Dope and Bore Fluid Balance in Hollow Fiber Spinning Machine
By LydiaPosted on August 1, 2025Category: Hollow Fiber
Balancing dope and bore fluid in a hollow fiber spinning machine determines the final properties of hollow fiber membranes. Operators observe that even minor changes in fluid composition or flow rates can alter membrane structure, leading to either increased selectivity or higher permeability. For example, adjusting the salt concentration in the bore fluid enhances selectivity while maintaining high permeance, a result not easily achieved with solvent-only mixtures. The hollow fiber spinneret enables precise control, ensuring that the interplay between fluid viscosity and water activity shapes the membrane’s performance. Consistent balance helps prevent defects and ensures reliable operation of the hollow fiber spinning machine.
Key Takeaways
Balancing dope and bore fluid flow rates and composition is crucial to control hollow fiber membrane structure and performance.
Precise spinneret operation and flow rate control prevent defects like delamination and buckling, ensuring strong and efficient membranes.
Adjusting dope viscosity and bore fluid composition shapes membrane pore size, porosity, and selectivity for specific applications.
Real-time monitoring and regular calibration of spinning parameters maintain stable production and consistent membrane quality.
Optimizing spinning conditions supports reliable membrane properties, improving filtration, gas separation, and long-term durability.
Impact on Hollow Fiber Membranes
Structure and Performance
The balance between dope and bore fluid directly shapes the structure and performance of hollow fiber membranes. Operators observe that changes in dope composition, such as adding ortho-phosphoric acid, increase thermodynamic instability. This adjustment causes rapid solidification when water acts as the external coagulant, forming a highly porous membrane with a finger-like structure and a thin skin layer. When the bore fluid contains a weak non-solvent like an 80/20 NMP/water mixture, phase separation slows down. This delay produces a sponge-like layer without a skin, which reduces mass transfer resistance and enhances CO2 absorption flux. The membrane’s pore size, porosity, and morphology respond sensitively to these fabrication parameters. These changes improve the separation properties and wetting resistance, which are critical for the quality of hollow fiber membranes. The hollow fiber spinneret allows precise control over these variables, ensuring consistent membrane production and high-quality ultrafiltration membranes. Operators must monitor these parameters to maintain the desired properties and performance in every production batch.
Fiber Geometry
Fiber geometry plays a vital role in determining the quality and properties of hollow fiber membranes. Experimental results show that a 1:1 ratio of dope to bore fluid flow rate during spinning creates perfectly rounded asymmetric fibers. Deviations from this ratio affect the shape and surface characteristics, which influence membrane production quality. Increasing the bore fluid flow rate enlarges the internal diameter and enhances wall porosity by promoting interconnected pores. This change increases hydraulic permeability and helps maintain lumen shape, preventing fiber shrinkage. The hollow fiber spinneret ensures that the geometry remains stable throughout production. Operators find that insufficient bore fluid flow rates cause irregular lumen shapes due to vacuum conditions inside the fiber. Adjusting the bore fluid composition, such as adding NMP or glycerol, further modifies pore size, porosity, and hydrophilicity. These adjustments impact the mechanical strength and permeability of the membrane, which are essential for reliable membrane production and consistent properties in hollow fiber membranes.
Dope and Bore Fluid Basics
Dope Properties
The dope used in hollow fiber spinning consists of a polymer solution with carefully selected properties. Operators choose high-molecular-weight polymers, such as Torlon®, to ensure strong chain entanglement and high viscosity. The solution often contains N-Methyl-2-pyrrolidinone (NMP) as a solvent, which dissolves the polymer and controls phase inversion during spinning. Additives like water or alcohols adjust hydrogen bonding, affecting the network strength and morphology of the resulting fiber. The physical properties of the dope, including shear and elongational viscosity, play a crucial role in spinning. These properties determine how the polymer chains orient and relax under stress, which influences the formation of the selective layer and the overall membrane structure. The table below summarizes key dope solution properties and their effects:
Property Category
Specific Properties / Factors
Effect on Fiber Formation and Morphology
Chemical Properties
High-molecular-weight polymers, NMP, additives
Influence viscosity, solubility, and membrane morphology
Physical Properties
Shear/elongational viscosity, viscoelasticity
Control chain orientation, stress relaxation, and selective layer formation
Effects on Fiber Formation
Spinneret design, extrusion rate, phase inversion
Affect membrane microstructure, defect formation, and selectivity
Bore Fluid Role
The bore fluid type and its composition directly impact the internal structure of hollow fiber membranes. Operators select the bore fluid based on the desired membrane morphology. For example:
Using distilled water as the bore fluid type creates finger-like macrovoids that extend from the inner and outer surfaces.
Mixing NMP with water in the bore fluid composition results in a skinless inner surface and a sponge-like structure beneath the finger-like regions.
Increasing the NMP concentration in the bore fluid increases pore size and porosity, shifting the morphology from double-skin to single-skin structures.
These changes in bore fluid type and composition affect gas permeance and wetting resistance. Membranes with higher NMP content in the bore fluid show improved CO2 absorption flux, which benefits gas–liquid membrane contactors. The choice of bore fluid type and its properties remains a key factor in controlling the final performance of hollow fiber membranes.
Hollow Fiber Spinneret
The hollow fiber spinneret serves as the interface where dope and bore fluid meet during spinning. Its design determines the distribution and interaction of both solutions. Spinneret geometry and channel design influence flow patterns, shear forces, and molecular orientation within the polymer solution. For example, higher dope flow rates through the spinneret increase shear rate, which orients polymer chains and forms a more compact outer layer. Conical channel spinnerets tend to produce macrovoids, while straight channel spinnerets suppress them at high flow rates. Multi-orifice spinnerets allow for precise delivery of multiple fluids, enabling control over skin layer formation and outer surface structure. The spinneret’s dimensions also affect fiber size, morphology, and mechanical properties. Operators rely on the hollow fiber spinneret to maintain consistent spinning conditions, ensuring reproducible membrane properties and uniform fiber geometry.
Key Parameters in Hollow Fiber Spinning
Flow Rate Control
Precise control of flow rates in a hollow fiber spinning machine determines the geometry and performance of the resulting membrane. Operators use separate pumps for the dope and bore fluid to achieve independent and accurate regulation. The dope flow rate and bore fluid flow rate directly influence the internal diameter, wall thickness, and overall morphology of the hollow fiber. Maintaining a stable ratio between these two flow rates ensures consistent fiber formation and prevents defects.
The recommended dope flow rate typically ranges from 11.35 to 15.6 mL/min, while the bore fluid flow rate remains constant at 5 mL/min. This results in a dope extrusion rate (DER) to bore fluid ratio between 2.25 and 3.1. Deviations from these optimal spinning conditions, especially increasing the dope flow rate, raise shear stress inside the hollow fiber spinneret. This change reduces the internal diameter and increases wall thickness, leading to tighter molecular packing and a thicker outer skin. These morphological changes enhance selectivity but reduce water permeation and flux, impacting efficiency.
Outside range leads to unstable fiber formation, defects, poor efficiency
Operators monitor and adjust these spinning parameters to maintain efficiency and prevent defects. Stable spinning conditions support uniform fiber geometry and reliable membrane performance.
Viscosity and Composition
The viscosity and composition of both dope and bore fluid play a central role in hollow fiber spinning. The dope solution, typically a polymer dissolved in a solvent, must have the right viscosity to ensure smooth extrusion through the hollow fiber spinneret. High polymer concentration increases viscosity, which slows phase inversion and leads to thicker walls and smaller pores. Conversely, lower viscosity allows faster phase separation, resulting in larger pores and higher porosity.
The composition of the bore fluid, such as the ratio of solvent to non-solvent, also affects the spinning process. For example, increasing solvent content in the bore fluid reduces fiber diameter and wall thickness, while enhancing inner surface porosity and pore size. These changes alter the microstructure from large voids to finger-like microvoids, which impacts mechanical strength and biological performance.
Note: The PLGA/DMSO/water ternary phase diagram and Flory-Huggins theory help predict how changes in viscosity and composition affect membrane morphology and efficiency.
Operators must carefully select the composition of both dope and bore fluid to match the desired membrane properties. Adjusting these spinning parameters allows for control over pore size, wall thickness, and surface features, which directly influence efficiency and application suitability.
Pressure and Temperature
Pressure and temperature are critical spinning parameters in hollow fiber spinning machine. Although pressure effects on the dope and bore fluid interface remain less documented, temperature variations significantly impact membrane morphology and efficiency. Lowering the spinning temperature, for example by adding triethylene glycol (TEG), shifts phase separation from solid-liquid to liquid-liquid. This change produces well-connected porous structures and enhances water permeability.
The table below summarizes the effects of temperature and composition on interface stability and membrane properties:
Parameter
Effect on Interface Stability and Membrane Properties
Temperature Variation
Lower temperature promotes liquid-liquid phase separation, increases porosity
Bore Fluid Composition
Higher propylene carbonate (PC) ratio increases inner surface porosity and permeability
Pressure Variation
Not well documented in current studies
Operators must maintain consistent spinning conditions by controlling temperature and monitoring the interaction between polymer and diluent. Strong interactions lead to spherulite structures, while weaker interactions favor interconnected networks. These adjustments ensure the hollow fiber spinning machine produces membranes with optimal efficiency and desired characteristics.
Tip: Regular calibration of temperature and pressure sensors in the hollow fiber spinning machine helps maintain stable spinning conditions and high efficiency.
Imbalance Effects
Common Defects
Imbalances in dope and bore fluid during the spinning process often lead to several defects in hollow fiber membranes. Operators observe that even small deviations in flow rates or composition can cause significant changes in membrane structure. The hollow fiber spinneret plays a crucial role in maintaining stability, but improper settings still result in defects. The table below summarizes the most frequently reported issues:
Uniform porous bore, insufficient mechanical strength due to wall thickness variation
Buckling limits mechanical strength and permeability due to shape and thickness irregularities
Structural Defects
Variations in wall thickness and shape, unstable jet formation at certain bore compositions and air gap conditions
Leads to inconsistencies in membrane structure and performance
Dense Inner Layer
Low NMP content in bore fluid (50-60%)
Formation of dense inner layer, which may hinder coating and pore formation
Tearing of Dense Layer
High draw ratio causing stretching during spinning
Potential tearing of thin dense layer on bore surface
Insufficient Solidification
Low residence time in air gap
Causes buckling and delamination due to inner layer not solidifying before outer layer shrinkage
Operators find that these defects directly impact the quality of hollow fiber membranes. Delamination and buckling reduce mechanical strength, while dense inner layers limit permeability. Each defect results from specific imbalances in the spinning process, especially when the hollow fiber spinneret does not maintain proper flow and composition.
Performance Issues
Defects caused by dope and bore fluid imbalance lead to serious performance issues in hollow fiber membranes. Membranes with delamination or buckling often show reduced selectivity and lower flux. Dense inner layers block effective coating and decrease pore formation, which limits the membrane’s ability to separate gases or liquids efficiently. Structural defects create inconsistencies in membrane properties, making it difficult to achieve reliable results in industrial applications.
Operators notice that poor control over the hollow fiber spinneret settings often results in unstable fiber formation. This instability affects the uniformity of membrane properties, such as pore size and wall thickness. As a result, the overall quality of hollow fiber membranes declines. Membranes with these issues cannot meet strict industry standards for filtration or gas separation. Consistent monitoring and adjustment of spinning parameters remain essential for maintaining high membrane quality and achieving the desired properties in every batch.
Tip: Regular inspection of the hollow fiber spinneret and careful adjustment of dope and bore fluid parameters help prevent defects and ensure the production of high-quality hollow fiber membranes.
Process Control in Hollow Fiber Spinning Machine
Spinneret Operation
Operators in industrial hollow fiber spinning manage the simultaneous feeding of dope and bore fluid through the hollow fiber spinneret. This process requires precise coordination to maintain optimal spinning conditions. The system typically uses two peristaltic pumps. One pump delivers the dope solution, which is prepared in a mechanically stirred tank by blending polymer, additives, and solvent. The other pump supplies the bore fluid, often water, from a surge tank. Both fluids enter the spinning block, where the hollow fiber spinneret, fabricated with advanced techniques, ensures accurate geometry and flow paths.
The dope extrudes through the spinneret, while the bore fluid passes through spinneret needles to form the hollow core. Operators control key spinning parameters, such as dope and bore fluid flow rates, air gap length, coagulation bath temperature, and take-up speed. The extruded fiber then moves through a coagulation bath, where solvent exchange solidifies the structure. Guide rollers and a collecting winder gather the finished fiber. Rheological properties of the dope and flow stability remain critical for avoiding defects like irregular bore diameters or surface roughness. The entire hollow fiber spinning machine integrates these steps for continuous, uniform fiber production.
Tip: Operators should always verify the rheological properties of the dope before starting the spinning process to prevent defects.
Monitoring and Adjustment
Effective process control in hollow fiber spinning relies on real-time monitoring and adjustment. Operators use sensors and automated control systems to track temperature, pressure, and flow rates of both dope and bore fluid. These technologies provide continuous feedback, allowing immediate correction if spinning conditions deviate from set points. Sampling and testing, along with data logging, support ongoing quality control and process optimization.
Sensors monitor dope and bore fluid flow rates.
Automated systems adjust pump speeds to maintain balance.
Data analysis identifies trends and supports preventive maintenance.
Operators ensure that the hollow fiber spinning machine maintains stable spinning conditions, which leads to consistent fiber geometry and membrane performance. Regular monitoring and prompt adjustment help prevent defects and support high-quality hollow fiber spinning outcomes.
Optimization in Hollow Fiber Spinning
Adjusting Flow and Composition
Operators seeking to optimize hollow fiber membranes focus on precise adjustment of dope and bore fluid flow rates and composition. The hollow fiber spinneret enables accurate delivery of both solutions, which is essential for maintaining membrane quality and efficiency. Careful selection of bore fluid composition prevents premature phase inversion inside the fiber, allowing the phase inversion front to move from outside to inside. This control shapes the internal channels and supports uniform membrane production.
Operators often use NMP or DMF as polymer solvents, with small water additions (1–10 wt%) in the bore fluid to influence phase inversion dynamics.
Adjusting the carrier needle speed and residence time in the precipitator according to solution viscosity helps maintain stable spinning conditions.
Changing the dope solution composition, such as varying polysulfone concentration or adding PEG400, shifts membrane morphology from finger-like channels to spongy pores.
Increasing polymer concentration and additives in the dope solution leads to denser membrane structures, which improves membrane functionality and longevity.
The dry-jet wet-quench method requires precise control of dope and bore fluid compositions and flow rates. Operators maintain specific flow rates for bore, core, and sheath fluids to ensure uniform fiber formation. Temperature control and post-treatment steps further enhance membrane quality and reduce defect rates. Adjusting dope viscosity and water affinity by selecting appropriate solvents and non-solvents allows operators to tune the sequence of phase separation, which directly impacts membrane properties and efficiency.
Enhancing Membrane Properties
Process parameter optimization in spinning directly enhances membrane properties and efficiency for a wide range of applications. Operators use hybrid braid compositions, such as CA/PAN or PET/PAN, to balance fiber swelling and mechanical strength. Adjusting the injection rate of non-solvent during phase inversion influences membrane morphology, with higher rates producing uniform macrovoid distribution and improved permeability.
Surface modification of braid fibers with silane coupling agents and acrylate adhesives fills gaps between fibers, increasing tensile strength and wettability. Incorporating additives like ZnO nanoparticles into the dope solution improves infiltration into the braid, enhancing mechanical stability and reducing separation layer thickness. The choice of spinning method, such as electrospinning or non-solvent-induced phase inversion, and control of parameters like polymer concentration, flow rate, and coagulation conditions, affect fiber morphology, porosity, and membrane performance.
Operators find that optimizing polymer composition ratios in the braid and coating layers improves tensile strength, bursting strength, permeability, and fouling resistance. These improvements support high-quality membrane production and ensure that hollow fiber membranes meet the demands of diverse applications, from water treatment to gas separation. Maintaining the balance of dope and bore fluid flow rates during scale-up ensures consistent membrane properties and efficiency, supporting reliable membrane functionality across production scales.
Tip: Regular calibration of pumps and monitoring of spinning parameters help maintain high membrane quality and efficiency, ensuring optimal membrane functionality for all applications.
Conclusion
Operators achieve reliable membrane production by following key steps:
Synchronize polymer extrusion and winding speeds.
Adjust bore fluid pressure and air gap distance.
Use pure water for bore fluid and coagulant baths.
Maintain room temperature during spinning.
Employ the dry–wet spinning technique with syringe pumps.
Monitor dope and bore fluid flow rates and spinneret dimensions.
Step
Parameter Details
Dope Preparation
Controlled temperature and stirring
Degassing
8-hour undisturbed period in spinneret tank
Equipment Setup
Sealed, argon-purged dope tank
Temperature Control
55 °C for dope and bore fluid, 20 °C for bath
Flow Rates
1 g/min for both dope and bore fluid
Air Gap Adjustment
10–30 cm in 5 cm increments
Winding Speed
13 m/min
Post-Treatment
Sequential immersion and vacuum drying
Continuous monitoring and adjustment of dope and bore fluid parameters, especially at the hollow fiber spinneret, ensure membranes with strong structure and high permeate flux. These practices support consistent membrane quality, efficient production, and robust performance in diverse applications. Operators who optimize spinning parameters enable membranes to meet the demands of water treatment, gas separation, and other advanced applications. Ongoing process control remains essential for long-term membrane durability and success in all applications.
