Vent Bleed

There are several extrusion applications where vented extruders are used. The main purpose of a vent extruder is for devolitizing the polymer of any volatiles, air, and moisture. 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, so that resin dryer do not have to use in the process. Also, certain processes such as in-line sheet thermoforming of PP and light bulk density feedstock re-pelletizing applications where the main purpose is to remove air entrapment.

If the extruder was purchased as a vented extruder and it operated properly until a new replacement screw was installed, it is most likely that the screw design is incorrect.

In order for a vented extruder screw to operate properly, the second metering or second pumping section must out pump the first metering by a minimum of 25 to 30%. Therefore, typically the second metering will be about 30-50% deeper then the first metering. In the past, there was a general “rule of thumb” that the second metering had to be 60% deeper then the first metering. This “rule of thumb” did not take into consideration the viscosity of the polymer and the head pressure of the system.

If the screw is designed 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 out pump the first stage by the desire 25 to 30%. As can be seen in the figure above, 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 should have a melt pump, then the melt pump speed needs to be increased so to drop the suction pressure which in turn will reduce the head
pressure 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 culprit to the problem. If the diverter is not designed properly to allow the polymer to be “folder” back into the screw channel in the vent area of the screw, then the resin will be “pushed” out through the vent port.

So, the next time you have an issue with vent bleed on your extruder; don’t immediately blame the screw design. Do your investigation and determine if the
problem is actually the screw, diverter or head pressure.