Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

Abstract

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom-made electromagnetic dynamic plasticating extruder was adopted to extrude high-density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high-circumferential-strength pipes.RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial selfreinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide-angle X-ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology.

An alternative explanation of forming force reduction for forming process submitted to vibration: Influence of the waveform in the viscoplastic domain

“…Reduction in forming load is reported due to the effective reduction during the interval of ultrasonic excitation. Effect of vibration extrusion on the structure and properties of high-density polyethylene pipes (HDPE) has been analyzed by Kaiyuan et al (2009). Important finding from these research work predicts increase in bursting pressure of pipe, tensile yield strength, higher crystallinity, larger crystal sizes and more perfect crystals which are highly desirable in metal forming process.…”

Mechanical and Microstructural Properties of HDPE Pipes Manufactured via Orbital Friction Stir Welding

“…Since the probability p was lower than 0.05 for all the sample typologies reported in Table 2 , the differences between the samples welded with different parameters were considered highly significant. As a result of the macromolecular orientation during extrusion, polymer pipes had a higher axial strength than circumferential strength [ 23 ]. Through the FSW process, the circumferential strength of the HDPE pipes increased marginally.…”

The effect of multiple variables on tensile property of injection‐molded polypropylene through the combination of orthogonal design and variance analysis

“…Extensive studies have been performed on shear‐induced crystallization under various kinds of flows in industrial and scientific fields, which led the molding techniques related to shear flow, such as dynamic packing injection molding (DPIM),1–4 vibration extrusion,5, 6 and fiber spinning,7 became popular in past several decades. It is well known that shear flow field causes the crystallization of polymers into oriented structures 8–10.…”