● Pressure sensing: Use high speed transducers at the hollow fiber spinneret inlet; look for pump-synchronous ripple.

● Correlation test: Change pump speed; if defect period tracks, strong coupling is indicated.

● Rule out pump wear: Ensure ripple is not solely due to pump mechanical issues (gear mesh, wear).

● Root cause: System compliance (line stiffness/length), dope compressibility, and pump ripple form a resonance that amplifies pressure waves.

● SCC resistance: Chlorides drive SCC in stainless steels. If chloride exposure is possible (raws, cleaning water), prefer 316L; for extremes, use Hastelloy C-276.

● Basis: Choose materials with proven resistance to the anticipated solvents, oxidants, and SCC environment.

● Too high temperature: Thermal degradation; low viscosity; breaks in the air gap; cracks/voids in cross-section.

● Uniform temperature control: Produces dense, crack-free cross-sections; improves diameter uniformity and mechanics.

● Viscosity gradients: Hotter zones → lower μ → faster flow → smaller swell; cooler zones → higher μ → slower flow → larger swell; leads to hole-to-hole diameter differences.

● Phase-separation kinetics: Temperature directly affects solvent–non-solvent exchange; nonuniform temperature yields different pore sizes/porosities across the plate.

● Local hot spots: Lower μ → faster flow → thinner walls; coarser finger-like pores.

● Local cold spots: Delayed phase separation → thicker skin or sponge-like structure; lower flux.

● Circumferential ΔT: Causes wall-thickness nonuniformity, eccentricity, even helical fibers.

● Axial ΔT (TIPS): Premature solidification before exit causes unstable extrusion.

● 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.

● 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.