Solving Venting Problems on Twin-Screw Compounding Extruders Adam Dreiblatt Director of Process Technology CPM Century Extrusion (231) 421-6896 dreiblatta@centuryextrusion.com
Presentation outline Why is stuff (polymer/powder) coming out of vent? Atmospheric vent Side feeder (atmospheric) vent Vacuum vent How to fix this? Why isn t there more stuff coming out of the vent? How to improve devolatilization efficiency?
The vent ports appear to be clear, so why does polymer come out there?
Venting requirements (general) 1. Melt seal upstream of open vent (screw design) 2. Low degree-of-fill (screw design) 3. Proper vent insert design (extruder design) 4. Vapor/gas below critical velocity (typically 1m/sec)
Atmospheric vents (no side feed) Removal of air, moisture and/or volatiles Located downstream of plasticizing/melting zone Polymer may or may not be completely molten Atmospheric Vent Vacuum Vent
Why do we need a vent? POWDER + AIR Filled screw section created by reverse elements for melting polymer AIR FLOW When feeding large volume of powder in main feed barrel, the air must be vented somewhere
Why do we need a vent? POWDER + AIR Non-restrictive screw design does NOT create filled section, allows air to pass to downstream vent port AIR FLOW One option is to design the initial melting section to allow the air to pass through the kneading section
Atmospheric vents (no side feed) Melt seal (restriction element) prior to atmospheric vent
Atmospheric vents (no side feed) Low degree-of-fill in vent area (i.e. high pitch forward conveying elements)
Atmospheric vents (no side feed) Downstream mixing elements do not create back-up into vent area
Vent inserts When looking at the discharge end of the machine, if your screws turn clockwise the down-turning screw is the one on the right Different types of vent inserts are used to keep the molten material from coming out of the extruder screws. The up-turning screw wants to push material out of the machine. The vent inserts are designed to keep this screw covered. You should only see the down-turning screw in the vent port.
Atmospheric vents (side feed) Removal of air, moisture and/or volatiles Positioned upstream of side feeder - incorporated into side feeding barrel ( combi ) - combi vents provide limited open area - can also use standard vent barrel upstream Polymer MUST be completely molten Primary design philosophy for side feeding
Side feeding concept Feeder Side feeders are used to force-feed material at a downstream location into the twin-screw extruder. Special barrels are used to accommodate the side feeder and can include a top vent port to allow the air to vent out of the extruder. A metering screw feeder is required to control the rate into the side feeder. Image courtesy Extricom GmbH
Back-vented combi barrel SIDE FEEDER AIR Installing an open vent upstream of the side feeder helps to disengage air from the solids (i.e. filler) coming into the extruder With this arrangement, the side feeder must be installed on the down-turning screw
Atmospheric vents (side feed) Melt seal (restriction element) prior to side feeder vent
Atmospheric vents (side feed) Low degree-of-fill in vent area (i.e. high pitch forward conveying elements)
Atmospheric vents (side feed) Downstream mixing elements do not create back-up into vent area
Atmospheric vents (side feed) Polymer MUST be 100% melted prior to side feeder incomplete melting causes filler to come out of side feeder vent!
Why do we need more venting? AIR Talc + Air AIR Talc + Air AIR AIR For very low bulk density fillers there is a lot of air to remove from the extruder barrel. Using the (small) combi-barrel vent results in high exit velocity which tends to carry the fine powder out of the machine. Using a larger (additional) vent barrel upstream of the side feeder increases the feeding capacity by reducing the velocity of the air
Vacuum venting Removal of air, moisture and/or volatiles Melt seal required upstream and downstream Vent insert design for minimum vapor velocity Low degree-of-fill in vent area Residence time between filled sections Mass transfer = f(partial pressure in headspace)
Vacuum venting requirements Melt seal (restriction element) prior to vacuum vent
Vacuum venting melt seal When melt seal pressure is less than 1 atmosphere, high vacuum can pull air through machine (vent flow) If vent flow is caused by melt seal, this problem will only occur when vacuum is applied powder/glass fiber may appear in vacuum port
Vacuum venting melt seal Melt seal pressure determines the supportable vacuum level (e.g. must be >> 1 atmosphere) When melt seal pressure is well above one atmosphere, vacuum pressure can never pull air through the machine
Vacuum venting requirements Low degree-of-fill in vent area (i.e. high pitch forward conveying elements)
Vacuum venting requirements Downstream vacuum seal (e.g. die or screen pressure) does not create back-up into vent area
Vacuum venting Increasing screen/die pressure increases the back-up length
Vacuum venting Increasing screw and barrel wear also increases the back-up length
Vacuum venting If vent flow is caused by melt back-up from the screen/die, this problem will occur with or without vacuum
Vacuum venting Residence time under vacuum is between filled sections
Vacuum venting Residence time under vacuum therefore decreases when back-up length increases (e.g. screen fouling) perhaps leading to voids in pellets, etc.
Why is stuff coming out of the vent? Screw design Must have melt seal prior to vent Maximum screw pitch to minimize degree-of-fill Melt seal pressure (vacuum venting) Vent area Correct vent insert type (installed correctly) Maximum open area (minimize velocity) Additional vent barrel (side feeding)
Why is stuff coming out of the vent? Side feeding MUST have 100% melted polymer Vacuum venting Sufficient pumping length (minimize pressure) Monitor screw and barrel wear condition Vent stuffer When all else fails
Why isn t more stuff coming out of my vent? Improving vacuum vent efficiency Increase temperature/pressure at melt seal (improved thermodynamic flash) Extend residence time by moving melt seal upstream Additional vacuum vent ports (extending L/D) Use of stripping agents to reduce partial pressure Is vacuum pump volume-limited?
Questions? Thank You!