Dual layer hollow fiber membrane Spinning Spinneret single hole

5.Processing type: NIPS (Non-solvent Induced Phase Separation; the spinneret must precisely control the synchronized extrusion rates and interfacial bonding of two dopes, which directly determines interlayer adhesion and performance stability.)

6.Application: Environmental protection, water purification (use cases include: treatment of fluoride-bearing industrial effluents from electronics manufacturing—retaining fluorides and heavy metal ions; advanced municipal drinking water purification—removal of cyanotoxins and organics; and pretreatment for brackish/island desalination—retaining suspended solids and colloids.)

7.Specification: 1.6/1.3/1.1/0.6/0.4 (unit: mm. The five nested channels correspond to functional zones: 1.6 = outer dope channel for anti-fouling PVDF outer layer; 1.3 = inner dope channel for high-selectivity PVDF inner layer; 1.1 = outer bore-fluid/porogen channel to tune outer-layer pore size; 0.6 = inner bore-fluid/porogen channel to tune inner-layer pore size; 0.4 = central support porogen channel to ensure hollow fiber structural stability.)

8.Manufacturing capability: 1.600±0.002 mm, 1.300±0.002 mm, 1.100±0.004 mm, 0.600±0.002 mm, 0.400±0.004 mm; assembly concentricity 0.003 mm (the five-layer structure requires extremely high concentricity to avoid interface offset between the two dopes that could cause layer delamination; precision far exceeds standard single-layer spinnerets.)

9.Spinneret material: Stainless steel (304 stainless steel with solution treatment and electropolishing; surface roughness Ra ≤ 0.8 μm, balancing corrosion resistance to PVDF dope containing DMAC and smooth dope flow.)

10.Applying material: PVDF (Polyvinylidene Fluoride) (high-purity PVDF resin, purity ≥ 99.5%. The outer layer is modified to enhance anti-fouling (surface contact angle ≤ 65°), and the inner layer formulation is tuned for high selectivity (pore size 0.01–0.02 μm). The spinneret must precisely accommodate the different flow characteristics of the two PVDF dopes.)

11.Additional notes: The core challenge of a single-hole, dual-layer composite hollow fiber spinneret lies in "five-channel coordinated control"—ensuring synchronized extrusion of the two dopes (flow-rate difference ≤ 2%), preventing premature mixing of porogen and dope, and precisely controlling the thickness ratio of the two layers (customer requirement outer:inner = 1:1.2). It is a key device for high-end membrane R&D in environmental water purification using the NIPS process.

12. Detailed collaboration process and customer value realization

(1) Customer background and core pain points

Before its 2017 purchase, Century Water faced technical bottlenecks in Southeast Asian environmental water projects.  Its previously used single-layer PVDF hollow fiber membranes exhibited three issues when treating highly polluted waters (e.g., fluoride-bearing electronics effluents, eutrophic municipal waters):

● Insufficient anti-fouling: organics and colloids readily adsorbed on the membrane surface, causing a 35% flux decline within three months and requiring weekly chemical cleaning (citric acid + sodium hypochlorite), increasing chemical costs and shortening membrane life.

● Selectivity–flux trade-off: achieving higher retention of small-molecule contaminants (e.g., fluoride ions, cyanotoxins) required smaller pores, which reduced flux and could not meet high-load industrial wastewater demands.

● Poor layer stability: attempts to self-develop dual-layer membranes failed because conventional spinnerets could not precisely control interfacial bonding of the two dopes, leading to layer delamination.

To address these issues, Century Water proposed developing "dual-layer composite hollow fiber membranes," centered on a matching single-hole spinneret capable of: (i) precise synchronized extrusion of two different dopes to ensure tight interfacial bonding;  (ii) combining high selectivity (inner-layer pore size 0.01–0.02 μm) with high flux (≥ 55 LMH);  and (iii) enhanced anti-fouling.

(2) Customized technical solution and pain-point breakthroughs

In response, Trustech designed a specialized solution around "five-channel coordination" and "NIPS compatibility," resolving the core challenges:

● Precision control of the five-layer structure: a 5-axis precision machining center (positioning accuracy ±0.0002 mm) was used for integrated machining of the 1.6/1.3/1.1/0.6/0.4 mm channels.  Post-machining, each channel's diameter and roundness were verified via laser interferometry to ensure per-layer deviation ≤ 0.002 mm.  Using "datum-axis positioning + multi-dimensional calibration," five-layer assembly concentricity was controlled within 0.003 mm, preventing dope-interface offset.

● Coordinated dope flow optimization: CFD was used to simulate flow of the two PVDF dopes (e.g., outer modified dope viscosity 3500 cP; inner high-purity dope viscosity 30,000 cP), optimizing inlet angles and outlet geometry (elliptical outlet to reduce shear), keeping extrusion rate difference within 1.5% to ensure tight interfacial bonding.

● Anti-fouling–flux balance design: for the anti-fouling outer layer, the outer dope channel inner wall was polished to Ra = 0.08 μm to reduce flow resistance, yielding a smoother outer membrane surface (contact angle reduced to 62°).  Precisely controlling the inner porogen channel flow rate (relative to inner dope flow) tuned inner-layer pore size to 0.015 μm, achieving "high retention (fluoride ion rejection 92%) + high flux (62 LMH)."

(3)Application outcomes and deepened collaboration

After Trustech delivered the spinneret in August 2017, Century Water achieved significant breakthroughs in dual-layer composite membrane R&D and application.  The produced dual-layer membranes met targets: high fluoride and cyanotoxin rejection by the inner layer, outer-layer flux of 62 LMH at 0.1 MPa, and reduced flux decline when treating fluoride-bearing electronics wastewater.