With the wide use of polymer materials as pressure parts, people have started paying more attention to the safety and longevity of polymeric materials. Creep is one of the most important factors to evaluate materials. In this study, a self-designed oscillatory shearing injection molding (OSIM) device was utilized to prepare pure HDPE specimens with special morphologies. According to a comparison of the creep behavior of the OSIM specimens with conventional injection molding (CIM) specimens, the distinction between the resistivity to creep due to the special morphologies was observed. Two initial external stress levels (10 MPa and 15 MPa) and three temperatures (ambient temperature 25 °C, 40 °C and 60 °C) were employed in this experiment. Different morphologies resulted in different responses to creep. The deformation and compliance of the CIM specimens were triple or more than those found for the OSIM specimens under the same conditions. The instantaneous deformation of the OSIM specimens was 0.2% compared with 0.6% found for the CIM specimens under 10 MPa at 25 °C. The deformation of the OSIM specimens was 4% after creep for an hour, but the CIM specimens were already necked at less than 50 min under 15 MPa at 40 °C. At 60 °C, too much plastic deformation appears in the creep behavior of the CIM specimens and the creep behavior was nearly not observed under these conditions. In addition, the creep behavior of the OSIM specimens can be observed at 60 °C. According to our tests and analysis, the property of creep resistivity for the OSIM specimens was better than that found for the CIM samples, in both the amorphous phase and crystalline region. In addition, the creep behavior of the OSIM and CIM specimens was satisfactorily described using the generalized Kelvin–Voigt model with one retardation time.

With the wide application of polymer pipes in the world, people pay more attention to the longevity of the pipes. Slow crack growth is one of the most important factors that influence the longevity of materials, especially PE pipes. Thus, the study of slow crack growth has become deep and concrete. In this paper, slow crack growth in specimens with different morphologies was studied. A self-designed oscillatory shear injection molding (OSIM) device was utilized to prepare pure HDPE 5000s specimens with different morphologies. The process of slow crack growth in OSIM specimens was compared with that in specimens prepared by conventional injection molding (CIM); so it was possible to study the influence of the different morphologies in specimens on the process of slow crack growth under the same external conditions. Different morphologies have great influences on the ability of materials to resist slow crack growth. The time taken for the slow crack growth to fracture in the OSIM specimens is considerably longer than that in the conventional ones. Under the same conditions of 80 °C and 9.0 MPa, the time of complete fracture is 650 min in OSIM specimens, in contrast to 60 min in conventional specimens. Moreover, in the OSIM specimens, the initial stress of the brittle–ductile transition is increased. At 80 °C, the brittle–ductile transition takes place at 4.7 MPa for conventional specimens, whereas for OSIM specimens, it occurs at 5.6 MPa. Furthermore, the mechanisms of their slow crack growth processes are different. The SCG process in conventional specimens is that stress concentration initiates craze, which grows rapidly and becomes a crack. However, the process in OSIM specimens is a cyclic process, in which stress concentration initiates craze, the crack grows, a new craze appears, and the new crack grows until the specimens fracture.

A new homemade apparatus, i.e. vibration assisted extrusion equipment, is employed to extrude polypropylene. Vibration assisted extrusion is based on the application of a specific macroscopic shear vibration field. Reduction of apparent melt viscosity as a function of vibration frequency is measured at different screw speeds and die temperatures. The effect of the process is investigated by performing mechanical tests, differential scanning calorimetry studies, polarized light microscopy and wide-angle X-ray diffraction. It is found that, compared with conventional extrusion, vibration assisted extrusion could effectively improve the rheological properties of PP melt by incorporating an extra shear vibration field. Both the tensile strength and elongation at break increased under the shear vibration field. For vibration assisted extrusion samples, both the melting temperature and crystallinity increased, accompanied by remarkable grain refinement. Vibration assisted extrusion induced a significantly enhanced bimodal orientation with a high fraction of a*-oriented α-crystallites, while only a limited improvement in the flow direction orientation. A structural model, i.e. bimodal c-axis and a*-axis orientation of PP macromolecular chains, was adopted to explain the experimental results.

A small homemade device was used to study the influence of mechanical vibration on the crystal structure and morphology of isotactic polypropylene (iPP) under different melting temperatures, vibration times, vibration frequencies, and cooling rates. The crystallite size, crystal structure, and crystallinity of iPP under or without vibration treatment were investigated by means of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and polarized microscopy observation (PLM). The crystallization of iPP varied with the length of vibration time, vibration frequency, cooling rate, and melt temperature. Compared with the data of conventional samples measured by DSC, vibration could increase the crystallinity of iPP, make melting peak of α-crystal move toward higher temperature and make that of β-crystal shift to lower temperature. Meanwhile, WAXD measurements showed that the vibration could reduce the content of β-crystal evidently, particularly at the lower vibration frequency, lower cooling rate, and higher melting temperature. Furthermore, PLM measurements showed that the vibration made the spherulite size smaller.

Melt vibration technology was used to prepare injection sample of HDPE/nano-CaCO3 blend, whose mechanical properties were improved significantly. Compared with conventional injection molding, the enhancements of the tensile strength and impact strength of the sample molded by vibration injection molding were 41.2 and 43.2%, respectively. According to the SEM, WAXD, and DSC measurement, it was found that a much better dispersion of nano-CaCO3 in sample was achieved by vibration injection molding. Moreover, crystal orientation degree of matrix HDPE increased under the effect of melt vibration. The crystallinity degree of HDPE in vibration sample increased by 5.5% compared with conventional one. The improvement of mechanical properties of HDPE/nano-CaCO3 blend prepared by low-frequency vibration injection molding attributes to the even distribution of nano-CaCO3 particles and the orientation of HDPE crystals and increase of crystallinity degree under the influence of melt vibration.