The Barr Screw

The initial part of the barrier section is the same as the Maillefer screw. However, when the melt channel is sufficiently wide, the barrier flight starts to run parallel to the main flight. 

Interestingly enough, Willert s description is essentially identical to the patent of Hsu [32]. This patent was filed in January 1973 and issued January 1975. Screws covered by Hsu s patent used to be sold under the name Maxmelt Screw by the Plastics Machinery Division of Hoover Universal. Coincidentally, Hsu used to work at Hartig at the time when the basic concepts of the Barr screw were being devel- 

The melting performance of the Barr screw can be analyzed by the same procedure followed for the Maillefer screw. The initial portion of the barrier section can be analyzed just as a Maillefer screw. If Zji is the length of the initial Maillefer portion of the barrier section, the solid bed velocity at z = Zj, is approximately  

In the parallel portion of the barrier section, the width of the solids channel is constant. The highest melting performance will be reached if the width of the solid bed fills the entire width of the solids channel. If the solid bed velocity is assumed constant, the total melting length can be found by using the equations derived from the standard extruder screw see Eq. 7.116. The total melting length for the parallel barrier portion is simply  

The total melting of the barrier section is the sum of Eqs. 8.99 and 8.100. This results in the following expression for the melting length of the Barr screw  

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In comparison to tbe melting length of the ideal compression screw, the melting length of the Barr screw is ... 

The screw geometry is very similar to the Barr screw with the one major difference being an abrupt change in helix angle in the main flight at the point where the barrier flight is introduced. This allows the width of the solids channel to stay just as wide as the full channel width of the feed section. Obviously, this is done in an attempt to maintain the solids channel as wide as possible. However, this also causes an abrupt change in the direction of the solid bed velocity, and this can lead to instabilities. 

If typical values are used for cpb and cpf, the melting length of the Dray and Lawrence screw will about 10 to 20% longer than the ideal compression screw. The melting performance of the Dray and Lawrence screw is thus about the same as the Barr screw and slightly better than the Maillefer screw. A patent on this barrier screw... 

This means that the melting performance of the Kim screw is slightly lower than the Dray and Lawrence screw and the Barr screw. It has an advantage over the Dray and Lawrence screw in that the transition from feed to barrier section occurs more smoothly. However, at the end of the barrier section the same difficulty arises as with the Dray and Lawrence screw. 

Figure 14.5 Schematics for the Barr-ll and Barr-Ill barrier screw technologies a) full-length schematic of a Barr-ll barrier screw, b) expanded view of the barrier entry section of a Barr-ll screw, c) full-length schematic of a Barr-Ill barrier screw, d) expanded view of the barrier entry section of a Barr-Ill screw, and e) expanded view of the crossoverflow rearrangement section at the exit of Barr barrier screws (Schematics courtesy of Jeff Myers of Robert Barr, Inc.)...

There are many different barrier screw designs, each offering unique advantages. They include the Uniroyal (the original barrier screw) the MC-3 of Hartig the Efficient of New Castle Ind. the Barr 2 of R. Barr Inc. the VPB of Davis Standard the Willert II of W. H. Willert Inc. and the Double Wave of HPM Corp. 

Mixing screws, such as the Barr have been successfully used to complete the melting. 

Figure 8.21 Schematic for Maddock-style mixers a) a mixer with the flutes aligned in the axial direction, and b) a cross-sectional view perpendicular to the screw axis showing the clearance for the mixing flight (courtesy of Jeff A. Myers of Robert Barr, Inc.)...

Figure 8.23 Schematic of blister ring mixers a) a blister mixer positioned in a metering section, b) blister section positioned at the discharge end of the screw near the tip, and c) the cross-sectional view showing the region of annular flow between the screw and barrel wall (courtesy of ieff A. Myers of Robert Barr, Inc)...

Figure 8.30 Cross-sectional views [53] of the extrudate samples using a conventional screw with segmented tip for the testing of different types of dynamic mixers a) conventional screw with no mixer, b) Twente Mixing Ring, and c) Barr Fluxion ring mixer. The extruder was operated at a screw speed of 80 rpm and a letdown ratio of 220 1 of a white pigmented ABS resin to a black color concentrate...

Two screws were designed with Energy Transfer (ET) sections from Robert Barr, Inc. [31], and the screws were labeled by Dow as ET-1 and ET-2. The original... 

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. 

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. 

Barres [113] used NIRS to monitor the chemical modification (catalyzed esterification) of a molten ethylene-vinyl alcohol copolymer by octanoic acid in a twin screw extruder. Extrusion samples were characterized off-line, for calibration purposes, with the use of H-NMR and residual free acid titration. Multivariate PLS calibration was performed successfully to predict the analyte concentrations. 

Figure 7.23 Energy transfer and VB energy transfer screws. (Reproduced with permission from J.A. Myers and R.A. Barr in Proceedings of the 60th SPE Annual Conference - ANTEC 2002, San Francisco, CA, USA, 2002, Paper No.251.

Figure 12.3 (A) Melting mechanism of single-screw extruders as determined by Maddock. (B) Schematic diagram of barrier screw cross section showing separate solids and melt channels separated by the barrier flight as defined by Barr.

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],... 

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