Other Melting Screws

Other Melting Screws 7.6.1 Double Wave Screw... 

The form taken by the distortion varies between types of polymer but generally it is helical. With polyolefins a feature resembling the thread of a screw may appear, and with polystyrene the extrudate may form a spiral with other melts, ripples or repetitive kinks like bamboo may be seen. For all melts, at rates well above the critical point, the helical nature becomes obscured by severe distortion which looks quite random. 

Draw Resonance appears as a continuous variation in bubble diameter. As in other extrusion processes, such as profile, it occurs when the melt is stretched too quickly (i.e., a high take-up ratio). Solutions act to reduce the take-up ratio, for example increasing the melt (screw) speed. 

In addition to the screws discussed above, there are other patented screws such as the Fusion Screw [29] which was designed for injection molding application but may be evaluated for compounding polymer blends. Figure 5.47 shows a schematic of the melting, homogenization, and chaotic mixing section of the Fusion screw. 

The characteristics of the various barrier screws are summarized in Table 8.1. The Maillefer screw has many desirable characteristics despite the fact that its melting performance is not quite as good as the other barrier screws. The Ingen Housz screw clearly has the best melting performance however, this is at the expense of geometrical simplicity. 

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. 

After the melting mechanism was described in 1959 by Maddock, variable width barrier section was invented by Maillefer [47] and the constant width barrier section screw design was invented by Barr [48]. Other barrier screw designs were patented by Geyer [49], Lacher [50], and Dray [51]. A good review of screw designs can be found in Chung [8],... 

The pressure build-up which occurs along a screw is illustrated in Fig. 4.2. The lengths of the zones on a particular screw depend on the material to be extruded. With nylon, for example, melting takes place quickly so that the compression of the melt can be performed in one pitch of the screw. PVC on the other hand is very heat sensitive and so a compression zone which covers the whole length of the screw is preferred. 

In the intermittent processes, single or multiple parisons are extruded using a reciprocating screw or ram accumulator. In the former system the screw moves forward to extrude the parisons and then screws back to prepare the charge of molten plastic for the next shot. In the other system the screw extruder supplies a constant output to an accumulator. A ram then pushes melt from the accumulator to produce a parison as required. 

As the screw turns, it conveys the pellets from the feed zone towards the melting zone. A combination of external heating and mechanical work melts the polymer as it is transported towards the metering zone. The metering zone pumps a uniform stream of molten polymer to a forming device, such as a profile die. Other types of extruders that employ two or more screws are commonly used for compounding polymer blends. The principles of twin screw extrusion will be discussed in Chapter 12. 

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