在低频振动场中注塑成型iPP试样的结构与性能研究

利用自行研制的低频振动注塑成型装置进行等规聚丙烯(iPP)试样的结构与性能研究.实验中对常规注射和振动注射成型的试样力学性能和微观形态进行了对比实验.采用低频振动注塑成型工艺实现了IPP试样的自增强,在190℃下进行注射,强度由常规试样的41.3 MPa最大提高到振动试样的48.4 MPa(振幅PA=59.4 MPa,振频FR=0.7 Hz),强度提高了17.2%;SEM显示常规试样芯层结构主要由球晶构成,振动注射使球晶在流动方向上变形、取向,晶粒尺寸得到细化;DSC表明振动注射促进熔融峰向高温漂移,晶体结晶更加完善,结晶度最大提高了12.1%;WAXD显示低频大振幅振动注塑有利于γ晶型的生成,γ晶型有利于试样实现自增强.

STUDIES ON STRUCTURE AND PROPERTIES OF iPP INJECTION-MOLDED IN LOW FREQUENCY VIBRATION FIELD

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.