth mixer is the double wave screw shown in Fig. 8.81 and the energy transfer screw see Fig. 12.23. Another example is the Pulsar mixing section shown in Fig. 8.103. 

Other Melting Screws 7.6.1 Double Wave Screw... 

Figure 7.22 Double wave screw flow arrangement.

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