Based on the strong relationship between ultrasonic speed and properties of the propagation medium and the rapid response of ultrasonic speed to the medium properties

the theoretical and experimental analysis were made to conclude single valued correspondence of ultrasonic speed with density of polymer melt.Considering very short molding cycle for the injection process

an improved least square method by reducing the independent variable within the scope of 0 to 1 was proposed for the purpose of increasing computation speed.An online soft measurement model of polymer density according to single valued ultrasonic speed was established and the soft measurement model found application in precise control of injected weight of the article in the process of injection molding.Integrating the product of density of polymer melt determined by soft measurement model and cross-sectional area of the screw to stroke distance of injection

more precise control of the weight of molded article could be realized.The superiority of the weight control method based on soft measurement of melt density to the traditional volume control method was confirmed to the benefit of improving repeat weight accuracy of molded articles.