HDPE Pipe Extrusion: Sag, Ovality and the Cooling That Decides Both

The line in one paragraph

HDPE pipe extrusion is a single-screw process feeding a spider or basket die, followed immediately by vacuum sizing, spray or immersion cooling, haul-off and cut-off. The melt leaves the die at 190 to 220 °C as a tube that is still soft enough to deform under its own weight, and the rest of the line exists to stop it from doing so.

What makes HDPE different from PVC pipe is that there is no calibration sleeve holding the shape by contact. PVC pipe is sized against a water-cooled sleeve that sets the surface within a few centimetres. HDPE is sized by vacuum against a perforated sleeve while the wall is still molten through most of its thickness, which means the roundness you get depends on how evenly the vacuum pulls and how fast the wall stiffens.

Sag: a race between gravity and crystallisation

Sag is the downward movement of the tube between the die and the first cooling point. It is not a surface defect; it is a change in wall thickness, thick at the bottom and thin at the top. A pipe with 5 percent sag has a bottom wall 10 percent thicker than the top and an ovality that no sizing sleeve will correct, because the asymmetry is already frozen in.

Three variables drive it. Melt temperature sets how soft the tube is. Line speed sets how long it hangs. Wall thickness sets how much mass is hanging. The relationship is not linear in any of them: doubling the wall roughly quadruples the sag, because mass and thermal resistance both increase.

The practical rule used on most lines is to keep the unsupported distance between die and first cooling under 150 mm for pipes below 160 mm, and under 80 mm above 315 mm. Beyond that, the tube is hanging long enough to move regardless of what the vacuum does.

Table 1 — Unsupported length between die and first cooling, by diameter. Above 400 mm the internal air pressure becomes a shaping tool, not just a support.

Cooling is the only lever that actually works

You cannot raise the melt temperature to improve flow and then complain about sag; the two are the same decision. What you can do is remove heat faster, and that means more tank, colder water, or both.

The cooling length required grows faster than line speed. In practice, going from 0.5 to 1.0 m/min on a 250 mm pipe needs roughly 2.5 times the tank length, not twice, because the wall has to shed heat through an increasingly thick frozen layer. A line that was commissioned with an 8 m tank and then pushed to double speed will produce oval pipe and the operator will blame the vacuum.

Water temperature matters more than most operators expect. HDPE gives up heat slowly, and a 25 °C tank against a 15 °C tank changes the achievable line speed by 15 to 20 percent on thick walls. Chillers pay for themselves on anything above 315 mm.

Spray cooling is more effective than immersion for large diameters, because the water that has already absorbed heat runs off instead of insulating the pipe. The best lines use a short immersion section to set the surface and then spray for the rest of the length.

Sizing: holding a round that wants to be oval

Vacuum sizing works by pulling the still-soft tube against a perforated sleeve or into a ring of water. The vacuum level is not the interesting number; the distribution is. A sleeve with blocked holes on one side pulls unevenly and produces an oval that follows the blockage pattern.

For pipes above 250 mm, internal air pressure is used together with external vacuum. A slight positive pressure inside the tube pushes the wall out against the sleeve, which improves roundness and reduces the vacuum needed. The pressure is small — 0.02 to 0.05 bar — and it is regulated, not guessed.

The sizing sleeve diameter must allow for shrinkage after the pipe leaves the line. HDPE pipe shrinks about 1.5 to 2.5 percent in diameter over the first 24 hours, depending on wall and cooling. A sleeve cut to the nominal diameter will produce pipe that measures under nominal the next day, which is why sizing sleeves are cut oversize by a factor that is specific to the line.

SDR, wall and the pressure rating nobody reads

SDR is the ratio of outside diameter to wall thickness. It is the only number that matters for pressure rating, because the hoop stress in a pressurised pipe depends on that ratio and not on the absolute size. A 32 mm SDR 11 pipe and a 630 mm SDR 11 pipe have the same pressure rating.

The rating itself comes from a long-term hydrostatic test at 20 °C, extrapolated over 50 years. For PE100 that gives a design stress of 8 MPa, and the rating follows as 2 × 8 / (SDR − 1). SDR 11 gives 16 bar, SDR 17 gives 10 bar, SDR 26 gives 6 bar.

Table 2 — SDR against wall and rating for a 250 mm PE100 pipe. The rating assumes water at 20 °C; derate for temperature and for aggressive media.

The number that gets missed is the temperature derating. PE100 at 40 °C is rated at about 0.74 of its 20 °C value, and at 60 °C about 0.52. A line specified for 16 bar cold service and then used for 60 °C process water is running at roughly 8 bar of actual capacity, and it will fail at a joint long before the pipe bursts.

What goes wrong above 250 mm

Small pipe is forgiving. Large pipe is not, because every problem has more time and more mass to express itself.

Table 3 — The six defects that account for most large-diameter HDPE scrap.

The one that hides until installation

Residual stress does not show on the line. It shows when the pipe is cut — the end opens or closes — or when it is jointed and the fusion face does not line up. A pipe that passes every dimensional check and then curls when cut was cooled too fast on the outside and too slowly on the inside. The fix is a longer tank or warmer water, not a straighter rack.