Undaunted by typically held beliefs that “there could also be nothing further which may be accomplished to reinforce single-screw extrusion,” and with out finish the tinkerer, Keith Luker has for the ultimate twenty years been engaged on the next iteration of his SFEM Elongational Mixer. What the founder and president of Randcastle Extrusion has possibly stumbled onto is a novel system that is not solely a less complicated mixer nonetheless, in some circumstances, improves supplies bodily properties and makes pre-drying pointless.
Luker has dubbed it the Molecular Homogenizer (MH). It’s a model new system that he says could have broad enchantment all through the material present chain — beginning with resin producers. It’s so named, he maintains, on account of it mixes supplies on the “molecular stage.” He elaborates, “The mixing is so high-quality that the properties of polymers change.” “Bodily properties of polymers (virgin polymers processed by the use of the screw) improve. In hygroscopic polymers, water-vapor absorption is postponed. Undried hygroscopic polymers current no bubbles. Rheological properties current unbelievable enchancment.”
Randcastle’s Molecular Homegnizer is a 36:1 L/D screw with seven mixing sections. Primarily based on the inventor, it has been confirmed to spice up mixing and likewise improve supplies properties in some circumstances. Inset displays single mixing part. Provide: Randcastle Extrusion
Luker has supplied papers on this mixing system in 2022 and 2023 at Plastics Experience’s Extrusion Conference, and further currently this earlier June on the SPE Extrusion Division Screw Design TopCon. Luker is the first to admit that further work, testing and evaluation should be accomplished to adequately make clear why the design is performing because it’s. That talked about, he has patents pending in 5 worldwide areas and Europe, and had the MH examined on quite a lot of provides, along with PLA, PMMA, PET, PET with 25% reclaim, PEEK, PVA, PVA with 3% reactive agent, PLA, SAN shade focus, PC, nylon and low chaff in LDPE. The mixer could be being utilized by on the very least one extrusion processor in a producing environment.
The examined screw had on the very least 1007 (100 trillion) mixing actions alongside the XYZ dimensions. “Such an increase resulted in mixing to the small molecule stage (corresponding to three atom water vapor),” he says. “On account of the sequence is orderly, the mixer imposes group to the messy, tangled mixture produced by a polymer reactor. We take into account this mixing changes the alignment of prolonged polymer chains; distributes small molecules (monomer, water vapor), elements, particulate, actives, and so forth. This ends within the enhancements we see on the macroscopic stage. The mixing may enhance, circuitously, diffusion.”
Mixing Forces
Luker notes that many typical mixers depend upon shear with compression forces to push supplies by the use of slim gaps. Compression forces, he contends, are antagonistic and counter mixing. That is, when compression is blended with shear or elongation, mixing effectivity is significantly impaired.
Pullout (left) of MH predecessor SFEM Mixer (C=Channel; P=Pump). C1 displays 2.5 striations, C2 higher than 25; C3 higher than 250. This ends in a 100-fold enchancment in mixing. Correct depicts new MH. Orange arrows current intensive free surfaces. Purple displays 1-dimensional shear in two areas. Black oval signifies 3D trilongation in three areas. Purple oval reveals 2D elongation in two areas.
Luker elaborates: “When compression is utilized equally and reverse in all three dimensions, it is known as pressure. After we apply compression or pressure to a cube, sphere or bubble, mixing does not occur. The drive is utilized in all three dimensions inwardly. If the cube or sphere had been product of loosely held particles, then compression or pressure will push these particles collectively inflicting agglomeration. When pressure is utilized to the bubble, it shrinks, turning into denser and this too is agglomeration. Agglomeration is the opposite of mixing and is attributable to compression and pressure. Take into account pressure as an antimixing drive.
Alternatively, reorienting or interrupting shear makes “order of magnitude enhancements” doable, he gives. Further, when a mixing transfer is oriented in a single route after which one different, “The orientation imparted by the first half is destroyed, and each ‘stage’ of the system behaves as a separate unit.”
In 2005, Luker developed the SFEM mixer that created vital elongational mixing. Dow Chemical supplied a paper at ANTEC 2011 which confirmed mixing outcomes that had been eight cases greater than their twin with immiscible polymer blends. This new mixer makes use of first principal preparations of interrupted shear, interrupted elongation and a mixing transfer known as inversive mixing.
“Until this affiliation, it was not understood that compression was so detrimental to extrusion mixing,” Luker says. “Acknowledged one different means, if there have been no compression blended in with shear and elongation, they’d mix vastly greater. Basically, that’s the reason the MH produces such gorgeous outcomes.”
The design affiliation, Luker maintains, “creates exponentially improved mixing such that gorgeous outcomes start to transform mundane.” As an illustration, processing by the use of the unvented Molecular Homogenizer has been confirmed to:
- Improve Bodily Properties: Improve tensile-at-yield higher than the virgin polymer.
- Improve Viscosity: Restore undried hygroscopic regrind to the viscosity of the unprocessed virgin polymer.
- Sluggish Water Absorption: Processing slowed water absorption from the on a regular basis 4 to 6 hours when uncovered to ambiance to over 72 hours
- Sequester Water Vapor: Moisture analyzer measurements confirmed the moisture content material materials of an unprocessed supplies stepping into the Molecular Homogenizer at 0.328%; it then reported that the Molecular Homogenizer processed polymer had 0.171% moisture.
How It Works
Randcastle Molecular Homogenizer has a 36:1 L/D and choices seven mixing sections. Primarily based on Luker, this design promotes a sturdy dispersive mixing drive known as trilongation, the place all the transfer is stretched in three dimensions. Each of the mixing elements of the system creates a repetitive mixing sequence twice: 3D trilongation, 2D elongation and 1D shear. Each mixer is assumed to reinforce the mixing by higher than 100 cases, Luker says.
He says that in distinction to a twin-screw extruder, the entire transfer strikes by the use of this sequence. Each time it does, the 100-fold enchancment is created, a bit like a static mixer multiplication moreover that’s dynamic. He says this creates higher than 100 trillion mixing events throughout the screw. On the end of each mixing part, the transfer is inverted the place the inside turns into the floor. This
reorientation permits the friction-warmed supplies on the screw to maneuver to the barrel the place cooling takes place sooner than the start of the next mixing part. In degassing capabilities, it strikes the trapped bubbles to the intensive flooring for rupture and venting.
Check out Outcomes
Undried hygroscopic polymers are recognized to supply seen bubbles in unvented extruders. Luker tales that in his lab the model new mixing screw processed PMMA, PET, PET with 25% amorphous reclaim, PEEK, PVA, PVA with 3% reactive agent, PLA, SAN shade focus (black and white), PC and low chaff in LDPE. None of these provides had been predried, and none confirmed any proof of bubbles (water). Further notably:
- PET With 25% Reclaim. Undried PET may very well be very well-known to take in water vapor and sort bubbles. Undried PET reclaim is amorphous and recognized to take in far more water. No bubbles had been seen.
- Reactive Extrusion of PVA: Undried PVA produces a bubbled extrudate. A reactive chemical was added to dried PVA and created a bubbled extrudate in a conventional screw — implying that the bubbles weren’t water vapor. However, in an undried PVA with 3% reactive agent, no bubbles had been seen throughout the Molecular Homogenizer extrudate.
- Espresso Chaff: Espresso chaff is the thin papery pores and pores and skin that comes off the espresso bean. When heated with LDPE, the chaff breaks down, releasing gases comparable to carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), and quite a few unstable pure compounds (VOCs). The Molecular Homogenizer produced no bubbles throughout the extrudate.
- Black and White SAN Coloration Focus: SAN is hygroscopic. Every carbon black and TiO2 are hydrophilic and likewise take up water vapor. The undried provides had been every pelletized. No bubbles had been seen throughout the strand decrease pellets. The pellets had been then processed in a conventional screw on a film line.
- Nylon: Nylon is a very hygroscopic polymer absorbing moisture throughout the 2%-9% differ. It is pricey to dry polymers. Throughout the Randcastle lab, Luker processed a nylon combine into film on a conventional screw for each week. Drying was important. Circumstances had been successfully established.
He then swapped out the screw with the Molecular Homogenizer and processed the similar nylon nonetheless didn’t dry it first. Bubbles had been nonetheless created. He then opened a single atmospheric vent, exposing the intensive free surfaces, and the film on one of the best was produced. He notes the output payment was the similar (rpm was elevated and extruder starve fed); the temperatures had been saved the similar as a result of the dried supplies; no die modifications had been important to supply the similar tolerance film — implying the viscosity was the similar; and pressure was the similar as a result of the dried supplies — implying the viscosity was the similar.
Proper right here, undried nylon was processed on the MH. Bubbles had been created.
- Undried PVA With Further Than 3% Reactive Agent: PVA with 3% reactive agent was one in every of many provides the place the strand did not current bubbles. Nonetheless, the experiment moreover blended 6, 9 and 12% reactive agent. At 6%, bubbles had been created. A single atmospheric vent was opened. No bubbles had been seen. The experiment proceeded and no bubbles had been seen at 9 or 12%.
Luker tales the Molecular Homogenizer has been confirmed to sluggish the velocity of moisture absorption, which he notes is very useful for regrind the place it takes higher than a number of hours to grind and reprocess the material. As an illustration, PMMA regrind will take up moisture in 4 to 6 hours, creating points (bubbles, lower viscosity, property degradation and roughened surfaces), he components out.
He remembers that the screw was used to course of dried PMMA by the use of a water trough and strand pelletizer on a moist, summer season Friday afternoon in New Jersey. The bag of pelletized supplies was left uncovered to the ambiance over the weekend, and it continued to rain. On Monday afternoon, the pellets had been extruded (undried) in a conventional screw. There have been no bubbles. This, Luker says, demonstrates that the time for processing regrind may very well be significantly extended for product enchancment.
After a single atmospheric vent was opened, exposing the intensive free surfaces, nylon film was produced with no bubbles on the similar payment.
In another check out, a Molecular Homogenizer and a general-purpose administration screw had been used to course of virgin dried PETG. The pellets had been then molded in a family mould and in distinction on the Pennsylvania College of Experience. Relating to properties of check out samples, says Luker:
• Elongation at Yield: The Molecular Homogenizer was 17.9% greater than the general-purpose screw and improved this property 6.7% vs. the virgin supplies.
• Izod Affect Vitality: The model new mixing screw was 22.4% greater than the general-purpose screw. The Molecular Homogenizer screw was 18% worse than the virgin pellets.
Who Would Revenue?
Luker states many capabilities which will revenue from the utilization of this new mixing screw experience. He notes, “Start with polymer producers. The MH has demonstrated postreactor enchancment in bodily properties and rheology — no matter an additional processing historic previous. Given that reactor operation already contains a pelletizing processing historic previous, we depend on the similar or greater outcomes from a Molecular Homogenizer positioned on the end of a polymer reactor for pelletizing. This may create polymer with greater properties and lessened or no need for drying.”
For compounders and other people involved in direct extrusion (bypassing the pelletizing step), Luker says the MH serves as a safe, high-pressure pump that eliminates the need for a gear pump. In reclaim operations, meantime, he asserts that the lower water absorption fees demonstrated translate into elevated time for processing with out the need for drying. Viscosity enhancements with reclaim moreover indicate further fixed die transfer, he gives. Bigger top quality and improved viscosity normally lead to elevated output, Luker notes.
Completely different capabilities that Luker believes to be successfully suited to the Molecular Homogenizer embrace: processing of PP, PFA and rigid PVC powder, which he says have been confirmed to run at bigger fees with the model new screw; evaluation and enchancment, as higher mixing effectivity could pave the way in which through which for model new supplies combos; and prospects of twin-screw extruders, the place the screw could be utilized as an add-on.