Poor screw design is usually to blame for vent bleed on a two-stage vented extruder. But it may not be the culprit. If during the screw-design phase all of the proper process data is collected, and a sample of the resin being processed is supplied so that a rheological study can be done, it is very doubtful the vent bleed is due to the screw design. This is especially true if the screw if installed into an existing vented barrel. Instead, have a process engineer inspect the vent diverter to make sure that it has been correctly configured.. If the configuration of the diverter is incorrect, or if the vent port is not located in the proper location, then vent bleeding can occur.
The port should be located approximately 2/3 of the way between the feed port and the discharge end of the barrel. Also, the vent port on a right-hand turning extruder needs to be located so that the feed port is to the right, the discharge of the extruder is to the left and you are looking into the vent port when standing beside the extruder.
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; firstname.lastname@example.org; twwomer.com