Screen packs

After extmsion, molten polymer is filtered through screen packs. The polymer may be separated iato different melt flow ranges to produce more uniform product grades. 

Film. Nylon film can be pioduced as eithei tnbulai oi cast film. In tubular film, melt is extmded through a screen pack and a tubular die, and a... 

Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

Figure 9-24B. Packing factors (screen packing and random dumped packing). Used by permission of U.S. Stoneware Co. (now, Norton Chemical Process Products Corp.).

Fig. 8-28). The screen pack is a filter that restricts unmelted plastic and/or contaminants from entering the die (6). 

The build up of pressure is aided by the use of a breaker plate, which is a thick disk of perforated metal positioned just downstream of the tip of the screw. A screen pack, comprising layers of woven metal mesh, is placed on the upstream side of the breaker plate. The screen pack increases back pressure within the metering zone and acts as a filter to catch any extraneous material that may have entered the extruder. 

In this test a specific amount of pigment in prepared form is forced through a fine screen pack by an extruder and the pressure rise upstream of the screen pack is measured. The calculation is done by the formula ... 

A 203.2 mm diameter smooth-bore extruder running a PS resin experienced low rates when extruding to a complicated downstream sheet line. The extrudate flowed from the extruder to a screen pack system and then to a sheet die. The line was able to produce acceptable product but at reduced rates. The reduced rates increased the cost of manufacturing the product and caused the plant to miss shipment dates to its customers. At a screw speed of 110 rpm, the rate was measured at 1750 kg/h for a specific rate of 15.9 kg/(h-rpm). Under all processing condi-... 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

It is important to be able to predict at just what level of t in the inlet cone the slurry begins to consolidate into an ice bed with brine flowing through it. If the zone at which consolidation begins is well below the level of the screens, packed ice may back up and fill the inlet pipe. The pressure drop through the ice packed in the inlet cone is important for specification of the pumping head required. 

Gas-side mass transfer in rotating packed beds does not show the same level of enhanced performance as liquid-side mass transfer. Average volumetric gas mass transfer values for a wire screen packing increased with gas flow rate but decreased with increased rotor speed. Compared to a packed tower, the RPB... 

In another study of gas-side mass transfer-limited absorption involving S02 absorption into a sodium hydroxide solution using a wire screen packing, the overall mass transfer coefficient was found to be lower than reported data for packed towers (16). Replacing the wire screen packing with two parallel rotating plates significantly enhanced the mass transfer performance. 

Such a level of unmelted polymer can frequently be tolerated, because the screen pack and the die itself provide additional opportunities for melting and mixing. 

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