ORIGINAL CONTRIBUTION Morphology and mechanical property of high-density polyethylene parts prepared by gas-assisted injection molding Long Wang & Bin Yang & Wei Yang & Nan Sun & Bo Yin & Jian-Ming Feng & Ming-Bo Yang Received: 29 May 2011 /Revised: 28 June 2011 /Accepted: 4 July 2011 /Published online: 29 July 2011 # Springer-Verlag 2011 Abstract The crystal morphology, melting behavior, and mechanical properties of high-density polyethylene (HDPE) samples obtained via gas-assisted injection molding (GAIM) under different gas pressures were investigated. Moreover, the non-isothermal crystallization kinetics of HDPE under different cooling rates was also studied. The obtained samples were characterized via differential scan- ning calorimetry, two-dimensional wide-angle X-ray scat- tering (2D-WAXS), tensile testing, dynamic mechanical analysis (DMA) and scanning electron microscopy techni- ques. It was found that the properties were intimately related to each other. Macroscopically, the flow-induced morphology of the various HDPE samples was character- ized with a hierarchical crystalline structure, possessing oriented lamellar structure, shish–kebab structure, and common spherulites in the skin, sub-skin, and gas channel region, respectively. The 2D-WAXS results demonstrated that the degree of orientation of the high gas pressure sample was larger than that of the low pressure sample at the corresponding layer. The tensile testing results of GAIM parts showed that the mechanical properties of the GAIM parts were improved with an increase of the gas pressure. Furthermore, the DMA was utilized to obtain the dynamic mechanical properties of the GAIM samples, and the results indicated that significant improvement of the orientation was observed with an increase of the gas pressure. Keywords High-density polyethylene . Gas pressure . Morphology . Mechanical properties Introduction As an innovative injection molding process, gas-assisted injection molding (GAIM) has attracted wide attention in the past decades. During the GAIM process, the cavity is partly filled with a pre-determined amount of the polymer melt. Subsequently, after a period of delay, gas is injected at high pressure through a special gas injection nozzle fitted to the machine barrel. Finally, the gas pressure remains until all polymer material solidifies after the completion of melt filling stage [1, 2]. A typical GAIM cycle is depicted in Fig. 1. By comparing with conventional injection molding (CIM), GAIM requires lower clamping tonnage, smaller cooling times, lower material consumption, etc. [3–5]. Moreover, part qualities can also be greatly improved by reduction in sink marks, residual stress, warpage, shrinkage, and so forth. However, during the GAIM process, the melt is confined both by the mold wall and the compressed gas, with new processing parameters involved in the operation as well. Consequently, the mold designs and process controlling of the GAIM process become more complicated and critical than those of CIM. In view of its intrinsic advantages, a great deal of work has been carried out on GAIM. Up to now, the mechanical properties of GAIM parts have been investigated by several researchers [6–18]. The mechanical strengths of GAIM parts can be improved as compared with CIM parts; nevertheless, there are not enough experimental data for quantitative conclusion until now. A great deal of studies [8–16] have focused on the effect of gas channel shape on the mechanical properties of GAIM parts and found that the L. Wang : B. Yang : W. Yang : N. Sun : B. Yin : J.-M. Feng : M.-B. Yang (*) College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’ s Republic of China e-mail: yangmb@scu.edu.cn Colloid Polym Sci (2011) 289:1661–1671 DOI 10.1007/s00396-011-2483-z