Effect of hdpe pipe forming process conditions on stress cracking

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HDPE pipe has many excellent properties, such as light weight, corrosion resistance, smooth inner wall, long service life, etc., and its high strength is applied to pipes carrying various media under pressure, but its environmental stress cracking performance is poor. Some HDPE pipelines will experience stress damage under long-term load and after a certain period of time.

1.1 Effect of cooling rate

Temperature is the most sensitive factor in the crystallization process of HDPE. Different degrees of crystallization have a great influence on the mechanical properties of the pipe. A factory HDPE pipe is formed in a water-cooled vacuum sizing box. The pipe wall section has different thermal history. Different thermal history has different crystallization behaviors for HDPE, forming a different crystallinity aggregation structure. The outer surface of the pipe is rapidly cooled by water to quickly cross the optimum crystallization temperature to form a layer of aggregated structure with a lower degree of crystallinity. The outer layer and the intermediate layer have a crystal layer which can obtain the most favorable number of crystal nuclei and growth rate, and the crystal grows well and the aggregate structure is stable. If a certain water temperature and corresponding traction speed are controlled at this time, the thickness of the layer can be increased. Due to the slow thermal conduction of HDPE, the optimum crystallization temperature stays longer in the middle layer and the inner layer, and the crystallization can be carried out under sufficient conditions, resulting in a small number of aggregated structure regions with larger particles. The unevenness of the inner and outer structures has a great influence on the mechanical properties of the pipe. The low crystallization zone makes the pipe tough, the high crystallization zone has rigidity, and the brittleness increases with the increase of crystallinity. When the flattened to 2/3, cracks appear on the inner surface; continue to flatten and the crack spreads to the outer surface. We believe that the use of high density homopolymer extruded tube should pay more attention to the molding process.

1.2 Effect of melting temperature

The melting temperature during molding and the time of residence at this temperature affect the amount of residual nuclei. The presence or absence of nuclei and the size of the nuclei have a great influence on the crystallization rate during molding. If the melting temperature is low, melting The time is short, the residual core is not damaged, the crystal nucleation speed is fast, the crystal size is uniform, and a relatively stable aggregate structure can be formed. If the melting temperature is high, the melting time is long, the original structure is destroyed more, and the residual The less the nucleus, the longer the nucleation time, the slower the crystallization rate, the larger the crystal size, and the unstable structure, which affects the strength of the tube. The melt temperature is high and the stress cracking time is very short. We have used the melt-forming tube in the head with long preheating time and high preheating temperature for short-term hydrostatic strength test, which is only 4.3h. The melting temperature also has a large effect on the melt fracture during HDPE molding. The melt is subjected to shear stress during molding, and when the shear stress reaches or exceeds a certain critical value, the melt ruptures. The melt temperature is different and the corresponding shear rate is also very different. In order to ensure the mechanical properties of the pipe, the molding temperature should be controlled during molding to avoid melt fracture.

1.3 Influence of traction speed

During the pipe forming process, the speed of the melt extrusion die should be coordinated with the speed of the traction pipe. If the traction speed is greater than the extrusion speed, some of the macromolecules are oriented along the traction direction. Under the action of the traction force, the elasticity is not restored and is cooled and shaped, especially when the die is fixed, and the traction speed is adjusted to process the thin film. Wall tubes, such tubes are more susceptible to cracks in the axial direction.

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