Stainless Steel Capillary Tube Manufacturing Process: From Raw Material to Finished Product

Overview of Capillary Tube Manufacturing

Stainless steel capillary tube manufacturing is a precision process that transforms solid stainless steel rod into ultra-thin walled tubes with tight dimensional tolerances. The process involves multiple stages of forming, drawing, annealing, and finishing to achieve the required specifications.

Typical capillary tube dimensions:

• Outside Diameter (OD): 0.3mm – 6.0mm (0.012″ – 0.236″)
• Wall Thickness: 0.05mm – 0.5mm (0.002″ – 0.020″)
• Length: Up to 30m in coil, or straight lengths
• Tolerance: OD ±0.01mm, Wall ±10%

Step-by-Step Manufacturing Process

Step 1: Raw Material Selection

High-quality stainless steel rod/billet is selected based on the required grade (304, 316L, 420, etc.). Material must meet:

• ASTM/EN/JIS material standards
• Chemical composition certification
• Mechanical property verification
• Surface quality inspection (no scratches, pits, or inclusions)

Step 2: Piercing (Tube Formation)

The solid rod is converted into a hollow tube through one of these methods:

Rotary Piercing (Mannesemann Process)

• Heated rod (1100-1200°C) is rotated and pressed against two rolling mills
• A piercing mandrel creates the hollow center
• Produces a “mother tube” with larger OD and wall thickness
• Suitable for larger diameter tubes (OD > 10mm)

Extrusion

• Cold or hot extrusion through a die with a mandrel
• Better surface finish and dimensional control
• Suitable for smaller diameter tubes

Drilling (for micro capillary tubes)

• Precision drilling for very small diameter tubes (OD < 1mm)
• Higher material waste but excellent dimensional control

Step 3: Cold Drawing

The mother tube is drawn through a series of dies to reduce diameter and wall thickness to final specifications:

• Die Drawing: Tube is pulled through a carbide or diamond die
• Mandrel Drawing: Internal mandrel controls wall thickness
• Multiple Passes: Typically 5-15 drawing passes with intermediate annealing
• Lubrication: Special drawing lubricants reduce friction and prevent galling

Each drawing pass reduces the diameter by 10-20% and work-hardens the material. Intermediate annealing is required to restore ductility.

Step 4: Intermediate Annealing

After several drawing passes, the tube is annealed to:

• Relieve internal stresses
• Restore ductility for further drawing
• Recrystallize the grain structure
• Improve surface quality

Annealing is performed in a controlled atmosphere furnace (nitrogen/hydrogen) at 1040-1120°C for austenitic stainless steels, followed by rapid cooling.

Step 5: Final Annealing

The final annealing step ensures:

• Consistent mechanical properties
• Optimal corrosion resistance
• Stress-free condition
• Bright, oxide-free surface

Bright annealing in a controlled atmosphere produces a surface ready for most applications without additional finishing.

Step 6: Surface Treatment

Depending on the application, additional surface treatments may be applied:

Pickling

• Removes oxide scale and surface contaminants
• Typical pickling solution: Nitric acid + Hydrofluoric acid
• Followed by water rinsing and passivation

Electropolishing

• Electrochemical removal of surface material
• Produces ultra-smooth surface (Ra ≤0.2μm)
• Enhances corrosion resistance
• Common for medical and food applications

Passivation

• Chemical treatment (nitric acid or citric acid) to enhance chromium oxide layer
• Improves corrosion resistance
• Standard for all stainless steel products

Step 7: Precision Cutting

Tubes are cut to specified lengths using:

• Laser Cutting: High precision, burr-free, suitable for micro tubes
• Rotary Cutting: Mechanical cutting for larger tubes
• Waterjet Cutting: Cold cutting process, no heat-affected zone

Cutting parameters are optimized to minimize burr formation and maintain dimensional accuracy.

Step 8: Forming and Fabrication

For specialized applications, additional forming operations may include:

• Bending: CNC bending to precise angles and radii
• Flaring: Expanding tube ends for fittings connections
• Reducing: Tapering tube ends for specific applications
• Swaging: Reducing tube diameter at specific points
• Welding: Laser welding, TIG welding for assemblies

Step 9: Quality Inspection

Comprehensive quality control at every stage:

Step 10: Cleaning and Packaging

Final cleaning and packaging ensures product quality during storage and transport:

• Cleaning: Ultrasonic cleaning to remove oils and particles
• Drying: Hot air or vacuum drying
• Coiling: Precision coiling for flexible packaging
• Packaging: Protective wrapping, sealed bags, labeled boxes
• Documentation: Material test reports, inspection certificates, traceability records

Quality Standards

Our manufacturing process complies with:

• ASTM A269/A269M (Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service)
• ASTM A213/A213M (Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger Tubes)
• EN 10216-5 (Pressure equipment – Seamless steel tubes)
• ISO 9001:2015 (Quality Management System)
• ISO 9001:2016 (Medical Application Quality Management – for precision grade)

Why Our Manufacturing Process Stands Out

• Precision: Tight tolerances (OD ±0.01mm, Wall ±10%)
• Consistency: Batch-to-batch uniformity with full traceability
• Flexibility: Custom sizes, lengths, and surface treatments
• Quality: 100% inspection with comprehensive documentation
• Speed: Fast turnaround for standard and custom orders

Contact us for detailed specifications and custom manufacturing capabilities.