● High-T TIPS (200–250°C): Use high temperature alloys (Hastelloy, Ti) to avoid thermal deformation; integrate cooling channels (annular passages) to remove heat and prevent degradation; flow-paths designed to limit thermal aging.
● Practice note: SUS304 and SUS316 have been used long-term for 32-hole TIPS lines without quality issues.
● High-T outlet free span: Extend to 15–20 mm for melt relaxation and to mitigate die swell–induced wall jumps.
● Thermal control: Lower thermal conductivity requires multi-zone heaters to keep axial ΔT < 1.5°C.
● Design allowance: Compute ΔD = ΔT × (α₁ − α₂) × D, and pre-clearance (e.g., ~0.02 mm) so that concentricity is optimal at operating temperature.
● Heat-up/cool-down rate: Recommend ≤30°C/h to keep thermal stresses <50 MPa and avoid micro-slip/scratching at seals.
● For same-material assemblies, effects are smaller.
● Needle damage: Traditional designs are hard to disassemble; bore needles are easily damaged and scrapped.
● Precision loss: Concentricity may shift after reassembly, requiring re-calibration.
● FCT Gen-5 advantage: Independent, pinless modular inserts enable quick change without damaging the body; reduces mechanical damage and precision drift, extending service life.
● Phase separation impact: Incompatible residual solvents (e.g., DMF) change coagulation rate—oversized pores or defective skin—lowering flux/selectivity.
● Material contamination: Residual corrosives (strong acids/bases) react with membrane (e.g., PA hydrolysis); metal fines can embed and reduce rejection.
● Biocompatibility: Medical membranes require very low residuals (e.g., ethanol ≤10 ppm) to avoid adverse reactions.
● Corrosion acceleration: Residual Cl⁻ (e.g., from HCl cleaning) induces pitting in stainless steel.
● Dedicated: Tailored to a specific material—e.g., solvent-resistant coatings for PVDF; high-strength materials for PA; ceramic coatings for CTA; titanium for PA to prevent hydrolysis. Higher precision/stability but requires frequent changeover in multi-product plants.
● Recommendation: For high-volume, choose universal to reduce cost; for high-precision (e.g., hemodialysis), choose dedicated to ensure performance.
● Rheology impact: Adjust temperature/pressure accordingly.
● Thermal sensitivity: Some additives decompose at high T, forming gels in dead zones—leading to clogs.
● Insoluble additives/nanofillers (TiO₂, CNTs, activated carbon):
● High sensitivity—primary risk is clogging. Particle size ≤1/5 of orifice (e.g., ≤20 μm for 0.1 mm rifice), with excellent dispersion (no agglomeration).
● Abrasion: Hard fillers wear orifices and capillaries, shifting critical dimensions.
● Agglomeration: Accumulates at contractions/orifices, causing blockages.
● Sedimentation: Dense fillers settle in low-shear dead zones.
● Safety factor:
● Industry practice: 3–4 for continuous precision chemical equipment.
● Formula (simplified): Allowable working pressure ≈ yield strength / safety factor.
● Example: 304 SS yield at temperature ~240 MPa; with SF = 4, allowable ≈ 60 MPa. Actual design set well below for safety margin.
● Nitriding (e.g., ion nitriding): Raises surface hardness (HV ≥ 1000) and corrosion resistance; suitable for high temperature TIPS and chlorinated media; good for PSf/PES dopes. Less resistant to fluorinated solvents; hydrophobicity moderate; hang-up reduction less than DLC; may slightly increase roughness.
● General principle: For strong solvents (e.g., DMF), prefer DLC; for high temperature/pressure, prefer nitriding. Always test coating compatibility to avoid chemical failure.
● Material-first: Prefer SUS304/316L; only consider coatings for extreme conditions (high solids/high wear) after long-term validation.
● Not strictly “the lower the better”:
● Diminishing returns below Ra ~0.4 μm with sharply rising cost.
● Exceptions: Ultra-smooth walls can alter wall slip of highly elastic melts; high-solids (>30%) systems may show slip–stick pulsation. Some systems exhibit an optimal roughness window.
● Over-polishing can reduce beneficial lubricating films; in viscose-like charged colloids, adsorption can worsen spinnability.
● Hang-up: Rough surfaces increase friction, leaving residual dope that forms gels and contaminates subsequent runs.
● Optimization: Aim for low roughness (Ra ≤ 0.8 μm) via polishing to reduce fouling and hang-up.
● Eccentricity: Damage can induce uneven clamping loads, generating small bending moments that distort internal flow alignment and cause eccentric fibers.
● Wear effects: Orifice enlargement, edge rounding, microcracks cause:
● Higher flow → thinner walls.
● Lower shear → looser skin.
● Higher eccentricity → lower strength.
● Batch variability: Even identical drawings yield micro-variance in orifice/gap, concentricity, and finish—changing spinning dynamics and membrane performance (flux, rejection, strength).
● Quality management: Treat hollow fiber spinnerets as critical assets; log use history against product performance; scrap/refurbish when beyond tolerance.
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