Quality Of Micro-holes Research Articles

Study of Behavior of EC Discharges during PMMA Machining by ECDM Process

Poly (methyl methacrylate)-PMMA material is an essential and useful material for the majority of microfluidic devices. The superior properties of PMMA, namely, optical transparency, biocompatibility and chemical inertness make the PMMA a worthy candidate for microfluidic devices. Micro-holes are one of the significant micro-features used in microfluidic devices fabricated in PMMA material. The various techniques of PMMA machining includes laser machining, hot embossing, injection molding, and mechanical cutting. Nevertheless, the usage of such processes is limited due to limitations such as higher cost, processing difficulties and machining quality. In recent times, the electrochemical discharge machining (ECDM) process has emerged as a efficient process for machining electrically non-conductive material such as glass, alumina etc. The present work is an attempt to perform micromachining in PMMA using the ECDM process. The primary mechanism of material removal in ECDM process consists of exhaustive and elevated temperature heating of the PMMA material due to the electrochemical (EC) discharges occurring at tool electrode tip in the EC region that leads to melting as well as vaporization of the material. Furthermore, the increment in the local machining region temperature enhances the rate of chemical etching which produces the smoother sidewalls of the microfeatures. The work material-PMMA, stainless steel tool electrode is kept in the close vicinity of each other in an electrochemical cell which also includes aqueous KOH electrolyte with counter ring electrode. The micro-hole quality factors such as size, depth, circularity etc. depend on the heat energy produced by EC discharges. Hence, the behavior of EC discharges greatly affects the machining process and micro-hole quality and it’s study is critical in order to achieve the micro-holes of appropriate quality for different microfluidic applications. The heat energy(q) generated, and EC discharges behavior depends majorly on the applied voltage (Va) and mean EC discharge current (Id). EC discharge behavior was evaluated by analyzing the Id values, the discharge frequency (f) with the help of current-time graphs which show the current peak stabilities and uniformities. Moreover, the stable and unstable EC discharge zones were also identified using the electrolysis current, discharge current and Va values recorded during the actual machining. The Va values up to 26 V indicated the instability region of EC discharges. The current peaks recorded for 26 V indicated the non-uniformity resulting in the micro-holes with inferior quality (higher size, lower depth, less circularity). The Va values greater than 28 V showed a higher stability as well as uniformity of EC discharges. Based on the outcome of the experimental results, superior quality micro-holes of varied sizes have been fabricated and characterized. The appropriate quality micro-holes of average opening size of ~181±5 µm with average depth of ~142± µm were achieved at the optimal range of Va i.e. 28 V to 30 V. Figure 1

Improving the quality of micro holes drilled with a current detection plasma arc device

Improving the quality of micro holes drilled with a current detection plasma arc device

Micro-Fabrication of PMMA Using Electrochemical Discharges

The Poly (methyl methacrylate)-PMMA material also recognized as acrylic is an significant material for the different microsystem’s technologies. PMMA properties such as optical transparency, high hydrophilicity, biocompatibility, chemical inertness and lower autofluorescence, makes it a suitable candidate for the microfluidics devices These devices consist of the various micro-features i.e., micro-holes as well as the micro-channels. In current times, fabrication methods such as laser machining, hot embossing, injection molding, and mechanical cutting are predominantly used for the microfabrication in PMMA material. Nonetheless, the major processes such as laser machining and mechanical cutting exhibit some limitations due to issues like higher processing+setup costs, higher thermal damage, edge burr formations etc. Recently, a method i.e. machining with electrochemical discharges emerged as the potential alternative to the existing methods to machine the PMMA substrates. This method also called ‘electrochemical discharge machining (ECDM)’ considered to be the hybrid combination of microfabrication based on electric-discharge and electro-chemical processes. ECDM is a relatively simple, economical process which, in principle, can perform micro-fabrication in various materials irrespective of its electrical-conductivity, hardness as well as brittleness. In ECDM heat energy generated by electrochemical (EC) discharges in an electro-chemical cell (consists of a micro tool rod/electrode, a big-sized counter electrode and an aqueous alkaline electrolyte).In ECDM process, when voltage (>2 V) is provided between the smaller size tool (cathode) and bigger counter electrode (anode) which are kept in the aqueous KOH. The applied voltage results in the electrolysis of aqueous KOH electrolyte and produces gas bubbles of hydrogen (H2) at cathode point and of oxygen at anode point respectively. At the point of cathode (tool electrode) gas bubble production is very higher due to higher current density at the smaller tool surface. The subsequent increase in the applied voltage starts the process of merger of bubble which forms an envelope of gas film around the smaller tool electrode. At this stage, tool electrode gets complete isolation from surrounding electrolyte. Breakage of this gas film envelope happens at elevated voltage (> 24±0.1 V) and discharges are generated which produces heat energy. The material removal from the PMMA workpiece happens which is kept below the discharge producing tool. The material removal is result of melting, evaporation and etching due to heat energy.The most of research on ECDM is demonstrated for the glass and alumina ceramics in past decade which are electrically nonconductive workpieces. Nevertheless, the ECDM process has not been completely explored for the polymers, such as PMMA. Bhargav et al. have recently explored this process for the micro-channel formation in PMMA using NaOH electrolyte with 0.7 mm titanium tool electrode having cylindrical shapes. It is very much clear from the known literature that work related to PMMA micro-structuring using ECDM is scarce. Furthermore, the experimental analysis was directed on the micro-channel (> 700 µm) formation only. Consequently, there is a wide research gap for micro-structuring of PMMA for formation miniaturized micro-holes (> 300 µm) of different shapes such as circular, slotted. Analysing EC discharge behaviour, tool electrode condition, and micro-hole geometric features are the key tasks which are performed.The present work is performed to investigate the EC discharge behaviour and identification of the appropriate applied voltage on the micro-hole formation as well as the geometric characteristics such as hole size, depth, heat affected zone (HAZ) and circularity. Also, the tool condition after usage and its effect on the EC discharges and micro-hole quality have also been explored. The identification of Vc i.e. critical voltage has been carried out. The EC discharges stability for the range of applied voltage has been analyzed. It has been found that the applied voltage needed for the appropriate machining quality with uniform EC discharges should be in the range of Vc+4 to Vc+6. The lower values of applied voltage Vc+2 indicated the region of the unstable and non-uniform EC discharges affecting the micro-hole quality. The worn-out tool also affects the hole quality and EC discharge stability. Figure 1

Investigation of Microhole Quality of Nickel-Based Single Crystal Superalloy Processed by Ultrafast Laser

The geometric accuracy and surface quality of thin-film cooling holes have a significant impact on the cooling efficiency and fatigue life of aeroengine turbine blades. In this paper, we conducted experimental research on the processing of nickel-based single-crystal high-temperature alloy DD6 flat plates using different femtosecond laser processes. Our focus was on analyzing the effects of various laser parameters on the geometric accuracy results of microholes and the quality of the surfaces and inner walls of these holes. The results demonstrate that femtosecond laser processing has great influence on the geometrical accuracy and surface quality results of film cooling holes. Notably, the average laser power, focus position, and feed volume exert a significant influence on the geometric accuracy results of microholes. For instance, a higher laser power can damage the microhole wall, thereby leading to the formation of tiny holes and cracks. Additionally, microholes exhibit optimal roundness and taper values when using a zero defocus volume. Moreover, increasing the feed distance results in enhanced entrance and exit roundness, whereas scanning speed has a negligible impact on microhole roundness.

Editors' Choice—Improving Quality of EDMed Micro-Holes on Titanium via In Situ Electrochemical Post-processing: A Transient Simulation and Experimental Study

Electrical discharge micromachining (EDM) poses challenges to the fatigue-life performance of machined surfaces due to thermal damage, including recast layers, heat-affected zones, residual stress, micro-cracks, and pores. Existing literature proposes various ex situ post-processing techniques to mitigate these effects, albeit requiring separate facilities, leading to increased time and costs. This research involves an in situ sequential electrochemical post-processing (ECPP) technique to enhance the quality of EDMed micro-holes on titanium. The study develops an understanding of the evolution of overcutting during ECPP, conducting unique experiments that involve adjusting the initial radial interelectrode gap (utilizing in situ wire-electrical discharge grinding) and applied voltage. Additionally, an experimentally validated transient finite element method (FEM) model is developed, incorporating the passive film formation phenomenon for improved accuracy. Compared to EDM alone, the sequential EDM-ECPP approach produced micro-holes with superior surface integrity and form accuracy, completely eliminating thermal damage. Notably, surface roughness (Sa) was reduced by 80% after the ECPP. Increasing the voltage from 8 to 16 V or decreasing the gap from 60 to 20 μm rendered a larger overcut. This research’s novelty lies in using a two-phase dielectric (water-air), effectively addressing dielectric and electrolyte cross-contamination issues, rendering it suitable for commercial applications.

Process modeling and optimization in laser drilling of bulk metallic glasses based on GABPNN and machine vision

It is a challenging to ensure the quality of microholes during the laser drilling of bulk metallic glasses (BMGs), where the defects such as heat-affected zones, burrs, and low smoothness are usually obtained. Based on machine vision, back-propagation neural network (BPNN), and genetic algorithm (GA) in artificial intelligence technology, a relationship model between the microhole features and laser processing parameters was proposed, so as to predict and optimize the processing parameters during the laser drilling of BMGs. Firstly, the geometric parameters and texture information of microholes were obtained using fractal theory and machine vision techniques, including the gray-level co-occurrence matrix (GLCM) algorithm, image binarization, and edge extraction. A comprehensive numerical characterization of various microhole features was conducted, providing the basis for subsequent prediction and optimization. Secondly, the previously-extracted microhole features were set as input layer, and the processing parameters were set as output layer. After proper training, the BP neural network optimization model (GABPNN), based on the genetic algorithm, was established. As compared to the original BPNN model, the GABPNN model exhibited improved performance, reducing the maximum prediction error from 4.860% to 2.285%. Thirdly, with a set of input layer parameters, the required processing parameters were predicted by the model. The predicted results were verified in experiments, where good surface quality of the laser-drilled BMG microholes with good roundness and small ablation was obtained. This shows that it is feasible to predict and optimize processing parameters by GABPNN based on feature parameters extracted by machine vision.

Quality Analysis of Micro-Holes Made by Polymer Jetting Additive Manufacturing.

Material jetting technology is gaining popularity, especially in polymer science, because of their high accuracy for additive manufacturing (AM) products. This paper aims to investigate the quality of micro-holes that are oriented in three basic directions, and manufactured by the material jetting AM process. This paper proposes a novel methodology to evaluate the accuracy of micro-holes features by using a transparent artifact. A test artifact with horizontal and vertical micro-holes in it, with industrial applications, was designed. Micro-holes were placed on planar and curve surfaces. Samples were manufactured by PolyJet technology from a translucent photopolymer resin which allows a facile investigation (by microscopy) of the inner structure of the micro-holes. The features of ten micro-holes printed in matte and glossy finish type, with diameters in coarse and medium options, according to ISO/ASTM 52902, were analyzed. Quality analysis of the micro-holes features was performed by microscopy investigations. The effects of main factors on the deviation of the micro-hole diameter were investigated by using the statistical design of experiments, and four control factors were considered. The best results were obtained for sample printed in matte finishing with the micro-holes oriented along the x-axis and z-axis. The smallest diameter of the micro-holes obtained by PolyJet technology on an EDEN 350 machine was 0.5 mm, but in industrial applications for a facile post-processing, a higher diameter is recommended to be used. A confirmatory experiment on a wing sample, with a number of micro-holes of the same diameter and a large length to diameter ratio of the micro-holes, was performed, and the repeatability of the results was confirmed.

Parametric optimization of hole taper control in ultraviolet nanosecond laser micro-drilling of copper foil

While hole taper is inevitably formed in the laser micro-drilling of through micro-holes, the formation propensity is largely determined by the interaction between laser processing parameters. In the present work, we perform numerical simulations and experiments to investigate the ultraviolet nanosecond laser micro-drilling of 1 mm diameter through micro-holes on copper foil with a thickness of 280 μm, with an emphasis on the coupled influence of laser processing parameters on both the hole taper and circularity. Firstly, the characteristics of formed micro-hole quality are experimentally characterized, and the formation mechanisms of hole taper are further revealed by finite element simulations of multi-pulse laser ablation. Then, the relational degree of individual laser processing parameters in determining the hole taper and circularity is discovered by experimental investigation based on the gray relational analysis model. Finally, the coupled effect between significant parameters of laser power and repeat scanning number on the hole taper and circularity is further investigated by utilizing the response surface methodology, which derives the optimum laser processing parameters for manufacturing 1 mm diameter through micro-hole with a taper degree of 2.556° and a circularity of 0.992 on the copper foil.

Effect of input process parameters on MRR in micro EDM drilling of CFRP sheet using RSM

In recent years, composite materials are replaced most conventional (industrial) materials such as steel, aluminum, copper, etc. since, CFRP material is one of the most widely used composite materials due to its excellent mechanical as well as physical properties, including strength, modulus, and compressive strength. EDM micro-holes in CFRP composites present a challenge due to fiber orientation, uneven distribution of conducting fiber, and delamination, which affect material removal rate (MRR) and micro-hole quality. This article aims to optimize input process parameters for optimum MRR. Micro EDM input parameters such as voltage, capacitance, and electrode rotation speed are examined on the MRR of micro-holes by using response surface methodology. Experiments were conducted based on the Box-Behnken design of experiment. In the experiment, capacitance was found to be the most significant factor among all input process parameters affecting MRR. Regression equations and response surfaces are developed and the results are that capacitance and rotation speed of the electrode are increased initially, and the MRR increases, however, its value decreases as the capacitance and voltage are increased further. The optimum MRR is obtained in the range of 150–250 nF capacitance, at 100 V.

Molecular dynamics simulation and experimental study on formation mechanism of micro-hole and cracks in nano-imprinting diamond

Diamond nano-imprint microhole forming technology is a new type of microhole forming method. However, there will always be some unqualified microholes and cracks in the manufacture of microholes. In order to obtain better microhole quality, it is crucial to study the mechanism of microhole formation. In this paper, the indentation process of single crystal copper by a conical indenter is simulated by molecular dynamics. After the indentation is completed, microholes and cracks will form on the surface of the copper sheet. Dislocation analysis will be conducted on the microholes and surrounding cracks, and then the microstructure and morphology will be demonstrated to explain the formation mechanism of microholes and cracks. The influence of different indentation speeds and temperatures on the energy change of the system is discussed. In addition, this article established an experimental device for diamond indentation of copper sheets, and conducted indentation microholes experiments to observe microholes and cracks through scanning electron microscopy. The results show that when the diamond indenter is pressed down, the number of atoms in contact between the diamond indenter particles and the copper surface gradually increases, causing local stress concentration and the formation of new dislocations. Among them, 1/6<112> shockley dislocation is the main dislocation type. When the strain energy stored in the lattice increases beyond a certain value, the lattice structure of the copper atoms in the contact area is broken, resulting in internal defects, gradually forming microholes and surrounding cracks. The increase in the imprinting speed will accelerate the plastic deformation of the copper sheet. This paper reveals the mechanism of the formation of microholes and cracks, laying the foundation for high-quality micropore manufacturing.

Fabrication of Micro-Hole in 3D Printed CFRP Composite by Electrical Discharge Machining Process

Carbon fiber-reinforced polymer (CFRP) composite has extensive application in several industries, including the automotive and aerospace industries. Fabrication of micro-features in CFRP composite is an important process that is frequently performed for various applications including for micro-products and micro-components. The fabrication of precise micro-features in CFRP composite is difficult to achieve by conventional means, even by additive manufacturing (AM). Thus, in this study, the machinability characteristics of the CFRP composite were experimentally investigated. CFRP composite was fabricated by using the AM technique. The different process parameters of 3D printing (infill density and layer sequence) and micro-EDM (voltage, capacitance, and tool rotation) were considered to investigate their effect on the hole quality and productivity. The optimization of the different parameters chosen for the purpose of the investigation was also carried out. The important process parameters that have an impact on the process were identified and analyzed. The better machining performance of CFRP composite was noticed at higher infill density and alternative fiber arrangement. The capacitance was the most significant parameter among all the EDM parameters to produce good quality micro-holes in CFRP composite.

Simulation Research on Variable Parameter Drilling of Glass Fiber Reinforced Composite / Copper Foil Laminates

Micro-holes play an important role in interconnecting different signal layers of printed circuit boards(PCBs), and their surface quality is critical to signal transmission performance. As an important parameter, drilling force affecting the quality of micro-holes, but its distribution is difficult to obtain because the size of micro-holes is tiny and the drilling process is usually in a semi-closed state. Therefore, simulation of the micro-hole drilling process is studied by Abaqus in this paper. The material of printed circuit board can be regarded as laminated material composed of glass fiber composite and copper foil. The whole simulation process is the simulation of alternating drilling of the two materials. Via simulation, the influence of different drill parameters and processing parameters on the output drilling stress, axial force and drilling quality is obtained, and the results are consistent with the experimental results in the literature. This simulation method can be used as a supplement to the experiment and can provide guidance for the drilling process of printed circuit boards.

A Miniaturized Ultrasonic Micro-Hole Perforator for Minimally Invasive Craniotomy.

Micro-hole perforation on skull is urgently desired for minimally invasive insertion of micro-tools in brain for diagnostic or treatment purpose. However, a micro drill bit would easily fracture, making it difficult to safely generate a micro-hole on the hard skull. In this study, we present a method for ultrasonic vibration assisted micro-hole perforation on skull in a manner similar to subcutaneous injection on soft tissue. For this purpose, a high amplitude miniaturized ultrasonic tool with a 500 μm tip diameter micro-hole perforator was developed with simulation and experimental characterization. In-depth investigation of micro-hole generation mechanism was performed with systematic experiments on animal skull with a bespoke test rig; effects of vibration amplitude and feed rate on hole forming characteristics were systematically studied. It was observed that by exploiting skull bone's unique structural and material properties, the ultrasonic micro-perforator could locally damage bone tissue with micro-porosities, induce sufficient plastic deformation to bone tissue around the micro-hole and refrain elastic recovery after tool withdraw, generating a micro-hole on skull without material. Under optimized conditions, high quality micro-holes could be formed on the hard skull with a force (<1 N) even smaller than that for subcutaneous injection on soft skin. This study would provide a safe and effective method and a miniaturized device for micro-hole perforation on skull for minimally invasive neural interventions.

Temporally modulated laser power dynamic drilling strategy with comprehensive optimization of quality and efficiency

Motivated by the need to comprehensively optimize the quality and efficiency of laser drilling, a dynamic drilling strategy wherein laser power is temporally modulated is proposed, the laser power of which increases with the number of pulses to match the dynamics of the drilling process. Taking nanosecond laser drilling as the specific research process, the proposed dynamic drilling strategy is successfully applied. A continuous hydrodynamic model is first established to reveal the mechanism and driving force of material removal. Subsequently, linear and hybrid power growth profiles are proposed to improve the driving force of melt ejection and evaporation intensity during the drilling process. By comparing the variations in crater depth, ablation rate, recoil pressure, and recast layer thickness with the number of pulses, the effect of dynamic drilling modes on the drilling process and micro-holes performances was revealed. Moreover, the effect of power growth rate on the dynamic drilling process and final micro-hole quality is discussed in detail to provide a basis for formulation and optimization of the power growth profile. Finally, an experiment using an aerospace Ni-based superalloy verifies the reliability of the dynamic drilling strategy and the material removal rate, taper, and recast layer thickness of the micro-holes are comprehensively optimized. The proposed dynamic drilling strategy provides an effective solution to the universal problems of micro-hole quality and efficiency in the field of laser drilling.

Electrochemical discharge machining of a high-precision micro-holes array in a glass wafer using a damping and confinement technique

Due to the unstable gas film, it is still a big challenge to achieve high repeatability and quality in electrochemical discharge machining (ECDM) of micro-hole arrays in glass. Based on our previous research on ECDM in micro channels, the present work uses a non-Newtonian fluid electrolyte (non-NTF electrolyte) in ECDM to further achieve a high uniform precision and quality micro-holes array in glass through the damping and confinement effect. The results revealed that an average entrance diameter of 343.8 ± 3.47 μm (mean ± standard deviation) and average heat-affected zone (HAZ) width of 18.01 ± 1.52 μm were successfully fabricated in a 300-μm-thick glass wafer. As compared to the conventional KOH electrolyte, the entrance overcut and the HAZ width of micro-holes were reduced by 43.84 %, and 64.81 %, respectively, while the repeatability improved by 67.92 %. The non-NTF electrolyte concentration and the tool rotation speed were also found to play a significant role in the damping and confinement effect, significantly affecting the geometrical properties of the micro-holes. Furthermore, the micro-holes array was filled with copper to form through glass vias (TGVs), and a standard deviation of the Kelvin resistance of TGVs was only 5.35 mΩ, further demonstrating an excellent repeatability and localization of ECDM micro-holes using a non-NTF electrolyte. The results illustrate that employing a non-NTF electrolyte is a simple way to increase the stability of the gas film and to improve the repeatability and localization of ECDM micro-holes.

Gas bubbles entrapment mechanism in the electrochemical discharge machining involving multi-tip array electrodes

This article addresses the gas bubble entrapment issue for the first time when a multi-tip tool array (MTA) is used in electrochemical discharge machining (ECDM). During the simultaneous creation of microholes by the MTA tools, the electrolyte vapor evaporates and condensates on the tool shank. The electrolyte droplet gradually grows in size and then drops, thus, interrupting the electrochemical discharge. The material removal process becomes intermittent, leading to the formation of poor-quality microholes and subsequent breakage of the glass substrate. The ECDM behavior was characterized by analyzing the gas film formation time and the variation in the mean discharge current. The experimental analysis shows that the gas bubble entrapment phenomenon occurs when the average entrapped gas bubble size and electrolyte condensate sizes are larger than the MTA tool electrodes' effective base size and tip length. Increasing the electrode lengths and pitch size reduced the bubble entrapment frequency by >60 %. Increasing the pitch size from 1 mm to 1.5 mm also reduced the bubble entrapment frequency and improved the uniform electrochemical discharges, which is essential to get good quality microholes.

Research on the Quality of Micro-holes Processed by Laser and Backside Electrolytic Hybrid Machining

Research on the Quality of Micro-holes Processed by Laser and Backside Electrolytic Hybrid Machining

Study on the mechanism affecting the quality of micro-hole in ultrasonic-assisted drilling of high-speed circuit boards

High-speed circuit boards are created to meet the high-speed signal transmission requirements of 5G communication technology, but the non-polar resins, flat glass fibers, and multiple and hard fillers used for this purpose have posed new challenges to their micro-hole processing. The quality of micro-hole has always been a decisive factor in board performance. Therefore, this paper aims at proposing a new processing method for improving the micro-hole drilling quality of high-speed circuit boards. By establishing an ultrasonic-assisted drilling tool motion model, analyzing the changes in drilling method, material deformation, chip breakage, and chip removal during ultrasonic-assisted drilling of printed circuit boards, the influence mechanism on micro-hole quality during ultrasound-assisted drilling is studied prudently. Besides, an experimental platform for ultrasonic-assisted drilling is designed and built, and single-factor experiments for verification of ultrasonic effects, optimization of drilling parameters, and orthogonal experiments for ultrasonic-assisted drilling of high-speed circuit boards are conducted on this platform. The experimental results show that the loading of ultrasonic vibration has made an obvious improvement on several machining defects including hole wall roughness, entrance burr, and nail head in micro-hole drilling of high-speed circuit boards. In addition, the influence order of each processing parameter and a better combination of them are discovered, which provides a theoretical research basis and instructions for the improvement of micro-hole quality of high-speed circuit boards.

Prediction of EDMed micro-hole quality characteristics using hybrid bio-inspired machine learning-based predictive approaches

Prediction of EDMed micro-hole quality characteristics using hybrid bio-inspired machine learning-based predictive approaches

Effect of ultrasonic vibration on forming force and forming quality in micro-punching with a flexible punch

In this paper, a special ultrasonic microforming method, Micro Ultrasonic thin Sheetmetal Forming using molten plastic as a flexible punch (short as Micro-USF), was used to conduct micro-punching experiments on stainless steel sheet with thickness of 10 μm and 20 μm. The influence of ultrasonic vibration on forming force and forming quality were investigated. The experimental results showed that the forming force required for punching thin sheet metal decreased gradually as the ultrasonic time or ultrasonic power increased. By applying the ultrasonic vibration effect, the forming force could be decreased dramatically and the maximum value of forming force drop could reach 86%. Moreover, with the application of ultrasonic vibration, the size accuracy and shape accuracy of micro-holes could be increased by 36.92% and 22.65%, but the cross-section quality of micro-holes were not significantly improved.