| Recent years, the plastics pipes have been widely used all over the world. Unfortunately, pipes made in the traditional way perform better in the axial strength rather than in the circular one, which is opposite with the requirement of productions. The online biaxial stretch forming technology has been proved to be effective in ascending the circumferential performance of pipes. In this paper, an online biaxial stretch forming set based on the internal pressure dilation method was introduced to produce two-way stretch polyolefin pipes. The influence of applying circumferential stretch and changing the axial draw ratio to the properties of pipes were studied. And the self-reinforcement mechanism of the two-way stretch method was discussed.The applying of circumferential stretch enhanced the circumferential blasting stress significantly. Increment ranged from 2.63%~32.74% and 17.94%~41.65% could be observed in HDPE pipes and PPR pipes respectively. Against this, axial tensile strength and modulus fell slightly, with the maxmum decline of 4.79% and 13.83% respectively in HDPE pipes. However, the tensile elongation at break was improved, either in HDPE and PPR pipes.The degree of crystallinity of the drawing direction was heightened with the increase of draw ratio. The X-ray diffraction peak intensity of crystal plane(110) in HDPE and(110),(040),(130),(111) in PPR became stronger in the direction of stretch, and dropped in the relatively vertical direction. DSC curves revealed an enhancement of the mass crystallinity, which could be attributed to more deformation caused by the additional circumferential stretch. For instance, with axial draw radio fixed as 1.924, the crystallinity of HDPE raised from 39.35% to 58.46%. An increase from 21.60% to 28.07% could also be observed in PPR. The SEM pictures revealed the microstructure of polyolefin pipes. It’s easy to find a decline of axial orderliness duo to applying of the circumferential stretch. Furthermore, the disorder of microstructure in the stretch plane was enhanced, leading to isotropous properties. The air pillow caused by the air pressure expansion method brought the pipes with a smooth inner surface. Contrast with uniaxial stretch pipes, the inner surface of the biaxial stretched pipes was much smoother, with no obvious scratch existing on which. When the axial draw radio was fixed as 2.836, the slip-starting angle of water on uniaxial stretch HDPE’s inner face was 23.81°. And the corresponding value of biaxial stretch HDPE pipes was 19.29°. |
