Viscous dissipation influencing viscosity of polymer melt in micro channels

Determination of melt rheological behavior within micro-structured geometry is very important for the accurate simulation modeling of micro-molding. Yet studies on the rheological behavior of polymer melts, flowing through micro channels, are complicated due to a large number of factors affecting the melt viscosity. One factor, viscous dissipation, is investigated in the current work through a novel experimental technique to determine the viscous dissipation of a polymer melt flowing through several micro channels with identical aspect ratio. Relative tests are conducted with the melt of high density polyethylene (HDPE) at different temperatures being extruded through the capillary dies with diameters 1000μm, 500μm and 350μm, respectively. It was found that the temperature rise due to viscous dissipation decreases significantly with the reduction of the characteristic size of micro channel at the same shear rate. In addition, based on the suggested model of radial temperature distribution, the influence of viscous heating on the melt viscosity is investigated. The results indicate that viscous dissipation does not play a significant role.     

Numerical investigation on frictional pressure loss in a perfect square micro channel with roughness and particles

A numerical study is performed to investigate the effect of inner surface roughness and microparticles on adiabatic single phase frictional pressure drop in a perfect square micro channel. With the variation of particles sizes (0.1 to 1 μm) and occupied volume ratio (0.01 to 10%) by particles, the Eulerian multi-phase model is applied to a 100 μm hydraulic diameter perfect square micro channel in laminar flow region. Frictional pressure loss is affected significantly by particle size than occupied volume ratio by particles. The particle properties like density and coefficient of restitution are investigated with various particle materials and the density of particle is found as an influential factor. Roughness effect on pressure drop in the micro channel is investigated with the consideration of roughness height, pitch, and distribution. Additionally, the combination effect by particles and surface roughness are simulated. The pressure loss in microchannel with 2.5% relative roughness surface can be increased more than 20% by the addition of 0.5 μm diameter particles.     

Micro laser welding of polymer microstructures using low power laser diodes

The use of laser welding for joining micro parts has experienced a substantial increase in popularity during recent years. Specifically translucent microfluidic devices are assembled using laser welding; however, a major issue is the laser beam size of commercially available laser-welding equipment and thus the resulting welding seam size, which may be orders of magnitude larger than microfluidic channels and structures. We have successfully achieved extremely small welding seams using focussed low-power laser diodes. Commercial laser welding stations for polymer assembly will typically operate in the power-region 15–50 Watts. The focussed laser beam will have a size of typically 500 μm × 500 μm and may, depending on optical configuration, be up to several mm². The resulting welding-seam will thus be in the area of 300–600 μm depending on beam energy distribution; additionally the melt will spread to unheated areas due to capillary forces. As microfluidic channels are in 20–100 μm regions, even a very limited amount of stray melt may completely fill a part of a channel and thus render it useless. We have used commercially available “single-die” laser-diodes of optical power 200–500 mW. The beam has been focussed and directed using simple optical installations, resulting in a beam-size in the area of 50 μm × 5 μm full width half maximum (FWHM) We have achieved firm welding seams of width <10 μm, with a welding speed of 15 mm/s and with virtually no noticeable spread of melt.     

Manufacturing of multi-functional micro parts by two-component metal injection moulding

Several metals and alloys can be used to enhance the mechanical and physical properties of micro parts and components for micromechanical, micro-chemical or sensor applications. Such parts can be produced in series by the powder metallurgical process of micro metal injection moulding (μ-MIM). This paper describes a novel manufacturing route for metallic multi-material micro components, two-component micro metal injection moulding (2C-μ-MIM). Similar to “two-colour” injection moulding of plastics, the process allows the integration of multiple functions in a micro part by simultaneously injecting and joining two materials in one mould. Net-shape parts with solid material interfaces are obtained. In this paper, the 2C-μ-MIM process is exemplified for the combination of a non-magnetic and a ferromagnetic stainless steel (316 L and 17-4PH). It is shown that intact material interfaces of less than 500×500 μm² can be achieved by careful selection and tailoring of metal powders, injection moulding and co-sintering parameters.     

Rapid prototyping and testing of 3d micro rockets using mechanical micro machining

The trend of miniaturization has been applied to the research of rockets to develop prototypes of micro rockets. In this paper, the development of a web-integrated prototyping system for three-dimensional micro rockets, and the results of combustion tests are discussed. The body of rocket was made of 6061 aluminum cylinder by lathe process. The three-dimensional micro nozzles were fabricated on the same aluminum by using micro endmills with φ100μm~φ500 μm diameter. Two types of micro nozzle were fabricated and compared for performance. The total mass of the rockets was 7.32 g and that of propellant (gun powder) was 0.65 g. The thrust-to-weight ratio was between 1.58 and 1.74, and the flight test with 45 degree launch angle from the ground resulted in 46 m~53 m of horizontal flight distance. In addition, ABS housing for the micro machined rocket was fabricated using Fused Deposition Modeling (FDM). A web-based design, fabrication, and test system for micro nozzles was proposed to integrate the distributed hardware resources. Test data was sent to the designer via the same web server for the faster feedback to the rocket designer.     

Influence of Tool Material and Surface Roughness on Galling Resistance in Sliding Against Austenitic Stainless Steel

Transfer and accumulation of adhered sheet material, generally referred to as galling, is a major cause for tool failure in sheet metal forming. In the present work, the galling resistance of three different tool materials was evaluated in lubricated sliding against austenitic stainless steel using a SOFS tribometer. All tool materials were prepared to four different surface roughnesses, ranging from a polished surface with R _a = 0.05 μm to a ground surface with R _a = 0.3 μm. The overall best performance was obtained for polished nitrogen alloyed powder metallurgy (PM) tool steel, where galling was detected only at the highest load evaluated, 700 N. However, for both the D2 type tool steel and nodular iron, best performance was observed for the surface possessing a surface roughness of 0.1 μm. The improved galling resistance for the rougher surfaces was related to filling of grinding scratches with sheet material during the initial stage of sliding, prolonging the development of protruding sheet material on the tools surface. Similar trend was not observed for the PM steel, which was related to width of the scratches originating from the surface preparation, in relation to tool microstructure.     

Modular tool concept and process design for micro impact extrusion

Micro impact extrusion is an appropriate mass production technology for micro structuring of sheet metal. The technology was applied to form sample geometries consisting of ten precision cavities which are intended to be collets for the form- and force-locked integration of piezo rods for later usage of the sheets as “smart sheet metal”. For reasons of flexibility, a modular tool concept was studied and applied. It allows a flexible arrangement of geometry elements but introduces new aspects which need to be considered. This study investigated the influences of tribological- and process parameters onto the microforming process. Therefore forming experiments were performed with different die coatings (titanium nitride, titanium carbon nitride, amorphous carbon), different lubricants (common oil, forming lubricant and without lubrication), as well as two die materials. In contrast to forming processes with established monolithic die configurations, phenomena like tilted structures and the appearance of unwanted burr were investigated. For creating structures with minimal tolerances, these effects need to be considered. The investigation of the influence of process parameters showed, that the lowest surface roughness was achieved without lubrication. Furthermore low-friction coatings and harder die materials improved the forming results.     

Development of microelectrodes for electrochemical micromachining

Electrochemical micromachining (EMM) has become more and more important in micro machining in recent years. Microelectrode as the tool of EMM is an essential cell in the machining process. In this study, microelectrodes with various end shapes are fabricated by different processing techniques. First, the different end shape forming methods for microelectrode, such as electrochemical etching, single electric discharge, and electrochemical micromachining are investigated. Second, microelectrodes with various end shapes fabricated above are simulated, analyzed, and then used in EMM process. At last, micro holes array with diameter of less than 10???m, three micro holes with no taper and a 3D microstructure are machined on metallic materials by above three types of microelectrodes.     

Comparison of bonding procedures for 3D-structured polyimide films on silicon substrates applied to ink-jet cartridges

Established bubble-jet printheads consist of an assembly of three layers, which are a CMOS substrate, a channel layer and a nozzle layer. The aim of the presented work was to simplify the setup of bubble-jet printheads by means of integrating channel and nozzle structures in one single three-dimensional laser-structured polyimide nozzle plate. Different bonding techniques for an assembly onto 1/3 inch standard CMOS printhead substrates are validated. The main challenges are a variety of bonding materials, an alignment accuracy of <5 μm and the prevention of blocking the 20 μm deep fluidic channels in the polyimide which have minimal lateral dimensions of 10 μm and a minimal pitch of 15 μm. In total, three bonding techniques, with and without additional adhesion layers, were developed and evaluated. One method applies a 4 μm thick layer of Epotec 353ND (Polytec), a standard two-component epoxy, in a specifically adapted rolling manner onto the film that is subsequently aligned to the silicon chip using a flip-chip-bonder. Screen-printing and dispensing processes of adhesives were investigated but failed due to insufficient structural resolution. The second method uses photolithographic processes to produce structured adhesion preforms in SU-8 resist. With a layer thickness of 3 μm and an adapted curing schedule, promising results concerning resolution and contour accuracy were obtained. Thirdly, bonding without additional adhesion layers was achieved in a micro-sealing process that takes advantage of the highly defined thermoplastic softening of polyimide KJ (DuPont). The different processes were compared regarding to yield, printing behavior of the assembled printheads and applicability to high volume productions. The hereby developed adhesion technology was applied on the assembly of large one inch printheads for special applications.     

Tribological behavior of thin electroplated and chemically deposited Ni-P coatings on copper substrates

The paper deals with the study of the tribological behavior of thin (1 and 5 μm thick) Ni-P coatings formed on plates made of electrotechnical copper by electroplating and chemical deposition. It has been found that after heat treatment, owing to diffusion processes, the coating components are capable of penetrating into the substrate material—to a depth of 15 μm for electroplated coatings and of 35 μm for chemically deposited coatings)—and influencing the mechanical properties of a specimen such as the hardness and friction coefficient. A correlation between the wear mode of the coating and the method of its deposition has been found. The results show that thin (one μm thick and lesser) Ni-P chemically deposited coatings are promising for being used in friction units of precise mechanical devices.     

Investigation of single phase frictional pressure loss in circular micro tubes

Single phase pressure drops in micro tubes were investigated through an experimental measurement and a numerical simulation. Experimental Po was obtained in circular micro tubes with 87 and 118 μm diameter with distilled water. Experiments were carried out in laminar flow region with varying the Re 15–450 for the 87 μm diameter tubes and 60–1300 for the 118 μm diameter tube. No early transition from laminar to turbulent flow was detected for the experimental range. The computational estimation of pressure drop in the 87 μm diameter tube was performed with the aid of CFD software. Boundary conditions from experiments were used for the numerical simulation. The results of experimental and numerical studies showed a good agreement with the conventional macro theory.     

Vacuum suction aid for microlens array formation using LIGA-like process

Microlens array fabrication using a vacuum suction process combed with the LIGA-like process is presented in this paper. The circular patterned array was designed on a photomask and transferred onto a substrate using photoresist patterning. Electroforming technology was used to convert the photoresist patterns into a metallic molds with an array of nozzles. Liquid JSR resist was spun onto the substrate joining the metallic mold to remove microlens array under vacuum conditions. The exposure energy and vacuum pressure were essential parameters in the microlens array manufacturing process. Microlens arrays with 50 μm in diameter at -50 cm-Hg vacuum pressure and 100 μm in diameter at -60 cm-Hg vacuum pressure were successfully formed. The produced microlens arrays presented smooth measured surface profiles coincident with the optical lens geometry.     

Some aspects of fabrication of micro devices by electrochemical micromachining (ECMM) and its finishing by magnetorheological fluid

Electrochemical micromachining (ECMM) is an advanced machining process for machining of electrically conducting materials. In the present work, a micro nozzle and a fluidic mixer having complex structures are made using masking technique by ECMM process. Mask is made of 50-μm transparent sheet and the micro nozzle and micro mixer are fabricated on an 800-μm thick copper sheet. The resulting rough inner walls and bottom surface of micro nozzle are finished using magnetorheological fluid-based finishing process. Surface finish of the nozzle is significantly improved after finishing. A comparison of width, depth, and surface roughness of the micro nozzle is also carried out before and after finishing. The mixing behavior of two fluids is visualized by microscope in micro mixer. The rough inner walls of the mixer’s channels act as obstructions and result in zigzag path of flowing fluid. Hence, mixing occurs at microscopic level because of rapid molecular diffusion.     

A technique for preparing samples of metal nanowires for electron and atomic-force microscopy

Methods for preparing nanowire samples for experiments with transmission electron and atomic-force microscopes are described. In the method used for transmission electron microscopy investigations, metal is electrochemically deposited into etched channels of a track membrane kept close against the end surface of a thin metal foil. A method for forming nanowires in a slit of a thin epoxy film is proposed for atomic-force microscopy examinations. Original Russian Text ￠ V.F. Reutov, M.F. Miklyaev, B.V. Mchedlishvili, 2007, published in Pribory i Tekhnika Eksperimenta, 2007, No. 3, pp. 144–147.     

Fabrication and measurement of the performance of a printed EMI shielding mesh filter on PET film

In this paper, Electromagnetic interference (EMI) shielding mesh filter films are fabricated using the screen printing method. An EMI shielding mesh filter requires fine line patterns under 30 μm, therefore, the screen masks with fine-line screen mesh, as well as suitable ink composed of fine metal particles are required. Moreover, because the printed EMI shielding filter has to be fabricated on a PET film, the ink should be sintered at a temperature below 150°C not to deform the film. For the purposes of this study, a screen mesh with a mask of 20 μm line, and ink made nano silver paste with a sintering temperature of 140°oC, which could guarantee fine-line patterning on the PET, were used. The ink had a viscosity of 90,000–12,000 cps and metal content of 85% in weight. The printed EMI shielding mesh had a line width of about 24 μm and a thickness of about 2.5 μm. The measured surface resistance was 0.5∼0.7 ω/□, offering good electrical performance as an EMI mesh. A simple measurement system was used to evaluate the electrical shielding performance of the printed EMI shielding mesh. A comparison of the printed EMI shielding mesh with pure film without mesh and copper sheet revealed that the printed EMI shielding mesh is indeed capable of providing good electrical performance.     

Electro-chemical micro drilling using ultra short pulses

Electro-chemical machining (ECM) has been rarely applied in micro machining because the electric field is not localized. In this work, ultra short pulses with tens of nanosecond duration are used to localize dissolution area. The effects of voltage, pulse duration, and pulse frequency on the localization distance were studied. High quality micro hole with 8 μm diameter was drilled on 304 stainless steel foil with 20 μm thickness. Localization distance can be manipulated by controlling the voltage and pulse duration, and various hole shapes were produced including stepped holes and taper free holes.     

Generation of Janus droplets for enhanced mixing in microfluidics

microfluidic channel system to generate Janus droplets is designed and fabricated, where the term Janus droplet refers to a chemically biphasic droplet. It is demonstrated that Janus droplets are formed from elongational breakup of coflowing core fluids which are constrained by a sheath fluid on both sides of them. Rhodamine B is adopted as an indicator to indentify generated Janus droplets. Monodisperse Janus droplets have been generated in a controllable manner such that those with average diameters of 26 ± 1.24 μm, 31 ± 1.44 μm and 34 ± 2.28 μm are formed in accordance with flow rate ratios between the sheath fluid and the core fluids, 30.7, 36.4 and 44.4, respectively. Generation of Janus droplets, demonstrated in the present study, has seen a new application in the areas of biotechnology and bioengineering, where enhanced mixing inside the micro bubbles can be utilized without the aid of other means of droplet generation and merging.     

汽车覆盖件冲压成形仿真研究进展

汽车覆盖件冲压成形仿真技术的发展,突破了原有汽车冲压件模具及工艺设计的设计方法,对保证工件质量、减少材料消耗、缩短产品开发周期、降低制造成本具有重要意义.概述了目前汽车覆盖件冲压成形仿真所涉及到的热点领域,如摩擦与接触、回弹分析、模具系统和工艺参数、材料屈服模型和板料形状设计,讨论了这些领域的研究进展和进一步研究的发展方向.     

激光诱导空化微孔冲裁试验研究

针对微尺度下金属箔模具冲孔存在模具挤压磨损、对中困难,激光打孔存在烧蚀、吸收层无法补偿等问题,提出通过激光诱导空泡对金属箔进行加载来实现冲裁小孔的方法。研究了不同激光焦点位置(H=0~4 mm)、激光能量(E=10.3~50.8 mJ)和铜箔厚度(T=20~70μm)对铜箔冲孔的影响,发现激光焦点位置影响明显,当铜箔变形平均深度达到147.0μm后,铜箔发生剪切断裂,可实现冲裁,并且制备的小孔边缘正表面无烧蚀、毛边、裂纹和卷边等缺陷。同时,利用高速摄影仪对激光诱导空化微孔冲裁过程进行研究,结果表明激光诱导空化微孔冲裁过程是激光等离子体冲击波、空泡溃灭冲击波和微射流共同加载的过程。     

New methods of track membrane treatment in the preparation of samples for further observation with scanning electron microscopy

Track membranes are porous systems consisting of a polymer foil with thin channels (i.e. pores) piercing it from surface to surface. The creation of non‐cylindrical pores in a track membrane is important for the optimization of membrane characteristics, i.e. the highest productivity at the required selectivity. A new method of cleavage preparation (the irradiation of track membrane samples with accelerated electrons) for the observation of channel shapes directly in the membrane cross‐sections is presented. Diagrams showing the tensile and burst strengths as a function of the irradiation dose, and images of surfaces and cleavages of track membrane samples are presented in this work. The changes in the pore sizes and shapes along the channel were clearly seen. These results can be used for the optimization of track membrane production.