HDPE pipe has many excellent properties, such as light weight, corrosion resistance, smooth inner wall, long service life, etc., but also with its high strength applied to pressure-conveying pipelines of various media, but its poor environmental stress cracking performance , Some HDPE pipes will be damaged due to 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 degree of crystallization has a greater impact on the mechanical properties of the pipe. A factory HDPE pipe is formed in a water-cooled vacuum sizing box. The wall sections have different thermal histories. The thermal history has different crystallization behaviors for HDPE, forming an aggregation structure with different crystallinity. The rapid cooling of the outer surface of the tubing can quickly cross the optimal crystallization temperature and form a layer of aggregate structure with lower crystallinity. The outer surface layer and the intermediate layer have a crystal layer which can obtain the most favorable number of crystal nuclei and the growth rate. In this region, the crystals grow well and the aggregate structure is stable. If a certain water temperature and corresponding pulling speed are controlled at this time, the layer can be increased. Thickness. Due to the slow heat transfer of HDPE, the optimum crystallization temperature stays in the middle and inner layers for a long time, and the crystallization can be performed under sufficient conditions. The result is a smaller number of aggregated structural regions with larger particles. The heterogeneity of the internal and external structure has a great influence on the mechanical properties of the pipe. The low crystalline area makes the pipe tough, the high crystalline area has rigidity, and the brittleness increases as the crystallinity increases. When flattened large cracks appeared in the surface of 2/3; Continue compressed, we believe that crack extension outer surface of homopolymer high density pipe extrusion molding process should pay attention to.
1.2 Effect of Melt Temperature
The melting temperature during molding and the residence time at this temperature will affect the number of remaining crystal nuclei. The presence or absence of crystal nuclei and the size of crystal nuclei have a great influence on the crystallization rate during molding. If the melting temperature is low, melting occurs. The time is short, the residual nucleus is not damaged, the crystal nucleus crystallizes fast during molding, the size of the crystal is uniform, and a relatively stable aggregate structure can be formed; if the melting temperature is high, the melting time is long, and the original structure is destroyed more, the residual The smaller the number of crystal nuclei, the longer the nucleation time, the slower the crystallization rate, the larger the crystal size, the unstable structure, and thus the strength of the tubing. The melt temperature is high and the stress cracking time is very short. The short-term hydrostatic strength test of melt-formed pipes in heads with long preheating time and high preheating temperature was used. The time was only 4.3 hours. The melting temperature also had a great influence on the melt fracture during HDPE molding. When the melt is subjected to shear stress, when the shear stress reaches or exceeds a certain critical value, the melt will break. The different melt temperatures, the corresponding shear rate is also very different. To ensure the mechanical properties of the pipe, During molding Control the molding temperature, to avoid melt fracture.
1.3 Influence of Traction Speed
In the process of pipe forming, 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 has not recovered. Cooled and shaped, especially when the die has been set, the thin-walled tube can be processed by adjusting the pulling speed. Such a pipe is more likely to produce cracks in the axial direction.