Role of gas delay time on the hierarchical crystalline structure and mechanical property of HDPE molded by gas-assisted injection molding

Abstract

The relationship among the processing parameters, crystalline morphology, and macroscopic properties in injected molded bar becomes very complicated due to existence of temperature gradient and shear gradient along the sample thickness. To enhance the shear strength, gasassisted injection molding (GAIM) was utilized in producing the molded bars. The aim of our research was to explore the relationship between processing conditions and the spatial variation of the hierarchy structure as well as the mechanical properties of high-density polyethylene (HDPE) obtained via GAIM. In our previous work [Wang L, Yang B, Yang W et al (2011) Colloid Polym Sci 289:1661-1671], we found that the enhancement of the gas pressure can remarkably increase the degree of molecular orientation in the HDPE samples, which turns out to improve the mechanical performances of GAIM parts. In this work, the hierarchy structure, orientation behavior, and mechanical properties of molder bars under different gas delay time were investigated using a variety of characterization techniques including rheological experiments, scanning electron microscope, tensile testing, differential scanning calorimetry, and two-dimensional wide-angle X-ray scattering. Moreover, the temperature field during the short shot stage of GAIM process was simulated using an enthalpy transformation approach. Our results indicate that these properties were intimately related to each other, and with prolonged gas delay time, GAIM samples with higher degree of orientation and improved mechanical properties were obtained.

Study on the solidification kinetics of high-density polyethylene during thin-walled injection molding process

“…Both the ITD and the STD in Case D are higher than those in Case C, and the cooling rate of Case D is thus higher than that of Case C. When the dimensionless cooling time ( τ ) reaches 1.0, the cooling rate ranks as A > B > D > C ( Figure 1D), which is consistent with the consequence of STD. From the comparison among Figures 1A -1D, it was concluded that the cooling rate of Case D is higher than that of Case C, wherein Case D has higher ITD than Case C. Although Cases A and B have the same ITD, the cooling rate of Case A is obviously higher than Case B as Case A has a lower T w , indicating that the mold temperature exerts more significant influence on the control of the cooling rate in the plastic IM process [35,42] . It was found that the cooling rate is closely related to the thermal gradient field with phase change [39] .…”

“…In our recent studies [17, 35 -39] , the cooling stage of IM was numerically investigated using the enthalpy transformation methodology (ETM) [10,40] , which is a useful way to handle the multidimensional phase-change issues [39,41] . Our findings suggested that the mold temperature exerts more significant influence on the cooling rate compared with the melt temperature [35,42] .…”

“…The time over which phase transition extends at χ = 1.0 ranks as Δ τ D > Δ τ C > Δ τ B > Δ τ A . The mold temperature is an important processing variable for controlling the cooling rate [35,42] . When the mold temperature is relatively low, the length of phase transition plateau will be considerably shortened primarily due to increased cooling rate.…”

Large scale formation of various highly oriented structures in polyethylene/polycarbonate microfibril blends subjected to secondary melt flow

“…After being pelletized, the pellets were molded by both a gas-assisted injection molding (GIAM) and common injection molding (CIM) processing. For GAIM processing, its main feature is to impose to a strong shear field on the polymer melt, and provide a fast cooling rate of polymer, which immensely increase the relaxation time of polymer chains [24]. The temperature profiles of the injection molding machine were 170,190,190, and 190 C from hopper to nozzle.…”

Hierarchical crystalline structures and dynamic mechanical properties of injection‐molded bars of HDPE: attributes of temperature field

“…In previous work, we presented the spatial variation of hierarchy structures and mechanical properties of HDPE samples produced via GAIM under different gas pressures and gas delay times . In order to systematically investigate the relationship between processing conditions and flow‐induced microstructure in GAIM as a long‐term project, research on HDPE with series of melt temperature is one of the necessary subjects.…”