Plasticator metering zone

The plastic has a viscosity of 200 Ns/m. Calculate also the shear rate in the metering zone. 

Devolatilization can be used to remove up to 5 percent of volatile impurities from the plastic melt. The first melt conveying (metering) zone builds up melt pressure. Then channel depth is increased abruptly in the vent zone, the melt is decompressed, and volatiles escape through the vent. After this the melt enters a second metering zone, which builds up melt pressure again, and feeds it to the die. 

Transition Zone-. It is the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone. 

Therefore, the plasticating extruder has three zones (1) the feed zone, wherein the conveying of the solid particle is carried out (it is usually cooled in order to eliminate sticking) (2) the compression zone, in which the principal melting takes place and (3) the metering zone, in which the liquid melt... 

The compaction zone has an increasing screw diameter from the small diameter in the feed zone to the larger diameter in the metering zone. The gaps between the screw and the barrel are reduced in the compaction zone. The plastic is melted in the compaction zone due to increased temperature and shear heating from the small clearance between the screw and the barrel. 

The metering zone has a constant diameter and gap between the screw and barrel. Plastic is transported with the rotating screw at a rate specified in the machine process parameters. 

The nozzle is at the end of the metering zone. The melted plastic accumulates at the end of the metering zone with the rotation of the screw. The screw is pushed forward as the plastic is injected through the nozzle and then into the runner of the mold, and finally into the part mold. 

The temperature is increased in the compaction zone where the solid plastic is melted from shear heating and from the increased temperature. The temperature is increased in the metering zone and the nozzle. 

From the compression zone, the melt enters the metering zone. Here the melt temperature is the highest, usually higher than the cylinder barrel surface due to the shearing forces within the plastic mass. 

The metering zone conveys the melt through an antibackflow valve and across the nose cone tip to the front of the barrel. The pressure buildup, from the melt in front of the screw, forces the rotating screw back until it trips a limit switch which stops the rotation. The melted plastic is now ready to be injected into the mold by hydraulic pressure applied to the injection ram. 

The screw has three zones—the feed, compression (or transition), and metering. The feed zone picks up the material from the hopper and feeds it at a pressure sufficient to assure proper functioning of the compression zone. Feed zone conveying is optimized by a deep screw channel and a helix angle appropriate for the coefficient of friction between the polymer and screw. A helix angle of 17.7 deg has been used for many plastics. The channel depth of the feed zone may be approximately three times that of the metering zone. ... 

The metering zone provides the extruder head with plastic melt at constant flow rate and pressure. Ideally, all of the pressure buildup along the... 

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