We've already covered vent bleed in another installment of "tips." Let's delve into the issue in some more detail.
The main purpose of a vent extruder is remove any volatiles, air, and moisture from the polymer. There are two main reasons that the vent port on the extruder will “bleed” or allow polymer to be expelled out through the vent hole. The first reason can be a poor screw design and the other is a poorly designed vent diverter.
The typical resins that are vented are HIPS and ABS, making use of resin dryers unncessary in this applications. Also, venting is used in certain processes, such as in-line sheet thermoforming of PP and light bulk density feedstock re-pelletizing applications, where the main purpose of the vent is to remove entrapped air.
There is a simple way to determine the cause of vent bleed. If the extruder was purchased as a vented extruder and it operated properly until a replacement screw was installed, it is most likely that the cause of the bleed is incorrect design of the new screw.
In order for a vented extruder screw to operate properly, the second metering or second pumping section must outpump the first metering by a minimum of 25 to 30%. Therefore, typically the second metering section will be about 30-50% deeper than the first metering section. In the past, there was a general “rule of thumb” that the second metering section had to be 60% deeper then the first metering section. This “rule of thumb” did not take into consideration the viscosity of the polymer and the headpressure of the system.
If the screw is design properly, and during the design stage of the screw the drag flow-pressure flow equation is used, it can be determined if the second stage of the screw can outpump the first stage by the desired 25 to 30%. The screw channels in the vent section of the screw must only be about 50% full so that there is an area in the screw channel for the volatiles to escape.
If the system has a melt pump, then the melt pump speed needs to be increased so to drop the suction pressure, which in turn will reduce the headpressure on the screw. This will normally stop the vent bleeding.
But, the screw design may not always be the source of the problem. If the extruder is a new system or if it is an existing extruder which is being converted over for a vented application, the vent diverter may be the cause of the problem. If the diverter is not designed properly to allow the polymer to be “folded” back into the screw channel in the vent area of the screw, then the resin will be “pushed” out through the vent port.
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com;.
