Vent flow occurs when polymer and/or compound flow out of a vent opening preventing air/gases from escaping out of the extruder. Not only can vent flow cause product defects but it can create an undesirable mess and can sometimes be an operator’s toughest challenge. It is highly recommended to use a wood or plastic tool to clear out a vent, using steel screw drivers/prybars or scrapers to clear out a vent opening can potentially cause a catastrophic failure when the metal tool is pulled into the extruder by the rotating screws.
The following situations describe when a vent is vulnerable to compound flow:
- When a vent is located near the exit of the extruder and the die or exit pressure is too high.
- When a vent is located upstream of and near a restrictive mixing element, for example, a reverse screw element or a neutral kneading block.
- When a formulation is altered during a production run and the compound viscosity is reduced.
- When the feed system is unstable, compound viscosity changes may occur.
- When starting the extruder, residual material in the extruder may act as a restriction along with typical start up instabilities and viscosity changes.
- When extruder torque suddenly rises significantly or is consistently high, this can be an indication that the fill level of the extruder is too high.
- When raising feeder throughput to the extruder without increasing the extruder screw speed, the fill level of the extruder increases.
- When using poorly designed or incorrectly installed vent inserts.
The following actions should be considered to minimize or mitigate vent flow:
- Increase extruder screw speed to reduce the fill level of the extruder and improve pumping efficacy.
- Lower feeder throughput to reduce the fill level of the extruder and reduce exit pressure.
- Replace worn screws and barrels located downstream of the vent to improve extruder pumping efficacy.
- Alter the pitch of the convey screw elements downstream of the vent and/or under the vent to one diameter pitch elements, in other words, for a 70mm diameter extruder the pitch would also need to be 70mm.
- If possible, increase the mesh size of filtration screens to reduce exit pressure.
- Increase the diameter of die holes and/or add more die holes to reduce exit pressure (die hole diameter has a more significant impact than number of die holes)
- For underwater pelletizing, purge out frozen die holes to reduce the exit pressure. Also, if possible, raise the temperature of the water to reduce exit pressure and minimize the risk of freezing die holes. During a formulation transition, consider diverting to purge to reduce exit pressure.
- Increase the temperature of the die and adaptors to reduce exit pressure.
- For a vacuum vent, if possible, lower the level of vacuum.
- If possible, alter the screw configuration and move restrictive mixing elements further downstream from a vent.
- If possible, monitor the vent during an initiated process change or as quickly as possible during an unexpected process change.
- The vent opening is located above the correct screw. It should be above the screw side that is rotating downward not the screw side that is rotating upward.
- The vent insert is bolted down evenly with equal torque applied to each bolt.
- Stabilize feeder/s by checking feeder settings, calibrations and sizing the feeder screw and tube correctly.
- Purge out residual material at start up. Start the extruder at higher than targeted screw speed and/or lower than targeted throughput. Monitor the vent during start up. Once stabilized, alter set points to targeted screw speed and throughput.
A clear or better understanding of what causes vent flow and when the extrusion system is vulnerable to vent flow could prevent or provide a rapid response to a vent flow situation. Running a lower vacuum to prevent vent flow may not always be an option. Installing a vent stuffer allows higher vacuum draw, vent stuffers act like a pump preventing the polymer/compound from escaping the extruder while allowing volatiles to escape.