Vented barrel

Barrels and Heaters These are also similar to those in extruder machines. In recent years, vented barrels have become available to facilitate the moulding of water sensitive plastics without the need for pre-drying. Water sensitivity in plastics can take several forms. If the plastic absorbs water then dimensional changes will occur, just as with wood or paper. The plastic will also be plasticised by the water so that there will be property changes such as a reduction in modulus and an increase in toughness. All these effects produced by water absorption are reversible. 

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

The blends that do not contain hygroscopic components, such as PPE/HIPS in principle do not have to be dried. However the material, when exposed to high humidity, adsorbs surface moisture and small voids and bubbles can appear after processing. These flaws can be avoided and better product appearance and performance can be obtained if the blend is thoroughly dried before or during e.g., using vented barrel) processing. 

In the case of HIPS/PPE blends Vestoran ), neither component is hygroscopic. In the majority of cases, pre-drying is not needed. Vented barrel processing is sufficient. 

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). 

VB vented barrel VLS vapor-hquid-solid (process)... 

Bums fi om hot plastics and gas from the vents on vented barrel machines. 

Volatiles. Many plastics contain small quantities of material that boil at processing temperatures or, they may be contaminated by water absorbed from the atmosphere. These volatiles may cause bubbles, a scarred surface, and other defects. See Chapters 2 and 9 for methods of removing volatiles (vented barrels and dryers). 

Moisture retention in and on plastics has always been a problem for all processors. Surface moisture or moisture absorbed within the plastic can cause splay, an unsightly surface defect of the molded part, and reduce mechanical properties. The increased use of hygroscopic plastics (see Chapter 9) also requires care and the assurance of proper drying of material via the usual technique, using dryers and/or vented barrels (Fig. 2-11). There are advantages of using vented barrels as opposed to the more familiar dryers (100) ... 

During start-up and operation of a vented barrel, make sure that the barrel in front of the vent is at a temperature above the freezing point of the plastic. To help retain heat in the metal barrel, thermal insulation should be used to cover exposed metal surfaces (this action should be taken even if the barrel is not vented). 

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...

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