Second flight

Barrier melting sections are constructed by positioning a second flight (or barrier flight) in the transition section such that the solids are maintained on the trailing side and the molten resin on the pushing side. A schematic of a cross section of a barrier melting section is shown in Fig. 6.22. The resin is melted as discussed in Section 6.3.1 in the solids channel of the device. The resin that is melted near the... 

British 4-in. trench mortars, called Stokes mortars (Figure 2-11), provided a solution to some of the problems with Livens projectors. The Stokes mortar did not require extensive preparation and could be moved as needed. Since it was not rifled, the range was only 1,200 yd, which meant about a 14-second flight time. The small size of the shell only held about 6 to 9 lb of agent, but experienced gunners could fire 25 rounds per minute. American troops used both Livens projectors and Stokes mortars during the war. Ordnance officers tried making their own Stokes mortars, but none reached the front before the end of the war. 

At the beginning of the transition (compression), a second flight Is started. This flight Is called the barrier or Intermediate flight, and It Is undercut below the primary flight OD. This barrier flight separates the solids channel from the melt channel. 

For this reason, many blown film screws today include a barrier flight. This second flight in the melting section of the screw helps to ensure that no unmelted material exits from the barrel. Because blown film products are very thin, unmelts can be quite problematic, both structurally (as stress concentrators) and aesthetically. 

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. 

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