Barrier Screw Developments

The general form of the barrier screw has in general become established. They usually have a wide shallow solids melting channel and narrower relatively deep melt channel. The general needs of start and termination of barrier flight have also been established. Compression ratio is 2 to 2.5 [20], and the barrier gap (0.5 mm). Shear and pin elements are added as necessary. It has been generally established that the result will be higher and consistent output rates in most cases, combined with reduced melt temperatures. 

With the apparent approaching maturity of the Maillefer and Uniroyal derived barrier screw developments, further advances will most probably be in more radical innovations. Their potential diversity is illustrated by the following examples. 

1 Barrier Screw with Divided Solids Melting Channel 

2 Barrier Screw with Spiral Barrel Grooves for Overall Length [27, 28] 

This barrier screw appears similar to that in descrihed [13]. With screw pitch decreased within the feed zone, air cooling is sufficient to control the feed zone temperature. Shearing elements plus a pin/pineapple mixer follows the barrier melting zone. 

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In the past barrier screws were used only with PET pellets because pellets are not susceptible to air entrapment. Powders, however, are susceptible to air entrapment and do not process well on conventional barrier screws. The internally vented barrier screw developed by Eastman achieves higher throughputs of PET powder. This screw design can also be used for other hygroscopic resins like ABS. 

An important development in screw design was the barrier screw. The primary reason for a barrier screw is to eliminate the problem of solids bed breakup for more efficient melting. They have been around for over a quarter century. Original developments were for extrusion, but latter they were used to solve problems in injection and blow molding. There are many different patented barrier screw designs that under the broad claims of the Geyer or Uniroyal U.S. Patent No. 3,375,549 that expired in 1985.3>143... 

The following analysis of the melting performance of various barrier screws is based on the analysis developed by Meijer and Ingen Housz [27]. This analysis provides a clear and logical approach to the determination of the melting capacity as a function of the barrier section geometry. 

With a demand over the intervening years for higher output rates, the possibility of incomplete melting due to solids bed break-up needed attention. This lead to the development of melting devices and barrier screws. 

With the original situation that the barrier screw was patented in Europe by Maillefer, a Swiss cable machinery manufacturer, there was little interest in Europe in this technology other than by insulated wire and cable producers. When the patents lapsed, interest focused on designs emanating from developments which had taken place in the USA. 

With the barrier screw patented in the USA by Uniroyal, a rubber product manufacturing company, there was scope for development by screw supplying companies, although perceived infringements were vigorously pursued [8]. Compared with a conventional single channel screw, the introduction of a second channel introduced a large number of variables to choose from ... 

The first successfiil tests to combine barrier screws (1) with grooved feed zone barrels were carried out at the end of the seventies, begin of the eighties (2), see figure 1. Since then this system has been continuously fiuther developed and analyzed. The specific, screw speed related throughput til /n, and the relative length L/D (3) and the screw speed (4) have been increased. 

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. 

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). 

Single-screw extruders have been widely used for blend preparation however, they do not offer sufficiently high stress levels to improve mixing thus, special designs of screws have been developed such as those with mixing heads or barrier zones that increase residence time and enhance blend mixing. 

A modified melt blending method has been developed for preparing exfoliated nanocomposites of poly(m-xylylene adipamide) with sodium montmoril-lonite [100]. There, an aqueous solution of sodium montmorillonite was blended with the polymer in a twin-screw extruder. This kind of mixing ensures that the silica nanoparticles are exfoliated in the polymer matrix through fixing the nanoparticles within the polymer matrix just as they are in water. Oxygen permeation data show enhanced the barrier properties of the nanocomposites. 

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. 

PLA and its copolymers have several end-use applications. The major applications include short shelf-life products such as plastic bags for carrying household waste, overwrap and lamination films, barriers for sanitary products and diapers, planting, disposable cups and plates, and so on [120]. Hydrolytic degradability of PLA has generated a lot of interest for biomedical applications such as drug delivery systems [134-142], protein encapsulation and delivery [143-145], development of microspheres [146-154], hydrogels [155], bone screws [156], sutures [157-161], scaffolds [162], and so on. However, the unreinforced PLA has certain limitations and that is why a lot of researches have been dedicated to... 

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