Vented barrel/screw

Figure 6.11 Typical single-screw extruder with a vented barrel.

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. 

For PET/elastomer blends (Rynite ), general purpose, gradual compression screws, with L/D of 18 are suitable. Three zone heating control and separate temperature control for the nozzle are recommended. Proper drying provides a better solution than vented barrels for ensuring adequate absence of moisture. 

IM process. Important in practically aU operations are factors such as using unworn screws and properly dried plastics. Special dryers vented barrels are required in drying the TP hygroscopic materials (such as PC, PMMA, PUR, PET). 

Changing the initial melt temperature in either direction may solve the problem. With a barrel and screw, it is important to study the effects of temperature changes. Another approach is to increase the pressure in processes that use process controls. Particle size, melt shape, and the melt delivery system may have to be changed or better controlled. A vacuum hopper feed system may be useful. With screw plasticators, changes in screw design may be helpful. Usually, a vented barrel will solve the problem. 

Figure 2.25 Two-stage screw and vented barrel used for degassing...

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. 

When single-screw extruders are starve fed (Fig. 5.12d), plastic particles do not immediately fill the screw channel. As a result, the first few channels of the feed zone lack the pressure required to compact the polymer particles. Particle conveyance in the imfilled channels is not as steady as transport with filled channels. Consequently, metered feeding is seldom used with single-screw extruders. Such feeding can be used to reduce the motor load, limit temperature rises, add several components through the same hopper, improve mixing in singlescrew extruders, control flow into vented barrel extruders, and feed low-bulk-density materials. 

There are many manufacturers (over 50) of twin screw compounders. These manufacturers offer a variety of ancillary equipment such as loss-in-weight feeders, twin screw side feeders, vent staffers, screw segment disassembly units, screen changers, etc. There are many different screw and barrel materials available with special wear and/or corrosion-resistant characteristics. This is important because TSEs are often used with abrasive and corrosive materials. 

There are two main considerations particular to PVC extrusion. First, in most cases, the feedstock will be dryblend, and there is a need to remove air and volatile components as the material fuses. To accomplish this, a vented barrel was developed in the early 1960s. The screw had two compression areas, one before the vent to start the fusion and the other after the vent to finish the melting and push the melt through the die. Both single- and twin-screw extruders today for PVC dry blend have a vent connected to a vacuum for gas removal. 

Vented barrels typically have slower screw recovery since there is a decompression zone in the middle that impedes resin conveyance forward. (See machine/screw supplier guidehnes for optimum barrel profile to minimize the effect of the vented barrel.)... 

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