Screw venting ratio

For ASA/PVC blends (Geloy ), single screw vented extmders are recommended with L/D = 24, and compression ratio = 2.1-2.5. PVC degrades rapidly, thus residence time at 200°C should never exceed 1 min. For normal processing at temperamres well below 200°C, residence time should not exceed 5 min. 

The above dr5dng instructions apply to extrusion grades. Single-screw, vented extruders are recommended (L/D = 24 l)with2.1-2.5 1 compression ratio. Melt temperature 180-195°C. 

Improper pump ratio for a two-stage screw Flow of resin out the vent opening 13.1... 

Single-screw extruders for compounding have relatively long L/D ratios (from 32 1 to 40 1), usually with several barrier sections Venting is almost always employed to remove the molten polymer of unwanted vapors (Chapter 3). 

The vent in the extruder used in the HIPS extrusion process allows the removal of volatiles from the melt. The classical vented screw extruder optimized for HIPS has a length-to-diameter ratio of 36 and a compression ratio of 2-2.5. 

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. 

The channel depth decreases in the extruder compression section. Screws are designed with different numbers of turns of flight, and different compression ratios (the ratio of the channel depth in the feed section to that in the metering section) to suit the rheology of the polymer being extruded. The pressure generated here either squeezes out any gas bubbles, or causes gas to dissolve in the melt. Unless a foamed extrusion is required, bubbles must not be allowed to reform in the melt when it returns to atmospheric pressure after the die. Consequently, there may be a vent to the atmosphere or to a vacuum line just before the compression section to aid degassing. 

A vented screw and barrel assembly may be used to eliminate drying typical L/D ratio for a vented screw is 20.1 or greater. In order to melt the polymer prior to the vent, the temperature of the feed section should be set as high as material feeding will allow. As a rule the temperature set at the vent section is lower than that of the feed and nozzle sections, so as to restrict the heating effect on the molten polymer and to allow the volatiles to escape through the vent. 

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