A Study Of New Rheological Measuring Method Of Thermoplastics

In this thesis, based on150tons injection machine, mold temperature controller,Archimedes spiral mold built-in pressure sensors and the CAE software MOLDFLOW,with the help of a data collecting system, pressure curves were obtained in the process ofinjecting of plastics melt in Archimedes spiral slot mold, and the rheological mechanism ofplastics melt was analyzed. A new method of evaluating rheological characteristics ofthermoplastics was developed.Firstly, injecting experiment of thermoplastic PP and HDPE were carried outrespectively in the injecting machine built-in Archimedes spiral slot mold. Factors whichinfluence the injection length of thermoplastic melt were discussed, including thethicknesses of Archimedes spiral cavity, injection pressure and mold temperature. Theresults show that the injection temperature of PP plastic within205℃~220℃should beadvisable; Injecting lengths change with the pressure of melt; Injecting lengths are highlyaffected by the thickness of the mold cavity, however, with the increase of thickness, thespeed of increasing of Injecting lengths reduces. The effects of mold temperature onInjecting lengths are not so obvious; Molding coefficient K’ can comprehensively reflectthe flowing rate of some thermoplastic melt under certain injecting conditions, the higherthe values of K’ are, the better the liquidity of the plastics would show.Secondly, the factors which would influence the time difference and the pressuredifference between the two sensors built-in the spiral slot mold were also studied. Thevalues of volume flow rate were calculated and the thicknesses of solidified layer ofplastics melt were taken into consideration and finally the apparent shear rate and shearviscosity were calculated. The results show that the filling time of melt reduces with theincreasing of melt temperature, the values of filling time and pressure difference alsoreduce with the filling speed in the spiral mold; It’s necessary to modify the volumeflowing rate of the melt according to the thickness of the solidified layer, for thesolidification layer would occupy the melt filling space. Even if the thickness of solidified layer is small, it would be a larger impact on the result of the melt volume flow rate in theprocess of filling.Finally, the constitutive equations of the melt apparent shear rate and shear viscositywere established. The methods of testing rheological properties of thermoplastics in thethesis were compared respectively with the results of CAE simulation analysis and testingresults of rotational viscometer. The results show that the values of injecting pressure andinjecting length obtained by CAE simulation which controlled by flow rate match with thecorresponding results of experiments, which shows that the modified melt flow rate isreliable. The apparent shear rates based on the model of the thesis match well with thevalues based on Cross-WLF model of MOLDFLOW under the condition of apparentshear rate less than1500s-1. The curves of apparent shear viscosity calculated in the thesisalso have a high similarity with CAE simulating results. The curves of dynamic viscosityof HDPE tested by rotational viscometer of parallel plate have the same trend with thecurve of melt apparent viscosity calculated in the thesis.