Extruder screen pack

Extruder Screen pack Extrusion conditions Zone 1 Zone 2... 

Rotating microscreen batch, fluid loading 3 to 6 L/s m of submerged area usually 66% area submerged solids loading 0.05 to 0.1 g/s m headless 7 to 14 cm to max. of 45 cm. Clean by backwash at 2 to 5 % volumetric throughput capacity. Screen pack downstream of extruders screen pack area directly proportional to rate of extrusion. Cycle time for the screen to blind > time between shutdowns. For example, screen pack area 0.17 m s/kg and screen pack diameter 20 cm s/kg but must be custom selected for the polymer. 

Variable bottle weight Finer extruder screen pack Increase screw cooling Raise rear extruder heats Decrease extruder rate... 

Variable bottle weight 1. Finer extruder screen pack 2. Increase screw cooling 3. Raise rear extruder heats 4. Decrease extrusion rate... 

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. 

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]...

Film. Nylon film can be produced as either tubular or cast film. In tubular film, melt is extruded through a screen pack and a tubular die, and a... 

Against the forces exerted by the melt flow, the screen packs are backed by a thick, densely perforated steel disk called a breaker plate. The outer rims of the breaker plate and of the screen pack fit into a round recess in the end of the extruder barrel and are clamped in place by the adapter flange of the adjoining piece of equipment, usually that of the extrusion die. To change a clogged screen pack, the die adapter flange has to be removed, the old pack taken out and replaced with a new one, and the equipment reassembled. 

If it is at all possible to do without screen packs they should not be used. Various reasons exist. Complete and continuing displacement of melt from all points in the screen pack is rather difficult. Hundreds of small dead spots are filled with melt as soon as the pack is put into service, and the material in these spots is moved only very slowly, if at all, by the drag of neighboring melts. This action can cause contaminating and degrading of the extrudate. 

If a screen pack with breaker plate is used, disconnect the crosshead (or die) from the extruder and remove the breaker plate and screen. If necessary, appropriate action is taken to clean them (Chapter 17). 

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