Volatile Removal in Vented Extrusion
Vented extrusion applications are characterized as utilizing a multiple stage screw and vented barrel. The use of these designs are only considered when volatile removal is essential during processing for proper operation. When processing many recycled materials, various compounding applications, and extremely hygroscopic resins, vented extrusion is an excellent option. However, there are several design features and limitations that must be considered for satisfactory performance. Without adequate equipment and operating conditions, insufficient degassing, contamination, and even vent flooding will occur. These downfalls can be very costly and in the case of vent flooding, very labor intensive to remedy.
How do Vented Extrusion Screws Work?
First, it is important to understand the fundamental function of vented extrusion. A vented screw works by reducing the melt pressure to atmospheric pressure, so that there is no tendency for the molten resin to pump out of the vent opening in the barrel. It is generally understood that the feed screw must adequately transport, melt, and develop the ideal maximum pressure at a desired temperature to achieve a desired output. With a two stage vented screw, there are essentially two screws stacked end on end. The first stage being a conventional screw that performs the expected operations. However, rather than overcoming the downstream head pressure, savvy screw designers build in a head pressure in the first stage in the form of a mixing element prior to venting.
The second stage again acts as a second screw on the same shaft. This second stage performs the same duties as the first, this time being melt-fed by the previous stage. Also, the second stage must overcome the downstream head pressure designated by the die and any secondary equipment that is inline. In order to achieve atmospheric pressure at the vent port, the screw channels in the second stage venting zone must be sufficiently deep to be partially filled. Also, the second stage metering section must be designed to accommodate a higher output capacity than the first stage. In special circumstances, even three stage screws can be utilized with two vent ports. Much like two staged screws, a three stage screw acts as a conventional screw with two melt-fed screws stacked end on end on the same shaft.
The Trouble with Limited L/D
One of the most obvious concerns for vented extrusion, aside from optimal volatile removal, is the limitations in output vs a traditional screw of the same L/D. Much longer L/D screws must be utilized to provide sufficient axial length to accommodate the vent section of the screw. Even still, reducing the melt pressure to atmospheric pressure then back to an ideal maximum pressure is an added processing requirement that does not positively impact output. Moreover, a high quality vented screw must have adequate mixing in the first stage to ensure the polymer is fully melted prior to venting. If the polymer is not adequately melted, volatiles will not be readily extracted. In many cases, a second mixing section is incorporated at the end of the last stage to promote a more isothermal melt. Again, this omits more axial length from the already limited L/D of the screw necessary for pumping and high throughput. If the final head pressure is too high, or the second metering section isn’t properly filled after volatile removal, the vent port will be prone to flooding.
Traditional vent sections in screw design simply consist of a deep, consistent channel, similar to a feed section in a conventional screw design. The channel must be sufficiently deep such that the polymer does not overcome the vent port. By design, these venting sections have areas of low flow, unfortunately this can lead to material hang up. Furthermore, volatile removal is inefficient with this approach because the resin in these deep channels is simply a blob with low surface area exposed for degassing. To aid in volatile removal and to reduce the risk of hang up, multiple flights can be incorporated in a vent section. Multiple screw channels help with separating the melt into ropelike strands, increasing the exposed surface area of melt, naturally improving volatile removal.
SpiraMelt Technologies has developed a modern approach to vented extrusion screw designs with the VR Pro Screw series. The VR Pro includes a unique helical vent section with modified channel geometry to maximize volatile removal in both two stage and three stage vented screw designs. The unique channel geometry increases surface area of the melt without dead spots, allowing for significant improvements in volatile removal and overall throughput. Upstream of the VR Pro vent section is a spiral mixing section that adequately melts any remaining agglomerates prior to venting. This not only improves overall melt quality and stability, but further improves volatile removal downstream. The VR Pro series can be tailored for any vented extrusion application: two stage, three stage, and even high-performance vented applications that leverage the SpiraMelt MVP Barrier section.
The Importance of Flow Diverters in Vented Extrusion Operation
Aside from proper screw design, vented barrels must be equipped with a properly designed flow diverter to avoid vent flooding. Most single screw extruders rotate counter-clockwise during operation. Because of this, optimal vent locations are either the top of the barrel, or more commonly the left side of the barrel (when facing downstream). With extruders that rotate clockwise, side venting would be best suited for the right hand side. Although top vent placement is very effective, it can be very cumbersome for operators to view or clean these ports. Meanwhile, side venting is easy to clean and access under most conditions. The barrel must be equipped with a tight fitting flow diverter that acts as a plug with an ID that precisely matches the barrel ID with a smooth, contoured relief area. When the screw rotates, resin readily sticks to the barrel wall while the advancing flight wipes the resin, conveying it forward. With the vent port open, the resin that sticks to the barrel wall will accumulate and pump out the hole. With a proper flow diverter, the contoured relief area will accommodate the melt, preventing it from stacking up and flooding the port. If a vent is flooding at a continuous pumping rate, it is likely caused from inadequate screw design. On the other hand, if the vent is flooding with a pulsing or burping action that corresponds with screw rpm, the flow diverter is likely to be improperly designed.
With the more common counter-clockwise rotating screws, left handed venting works well because of gravity. As the melt is conveyed forward past the vent opening, the resin will accumulate in the relief area of the flow diverter. The molten polymer tends to sag back to the bottom of the barrel, clearing the relief area as the advancing flight passes more resin forward. The opposite is true for clockwise rotating screws. With top venting, the same mechanism is present, however gravity readily pulls the entire melt down from the relief area. Top venting is the most efficient at keeping the vent port clear, but is less user-friendly. If a top vent port is plugged for whatever reason, the molten polymer will pump out, cascading down the exterior of the barrel, onto heater bands and wiring. With side venting, this is far less of a concern and easier to clean up. Also, side venting allows easy line of sight for the operator to observe operating conditions.
Final Thoughts
Vented extrusion operation is a necessary approach for certain processing conditions. This process can significantly improve part quality and profits by better utilizing recycled materials or hygroscopic resins without drying. However, it is clear that careful consideration must be taken when choosing the right equipment. Experienced screw designers must be able to not only design a high performance screw, but also provide a flow diverter design that can accommodate the rigors of processing. If the system is not optimized for volatile removal, your extruder will be completely inoperable. With vented operation, there is most certainly no “general purpose” approach to design. At SpiraMelt Technologies we specialize in custom, high-performance feed screw designs. Contact us today to learn how a tailored screw design can maximize your production!
