Polytetrafluoroethylene (PTFE) tubes, renowned for their "king of plastics" properties of exceptional temperature and corrosion resistance, are widely used in chemical processing, medical devices, semiconductors, and other high-stakes industries. Despite their deceptively simple appearance, producing a smooth, uniform, defect-free PTFE tube from raw resin involves a sophisticated sequence: resin compression molding → sintering → extrusion → cooling. Each step’s process parameters directly dictate the final product’s performance and yield. This article breaks down the critical controls in these core processes, revealing how precise engineering resolves common issues like rough inner walls, uneven wall thickness, and cracking.

1. First Step: Resin Compression Molding – Laying the Foundation for Billet Density and Uniformity

PTFE resin is a linear polymer polymerized from tetrafluoroethylene monomers, with molecular chains arranged in a helical conformation. Unlike common thermoplastics, it does not melt or recrystallize with heat—making direct injection or extrusion impossible. Instead, PTFE tubes require a "compression molding-sintering" process to create dense preformed billets for secondary processing.

Key Control Parameters

1. Resin Selection: Prioritize suspension-polymerized resin with uniform particle size (20–40 mesh) and water content <0.01%. Impurities or moisture cause bubble formation during sintering, leading to internal defects.

2. Molding Pressure: Divided into pre-pressing(5–10 MPa) to expel air between particles and prevent voids, and final pressing(15–25 MPa) to achieve high density (target: ≥2.1 g/cm³, close to PTFE’s theoretical density of 2.3 g/cm³).

3. Molding Temperature: Maintain 30–50°C (slightly above room temperature). Too low, and friction between particles resists compaction; too high, and premature cross-linking causes inconsistent shrinkage during sintering.

   
   

Common Issues & Solutions

Uneven billet density leads to wall thickness variation during extrusion. One manufacturer once saw ±15% wall thickness deviation due to insufficient pre-pressing pressure (loose center in billets). Fixes: Extend pre-pressing time (10–15 minutes) and add exhaust grooves to the mold to vent trapped air.

2. Second Step: Sintering – Activating Molecular Chain Cross-Linking to Form a 3D Network

Sintering transforms the compacted billet into a dense, insoluble solid via a physicochemical reaction: linear PTFE chains break, rearrange, and form a 3D network while releasing by-products (e.g., perfluoroisobutylene). This step defines PTFE’s mechanical strength, temperature resistance, and dimensional stability.

Key Control Parameters

1. Heating Rate: Follow a "slow-fast-slow" gradient: ≤5°C/min initially (avoid thermal stress cracking from internal temperature gradients), increase to 10–15°C/min at 250°C (when PTFE softens), and slow back to 5°C/min near 380°C (peak sintering temperature) for full melting.

2. Sintering Temperature: Hold at 375–385°C (just above PTFE’s 327°C melting point). Too low, and chains under-cross-link (soft, high-shrinkage billets); too high, and degradation (toxic gases, reduced strength) occurs.

3. Soaking Time: 2–4 hours at peak temperature, extended for thick-walled billets to ensure center sintering.

Common Issues & Solutions

1. Shrinkage Cracking: Caused by fast heating or insufficient soaking—solve by reducing heating rate and extending soak time.

2. Residual Bubbles: From moisture or unexpelled gases—add a vacuum system to the sintering furnace or pre-dry resin at 120°C for 2 hours.

3. Third Step: Extrusion Molding – Determining Tube Dimensions and Surface Quality

The sintered billet is reheated to a molten state (~350–380°C) and forced through a die to form the tube. This step dictates final dimensions and surface finish.

Key Control Parameters

1. Extruder Temperature Profile:Feeding Section (250–300°C): Prevents premature melting/clogging.

   Compression Section (320–360°C): Unwinds molecular chains for flow.

   Homogenization Section (370–380°C): Ensures uniform melt.

   Die Head (360–370°C): Avoids overheating/degradation.

2. Screw Speed & Pressure: Match screw speed to traction speed (linear speed: 0.5–2 m/min). Excess speed causes shear overheating (>400°C, leading to "melt fracture"—sharkskin-like roughness); too slow reduces output. Hold die pressure at 10–20 MPa to stabilize wall thickness.

3. Die Design: Use a capillary die—the gap between the die orifice and mandrel sets wall thickness (e.g., 1.05–1.1 mm gap for 1 mm target thickness, compensating for cooling shrinkage). Polish dies to Ra ≤0.2 μm to prevent inner wall scratches.

Common Issues & Solutions

1. Rough Inner Wall: From unpolished dies or fast extrusion—replace with high-polish dies, reduce screw speed (e.g., 5→3 rpm), and increase die pressure.

2. Wall Thickness Deviation >±5%: Caused by misaligned die/mandrel or unstable traction—calibrate coaxiality (<0.05 mm error) and use servo motors for traction control (linear speed fluctuation <±0.1 m/min).

4. Fourth Step: Cooling & Shaping – Fixing Dimensions and Eliminating Internal Stress

The molten tube must cool to solidify and retain shape. Improper cooling causes deformation, wrinkling, or stress concentration.

Key Control Parameters

1. Cooling Medium & Temperature: Use deionized water (20–30°C) to avoid contamination. Too cold (<10°C) increases internal stress; too hot (>40°C) causes warping.

2. Cooling Length & Speed: Ensure uniform cooling (water tank length = 5–10× tube diameter). For a φ20mm×1.5mm tube, use a ≥1.5m tank to cool to <50°C before exit.

Common Issues & Solutions

1. Tube Bending: From uneven cooling (e.g., unilateral low flow)—check nozzle distribution or adjust traction direction for balance.

2. Internal Stress Cracking (Long-Term): From rapid cooling (surface shrinks faster than interior)—use gradient cooling (30°C warm water → 20°C cold water) to reduce stress.

5. Fifth Step: Post-Processing & Testing – Ensuring Finished Product Reliability

Even after molding, post-processing and testing are critical to remove defects and validate performance.

Key Controls

1. Thermal Setting: For high-precision tubes (e.g., medical grade), reheat to 350°C for 1 hour to eliminate residual stress—shrinking deviation from 3–5% to <1%.

2. Surface Treatment: Reduce inner wall friction (for high-viscosity fluids) with plasma treatment or a <0.1 μm silicone oil coating.

3. Full Inspection:

Wall thickness (micrometer).

Burst pressure (≥2× design pressure).

Surface roughness (profilometer, Ra ≤0.8 μm).

Microbial limit (medical grade: ≤10 CFU/cm²).

Conclusion: The "Synergy Philosophy" of Process Control

• PTFE tube extrusion is a systematic engineering process where every step is interdependent:

• Compression molding gives the billet "inherent quality."

• Sintering activates the "molecular network."

• Extrusion shapes the "external form."

• Cooling fixes the "performance boundaries."

Even minor parameter deviations (e.g., uneven billet density → wall thickness variation → cooling cracks) can cascade into defects. For manufacturers, the path to consistency lies in data-driven monitoring (online thickness gauges, temperature sensors) and standardized SOPs that define parameter ranges for each step.Only through this precision can high-quality PTFE tubes—smooth (Ra ≤0.8 μm), dimensionally uniform (<±3% deviation), and defect-free—be produced to meet the extreme reliability demands of chemical, medical, and semiconductor industries.