Univ. Prof. Dr. Ing.

Bergs, Thomas

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Now showing 1 - 10 of 16
  • Publication
    An Analytical Model for Robot-Based Grinding of Axisymmetric Mold Inserts Using a Rotary Unit
    ( 2022)
    Tamassia, Eugenio
    ;
    Hähnel, Sebastian
    ;
    Pini, Fabio
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    ;
    Leali, Francesco
    The grinding of mold inserts used for injection molding aims to improve the surface roughness according to precise quality standards. The insert surface must also have a surface topography that facilitates the release of the plastic material at the end of the injection process. In particular, fine machining lines must be parallel to the extraction direction from the mold to avoid the sticking of plastic material and subsequent surface damages compromising the functionality of the finished product. However, this step in the production chain is most often conducted manually. This paper presents an analytical model to grind a truncated cone-shaped mold insert for the mass production of plastic cups. The automated solution consists of a flexible robotic system equipped with a rotating external axis to improve the accessibility of the tool to the surface to be machined. The tool path programming requires the development of an analytical model considering the simultaneous mot ion of the insert and the robot joints. The effectiveness of the developed model is evaluated in terms of final surface quality, grinding lines direction, and total process time. The automated strategy developed can be easily implemented with machine tools and applied to inserts with different axisymmetric geometries.
  • Publication
    Machine learning-based predictive modeling of contact heat transfer
    ( 2021)
    Vu, Anh Tuan orcid-logo
    ;
    Gulati, Shrey
    ;
    Vogel, Paul-Alexander
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    Heat transfer phenomena at the interface between two contacting solids are highly complex involving multiple influencing factors. Over the years, a large amount of experiments were carried out to determine the contact heat transfer coefficients between two dissimilar joint materials. However, there are still no existing theoretical or physics-based models that satisfactorily predict the contact heat transfer coefficients. By taking advantage of the existing data, in contrast, machine learning promises a powerful method, capable of predicting the contact heat transfer coefficients for different material pairs and contact conditions. This research introduces a robust machine learning-based model that succeeds in precisely estimating the heat transfer across the interfaces between glass and steel, a material pair widely used in hot forming of glass. The data used for training and validating the machine learning models were determined experimentally by means of infrared thermography. The datasets consisted of contact heat transfer coefficients with dependence on three factors - interfacial temperature, contact pressure, and surface finishes. Aim of this study is to analyze the prediction accuracy and interpretability of various supervised learning algorithms in order to realize the machine learning models that are able to capture the underlying physics governing the heat transfer phenomena at the glass-mold interface. Finally, the results were compared with those estimated by a theoretical model and a numerical simulation model. The comparison demonstrates enhancements in prediction accuracy enabled by the data-driven method. This study indicates accurate and efficient strategies for solving thermal problems in hot glass forming processes.
  • Publication
    Hartwalzen von Lagerringen zur Lebensdauersteigerung
    ( 2020-07-30)
    Jahnel, Kirk
    ;
    Hähnel, Sebastian
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    Zur Steigerung der Lagerlebensdauer von Hybridwälzlagern hat das Fraunhofer Institut für Produktionstechnologie IPT gemeinsam mit Cerobear GmbH, Hegenscheidt-MFD (A Member of the NSH-Group) und Schmitz-Metallographie GmbH an der Qualifizierung und Implementierung des Verfahrens Hartwalzen in die Prozesskette zur Herstellung von Hybridwälzlagern gearbeitet. Durch die Induzierung von Druckeigenspannungen in die Lagerlauffläche konnte die Lebensdauer nachweislich erhöht werden. Die Arbeiten wurden im Rahmen des geförderten Forschungsvorhabens "Hartwalzen von Lagerringen" durchgeführt, das aus Mitteln des europäischen Fonds für regionale Entwicklung (EFRE) gefördert wurde.
  • Publication
    Precision Glass Molding of infrared optics with anti-reflective microstructures
    ( 2020)
    Rojacher, Cornelia
    ;
    Vu, Anh Tuan orcid-logo
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    Highly precise infrared lenses are used in a broad range of optical systems such as night visions, thermal imaging or gas sensing. As most infrared materials (e.g. Germanium, Chalcogenide glass) suffer from high Fresnel reflection losses, the use of anti-reflective coatings is state of the art to overcome this issue. An alternative approach is the implementation of anti-reflective microstructures into molded infrared lenses. This shortens the process chain and enables many advantages for example regarding the monolithic optics design. Precision Glass Molding (PGM), a replicative manufacturing technology, allows the macroscopic lens molding and the replication of surface microstructures to be carried out simultaneously. While PGM is an established process for manufacturing glass optics in general, there is a lack of knowledge regarding the replication of microstructures. This leads to the necessity to further investigate the PGM process chain for molding microstructures. The current paper addresses the process chain of manufacturing anti-reflective optics by precision glass molding. Process simulations are presented by a multiscale approach. In order to prevent wear, a suitable anti-adhesive coating system for molding tools with regard to the special requirements of microstructured surfaces is introduced. The results of the molding experiments highlight the importance of a multiscale simulation approach and demonstrate the stability of the anti-reflective microstructure.
  • Publication
    Vibration-Assisted Face Grinding of Mould Steel
    ( 2020)
    Hähnel, Sebastian
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    ;
    Pini, Fabio
    ;
    Leali, Francesco
    This work investigates vibration-supported, force-controlled fine machining with elastic bonded mounted points for automated fine processing of mould steel samples. The aim is to compare conventional robot- or machine-tool-based face grinding with a vibration-supported grinding process. The influence of vibration support on the surface topography is investigated primarily to minimize kinematically caused grinding traces. First, the state of the art for the production of tool moulds and vibration-supported fine machining is explained. On this basis, the potentials for the reduction of grinding marks through vibration support for an increase in the degree of automation are derived and the experimental procedure is introduced. Subsequently, robot-based grinding tests with vibration support are carried out and compared with conventional grinding tests. After the tests carried out, the results are evaluated using tactile and optical measuring methods.
  • Publication
    Numerical and experimental determinations of contact heat transfer coefficients in nonisothermal glass molding
    ( 2020)
    Vu, Anh Tuan orcid-logo
    ;
    Helmig, Thorsten
    ;
    Vu, Anh Ngoc
    ;
    Frekers, Yona
    ;
    Grunwald, Tim
    ;
    Kneer, Reinhold
    ;
    Bergs, Thomas
    Heat transfer at the interfacial contact is a dominant factor in the thermal behavior of glass during nonisothermal glass molding process. Recent research is developing reliable numerical approaches to quantify contact heat transfer coefficients. In most previous studies, however, both theoretical and numerical models of thermal contact conductance in glass molding attempted to investigate this factor by either omitting surface topography or simplifying the nature of contact surfaces. In fact, the determination of the contact heat transfer coefficient demands a detailed characterization of the contact interface including the surface topography and the thermomechanical behavior of the contact pair. This paper introduces a numerical approach to quantify the contact heat transfer by means of a microscale simulation at the glass-mold interface. The simulation successfully incorporates modeling of the thermomechanical behaviors and the three-dimensional topographies from actual surface measurements of the contact pair. The presented numerical model enables the derivation of contact heat transfer coefficients from various contact pressures and surface finishes. Numerical predictions of these coefficients are validated by transient contact heat transfer experiments using infrared thermography to verify the model.
  • Publication
    Thermo-viscoelastic modeling of nonequilibrium material behavior of glass in nonisothermal glass molding
    ( 2020)
    Vu, Anh Tuan orcid-logo
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    Nonisothermal Glass Molding (NGM) has become a viable replicative manufacturing technology for the cost-efficient production of complex precision optical components made of glass. During the pressing stage in NGM, glass materials undergo a huge temperature change in the glass transition range. In this range, glass exhibits thermo-viscoelastic responses, and the temperature drop through the glass transition leads glass structure to depart from an equilibrium to a nonequilibrium state. Thermo-viscoelastic properties of the nonequilibrium glass material greatly depend on temperature and thermal history. This paper presents a phenomenological constitutive model developed for modeling the thermoviscoelastic responses of nonequilibrium glass. We propose a direct incorporation of the temperature -and thermal history effects into each parameter of the phenomenological model. This novelty allows the model to describe the nonequilibrium phenomena and to study temperature-dependent viscoelasticity of glass without assuming thermo-rheologically simple characteristics. Furthermore, the phenomenological model enables the coupling of structural -and stress relaxation phenomena. The model validation conducted for creep experiments demonstrates an enhancement in numerical predictions of the nonequilibrium material behaviors of glass over wide ranges of temperature and thermal history.
  • Publication
    PtIr protective coating system for precision glass molding tools
    ( 2020)
    Friedrichs, Marcel
    ;
    Peng, Zirong
    ;
    Grunwald, Tim
    ;
    Rohwerder, Michael
    ;
    Gault, Baptiste
    ;
    Bergs, Thomas
    During Precision Glass Molding (PGM), the molding tools have to withstand severe thermo-chemical and thermo-mechanical loads cyclically. To protect their high-quality surface against degradation and increase their service lifetime, protective coatings are applied on the molding tools. In this work, we designed four different PtIr protective coating systems, where the thickness of the PtIr layer and the adhesion layer were varied. Their lifetimes were evaluated and compared using an in-house built testing bench. Among all the studied coating systems, the protective coating, which consists of a 600-nm-thick PtIr layer and a 20-nm-thick Cr adhesion layer, showed the best durability. To understand the degradation mechanism of the coating during actual engineering production, an industrial PGM machine was used and emulation PGM tests were conducted. Detailed sample characterization was performed using an array of complementary techniques including white light interferometry (WLI), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Phenomena such as interdiffusion, oxidation, coating spallation and glass sticking on the coating were observed and are discussed in the context of optimization of the coating's performance and durability.
  • Publication
    Modeling of thermo-viscoelastic material behavior of glass over a wide temperature range in glass compression molding
    ( 2020)
    Vu, Anh Tuan orcid-logo
    ;
    Vu, Anh Ngoc
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    In glass compression molding, most current modeling approaches of temperature-dependent viscoelastic behavior of glass materials are restricted to thermo-rheologically simple assumption. This research conducts a detailed study and demonstrates that this assumption, however, is not adequate for glass molding simulations over a wide range of molding temperatures. In this paper, we introduce a new method that eliminates the prerequisite of relaxation functions and shift factors for modeling of the thermo-viscoelastic material behavior. More specifically, the temperature effect is directly incorporated into each parameter of the mechanical model. The mechanical model parameters are derived from creep displacements using uniaxial compression experiments. Validations of the proposed method are conducted for three different glass categories, including borosilicate, aluminosilicate, and chalcogenide glasses. Excellent agreement between the creep experiments and simulation results is found in all glasses over long pressing time up to 900 seconds and a large temperature range that corresponds to the glass viscosity of log (η) = 9.5 â 6.8 Pas. The method eventually promises an enhancement of the glass molding simulation.
  • Publication
    Approaches and Methodologies for Process Development of Thin Glass Forming
    ( 2019)
    Vogel, Paul-Alexander
    ;
    Vu, Anh Tuan orcid-logo
    ;
    Mende, Hendrik
    ;
    Grunwald, Tim
    ;
    Bergs, Thomas
    ;
    Schmitt, Robert Heinrich
    The steadily growing thin glass market is driven by a vast amount of applications among which automobile interiors and consumer electronics are, such as 3D glass covers for displays, center consoles, speakers, etc. or as part of optics within head up-displays. Today, glass manufacturers are suffering from challenges brought about by the increases of shape complexity, accuracy and product variants while simultaneously reducing costs. The direct manufacturing method via grinding and polishing is no longer suitable because of its limited machinability for thin glasses in respect to fracture and its cost insufficiency due to the length of the process chain. Instead, replication-based technologies or thin glass forming become promising manufacturing methods to overcome the aforementioned technical and economic challenges. For instance, thermal slumping is only able to satisfy the most basic requirements and is in particular limited regarding the deformation degree and shape complexity of thin glass products. Technologies such as vacuum-assisted slumping or deep drawing are currently in development at the Fraunhofer Institute for Production Technology IPT and promise additional cost benefits. This paper introduces all potential process variants for thin glass forming. The suitability of different methods for process development, specifically process modeling based on either experimental-, simulation- or machine learning approaches (white box and black box models), will be addressed and discussed. Furthermore, process efficiency is examined on both an economic and technical level, where molding time, suitable geometries and accuracy are the focus. The methodologies presented in this paper aim at developing a guideline for glass manufacturers on determining the optimal strategy for the process development of thin glass production.