Shrinkage of HDPE Pipes Outer Diameter

[rambabu0608]

Dear Forum Members,

We would like to hear from the forum experts that, Is HDPE pipe outer diameter (OD) will get shrink after manufacture with time and temperature? If so, what could be the problem?

We have received huge quantity of various sizes of pipes (OD 110 - 400 mm, SDR 36,) from a manufacturer around 8 months before for which the Outer diameter of pipes were found in specified standard limits during production and pipes were produced under controlled process conditions. The temperature of pipes was at almost ambient conditions (@ 25 - 30 Deg.C) when measured 100% pipes before despatch during manufacturing through Third-party inspectors (TPI) services. These pipes shall be used for lining services inside carbon steel spools where the OD of Pipes acts a major role.

But now , we have found reduction in outer diameter (1.5 - 3.0 mm) for all pipes irrespective of pipe size.
(Temp. at site was 15 - 18 Deg.C)

The reduction in Outer diameter of pipe is 1.0 mm for smaller size for 110 mm and 2.5 - 3.0 for 400 mm pipes..

Please lets know the technical justification for reduction of outer diameter of HDPE pipes.

Thanks in Advance.

RAM

 

[Compositepro]

I have no direct experience with this issue but it does not surprise me. When pipes are extruded they will be cooled with water on the outside to set the size and shape. The outside layers of polymer will freeze first and harden while inside layers are still soft. As the inside layers cool, they shrink. But this shrinkage is restrained by the already frozen outer layers. Thus there are internal stresses in the pipe which will slowly be relieved by creep of the polymer. The creep rate is is strongly dependent on the temperature. Annealing the pipe at 150F, or so, would relieve the stresses fairly rapidly.

 

[Demon3]

Things shrink when cooled. Perhaps someone smarter than me can calculate whether that shrinkage can be explained by the latter measurement being done at 10C lower temperature.

Chris DeArmitt - PhD FRSC CChem
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[dgallup]

HDPE CTE is 120 x 10^-6/degC so a 400 mm pipe will shrink about 0.48 mm per 10 degree C change in temperature. Doesn't look like enough to cover the discrepancy.

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[patprimmer]

HDPE is quite prone to post moulding shrinkage where the crystals continue to form well after moulding. As the crystals form into a tighter packed crystal structure the material continues to shrink.

Between moulding and one hour later it can be quite pronounced. The longer the time after moulding the slower the rate. If you graph size vs time after moulding, you will get an asymptote.



Regards
Pat
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[Demon3]

If you add a nucleating agent you your PE it will crystallize faster and you'll get less post shrinkage and better dimensional stability. As I work for a company that makes an HDPE nucleating agent for just that I'll say no more.

Chris DeArmitt - PhD FRSC CChem
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[patprimmer]

Chris.

I'm not sure what nucleating agents do to the processing properties when extruding pipe and trying to control size in the calibrator. Typically amorphous resins are easier to control.

Regards
Pat
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