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Plastics parts with micro-structured surfaces enable the development of innovative products such as optical components in sensors or light management systems for laser and LED applications. Moreover, micro-structured parts can be utilized in the medical and packaging industry for hydrophobic or antibacterial products. The production of micro-structured parts causes challenges in molding and demolding. Rough surfaces of the laser-structured mold inserts offer flow resistance during injection phase as well as increased demolding forces which cause failures of the replicated structures during ejection. Therefore, an innovative approach combines coated mold inserts by means of physical vapor deposition (PVD) and a highly dynamic laser tempering system to improve the replication of micro-structured plastics parts. Both uncoated and coated micro-structured mold inserts were used in a series of molding experiments by means of conventional and dynamic mold tempering. Based on the results, it can be shown that significant improvements of the replication of micro-structures of different sizes can be achieved by use of PVD mold coatings. This is attributed to the tribological interactions between coating and plastics melt. Furthermore, results indicate an influence of the thermal conductivity of PVD coatings to enhance replication quality.

The foam injection moulding is an established process to produce plastic components with reduced density. Due to the fragile foam structure, contactless welding methods such as laser transmission welding are suitable for foamed components. In a research project, the correlation between the injection moulding parameters and the resulting foam structure is determined in preliminary tests. Specimens with four different foam structures are produced, which differ in terms of their surface layer thickness and cell density. Measurements of the transmittance, as well as the intensity profile after irradiation of the transparent specimen, show that the transmission of the foamed test specimens is lower than compared to compact specimens. In particular, foam structures with low cell density and low surface layer thickness show a low transmittance. The weld strength is influenced only by the foam structure of the transparent joining partner. Additionally, a possible post-expansion of the enclosed gas during the welding process has to be taken into account. The temperature within the laser-absorbing joining partner is locally increased above the melting point of the plastic. Once the surrounding material is melted, the gas can expand due to the temperature increase and defects during the solidification of the melt are formed.

A flexible and individual component manufacturing process for thermoplastic composites (TPC) has been developed at the Institut fuer Kunststoffverarbeitung in Industrie und Handwerk an der RWTH Aachen (Institute of Plastics Processing (IKV) at RWTH Aachen University). The process consists of a quality controlled tape production and a combined forming and joining process with additive manufactured functional structures. This paper describes the requirements for the unidirectional (UD) tape properties and the quality controlled tape production line in order to allow for a flexible and individual component manufacturing of load optimised thermoplastic composite parts. Besides the UD tape geometry and fibre impregnation quality an even fibre distribution over the width of the UD tape is an important characteristic. Results of investigations regarding the online measured quality data (fibre distribution) and offline measured UD tape properties (local fibre weight content) are presented and discussed.

Light Emitting Diodes (LED) conquer the growing global market of lighting technologies. Due to their advantages regarding efficiency and lifetime, they are increasingly used in consumer products, for lighting applications in the home and in the mobility sector as well as in industrial applications. Conventionally, LED chips are encapsulated by so-called primary optics and the optical function is given by an optical head. In the context of a joint research project in cooperation with the partners Hella KGaA Hueck & Co., Lippstadt, Momentive Performance Materials GmbH, Leverkusen, all Germany, and ELMET Elastomere Produktions- und Dienstleistungs-GmbH, Oftering, Austria, an injection molding process of combined primary optics and optical heads made of liquid silicone rubber (LSR) was investigated. Due to the material-specific advantages of highly transparent LSR especially regarding the excellent high temperature resistance and the great freedom in design the optics can meet both the requirements for the encapsulation of LED chips as well as the realization of an optical function. An integrated injection mold concept was realized in order to reduce mounting steps, where a LED circuit-board is inserted and directly overmolded with LSR. Processing studies have shown that a reproducible replication of LSR optics with good optical properties and molding accuracy is possible. Moreover, for the use in an automotive application long term properties are crucial. Therefore, a temperature cycling test between −40°C and 125°C for 1,000 cycles as well as a continuous operation of the LED with permitted maximum current for 2,000 h were carried out. Additionally, the application of optical micro structured LED optics made of LSR was examined. For this purpose appropriate optics and mold inserts were designed and manufactured.

The importance of microstructured parts and extrudates is rising because of the increasing functional integration in many different fields of application, for example in the automotive industry and medical technology. Variothermal extrusion embossing is a continuous process that combines plastic film production with the application of large-area microstructures. The process utilizes an embossing roll and an external heating device to continuously imprint the desired structure into film during extrusion. Through this microstructuring, it is possible to integrate innovative functions into a film. Present efforts are focused on forming optical structures (micro-optics) on polycarbonate and poly(methylmethacrylate) films. This article deals with process control requirements that are necessary to successfully apply this technology, and also demonstrate how, through suitable process parameter selection, high-quality microstructures can be effectively embossed on the films. Different microstructures between 24 and 165 µm in height and between 100 and 200 µm in width are tested. With poly(methylmethacrylate) better microstructure reproduction could be achieved than with polycarbonate.

Plastics materials are nowadays used in many structural applications for the substitution of metals with respect to weight reduction. In order to utilize the high freedom of design and the light-weight potential of plastics materials in crash-relevant structural parts, so-called hybrid composites which combine the high rigidity and strength of steel with the advantages of plastics materials are investigated in the outlined research. Thereby, the joining of both materials as well as the design by means of numerical methods such as the finite element analysis (FEA) are challenges which have to be met. A new approach in joining is based on the modified arc welding process where metal pin structures are formed in one working step and subsequently welded onto the surface. The pins are formed with ball-shaped, cylindrical or spiky ends and produced directly from the welding wire without requiring additional pre-fabricated components such as studs or similar. This allows the small-scale surface structuring of metal components that can be adapted optimally for a form fit on the respective plastics structure. Subsequently, injection molding is used for the application of the plastics material onto the pin-structured metal part in order to generate a positive fit between metal and plastics in an intrinsic joining process. An additional joining process, which is carried out after injection molding, is not required. Within the framework of the research presented, comprehensive mechanical tests are presented to illustrate the suitability of pin-structured metal-hybrid composites in crash applications. In comparison to structures which are in particular exposed to static loads and therefore designed to exhibit maximum component strength, crash applications are designed to fail in a continuous process to achieve maximum energy consumption. The outlined research illustrates the enhanced failure behavior of pin-structured plastics/metal-hybrid composites and the increased energy consumption under impact loading. Moreover, a comparison between pin structuring and laser structuring with regard to the obtainable mechanical properties under impact loading is given. Concluding, the current potential and weak points in the simulation of plastics/metal-hybrid structures using FEA is presented and discussed.

Modern injection molding machines can reproduce machine values, such as the position and speed of the plasticizing screw, with a high precision. To achieve a further improvement of the part quality, adaption and self-optimization strategies are required, which is realized by the implementation of a model-based self-optimization to an injection molding machine. Within this concept, a pvT-optimization allows an online control of the holding pressure that is tailored to the plastics material, considering the relationship between pressure, specific volume and temperature. A control strategy is required that controls the cavity pressure with respect to the reference generated by the pvT-optimization. However, cavity pressure control, in contrast to pressure control in the plasticizing unit, is hitherto not possible without a time-consuming system parametrization. Due to the repetitive character of the injection molding process, the iterative learning control (ILC) is a suitable approach. The ILC uses information gained within the previous cycle and a model to generate the optimal controller outputs for the following cycle. Based on this iterative learning, the reference tracking of the cavity pressure can be improved over several cycles. Additionally, repetitive disturbances can be compensated automatically. To improve the convergence speed of the ILC, a process model can be used explicitly. Based on this premise, an ILC for cavity pressure control is developed and researched in injection molding experiments. It is shown that the flexibility of the control strategy can be improved without compromising performance.

The main criterion in the design of extrusion dies is to ensure a uniform velocity distribution of the plastics melt at the die outlet. In the case of spiral mandrel dies, the flow balance can be influenced negatively by thermal effects which are especially dominant in the pre-distributor. In previous works, it was shown that thermal inhomogeneity in a 2n-pre-distributor leads to an uneven melt distribution at the end of the pre-distributor. In the present study, the temperature influence on the melt distribution in a star-pre-distributor is investigated for three different polyethylenes with the help of flow simulations in Polyflow (Ansys). The simulation model depicts the whole pre-distributor and takes both the shear heating in the melt and the heat conduction in the pre-distributor into account. The simulative analysis shows that the dissipative shear heating leads to an uneven throughput distribution in the star-pre-distributor. In order to compensate this effect, in the next step of the simulations, heating cartridges are applied to the pre-distributor. In this way a significant homogenization of the pre-distribution can be achieved.

Die Vorteile langfaserverstärkter thermoplastischer Werkstoffe führen immer häufiger zum Einsatz in strukturell beanspruchten Bauteilen. Gerade das Potenzial hinsichtlich Energieabsorptionsvermögen macht diese Werkstoffe für Anwendungen im Crash-relevanten Bereich hochgradig interessant. Um die Möglichkeiten bei der Auslegung zu erweitern und das Verständnis der durch die lagen Fasern dominierten Werkstoffeigenschaften auszubauen, beschäftigt sich das Institut für Kunststoffverarbeitung (IKV) intensiv mit langfaserverstärktem Thermoplastbauteilen (LFT). Ausgehend von bestehenden quasistatischen Berechnungsmethoden für langfaserverstärkte Thermoplaste entwickelte das IKV daher im Rahmen eines öffentlich geförderten Forschungsprojekts ein Simulationsverfahren zur Berechnung spritzgegossener LFT-Bauteile unter kurzzeitdynamischer Belastung.

Polymer engineering & science pen.25614 (2020). doi:10.1002/pen.25614, Published by Wiley, Hoboken, NJ

Nucleating agents play an important role as additives in the production of injection-moulded components from semi-crystalline thermoplastics. To date, however, no work has been published in the scientific literature, which simulates the influence of nucleating agents on the formation of the microstructure of semi-crystalline thermoplastics. This research gap is to be closed within the framework of the research.

Polymer engineering & science (2020). doi:10.1002/pen.25383, Published by Wiley, Hoboken, NJ

The increasing scarcity of resources and growing environmental awareness require higher recycling rates for plastic waste. Common techniques to do that are mechanical recycling, thermal recycling and chemical recycling, which is also called feedstock recycling. From all three techniques, chemical recycling is the only one which can produce new materials that correspond to the quality of conventional virgin material. However, the technique is limited to suitable polymers, e.g. polystyrene, which can be depolymerised at elevated temperatures. For an efficient industrial scale-up, a continuous process is desirable. In our work, we present such a continuous process for the recycling of polystyrene from post-industrial waste. A co-rotating, tightly intermeshing twin-screw extruder in high-temperature design is used together with a vacuum separation system with three degassing openings. By determining a stable process point a continuous depolymerisation of polystyrene is technically realised.

The two-domain Schmidt equation of state (EoS), which describes the pressure-specific volume&ndash, temperature (pvT) behavior of polymers in both the equilibrium molten/liquid state and non-equilibrium solid/glassy state, is often used in the simulation of polymer processing. However, this empirical model has a discontinuity problem and low fitting accuracy. This work derived a continuous two-domain pvT model with higher fitting accuracy compared with the Schmidt model. The cooling rate as an obvious influencing factor on the pvT behavior of polymers was also considered in the model. The interaction parameters of the equations were fitted with the experimental pvT data of an amorphous polymer, acrylonitrile-butadiene-styrene (ABS), and a semi-crystalline polymer, polypropylene (PP). The fitted results by the continuous two-domain EoS were in good agreement with the experimental data. The average absolute percentage deviations were 0.1% and 0.16% for the amorphous and semi-crystalline polymers, respectively. As a result, the present work provided a simple and useful model for the prediction of the specific volume of polymers as a function of temperature, pressure, and cooling rate.

Laser transmission welding is one of various welding techniques used to join thermoplastics. Low heat introduction into the welded parts and a high welding speed are the reasons why laser transmission welding established itself as a joining process in the plastics processing industry. To minimise defective parts and maximise productivity, it is essential to determine a set of ideal welding parameters that allow maximum bond strength at the lowest possible cycle times. To facilitate this process, simulation models provide detailed analysis without the need for destructive and costly part testing. To predict the weld strength of two thermoplastic parts joined by laser transmission welding, the Institute for Plastics Processing has developed a model that combines the simulated thermal properties of the material during and after welding with the molecular behaviour of plastic melts. Based on the results of the thermal modelling of the welding process a mathematical model describing the movement of polymer chains is used to calculate the resulting bond strength depending on material properties as well as heating and cooling rates. The temperature data of nodes situated at the interface of both welding partners are extracted from the simulation for every time increment of the simulation. The model, which is based on the reptation theory of polymer melts, is then used with these data and the bond strength is calculated. The results are validated by tensile tests on welded parts with the same input parameters used in simulation. In first results, the calculated bond strength shows a good agreement with the values measured in tensile tests. Occasional deviations can be explained by the fact that the material decomposition occurring in experimental welds is not considered in the thermal simulation and the reptation theory.

Journal of Composites Science 122(4), 122 (2020). doi:10.3390/jcs4030122 special issue: "Special Issue "Discontinuous Fiber Composites, Volume II" / Special Issue Editors: Prof. Dr. Tim A. Osswald, Guest Editor; Dr. Christoph Kuhn, Guest Editor", Published by MDPI, Basel

21th IFAC World Congress, online, 11 Jul 2020 - 17 Jul 2020; IFAC-PapersOnLine 53(2), 10380-10387 (2020). doi:10.1016/j.ifacol.2020.12.2777 special issue: "21th IFAC World Congress / Edited by Rolf Findeisen, Sandra Hirche, Klaus Janschek, Martin Mönnigmann", Published by Elsevier, Frankfurt

Journal of applied polymer science (2019). doi:10.1002/app.48863, Published by Wiley, New York, NY [u.a.]

In thermoforming up to 90% of the product costs are related to the material. For this reason, the efficient use of materials is essential in the production. Material efficiency of thermoformed packaging products, especially cups, can be increased by using pre-stretching plugs. These plugs affect the wall thickness distribution of the product. Thin areas, which can cause problems concerning top-load stability, can be avoided. By adjusting the shape of the pre-stretch plugs and the thermoforming process the wall thickness distribution changes significantly. Thus the distribution can be homogenised and the wall thickness can be increased more than 100% in the thinnest area. The plug design is varied in wall angle and the radius which joints the side wall and the flat top of the plug. Using DoE the effects of different geometry combinations and sheet temperatures are analysed. It can be shown, that the wall angle of the plug and the sheet temperature have a big influence on the homogenisation. Top-load stability is an important feature of packaging cups. By adjusting the wall thickness distribution, the top-load stability can be raised significantly. This high top-load stability in turn allows a reduction of the sheet’s thickness, increasing material efficiency. Additionally, the required plug forces to pre-stretch the sheet are investigated, which can improve the process control or serve as a parameter for quality management.

Materials and design 196, 1-11 (2020)., Published by Elsevier Science, Amsterdam [u.a.]

The contact of plastic components and media in use is often unavoidable. In addition to the influence of the medium on the mechanical properties, the occurrence of environmental stress cracks (ESC) is one of the most frequent causes of failure of plastic components in use. A significant improvement in the durability of plastics under the influence of the medium using a fibre reinforcement was shown in preliminary tests. But so far, there have been no fundamental investigations in the literature on this topic. The presented investigations deal with the influence of media on the mechanical behaviour and the occurrence of stress cracks under consideration of fibre reinforcement and its orientation. Therefore, a new test bench as well as tensile tests under media influence accompanied by acoustic emission measurement were used. Various media in combination with polycarbonate and polystyrene were investigated. Furthermore, a multiscale simulation based on molecular dynamics simulation as well as process and structure simulation was developed and validated.

Missing Journal / Fehlende Zeitschrift 183, 108149 - (2019). doi:10.1016/j.matdes.2019.108149, Published by Elsevier Science, Amsterdam [u.a.]

The injection molding of thermoplastics is one of the most efficient manufacturing processes for autonomous manufacturing of plastic parts. The process is commonly divided into two main phases: the injection phase and the packing phase. During the injection phase the injection velocity is conventionally controlled. In contrast, the packing phase is pressure controlled. Due to the different control objectives within these two phases the control strategy is changed regarding a switch-over point, which leads to issues in process control. Therefore, a cross-phase control strategy is required which avoids the switch-over in order to optimize the process control strategy. In this contribution, a model-based predictive controller (MPC) is presented which considers the process behavior during the injection phase as well as the packing phase. Therefore, a physical-motivated model is developed for both phases. Afterwards, this model is continuously peace-wise linearized around the current operation point in each time instance. Furthermore, the measured signals are filtered and the required system states are estimated by an Extended Kalman Filter (EKF). Then, the presented approach is applied to a servo-electric injection molding machine in order to empirically validate the mentioned approach. The controller shows good tracking performance for both the injection phase and the packing phase.

Components made from fiber reinforced plastics are increasingly applied due to their high potential for lightweight design. However, the high material and production costs are critical for establishing components in cost sensitive markets like the automotive industry. A solution to reduce material costs is the additive 3D-fiber spraying process. This preforming technology enables cost-efficient and net-shaped manufacturing of long fiber (25 – 50 mm) preforms with minimal waste directly from the roving. Therefore, this preforming technology is interesting for parts with a complex geometry where textile preforming reaches its economical limitations regarding draping and nesting strategies. However, this technology is affected by process fluctuations. For reproducible properties, the 3D-fiber spraying process is integrated in a production unit. Within this unit, an inline quality assurance system based on detection of process fluctuations and a self-regulating compensation system is established. The inline quality metrology is realized by an optical system. Local deviations of fiber distribution and orientation are detected and used for individual compensation using continuous carbon fibers. The fiber pattern is shaped based on an inline simulation of the mechanical component properties. In this paper the suitability of the optical inline assurance system for the fiber spraying process is investigated.

Die Auslegung kurzfaserverstärkter thermoplastischer Bauteile kann heute als Stand der Technik angenommen werden. Dem gegenüber stellt die Auslegung von langfaserverstärkten Thermoplasten (LFT) Entwickler und Konstrukteure vor große Herausforderungen. Eine am Institut für Kunststoffverarbeitung (IKV) entwickelte integrative Berechnungsmethode zur Vorhersage des Langzeitverhaltens spritzgegossener LFT-Formteile zeigt eine Möglichkeit Abhilfe zu schaffen (Bild 1).

The thermal mold design and the identification of a proper cooling channel design for injection molds becomes more and more complex. To find a suitable cooling channel system with objective rules based on the local cooling demand of the part a new methodology for the thermal mold design based on an inverse heat transfer problem was introduced. Based on a quality function regarding production efficiency as well as part quality, additional aspects to model the injection molding process are discussed. Aim of those extensions is the improvement of the inverse optimization of the problem.

Journal of polymers 2016, 4734913 (2016). doi:10.1155/2016/4734913, Published by Hindawi, New York, NY

Cellulose acetate (CA) is a bio-based polymer suitable to replace foamed polystyrene (PS) in packaging applications. Foam trays can be produced by thermoforming of extruded sheets foamed with physical blowing agents. In this paper, the effects of various process settings and the calibration of the sheet on foam morphology and surface quality of extruded CA sheets are presented. Different contact cooling options were applied in order to investigate their influence on surface roughness, density, and morphology of the sheets. By adjusting cooling parameters, blowing agent formulation, and process settings, smooth foam sheets with a surface roughness below 10 µm and a density in the range of 150 kg m−3 were produced.

In the market of lighting technologies, light-emitting diodes (LEDs) gradually substitute conventional light sources. Because of their high energy efficiency and long lifetime, they are increasingly used in consumer products, interior and exterior lighting applications in the home and mobility sector as well as in industrial applications. The material properties in the surrounding area of the light-emitting semiconductor chip are crucial to the performance of LED. Although the energy efficiency of LED is higher compared to conventional light sources, temperatures exceed about 150°C close to the semiconductor chip. Especially in combination with high amounts of blue ultraviolet (UV) radiation, the materials for encapsulation cannot meet the requirements and reduce the lifetime of an LED significantly. Contrary to conventional materials, high transparent liquid silicone rubber (LSR) can resist high temperatures as well as UV radiation and offer a great freedom in design. This enables the combination of the encapsulation (primary optics) and the secondary optics in one component. The objective of an ongoing joint research project with various partners from the industry is the development of an innovative injection moulding process for high precision optics in LED applications made of LSR, which is analysed at the Institute of Plastics Processing (IKV), Aachen, Germany. Therefore, the LED board is placed in the injection mould and overmoulded with LSR. The goal is a highly integrated process with major emphasis on the reduction of components, mounting steps and costs. Furthermore, the combination of primary and secondary optics promises an improved effectiveness because losses in light power due to the transition of the primary and secondary optics are reduced.

The production of functional optical parts made of plastics is a growing market and the associated production processes have to be developed to fulfil the increasing quality requirements. In this paper an alternative injection compression molding process using the pvT-behavior of the plastics material is compared to the conventional processing method using an isobaric process path. For both control strategies a cavity pressure loop-control is implemented and used. According to measurements of the mold surface reproduction accuracy and internal properties of the molded part the processing method using the pvT-behavior results in a higher optical part quality in the same cycle time compared to the isobaric processing method.

The main goal of the presented work is to implement a robust framework for the computation of the crystallization kinetics of semi-crystalline thermoplastics by using a multiscale approach. The purpose of multiscale modeling is to assess parameters influencing microstructure formation that would otherwise require very time-consuming analysis with experiments. The numerical method, crystallization kinetics and their implementation into numerical software operating at macro- and micro-scale are described as well as the experimental data needed to prepare and validate numerical results. Keywords: Injection molding, Polymer, Crystallization, Numerical Simulation