● Control extrudate swell (Barus effect): Nonuniform shear causes twist/size instability.
● Adequate L/D: For annular slits, ensure sufficient land length; commonly L/D > 10.
● Pre-compression transition: Use gradual conical convergence into the annular gap to equalize inflow.
● CFD-guided optimization: Iterate geometry until a flat shear-rate profile at the outlet is achieved.
● Converging outlet: Conical/parabolic convergence (5°–15°) to avoid shear shocks.
● Concentric ring-gap uniformity: For bilayer dies, ring-gap tolerance ≤±2 μm around the circumference.
● Edge treatment: Micro-chamfer/radius to reduce edge-induced “sharkskin” instabilities.
● Match to rheology: Highly elastic dopes may need lower shear—use larger orifices or shorter capillaries.
● Avoid section jumps: Use gradual area transitions.
● FIFO principle: Promote forward flow with no recirculation.
● Tapered or coat-hanger manifolds: Ensure uniform widthwise pressure and smooth flow with minimal dead zones.
● Surface finish: Electro/mirror polish to Ra <0.8 μm to reduce adhesion.
● Match flow and volume: Avoid oversized manifolds at low flow; ensure sufficient shear to sweep surfaces.
● Minimize cavity volume: Reduce hold-up to shorten cleaning time.
● Tilt/vent: Prevent air-pocket dead zones with angled install or vents.
● Maintain velocity: Target ≥0.5 m/s via pressure increase or area reduction to prevent stagnation.
● Unstable lumen size: Bore flow variations directly alter inner diameter.
● Worsening eccentricity: Changes in bore viscosity/density disrupt interfacial tension balance.
● Visual cues: Variable lumen size, pinholes, frequent breaks; too much non-solvent in bore accelerates coagulation; too little delays it.
● Key analytics: Chromatography to quantify solvent/non-solvent ratio (±5% spec); monitor bore viscosity (≤3% variation).
● Process isolation: Hold shell parameters constant, normalize bore composition; if stability returns, root cause confirmed.
● Axial heterogeneity: Pore size/porosity vary along fiber when bore composition swings.
● Event correlation: Match instability to bore system events (tank changeovers, charging).
● Online monitoring: Install inline viscometer/densitometer if feasible.
● Exclusion test: Run with known-good bore while holding others constant to isolate cause.
● Pump health: Check pulsation, seals, trapped air.
● Outlet chamfer: Micro-chamfer (0.1–0.3 mm) or parabolic transition to prevent edge hang-up or tearing.
● Anti-disturbance geometry: Avoid abrupt steps near outlet to reduce bath backflow impact.
● Hole spacing: Increase inter-hole pitch or add shields to avoid sticking at high coagulation rates.
● Corrosion resistance: If bath contains acids/bases (e.g., regenerated cellulose), upgrade outlet materials.
● Wet spinning variants: For rapid coagulation, use minimal or no air gap (in-bath immersion), requiring robust immersion sealing.
● Streamlined outlet profile: Conical/streamlined exterior to reduce turbulence-induced vibration/stretching pre-bath.
● Bath flow direction: Layout multi-orifices with bath flow to avoid upstream boundary-layer effects on downstream holes.
● CFD: Simulate velocity/pressure fields and interface behavior to guide design.
● Design compensation: For large viscosity contrasts, shorten/widen high-μ paths to balance resistance.
● Flow-path shape: Use gradually converging or constant-area channels; avoid sudden expansions/contractions.
● Process tuning: Independently adjust metering-pump speeds/pressures to balance ΔP and achieve desired structure.
● Viscosity monitoring: Measure μ (e.g., rotational viscometer) and adjust pumps/valves as μ drifts to maintain balance.
● Lapped metal-to-metal faces clamped by bolts provide primary sealing.
● Add gaskets: Soft metal (copper, soft iron) or high-performance polymers (PTFE, PEEK). Under preload, gaskets plastically conform to micro-roughness.
● Radial seals (O-rings): For secondary joints or quick-disassembly, use chemical/heat-resistant O-rings (FKM/FFKM).
● Precise ratio control: Adjustable layer ratio (commonly 1:1 to 1:3) via independent valves/pumps.
● Synchronous formation: Both layers meet and wrap the bore fluid at the outlet to avoid delamination.
● Orifice matching: Outer-ring orifice I.D. 0.05–0.10 mm larger than the inner ring, tuned to layer viscosities (higher μ → slightly larger orifice).
● Channel width: Select per flow (e.g., 2–3 mm for higher flows) to limit pressure drop.
● Overall length: Fit equipment (typ. 50–100 mm); die-to-bath distance 5–10 mm to ensure proper bilayer merging.
● Biocompatible materials: Medical-grade stainless or titanium; surface Ra ≤0.5 μm; no metal ion leaching; compliant with ISO 10993.
● Structural reliability: Independent insert designs (e.g., Trustech FCT Gen-5) allow single-orifice replacement without full shutdown; dead-zone-free flow paths reduce residual contamination.
● Low-shear channels: Minimize protein adsorption/denaturation.
● Key parameter—die gap: The annular gap determines coating thickness; size/adjust per dope viscosity and desired thickness.
● Flow-path design: Ensure circumferentially uniform pressure to achieve thickness uniformity and avoid eccentric coating.
● Dead-zone-free flow paths: Avoid polymer degradation hot spots.
● High viscosity compatibility: Common polymers include PI and PSf; require large channels and strong pressure capability.
● Dry–wet compatibility: Often requires longer air gaps (5–20 cm) to form a dense skin.
● High temperature materials: PI spinning may require >200°C.
● Flow-path structure → formation: Manifold/equalization design governs dope distribution, avoiding “streaky” multi-orifice output.
● Material → stability: Insufficient wear resistance enlarges orifices; poor chemical compatibility causes corrosion—both degrade quality over time.
● Sealing and flow resistance → continuity: Leaks and unequal resistance cause breaks or lumen closure, reducing yield.
● Film-formation dynamics: At the outlet (NIPS/TIPS), extrudate swell and initial bore/shell interface set the stage for phase separation, impacting pore size, distribution, and porosity.
● high temperature: Nickel alloys, titanium alloys, SUS630 (17-4PH) for TIPS (100–260°C), with higher hot strength and wear resistance.
● Corrosion-resistant: Hastelloy and ceramic-coated materials for aggressive solvents (DMF, DMSO) and strong acids/bases.
● Auxiliary coatings: PTFE or polysiloxane coatings on orifice walls to increase hydrophobicity and reduce deposition.
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