Analysis of the Thermal Coupling of the Molten Mass, Structure, and Tool to Precisely Predict Schrinkage and Warpage in Injection Moulding Processes



Christian Hopmann



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Both polymer and metal processing are similar with regards to transformative processes in that a molten mass first flows into the tool mould's cavity that is to be cast and solidifies in the cavity. Temperature compensation processes between the molten mass, framework structures, and the tool mould determine the resulting state of order on the molecular and atomic level, which decisively shape the formation of the morphology as well as the origination of internal stress. In plastics components, which claim lower tolerance regarding the final contour, these internal characteristics often lead to grave issues during the manufacturing process, such as shrinkage and warpage. The underlying mechanisms have not yet been observed as a whole in polymer processing research. This is due to missing models of temperature compensation processes between the molten mass, framework structure, and tool mould as well as missing thermal material data in relation to the morphology and the elaborate determination of the thermal transfer coefficients in metal and polymer pairings. The research in this subproject will investigate the temperature compensation processes with the aim of improving the precise prediction of polymer component warpage and then modelling it in the following funding periods as well as considering it in the injection moulding simulation. Integrative simulation approaches up until now, in which the thermal interactions are only estimated with approximations, do not yet enable the complete avoidance of costly, sustainable adjustments to the injection mould. In order to improve the simulation, the thermal interactions that haven't been sufficiently observed so far must be analyzed, understood, and mastered. Because there is currently no possibility to measure the temperature field in the molten mass and solidified framework, a central point of the first project phase is the development, production, and operation of a new measurement device for the high-resolution determination of the temperature field in the molten mass and in the solidifying and solidified framework.

Different approaches are used as measuring methods. Aside from the measurement of the contact temperature using thermoelectricity, the temperature is also measured using infrared sensors. Additionally an ultrasound-tomographic measurement of the sonic speed is measured in the molten mass and then shot back to the temperature using changes in the sonic speed field. Due to the complex question, this work package also contains the further development and combination of different measurement methods. An existing measurement device is expanded for the polymer/metal material coupling on the instrument wall, in order to enable the measurement of thermal transfer coefficients dependent on pressure under injection-relevant framework conditions. Additionally, thermal materials data must be collected depending on the structure. Within the framework of this project, partial-cristalline polymers commonly processed in injection processes, such as polypropylene (PP) and polyoxymethylene (POM) are investigated further in different modifications.

With the new injection tool, the subproject offers the possibility for the first time for temperature field calculations in injection processes to be validated in high resolution with simulation appraoches, like they are developed in Subprojects B5 and B6 for example. The measured material values additionally serve as entry sizes for the material models and thus enable the development of new simulation approaches.

  Am IKV entwickeltes Werkzeugkonzept zur Durchführung ultraschalltomografischer Temperaturmessungen. Copyright: IKV

Within the scope of the Collaborative Research Centre 1120 "Precision from Melt", a method based on ultrasonic tomography was developed with which the temperature distribution can be determined both spatially resolved and contact-free. For this purpose, a special tool was developed at the IKV (see Fig. 1). 20 ultrasonic sensors are arranged around a cylindrical cavity with a diameter of 30 mm. This allows the temperature to be reconstructed from the transit times at over 200 locations in the measuring cross-section.

*** B4 -3- Videos, Animationen, Bilder ***

This video shows the development of the runtime signals for three different angles (18°, 90° and 180°). After the filling phase is finished, the runtime signals can be detected. While adjacent and opposite transducers provide a good signal intensity, the signal of the 90° transducer is detectable but strongly attenuated due to the unfavorable sound path. The end of the holding pressure phase is also clearly visible in the signal course. Due to the rapid change in pressure, the density of the material also changes, which significantly influences the speed of the ultrasonic wave. This effect can be seen most clearly on the opposite transducer. The acquisition of the signals can be achieved for any ultrasonic transducer combination, which shows the general applicability of ultrasonic tomography. However, the complexity of the run-time signals, which is due to the material structure, makes it difficult to determine the concrete run-time signal required for the temperature reconstruction.