Research Papers

J. Micro Nano-Manuf. 2016;4(2):021001-021001-4. doi:10.1115/1.4032323.

Electromagnetic forming (EMF) is a high-speed forming process that is already established in the macroworld. Due to its advantages like high deformation rate and cheaper tools, it is introduced to microforming. In this research, the replication of prismatic optical microstructures is investigated. EN AW-1050A (Al99.5) micrometal sheets with a thickness of 50 μm and 300 μm are electromagnetically micro-embossed. With this technique, it is possible to successfully replicate triangular cross section micro V-grooves of 86.6 μm in width and 24.1 μm in depth with an average surface roughness of Sa = 44 nm. The microstructures of the embossing tool are generated by diamond micro chiseling (DMC), a novel machining process to produce microstructures with discontinuous geometry, like miniature cube corner retro reflectors and V-grooves with well-defined endings.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2016;4(2):021002-021002-14. doi:10.1115/1.4032688.

The advances in the Terahertz (THz) technology drive the needs for the design and manufacture of waveguide devices that integrate complex three-dimensional (3D) miniaturized components with meso- and micro-scale functional features and structures. Typical dimensions of the waveguide functional structures are in the range from 200 μm to 50 μm and dimensions decrease with the increase in the operating frequency of the waveguide devices. Technological requirements that are critical for achieving the desired microwave filtering performance of the waveguides include geometrical accuracy, alignment between functional features and surface integrity. In this context, this paper presents a novel manufacturing route for the scaled-up production of THz components that integrate computer numerical control (CNC) milling and laser micromachining. A solution to overcome the resulting tapering of the laser-machined structures while achieving a high accuracy and surface integrity of the machined features is applied in this research. In addition, an approach for two-side processing of waveguide structures within one laser machining setup is described. The capabilities of the proposed manufacturing process chain are demonstrated on two THz waveguide components that are functionally tested to assess the effects of the achieved machining results on devices' performance. Experimental results show that the proposed process chain can address the manufacturing requirements of THz waveguide filters, in particular the process chain is capable of producing filters with geometrical accuracy better than 10 μm, side wall taper angle deviation of less than 1 deg from vertical (90 deg), waveguide cavities corner radius better than 15 μm, and surface roughness (Sa) better than 1.5 μm. The manufacturing efficiency demonstrated in this feasibility study also provides sufficient evidences to argue that the proposed multistage manufacturing technique is a very promising solution for the serial production of small to medium batches of THz waveguide components. Finally, analyses of the manufacturing capabilities of the proposed process chain and the photoresist-based technologies were performed to clearly demonstrate the advantages of the proposed process chain over current waveguide fabrication solutions.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2016;4(2):021003-021003-11. doi:10.1115/1.4032904.

In spite of its applications in macromanufacturing processes, water jet processing has not been extensively applied to the field of micromanufacturing owing to its poor tolerance and lack of effective control of the jet impingement position. This paper investigates the phenomenon of liquid dielectrophoresis (LDEP) using a localized nonuniform static electric field to deflect and control the jet's trajectory at the microscale for a water jet in air. A new analytical modeling approach has been attempted by representing the stable length of a water jet as a deformable solid dielectric beam to solve for the deflection of the jet under the action of the electric field. This method bypasses the complicated flow analysis of the water jet in air and focuses specially on the effect of the electric field on the trajectory of a laminar water jet within the working length. The numerical analysis of the phenomena for this electrode configuration was carried out using comsol. Preliminary proof-of-concept experiments were conducted on a 350 μm diameter sized water jet flowing at 0.6 m/s using a pin plate electrode configuration where a deflection of around 10 deg was observed at 2000 V. The results from the simulation are in good agreement with the results obtained in the preliminary experiments. This novel approach of modeling the water jet as a deformable dielectric beam might be useful in numerous applications involving precise control of the water jet's trajectory particularly in microwater jet material processing.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2016;4(2):021004-021004-6. doi:10.1115/1.4032757.

There is an increasing demand for product miniaturization and parts with features as low as few microns. Micromilling is one of the promising methods to fabricate miniature parts in a wide range of sectors including biomedical, electronic, and aerospace. Due to the large edge radius relative to uncut chip thickness, plowing is a dominant cutting mechanism in micromilling for low feed rates and has adverse effects on the surface quality, and thus, for a given tool path, it is important to be able to predict the amount of plowing. This paper presents a new method to calculate plowing volume for a given tool path in micromilling. For an incremental feed rate movement of a micro end mill along a given tool path, the uncut chip thickness at a given feed rate is determined, and based on the minimum chip thickness value compared to the uncut chip thickness, the areas of plowing and shearing are calculated. The workpiece is represented by a dual-Dexel model, and the simulation properties are initialized with real cutting parameters. During real-time simulation, the plowed volume is calculated using the algorithm developed. The simulated chip area results are qualitatively compared with measured resultant forces for verification of the model and using the model, effects of cutting conditions such as feed rate, edge radius, and radial depth of cut on the amount of shearing and plowing are investigated.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2016;4(2):021005-021005-7. doi:10.1115/1.4032891.

Vacuum venting is a method proposed to improve feature replication in microparts that are fabricated using micro-injection molding (MIM). A qualitative and quantitative study has been carried out to investigate the effect of vacuum venting on the nano/microfeature replication in MIM. Anodized aluminum oxide (AAO) containing nanofeatures and a bulk metallic glass (BMG) tool mold containing microfeatures were used as mold inserts. The effect of vacuum pressure at constant vacuum time, and of vacuum time at constant vacuum pressure on the replication of these features is investigated. It is found that vacuum venting qualitatively enhances the nanoscale feature definition as well as increases the area of feature replication. In the quantitative study, higher aspect ratio (AR) features can be replicated more effectively using vacuum venting. Increasing both vacuum pressure and vacuum time are found to improve the depth of replication, with the vacuum pressure having more influence. Feature orientation and final sample shape could affect the absolute depth of replication of a particular feature within the sample.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2016;4(2):021006-021006-9. doi:10.1115/1.4033344.

In micro electro-discharge machining (micro-EDM), it is believed that electrical conductivity of the dielectric modified by additives plays an important role in discharge initiation and electrical breakdown, thereby affecting the process characteristics including process accuracy, material removal rate (MRR), and surface finish. However, there has been a lack of systematic efforts to evaluate the effect of dielectric conductivity in micro-EDM. This paper investigates the role of electrical conductivity of the dielectric on the breakdown, plasma characteristics, and material removal in micro-EDM via modeling and experimentation. Experiments have been carried out at four levels of electrical conductivity of saline water, i.e., 4 μS/cm, 362 μS/cm, 1106 μS/cm, and 4116 μS/cm, to study electrical breakdown of the dielectric and resulting craters. A global modeling approach is employed to model the micro-EDM plasma in saline water and predict the effect of dielectric conductivity on electron density, plasma temperature, heat flux to anode, plasma resistance, and discharge energy. It is found from both experiments and model-based simulations that increase in the dielectric conductivity facilitates the electrical breakdown of the dielectric by lowering the minimum breakdown potential at a given interelectrode gap. Experimental results also show increase in the volume of material removed per discharge when dielectric conductivity is increased, which is attributed to the increase in anode heat flux predicted by the micro-EDM plasma model. The model also predicts increase in electron density, decrease in plasma resistance, and decrease in discharge energy as the dielectric conductivity increases.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Micro Nano-Manuf. 2016;4(2):024501-024501-8. doi:10.1115/1.4032324.

A comparison of the machining performance of micro-electro-discharge machining (EDM) milling and sinking is proposed considering the fabrication of microchannels with controlled sloped walls realized in a hardened steel workpiece. Adopting the fine-finishing machining regime for both micro-EDM techniques, the experimental results show that micro-EDM sinking is about ten times faster than milling in the worst case, though a lack of accuracy in the final microfeatures in the former case is detected due to not compensated tool wear. On the contrary, micro-EDM milling provides a better control of the microchannel dimensions. Finally, a microfilter mold for medical applications is machined in order to show the potential of the combination of both technologies.

Commentary by Dr. Valentin Fuster

Design Innovation Paper

J. Micro Nano-Manuf. 2016;4(2):025001-025001-12. doi:10.1115/1.4032302.

Just as in conventional injection molding of plastics, process simulations are an effective and interesting tool in the area of micro-injection molding. They can be applied in order to optimize and assist the design of the microplastic part, the mold, and the actual process. Available simulation software is however actually made for macroscopic injection molding. By means of the correct implementation and careful modeling strategy though, it can also be applied to microplastic parts, as it is shown in the present work. Process simulations were applied to two microfluidic devices (a microfluidic distributor and a mixer). The paper describes how the two devices were meshed in the simulations software to obtain a proper simulation model and where the challenges arose. One of the main goals of the simulations was the investigation of the filling of the parts. Great emphasis was also on the optimization of selected gate designs for both plastic parts. Subsequently, the simulation results were used to answer the question which gate design was the most appropriate with regard to the process window, polymer flow, and part quality. This finally led to an optimization of the design and the realization of this design in practice as actual steel mold. Additionally, the simulation results were critically discussed and possible improvements and limitations of the gained results and the deployed software were described. Ultimately, the simulation results were validated by cross-checking the flow front behavior of the polymer flow predicted by the simulation with the actual flow front at different time steps. These were realized by molding short shots with the realized molds and were compared to the simulations at the global, i.e., part level and at the local, i.e. feature level.

Commentary by Dr. Valentin Fuster

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