Holger Rühl

Zusammenfassung

In injection molding, the flow behavior of plastic materials in the mold cavity can be influenced by modifying the molding parameters as well as the condition of the mold surface in terms of roughness or material. Previous studies have reported on altered flow of plastics on physical vapor deposited hard coatings, which could potentially lead to an improved replication of micro-sized features in micro injection molding. The objective of the presented study was to investigate the filling of plastic material into microstructures for the surface condition of conventional tool steel and for the condition of a physical vapor deposited chromium nitride coating. For the experiments, two mold inserts of conventional tool steel were manufactured and polished to optical roughness. Truncated pyramid-shaped microstructures were created by micro-milling and were placed along the flow path of a disc-shaped component with a thickness of 1 mm. Subsequently, one mold insert was coated with a commercially available chromium nitride. The plastic material used in the molding experiments was polycarbonate. In order to ensure comparable flow conditions within the cavity for both surface conditions, injection molding was conducted at constant pressures, which were varied from 400 bar to 2000 bar. Furthermore, the mold and injection melt temperature were varied to study their influence on the filling for both surface conditions. The filling of the microstructures was evaluated using both light microscopy imaging and quantitative measurements of the replicated microstructures. The results indicate that different filling states can be observed depending on the surface condition and the processing parameters.

Zusammenfassung

Biologization of biomaterials with endothelial cells (ECs) is an important step in vascular tissue engineering, aiming at improving hemocompatibility and diminishing the thrombo-inflammatory response of implants. Since subcellular topography in the scale of nano to micrometers can influence cellular adhesion, proliferation, and differentiation, we here investigate the effect of surface roughness on the endothelialization of gelatin hydrogel scaffolds. Considering the micron and sub-micron features of the different native tissues underlying the endothelium in the body, we carried out a biomimetic approach to replicate the surface roughness of tissues and analyzed how this impacted the adhesion and proliferation of human umbilical endothelial cells (HUVECs). Using an imprinting technique, nano and micro-roughness ranging from Sa= 402 nm to Sa= 8 μm were replicated on the surface of gelatin hydrogels. Fluorescent imaging of HUVECs on consecutive days after seeding revealed that microscale topographies negatively affect cell spreading and proliferation. By contrast, nanoscale roughnesses of Sa= 402 and Sa= 538 nm promoted endothelialization as evidenced by the formation of confluent cell monolayers with prominent VE-cadherin surface expression. Collectively, we present an affordable and flexible imprinting method to replicate surface characteristics of tissues on hydrogels and demonstrate how nanoscale roughness positively supports their endothelialization.

Zusammenfassung

Inkjet printing for printed electronics is a growing market due to its advantages, including scalability, various usable materials and its digital, pixel based layout design. An important quality factor is the wetting of the ink on the substrate. This article proposes a workflow to evaluate the print quality of specific layouts by means of image analysis. A self-developed image analysis software, which compares a mask with the actual layout, enables a pixel-based analysis of the wetting behavior by the implementation of two parameters called over- and underwetting rate. A comparison of actual and targeted track widths can be performed for the evaluation of different parameters, such as the tested plasma treatment, drop spacing (DS) and substrate temperature. To prove the functionality of the image analyses tool, the print quality of Au structures inkjet printed on cyclic olefin copolymer (COC) substrates was studied experimentally by varying the three previously mentioned parameters. The experimental results showed that the wetting behavior of Au ink deposited on COC substrates influences various line widths differently, leading to higher spreading for smaller line widths. The proposed workflow is suitable for identifying and evaluating multiple tested parameter variations and might be easily adopted for printers for in-process print quality control in industrial manufacturing.

Zusammenfassung

Hard coatings can be applied onto microstructured molds to influence wear, form filling and demolding behaviors in microinjection molding. As an alternative to this conventional manufacturing procedure, “direct processing” of physical-vapor-deposited (PVD) hard coatings was investigated in this study, by fabricating submicron features directly into the coatings for a subsequent replication via molding. Different diamondlike carbon (DLC) and chromium nitride (CrN) PVD coatings were investigated regarding their suitability for focused ion beam (FIB) milling and microinjection molding using microscope imaging and areal roughness measurements. Each coating type was deposited onto high-gloss polished mold inserts. A specific test pattern containing different submicron features was then FIB-milled into the coatings using varied FIB parameters. The milling results were found to be influenced by the coating morphology and grain microstructure. Using injection–compression molding, the submicron structures were molded onto polycarbonate (PC) and cyclic olefin polymer (COP). The molding results revealed contrasting molding performances for the studied coatings and polymers. For CrN and PC, a sufficient replication fidelity based on AFM measurements was achieved. In contrast, only an insufficient molding result could be obtained for the DLC. No abrasive wear or coating delamination could be found after molding.

Zusammenfassung

Die Benetzung von Schmelze-berührenden Maschinenkomponenten sowie die Anhaftung von erstarrtem Kunststoff auf dem Formwerkzeug basiert auf Wechselwirkungen an der Grenzfläche zwischen Kunststoff und Werkzeugmaterial. Ein Ansatz zur Charakterisierung der Benetzbarkeit von Festkörperoberflächen ist die Bestimmung der freien Oberflächenenergie auf Basis von Kontaktwinkelmessungen. Bisherige Studien ermittelten die freie Oberflächenenergie von verschiedenen Hartstoffschichten meist durch Kontaktwinkelmessungen bei Raumtemperatur oder basierend auf einem statischen Messverfahren, welches nahezu keine Reproduzierbarkeit aufweist. Um die Benetzbarkeit von Hartstoffschichten für die Kunststoffverarbeitung zu bewerten, wurden in dieser Studie dynamische Kontaktwinkelmessungen von zwei Prüfflüssigkeiten auf poliertem Stahl sowie einer Titannitrid- (TiN), zwei Chromnitrid- (CrN) und einer amorphen Kohlenstoffbeschichtung (a-C:H:Si) im Entformungstemperaturbereich von 60 °C bis 140 °C durchgeführt. Anschließend wurden die freien Oberflächenenergien der Beschichtungen für den betrachteten Temperaturbereich nach der Methode von Owens, Wendt, Rabel und Kaelble bestimmt. Die Ergebnisse zeigen, dass die CrN-Beschichtungen die geringste freie Oberflächenenergie aufweisen. Im Vergleich zeigte sich für alle Hartstoffschichten eine Abnahme der freien Oberflächenenergie mit sich erhöhender Temperatur. Die chemische Zusammensetzung der Probenoberfläche scheint bei gleichbleibenden Messbedingungen für glatte Oberflächen der bestimmende Faktor für die Höhe der freien Oberflächenenergie zu sein. The wetting of melt-facing surfaces as well as the sticking of solidified plastic on molding tools is related to surface effects between plastic materials and mold material. One approach to quantify the wetting on solid surfaces is the determination of the free surface energy. Several studies determined the free surface energy of hard coatings used in polymer processing based on the drop shape analysis. These measurements were, however, mostly conducted at room temperature, which is not comparable to molding temperatures, or were limited to static contact angle measurements, which are nearly non-reproducible. In this study, the wetting of hard coatings used in polymer processing was investigated by measuring the advancing contact angle of two test fluids on a titanium nitride, two differently deposited chromium nitride and one diamond-like carbon coating at relevant demolding temperatures in the range from 60 °C to 140 °C. The results showed the smallest free surface energy values for the two chromium nitride coatings. The comparison of all investigated coatings revealed a decline of the free surface energy with increasing solid temperature. The chemical composition of the material seems to be a determining factor for smooth surfaces and constant measuring conditions.

Zusammenfassung

The demolding of plastic parts remains a challenging aspect of injection molding. Despite various experimental studies and known solutions to reduce demolding forces, there is still not a complete understanding of the effects that occur. For this reason, laboratory devices and in-process measurement injection molding tools have been developed to measure demolding forces. However, these tools are mostly used to measure either frictional forces or demolding forces for a specific part geometry. Tools that can be used to measure the adhesion components are still the exception. In this study, a novel injection molding tool based on the principle of measuring adhesion-induced tensile forces is presented. With this tool, the measurement of the demolding force is separated from the actual ejection step of the molded part. The functionality of the tool was verified by molding PET specimens at different mold temperatures, mold insert conditions and geometries. It was demonstrated that once a stable thermal state of the molding tool was achieved, the demolding force could be accurately measured with a comparatively low force variance. A built-in camera was found to be an efficient tool for monitoring the contact surface between the specimen and the mold insert. By comparing the adhesion forces of PET molded on polished uncoated, diamond-like carbon and chromium nitride (CrN) coated mold inserts, it was found that a CrN coating reduced the demolding force by 98.5% and could therefore be an efficient solution to significantly improve demolding by reducing adhesive bond strength under tensile loading.

Zusammenfassung

Hard coatings can be applied onto microstructured molds to influence wear, form filling and demolding behaviors in microinjection molding. As an alternative to this conventional manufacturing procedure, “direct processing” of physical-vapor-deposited (PVD) hard coatings was investigated in this study, by fabricating submicron features directly into the coatings for a subsequent replication via molding. Different diamondlike carbon (DLC) and chromium nitride (CrN) PVD coatings were investigated regarding their suitability for focused ion beam (FIB) milling and microinjection molding using microscope imaging and areal roughness measurements. Each coating type was deposited onto high-gloss polished mold inserts. A specific test pattern containing different submicron features was then FIB-milled into the coatings using varied FIB parameters. The milling results were found to be influenced by the coating morphology and grain microstructure. Using injection–compression molding, the submicron structures were molded onto polycarbonate (PC) and cyclic olefin polymer (COP). The molding results revealed contrasting molding performances for the studied coatings and polymers. For CrN and PC, a sufficient replication fidelity based on AFM measurements was achieved. In contrast, only an insufficient molding result could be obtained for the DLC. No abrasive wear or coating delamination could be found after molding.

Zusammenfassung

Molding techniques allow the replication of surface structures to enable improved or novel functions for optics, fluidics and further applications. In general, the molding process can be facilitated by means of antiadhesive-coated molds due to a reduction of demolding forces, wear or prevention of heat dissipation. Consequently, employing coated molds, an improved demolding of features with submicron dimensions and high aspect ratio can be expected. In order to achieve adequate results regarding the replication of nanostructures, accurate mastering techniques to structure the coating are required. One possible technique is focused ion beam (FIB) writing, which was investigated in this study regarding its suitability to mill structured surfaces of submicron dimensions into coated mold inserts. Mold inserts were coated with a thin-film diamond-like carbon (DLC) and chromium nitride (CrN) as well as electroless plated nickel-phosphorus (NiP). The coatings were examined by means of SEM imaging and roughness measurements regarding their applicability for the creation of nanostructures. Subsequently, a test pattern was written into the coatings using a FIB-SEM dual beam system. The test pattern consisted of the USAF test chart target groups 9, 10 and a Siemens star. The lateral dimensions were ≤ 980 nm. During ion beam milling, the beam parameters current and the number of scan repetitions were varied to investigate their influence on the structuring result. Subsequently, SEM measurements were conducted to characterize the milled nanostructures. Different lateral resolutions and depths of the pattern and effects like redeposition or the proximity effect were observed for the nanostructures depending on the coating and processing conditions. Lateral feature sizes down to 60 nm could be realized. In conclusion, the gained results approved FIB writing as a suitable technique to manufacture highly precise nanostructures into surface coatings for molding.