How Does the Cooling Rate During PFA Extrusion Change the Residual Stress Distribution in Heater Sheaths?

PFA extruded on a metal core leaves the die at 350-400°C and is cooled down to room temperature. The rate of cooling dictates how the polymer chains freeze in place, which directly controls the distribution of residual stress through the final heater sheath. The fast cooling rate (5-10 deg C/min) prevents relaxation of the chains and results in a low residual stress (1-3 MPa) evenly distributed through the wall thickness. Fast cooling (50-100 °C/min, e.g. water quench) fixes the chains in an orientated condition and results in considerable residual tension (5-10 MPa) concentrated at the inner and outer surfaces. The rapid cooling results in tensile stress of 6–8 MPa on the inner surface (PFA-metal interface) and compressive stress of 4–6 MPa on the outer surface for a PFA sheath with a thickness of 2 mm. This stress distribution accelerates environmental stress cracking and promotes delamination during thermal cycling. Heaters with great dependability require slow cooling.

Mechanisms of Cooling Rate and Stress Formation
As the extruded PFA cools, the outside surface solidifies first as it is in touch with the cooler air or water. The heated core causes the inside ( toward the metal center ) to cool more slowly , so it stays molten longer . This differential solidification results in a temperature gradient through the wall. As the outside skin hardens and contracts it drags the still molten interior outwards, tensioning the outer and compressing the inner. Finally the inner layer solidifies and shrinks, pulling back and reversing the stress condition. The final residual stress relies on the cooling rate and the limitation of the metal core.

For a quickly quenched sheath (water bath at 20°C, cooling rate 80°C/min) the outer skin solidifies in 2-3 sec. The inner layer cools down below 200 °C within 30–60 s. The outside layer freezes in tension (3-5 MPa), while the inner layer freezes under tension (5-8 MPa) from the contracting outer layer. The result was an inner surface in tension (poor) and an outside surface in tension (less terrible). The stress at the inner surface favours the crack initiation at the contact of PFA and metal.

For a gradually cooled sheath (air-cooling in a temperature controlled oven, 8°C/min) the temperature gradient over the wall is small. The wall solidifies almost all at once. The residual stress is minimal (1–2 MPa) and mostly compressive at the inner surface (excellent, as compression closes microcracks).

Quenching Rate and Residual Stress
Cooling Rate (°C/min) Cooling TechniqueInner surface stress (MPa) Outer surface stress (MPa)Type of interface stressDelamination Risk
oven cool, gradually (annealed)5–10 −1 to −2 (compressive) −1 to −2 (compressive)Compressive (good) Very low
Air cool (room temperature, still air) 15–25 +1 to +3 (tensile) 0 to +1 (tensile) Mild tensile Low Forced air (fan) 30–50 +3 to +5 +1 to +3 Moderate tensile Moderate
Water quench (cold) 60-100 +6 to +10 +3 to +6 High tensile High
Ice water quench 100–150 +10 to +15 +5 to +10 Very high tensile strengthExtremely high
Note: (+) positive values indicate tensile stress; (−) negative implies compressive stresses.

Influence of Residual Stress on Heater Performance
Fast cooling leads to high residual tensile stress at the PFA-metal interface, which has three detrimental effects:

Accelerated Environmental Stress Cracking With strong chemicals (acids, chlorinated solvents), the stressed contact fractures at lower applied stress. If the heater with 8 MPa residual stress may shatter in 30% HCl at 90°C in 500 hours, the same heater with 2 MPa residual stress lasts 5,000 hours .

Reduced thermal cycle life: Each heating/cooling cycle adds thermal expansion stresses to the residual stress . If the sum is more than the yield strength of PFA (12-15 MPa) the interface yields. Heater swiftly cooled (residual 8MPa) + thermal stress (5MPa) = 13MPa ~ yield. 7 MPa is safe with a slow cooled heater (residual 2 MPa) + thermal stress (5 MPa).

Premature delamination: The PFA separates from the metal core due to tensile stress, forming microgaps. Moisture seeps into the crevices and then heating turns the moisture to steam, scorching the sheath.

Detection and Designation
Residual stress can be determined by cutting a ring from the PFA sheath, making an axial slice, and measuring the resulting curvature (radius of curvature method). Fast cooled tubes curve into a tight circle (radius 20–50 mm), slow cooled tubes stay almost straight (radius >200 mm).

Specify in procurement . PFA sheath shall be slow cooled after extrusion with a cooling rate not exceeding 15°C/min from melt to 100°C. Request residual stress measurement according to ASTM D6991 (slit tube method) with acceptance criterion: curvature radius ≥150 mm for a 100 mm long ring.

Conclusion: Slow cooling reduces residual stress by 70-80%.
The PFA extrusion cooling rate determines the residual stress distribution in heater sheaths directly. Rapid cooling (water quench) induces high tensile stress (6–10 MPa) at the PFA-metal interface leading to stress cracking, reduction in thermal cycle life and delamination. Slow cooling (air or oven, 5–15°C/min) reduces residual stress to 1-3 MPa (compressive or low tensile) and increases heater life by 3-5× in aggressive usage. For any heater used for thermal cycling or corrosive conditions, specify slow cooling and confirm with residual stress testing. The processing time is longer (hours versus minutes) and increases the production cost by 10-20%, but is paid back by a longer service life. A quickly cooled heater is a recipe for a disaster. Slow it cool, low it stress, long it hot. That's the law of extrusion.