Vent zone

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

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. 

The optimum temperature prohle should be selected according to the ratio of shot size to machine capacity, screw design, mold and part design, as well as cycle time. Barrel temperature prohle should be set so the material melts gradually — lower near the feed, hotter near the die zone. However, for vented barrel machines, a relatively hat temperature prohle is recommended to ensure that the polymer is melted by the time it reaches the vent zone. Reverse temperature prohles are also sometimes used, primarily to compensate either for improper screw design or with short L/D ratios, and to reduce machine amperage or torque requirements. 

Contemporary extmders frequently have a venting zone composed of suitable outlets stationed at about 2/3 of the screw length, after which the melt is again pressurized in its advancement towards the die. It is also customary to cool the interior of the screw thus avoiding material sticking to the surface of the screw. (Once it sticks to the screw it will not advance to the exit.)... 

A vented screw and barrel assembly can be used for this material, so as to eliminate drying. However it is important to use the correct temperature profile on the vented barrel assembly, for example, by keeping the vent zone lower than the other zones i.e. 230 to 250° C (446 to 482° F) so as to ensure that the moisture is exhausted from the vent section in order to produce moldings free from the characteristic splash/splay marks. 

Po and Pb cycling in a hydrothermal vent zone in the coastal Aegean Sea was studied. 

Pb levels in sinking particles from the control site tend to be higher than those measured in the vent zone, as demonstrated by the lower Po/ Pb ratios (3.3-8.5) observed on particles in the control site. These Pb levels (0.01-0.23 Bq/g) are comparable with data reported for the northwestern Mediterranean Sea. The hydrothermal emissions in the shallow Aegean Sea off Milos Island do not have any major effect on the biogeochemical cycling of the natural radionuclides Po and Pb in the water column (Boisson et al. 2001). 

Extruder screw profile design is critical to HME devolatilization. For optimal devolatilization performance, the screw must completely melt the material fed into the extruder in a mixing zone of the screw, prior to the first vent zone, and this mixing zone must be filled with molten material. This melt seal upstream of the first vent zone will ensure that any vacuum applied to the first vent zone will not leak past the first mixing zone and pull powders from the feed zone up into the vent. The melt seal downstream of the first vent zone can be formed by adding another mixing... 

The extruders were of a similar size, but the one used for the HDPE/filler trials was longer to accommodate a vent zone and had an alternative conventional screw. The PBT/pigment trials used a screw with a mixing element following the barrier section. As the two investigations used screws of similar diameters and together covered a range of screw features, the essential details are compared in Table 13.1. 

With screw profile Bl, all ingredients (PP, PA6, MA, styrene and peroxide) are fed to the extruder together through the main hopper. The barrel temperature is set at 240 over the entire extruder. A venting zone (port 1) is placed near the die to remove volatile substances such as non-reacted residual monomers. 

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