Study On The Molecular Condensed State Structure And The Property Of Injection Molding Products During The Cooling Process

During the injection molding process,the temperature variation of the melt will directly affect the accumulative morphology of the molecular chains,which finally determining the condensed structure and the mechanical properties of the product.Firstly,based on the three conservation laws during continuous media movement process,and combine with the melt flow characteristics in the injection molding,the heat transfer and heat exchange during the process of melt flowing in the cavity are theoretically analyzed.Since the viscous dissipation effect and convection heat transfer effect have the greatest influence on the heat transfer of the melt,the formula for calculating the change of melt temperature after the combined effect of viscous dissipation and convection heat transfer is established.Secondly,aiming at the melt cooling process during the injection molding cycle,the temperature change of the melt in the cavity was recorded by the camera through the thermographic material combined with the visualized mold.And the temperature field variation of the parts were calculate by the image processing technology.The results show that there is an uneven cooling process on the part,the areas that around the part cooling first,and then is the central part,but the highest temperature position is not distributed in the geometric center of the part,it is lower than the geometric center instead.Further,the injection molding experimental platform with thermocouple sensors and ultrasonic vibration system is designed and constructed.And the melt temperature in different cavity positions were measured and analyzed under different injection speeds,injection pressures,melt temperatures,ultrasonic powers,and ultrasonic action times during the molding process of Polypropylene(PP)and High Density Polyethylene(HDPE)parts by single factor experiments.The results show that both the peak melt temperature and the cooling time of the two materials increase with the increase of the injection speed,injection pressure,and melt temperature;after the ultrasonic field is applied,the peak melt temperature and cooling time increases with the increase of ultrasonic power,but the effect of ultrasonic time on the peak melt temperature and cooling time is not obvious.Finally,the PP samples and HDPE samples obtained under different process parameters and ultrasonic conditions were detected and tested by X-ray diffraction(XRD)and the tensile testing machine.The results show that the crystallinity and the tensile strength of the sample increases with the increase of the injection speed and injection pressure,and decreases with the increase of the melt temperature.After the ultrasonic field is applied,the crystallinity and the tensile strength of the sample is obviously improved,the crystallinity and the tensile strength increase first and then decrease with the increase of the ultrasonic power.When the ultrasonic power is 600 W,the crystallinity and the tensile strength is the highest.The crystallinity of the whole parts under different ultrasonic powers also shows the same pattern.Also the crystallinity of the core part is significantly higher than that of the surface under the same conditions.However,the effect of the ultrasonic time on the crystallinity and the tensile strength of the part is not obvious.