A self-made low frequency vibration injection molding device was adopted to explore the mechanical properties and morphology for iPP injected moldings.The morphology and mechanical properties of samples produced by conventional injection molding(CIM) were used to compare with those obtained by vibration injection molding(VIM).For VIM the main processing parameters are vibration frequency and vibration pressure amplitude

the range of vibration frequency is 0～3 Hz

and the range of pressure vibration amplitude is 0～59.4 MPa.During the injection and pressure holding stages for CIM the injection and holding pressures are always constant

but for VIM an additional pulsing pressure vibration is exerted on the melt in the runner system and mold cavity

causing compression and decompression on the melt and shearing at the melt-solid interface

and it progresses from surface to core of the dumbbell specimen during solidification stage.In this work

the isotactic polypropylene material was plasticized and pumped into the melt chamber by a single screw extruder.During injection and pressure holding stages for the VIM

the melt was vibrated about 25 s in the dumbbell specimen mould and then cooled down in about 20 s.The melt injection temperature and mould temperature are set at 190℃ and 40℃ respectively.The injection pressure for CIM and the base pressure for VIM was 39.5 MPa.The VIM samples were injected at different vibration frequencies and pressure vibration amplitudes

respectively.To prepare VIM samples treated under different vibration frequencies

the vibration pressure amplitude was set at 19.8 MPa

and for VIM samples teated under different vibration pressure amplitudes

the vibration frequency was set at 0.7 Hz.With application of melt vibration technology the mechanical properties of iPP injection moldings were improved.The tensile strength and impact strength increase with the increasing of pressure vibration amplitude

while the elongation at break decreases.The tensile strength increases with increasing frequency in the low frequency range

but the breaking elongation decreases.When the vibration frequency is above 0.47 Hz

the elongation at break begins to increase with increasing vibration frequency

and the tensile strength is also simultaneously improved.Injected at 190℃ the mechanical strength increases from a conventional value of 41.3 MPa to 48.4 MPa obtained at 0.7 Hz of vibration frequency and 59.4 MPa of pressure vibration amplitude

and the corresponding increase percentage is 17.2%.DSC

SEM and WAXD were used to investigate the structure and morphologies of core regions of injection-molded samples.SEM studies show that the core region of samples mainly consists of sperulites

they may deform and orient along the flow direction and decrease their size in the vibration injection process.DSC curves display that the melt peaks of vibration injection moldings shift to high temperatures compared with those of conventional samples

and the maximal increase percentage of crystallinity for vibration samples is 12.1%.WAXD shows that γ-form crystals are obtained easily at low vibration frequencies and large pressure vibration amplitudes.The orientation of sperulites

increased crystallinity and existence of γ-form crystals are beneficial to improvement of the mechanical properties of iPP vibration injection-molded samples.