Research On Condensed State Structure And Mechanical Property Of Injection-molded Parts Based On Ultrasonic

With the development of polymer materials science,injection molded parts have been generally applied in almost every field of national economy.Especially,the use ration as key part in the fields of high technology is getting higher and higher.Thus,the quality and service performance of injection molded parts has to fulfill more stringent requirements.Many experiments on ultrasonic-assisted injection moulding have proved that the ultrasonic could reduce the melt viscosity,promote the flow of polymer melt,decrease the residual of parts,etc.,but the internal law of function and its mechanism are still not clear.For this problem,the research has been carried out as follows.Firstly,an ultrasonic-assisted injection mold,for rectangular shell of isotactic polypropylene with two standard tensile specimens at the bottom,was designed and manufactured.To ensure that the ultrasonic system could be integrated into the mold and ultrasonic energy could transfer into melt,ejection system,cooling system,ultrasonic system and the structure features to lower the loss of ultrasonic energy were theoretically analyzed and designed.Secondly,the experiment of injection moulding under different conditions,including melt temperature and mold temperature was carried out.The condensed state structure of the sample was characterized by polarized light microscopy and wide angle X-ray diffraction,and the mechanical property was evaluated by tensile test.Results clearly show that the enhanced molecule thermal movement induced by the rise of melt temperature is the main factor decreasing the degree of oriented crystallization,crystallinity level and yield strength;the increase of mold temperature,inducing a decrease of temperature difference between melt and cavity surface,slowing down the cooling rate of melt,allows higher crystallinity level and yield strength of samples;but higher mold temperature would reduce the yield strength of samples,due to a faster growth rate of spherulite size.It is thus clear that if the melt temperature or mold temperature is too high,the polymer parts with best condensed state structure and yield strength would be far harder to produce.Finally,the single factor injection moulding experiment setting the action time of ultrasonic and the ultrasonic power as the main experimental variables,and then the law of function and mechanism the ultrasonic played on the formation of the condensed state structure and mechanical property of samples were researched.The results show that ultrasonic applied during the filling and packaging stage could increase the level of crystalline,perfect the internal structure of crystalline,thus improve the mechanical strength of samples;ultrasonic applied during the filling,packaging and cooling stage could also increase the level of crystalline,but it would increase the defects of crystalline structure,and then reduce the yield strength of samples.