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Stresses caused during Pipe Manufacture

NEXOR Pipes R&D

July 27, 2006

 

Introduction

Frozen-in stress is present in almost any plastic end product. Frozen-in stress (also known as internal stress or residual stress or molded-in stress) is the result of the cooling of molten material. Differences in the cooling rates among the local volumes of any molten body unavoidably contribute to the formation of frozen-in stress.

There are two general types of residual stresses formed during pipe manufacture: longitudinal residual stress and circumferential residual stress.

Longitudinal Residual Stress

Longitudinal internal stress occurs in the longitudinal or lengthwise direction of the pipe. Generally, longitudinal stress tends towards compression. That is – there is a built in stress that tends to force the pipe to reduce in length.

During pipe manufacture, longitudinal internal stress is created when molten material exits the die head of the extruder and it is pulled by the haul-off machine. The process of pulling creates a tensile force in the longitudinal direction. With Newton’s 3rd law of Motion, an equal and opposite force is created. This means that there exists a compressive force in the lengthwise direction of the pipe. This compressive force is frozen-in the pipe as a result of the molten plastic material being solidified in the vacuum tank and the spray tank while the tensile force due to the haul-off is still in effect.

A way to determine how much longitudinal internal stress is present is to allow the pipe to relax. By heating the pipe to a temperature below its melting point, internal stresses are allowed to take effect. Thus, heating the pipe will cause it to contract in the longitudinal direction.

Determination of longitudinal internal stress of NEXOR pipes is done following the procedures prescribed by ISO 2505 Longitudinal Reversion Test.

The principle is as follows: “A pipe of specified length is placed in an air oven at a specified temperature for a specified time. A marked length of this portion of pipe is measure before and after heating. The reversion is calculated as a percentage of the change in length in relation to the initial length.” (ISO 2505:2005)

The higher the longitudinal reversion of the pipe, the more is the frozen-in stress and the worse the pipe is.

Circumferential Residual Stress

Circumferential Residual Stress is the result of uneven cooling of pipes between its outer layer and its inner layer. Conventional pipe extrusion has cooling on the external surface of the pipe in a water bath. The cooling imbalance between the inner and the outer wall causes compressive stress in the circumferential direction to develop on the outer surface; additionally, tensile stress is developed on the inner wall surface.

The faster the rate of cooling and the thicker the pipe wall thickness, the greater is the residual stress developed in the circumferential direction. While rapid cooling may lead to higher manufacturing productivity, fast cooling times may bring about unwanted residual stress. 

A pipe with high circumferential residual stress cut in the axial direction would show a gap in the circular direction – with the outer surface tending towards each other, while the inner surface edge moving away from each other. The extent of the distance between the edges provides a direct measure of the stress magnitude.

Conclusion

Residual stresses inherent in the extrusion of plastic pipe products may have long term adverse effects on product performance. In particular, ESCR or environment stress cracking is accelerated in pipes with high frozen in internal stresses. Additionally, dimensional changes may occur in PE pipes as the internal stresses are relaxed over time.

Unfortunately for the end-consumer of PE pipes, frozen-in stress is an invisible quantity. It is unseen by the naked eye and it can only be determined using laboratory techniques.

Too often manufacturers of poor quality PE pipes tend to favor the optimization of manufacturing speed as opposed to optimization of product quality, thereby creating unfavorable internal stresses.

NEXOR PIPES are guaranteed to pass longitudinal reversion test as prescribed in ISO 2505. In addition, NEXOR PIPES are also checked for circumferential residual stresses.

References

 

ISO 2505:2005 Thermoplastic Pipes – Longitudinal Reversion – Test Method and Parameters

 

ISO 4427 Polyethylene Pipes for water supply - Specifications

 

Janson, Lars-Eric. Plastic Pipes for Water Supply and Sewage Disposal. NESTE Chemicals, Sweden, 1989.

 

Chasis, David. Plastic Piping Systems. 2nd Ed. Industrial Press Inc. USA. 1988

 

Willoughby, David et al. Plastic Piping Handbook. Mcgraw-Hill Handbooks. USA. 2002