Molded Micro Research Articles

Nanoscale deformation of crystalline metals: Experiments and simulations

In the present work, we performed experiments and molecular dynamic simulations to study the atomic physical process and deformation mechanism involved in nanoscale deformation of crystalline metals. By contact deforming bulk metals using hard molds with different cavity sizes at various temperatures and different stress levels, the flow of metals (e.g., Au and Ag) into nanocavities is quantified by the aspect ratio of molded micro-/nanorods, based on which the dependence of deformability on cavity size, temperature and stress is revealed. It is found that the critical forming pressure is determined by the entering barrier. Once the entering barrier can be overcome, quantified by the aspect ratio of metal nanorods, the molding efficiency increases with the cavity size decreasing. Moreover, three deformation modes are uncovered in the nanoscale plastic flow of metals and directly related with the atomic physical processes, i.e. atomic diffusion, dislocation nucleation, multiplication and movement. In the dislocation dominated temperature regime, a transition of deformation mode from burst growth to continuous growth is observed as the cavity size increases. When the mold cavity size is very small (100-101 nm), the metal atoms flowing in a mold cavity will arrange amorphously even at low temperature, which usually only occurs at high temperature for large cavity sizes. Therefore, reducing the size shows the same effect as increasing the temperature, making nanomolding easier.

Peeling mechanics of film-substrate system with mutually embedded nanostructures in the interface

Through mechanical deformation of crystalline metals with hard molds, superplastic nanomolding provides a simple and high-throughput method to directly form nanostructures in the metal surface. The releasing of the molded nanostructures generally involves in chemical etching away the mold, in which metal nanostructures could also be chemically attacked. Therefore, mechanical demoulding is very promising not only for solving the above problem, but also for drastically reducing the cost by recycling molds. As a result, the mechanics underlying the mechanical demoulding is very important and highly desired since it can tell what kind of nanofeatures can be released without damaging the molds. In this paper, we develop a theory for the peeling mechanics of elastic film with its surface nanostructures deeply embedded in a substrate. Specifically, by analyzing the obtained micromechanical behavior in the interface and combining it into the mechanical framework of the macro-bending behavior of the film, we successfully derive a nonlinear governing equation for demoulding. By non-dimensionalizing the governing equation and further numerically solving it, we find out a simple expression for the apparent adhesion work. Our theory demonstrates that the apparent adhesion work is mainly contributed by the interface shear stress rather than the interface energy as revealed by the peeling of elastic film from flat or wavy interfaces, and the fillet radius at the edge of the cavity is found to be a crucial factor to determine the success or failure of demoulding. Finally, by recording the real-time peeling force of an actual peeling process, we observe that the theoretical prediction is in good agreement with the experimental results. This work not only provides theoretical guidance for mechanical demoulding of molded micro-/nanostructures, but also establishes a mechanical framework for studying the peeling mechanics of real material interface.

Numerical and experimental study on the optical performance of micro-pyramid functional surfaces

Micro-pyramid functional surfaces are currently widely used in various advanced scientific, technological, industrial, and engineering fields with their excellent performances of compactness, light weight and higher heat transfer performance. Many recent studies have introduced micro cutting as a method to achieve high-quality micro pyramid surfaces. This paper concerns a micro-pyramid with a height and slope of 60 μm and 30°. Firstly, numerical simulation of the optical effect related to micro-pyramid surfaces was carried out to reveal the mechanism of functional surface imaging and its typical and potential applications. The simulated optical effect shows that the micro-pyramid can form four virtual images on the same side of an object. Next, a novel and simple fabrication method for micro-pyramidal structures was developed by using cross cutting with a 120° V-tip diamond tool by ultra-precision raster milling (UPRM), which can obtain good surface quality, high form accuracy and efficient productivity in contrast to grinding, laser machining and etching. Finally, precision injection molding was employed to replicate a micro-pyramid on the PMMA for optical testing. Although the quality of molded micro pyramid pattern decreased, optical testing showed that clear images were obtained, and the experimental results were consistent with the simulation results.

Chloroform compatible, thiol-ene based replica molded micro chemical devices as an alternative to glass microfluidic chips.

Polymeric microfluidic chips offer a number of benefits compared to their glass equivalents, including lower material costs and ease and flexibility of fabrication. However, the main drawback of polymeric materials is often their limited resistance to (organic) solvents. Previously, thiol-ene materials were shown to be more solvent resistant than most other commonly used polymers; however, they still fall short in "harsh" chemical environments, such as when chlorinated solvents are present. Here, we show that a simple yet effective treatment of thiol-ene materials results in exceptional solvent compatibility, even for very challenging chemical environments. Our approach, based on a temperature treatment, results in a 50-fold increase in the chloroform compatibility of thiol-enes (in terms of longevity). We show that prolonged heat exposure allows for the operation of the microfluidic chips in chloroform for several days with no discernable deformation or solvent-induced swelling. The method is applicable to many different thiol-ene-based materials, including commercially available formulations, and also when using other commonly considered "harsh" solvents. To demonstrate the utility of the solvent compatible thiol-enes for applications where chloroform is frequently employed, we show the continuous and uniform production of polymeric microspheres for drug delivery purposes over a period of 8 hours. The material thus holds great promise as an alternative choice for microfluidic applications requiring harsh chemical environments, a domain so far mainly restricted to glass chips.

Effects of wall slip on ZrO2 rheological behavior in micro powder injection molding

The effects of wall slip on the rheological behavior of ZrO2 feedstock flowing through various channels were studied. As compared to conventional no-slip condition, a power-law wall slip model was established to simulate the filling behavior of ZrO2 feedstock flowing through the mold. The effects of wall slip on pressure distribution of the micro spool mandrils during filling was compared with the injection molded micro components. Experimental results verified that the simulation including wall slip yielded better prediction for the pressure gradient as well as crack propagation for the micro components. Increasing mold temperature not only enhanced the feedstock temperature flowing through the micro channels, but also reduced the temperature difference of the micro components and likely the ensuing thermal stress as well as cracks. Powder-binder separation is more sensitive to the mold temperature variation than melt temperature variation. Defect-free micro spool mandrils were injection molded using the optimized parameters.

Attempts to fabricate micro injection molding tools and assemble molded micro parts on same EDM machine

An EDM method for manufacturing plastic micro gear pumps is proposed. Micro gates and cavities for mating gears are machined by EDM in both sides of a stainless steel foil, respectively. The foil is then transferred to an injection molding machine, and fixed to a mold to form mating gears by injection molding. The mating gears formed on the foils are then re-positioned on the EDM machine, released from the foil, and placed in a second foil pre-machined by EDM for the pump housing, using an ejector pin fabricated by reshaping the tool electrode used to machine the two foils.

In-line computational shear interferometry of insert molded micro parts for optical application

Micro injection moulding is a mass production of micro optics that offers the possibility of a high functional integration within a single micro part by insert moulding. Since the functionality of the micro parts depends on the homogeneity of the overmoulded polymer layer, a robust, accurate and fast metrology system is needed for a quantitative quality assessment. Here, we demonstrate an in-line metrology approach for the optical inspection of insert molded micro parts. In contrast to standard interferometers, the proposed system has low demands regarding the coherence of illumination. Thus, an LED light source can be used instead of a laser, reducing the cost and increasing the safety of the production platform. In addition, the system is robust against mechanical distortions, since it is based on a common path approach. These advantages make the system a good candidate that fulfills the needs in regard to the in-line inspection of insert molded micro parts. As an example of application, the proposed system is used to inspect a cannula with overmolded thermoplastic as a light sleeve providing illumination over a specific area of surgery.

Layer-dependent characterization of wear behaviour on variothermal injection moulded micro parts using pin on disc measurements

In micro- and thin-wall injection moulding the process conditions affect the developed internal structures and thus the resulting part properties, e.g., wear behaviour. In this paper a dynamic mould tempering was used to affect the local morphology development and the resulting wear. Investigations with a pin on disc measurement on semi crystalline polymers reveal a distinct layer-dependent tribological behaviour of the injection moulded micro parts. Increased mould temperatures during injection moulding favour the morphological structure and the resulting part properties.

Influence of short fibre reinforcement on properties of injection moulded micro parts in dependence of fibre content and part dimensions

Varying part dimensions usually result in reduced usable material properties. The dependence of the resultant part properties with reduced part dimensions associated with fibre reinforcement is largely uninvestigated. This paper discusses the interactions between different part dimensions and varying short glass fibre reinforcement. The results show that fibre reinforcement can favour the use of mechanical material properties. However, the amount is dependent of the fibre content and the part dimensions available.

Mouldability of various zirconia micro gears in micro powder injection moulding

Abstract The mouldability of various zirconia micro gears was investigated based on different binder systems. Temperature window for feedstock preparation and injection moulding was set up based on thermal analysis. Rheological properties of the feedstocks and injection moulded micro gears of different dimensions (200 to 900 μm) were examined to study the mouldability. Experimental results show that the micro gears below 500 μm were difficult to mould due to sticking and short-shot. As the low-molecule binder component, bee wax (BW) provided the feedstock with better fluidity and stability than paraffin wax (PW) to replicate the small mould. As the backbone polymer, a combination of ethylene–vinyl acetate (EVA) and high-density polyethylene (HDPE) was beneficial to mould filling and demoulding. The micro gears as small as 200 μm were injection moulded and sintered with shape retention using the feedstock comprising the binder system of HDPE, EVA, BW and stearic acid.

Supporting design of primary shaped micro parts and systems through provision of experience

Nowadays, miniaturization is one observable trend in the development of new technical systems. Based on the miniaturization of existing macroscopic production technologies e.g. powder injection molding, a new process chain for the development of primary shaped microparts and systems is being developed in the collaborative research center 499 "design, production and quality assurance of molded micro components made of metallic and ceramic materials". Because of the constantly evolving miniaturization and growing complexity of the developed microparts and systems, new development processes and methods are needed. Because of the mostly restrictive influence of the downstream phases (production and quality assurance) over the design phase, micro-specific design processes require well-founded knowledge about micro-specific manufacturing techniques (e.g. restrictions) which may be stored in form of design rules. However, many micro-specific aspects cannot be represented in design rules. In this paper the authors propose a new approach to ensure this other kind of knowledge.

Ensuring functional reliability of molded micro components

Microsystems technology is an emerging technology that is used in ever more technical systems, such as inkjet nozzles or accelerometers. 3D micromechanical structures became feasible by miniaturizing macromechanical elements. One big challenge in downsizing mechanical elements is to adapt conventional manufacturing technologies to the conditions of microtechnology. Integrated design and production processes must be suited to the new requirements, in-depth knowledge of production technology is required to design micromechanical components. Since 2000, the Collaborative Research Center 499 "Design, Production and Quality Assurance of Molded Microcomponents made of Metallic and Ceramic Materials" has been focusing on a continuous and stable process chain for molded micro components (Albers et al. 2005, Microsyst Technol). The aim of prior funding periods was to develop the knowledge about how components, such as gearwheels, and systems, such as micro gears, can be designed. Present research is focused on how to design functional elements, i.e. shaft-hub joints to transmit torque. This paper will introduce an approach that generates know-how via testing several plain bearings and shaft-hub joints in order to derive the necessary know-how to develop simulation tools later on.

Warpage Measurement of a Micro Electrical Fan on Micro-Injection Molding

This study characterizes warpage of a micro-injection molded micro electrical fan using the Michelson interference method. This study conducts experiments to analyze different polymers-polypropylene (PP), polyamide (PA), acrylonitrile-butadiene styrene (ABS), ABS+ polycarbonate (PC), and polyoxymethylene (POM)-process parameters, such as mold temperature, injection temperature, injection pressure, injection time, packing time, and packing pressure, for a micro electrical fan. To obtain the optimum result (minimum warpage), this study assesses the effect (warpage) of each material on micro-injection molding. PA plastic is the very suitable material for micro electrical fan with Michelson interference analysis on micro-injection molding.

Elicitation of a reference process model for tool-based micro technologies for planning and controlling purposes and user support

The production of moulded micro components including design, manufacture and quality control is a highly integrated process. A lot of experts, machines, tools, etc. should be deployed meaningfully along this process. The process chain was never written down in detail or as a whole so that new products could be developed on this basis. All developments hitherto have been carried out partly intuitive and partly based on area-specific sub process chains. For an efficient and effective planning and controlling and user support during future tool-based micro product engineering processes, the authors propose a corresponding reference process model. For this purpose, implicit process knowledge must be retrieved from expert’s minds to consider area-specific sub-process chains. For implementation of the reference process model, the Integrated Product Engineering Model (iPeM) will be used.

Determination of Optimal Microminiature Powder Injection Molding Parameters by Taguchi Approach

The global manufacturing trend is now focusing towards miniaturization. Microminiature Powder Injection Molding (μPIM) is a viable technology to fabricate complex and high performance miniaturized components. The μPIM technique was used to produce the near-net shape micro components in this study. Fine stainless steel powder with particle size of 5μm was mixed with a ternary water-based binder system. Micro dumbbells with the largest dimension of 9mm were replicated. In order to obtain successful and well molded micro dumbbells, the Design of Experiments (DOE) technique was applied to investigate the optimal parameters in injection molding process. Injection parameters such as injection pressure (A), injection temperature (B), powder loading (C), mold temperature (D), injection time (E) and holding time (F) were optimized by using stainless steel feedstocks. Taguchi approach is chosen and the results were evaluated with signal-to-noise (SN) ratio and analysis of variance (ANOVA). The results show that the feedstocks could be replicated by using μPIM method with the application of Taguchi approach.

Methods for simulating and optimizing molded micro components and systems

Casting production of metallic and ceramic micro components is still in the research phase, and the components obtained with the current technologies are suffering of a certain lack of reproducibility. Within the scope of Collaborative Research Center 499 “Design, Production and Quality Assurance of Molded micro components made of Metallic and Ceramic Materials”, basics in a persistent process chain for micro components are acquired. The development, process preparation, mass production and material and components behavior investigation is understood under consistent chain. This work presents computer simulation based methods that contribute to optimize micro molded systems on both a component and a system basis. The introduction of such methods allows studying the behavior of components and systems with regard to their grain structure and the wide geometric tolerances required.

Effect of Protective Layer and Etch Process on Silicon Molded Micro Peltier Arrays

We used silicon molding and examined the protective layer on a silicon molded Peltier array. Both a large p-type array and n-type array were created with the protective layer. Because the conventional bismuth-antimony-telluride (BiSbTe) alloys react with XeF 2 etching gas rapidly, we need to place the protective layer at the interface between the silicon and the thermoelectric material using the water vapor thermal oxidization method. As the xenon difluoride selective etching ratio of silicon and SiO 2 is about 100:1, the protective layer is damaged if the removal ratio of silicon is high and the etching process time is long. Next we examined a new method involving both an anisotropic process using deep reactive ion etching (DRIE) and an isotropic process using XeF 2 etching, and we formulated an etching process that causes no damage to the protective layer.

Relationships between process, microstructure and properties of molded zirconia micro specimens

Due to size effects the mechanical behavior of micro-components with dimensions in the range of some 100 μm and structure details of about 10 μm differs markedly from those of larger components. This is a crucial aspect for the design of micro-components for applications where demands for high strength are critical. The present study, which was performed in the frame of the Collaborative Research Centre 499 (SFB 499), approaches this issue by investigating the relationship between production process, microstructure and the mechanical properties of micro-specimens made from zirconia using two different feedstocks. The specimens were produced by a sintering process. The sintering temperature was varied between 1,300 and 1,500°C. Mechanical and tribological behavior of the specimens was determined by three-point bending tests as well as static and sliding friction tests, respectively. Properties derived from these tests were then correlated to the surface states in the specimens such as porosity, edge radius and roughness. The strength of the micro-specimens was found to be significantly influenced by these surface features. Whilst low porosity alone is not sufficient for high strength, notch effects resulting from pores as well as surface roughness can lower the strength. With increasing edge radius the strength of the material also increases. The porosity, edge radius and surface roughness were mathematically correlated with the strength to allow for a forecast. Within the SFB 499 feedstocks with specific properties were designed and reliable processes were developed to guarantee desirable surface roughness and porosity in the specimens. A characteristic bending strength of about 2,000 MPa is realizable in the micro-specimens within a good statistical reliability. The tribological tests revealed that the wear properties of the zirconia micro-components are strongly dependent on the quality of the feedstock.

On the simulation of molded micro components and systems

Regarding micro components and systems, experimental work for characterizing materials' properties as well as components' and systems' behaviors have to be supplemented by numerical analyses. These analyses should cover component and system issues. On a component level, macroscopic approaches are extended by methods allowing consideration of the influence of components' grain structures including possible defects. On a system level, the high tolerances accepted for the individual components due to production inaccuracy and their effects on the expected load distribution capability of the system are taken into account. This paper presents approaches for simulation of micro components and systems using the finite element method and multi body simulation. Methods to overcome the abovementioned issues will be shown, as well as the effects of grain structure on the stress distribution in the individual components.

Rapid mold temperature variation for assisting the micro injection of high aspect ratio micro-feature parts using induction heating technology

Hot embossing and injection molding are popular methods to duplicate micro features formed during polymer micro-fabrication of MEMS devices. However, both methods face challenges in filling the polymer melt completely into a micro-featured geometry of a high aspect ratio. In this study, electromagnetic induction heating combined with water cooling is used to achieve rapid mold surface temperature control during the micro-feature injection molding process. A CAE simulation was also developed through integration of both thermal and electromagnetic analysis modules of ANSYS, and its capability and accuracy were verified experimentally. Efficiency evaluations of induction heating and the uniformity of mold temperature control were conducted on a micro-featured mold. This mold was designed with a micro channel array of 30–50 µm in width and 120 and 600 µm in depth, corresponding to aspect ratios ranging from about 2.4 to 12. The accuracies of the micro channels in molded PMMA parts can be used to evaluate the effect of mold temperature on replication accuracy. It was found that rapid mold surface heating with temperature rising from 60 °C to between 100 °C and 140 °C by induction heating requires 2–3.5 s, while the mold temperature returns to 60 °C in about 70–110 s. The simulated mold surface temperature results are consistent with measured results. Achieving the same temperature variation by switching circulation coolants of different temperatures requires at least 7 min. The simulation also reveals that the electromagnetic wave can penetrate into the bottom of the micro channel and results in only about a 2 °C difference in temperature uniformity. For mold temperatures of 100 °C, 120 °C and 140 °C, the molded channel depths were 94.9 µm, 105.4 µm and 116.0 µm, respectively, when the ideal channel depth was 120 µm. When the channel depth is 600 µm, the mold temperature must exceed 120 °C, so that reasonable accuracy in micro-feature replication can be achieved. Our results to date indicate that the aspect ratio for molded PMMA micro channels can be as high as 12. Efficient mold temperature variation by induction heating to improve the replication accuracy in molding micro features is successfully illustrated.