Printed Line Width Research Articles

Heterogeneous Transfer Learning of Electrohydrodynamic Printing Under Zero-Gravity Toward In-Space Manufacturing

Abstract As we continue to commercialize space and mature in-space manufacturing (ISM) processes, there is a strong need to transfer the knowledge we learn from experiments on the ground to zero-gravity environments. Physics-motivated manufacturing processes, like additive manufacturing, experience a shift in fabrication parameters due to the absence of gravity and the change of environments. Thus, we found traditional machine learning methods are not capable of addressing this domain shift and present a transfer learning scheme as a solution in this paper. We tested a kernel ridge regression model built for heterogeneous transfer learning (KRR-HeITL) on data from the electrohydrodynamic inkjet printing (EHD printing) process. EHD printing is a process that uses electrical force to control material flows, thus achieving the fabrication of electronics without requiring gravity. Our team has successfully conducted three rounds of parabolic flights to validate this technology for ISM. We trained on multiple datasets built from on-ground experiments and tested using zero-gravity printing data obtained from parabolic flight tests. Measurements of the Taylor cone both on-ground and in zero-gravity were taken and exploited as a part of the training data. We found that our method obtains good interpolation accuracy (MAPE 3.85%) compared to traditional machine learning methods (MAPE 16.84%) for predicting the printed line width. We concluded that the KRR-HeITL method is well suited for zero-gravity domain shifts of EHD printing parameters. This study paves the way for future predictions of ISM parameters when there are only on-ground experiments or very limited zero-gravity datasets for a given process.

A vibrating membrane ejector for direct ink writing of printed electronics

Direct ink writing (DIW) represents a technical branch of additive manufacturing technology, particularly suitable for prototyping or small-batch printing production of printed electronic components. However, the existing print heads required for near or sub-100 μm line width are quite sophisticated, limiting the accessibility and application of the DIW method. This paper reports the use of the vibrating membrane ejector (VME) as a new option for the print head of DIW. The structure of the VME-based print head was specially designed for this purpose. Finite element modeling and analysis of the VME’s vibration characteristics were performed to provide insights into the ejection conditions and behaviors. The factors influencing the size of printed structures were identified and analyzed through the printing of a metal–organic complex silver (Ag) ink. After optimizing several operational parameters to limit the spreading effects and suppress the satellite droplets, the DIW printed line width has reached about 100 μm. The effectiveness of the VME-based print head was further demonstrated through the DIW fabrication of interdigitated electrodes and microstrip transmission lines. This highlights the versatility of the VME-based print head as a practical tool for device prototyping and ink development in the field of printed electronics.

Enhancing the quality factor of aerosol jet printed RF spiral inductors through gold electroplating

Aerosol jet printing (AJP) outperforms inkjet printing by significantly reducing printed line width, effectively addressing issues such as bulging and surface irregularities. This technology allows for line widths as narrow as 10–100 μm with high aspect ratios, making it well-suited for radio frequency (RF) applications. Consequently, AJP emerges as a valuable tool for direct printing in RF applications. Among conductive inks, silver nanoparticle (Ag-NP) ink is preferred for its straightforward direct printing process and lower sintering temperature requirements. However, the conductivity of printed Ag NP traces falls markedly below that of bulk silver due to significant porosity, limiting its use in RF applications where a high-quality factor is essential. The quality factor of an inductor, indicative of its efficiency in energy storage and release, inversely correlates with its resistance. Our research combines AJP with selective electroplating to reduce the resistance of printed traces, thereby enhancing the inductor’s quality factor for RF applications. We fabricated spiral inductors on alumina substrates using silver NP ink and subsequently applied selective gold electroplating to these traces. This approach led to a significant increase in the inductors’ quality factor, improving it by a factor of 3–5 in the RF frequency range of 100–700 MHz.

Radio frequency air cold plasma pretreatment to enhance 3D printing performance of tapioca starch by altering its rheological behavior and water migration

This work investigated the effect of radio frequency air cold plasma (RFACP) pretreatment in enhancing 3D printing performance of tapioca starch. The results of 3D printing showed that the cold plasma (CP) treatment enhanced printing precision from 75.88% to 89.16% in starch. On the one hand, it could appropriately weaken the fluidity of starch ink to reduce the printing line width. On the other hand, it could enhance the self-supporting ability of starch ink and alleviate the sagging and collapse of 3D patterns. The results of rheological behavior further demonstrated that CP-treated starch exhibited greater gel structure characterized by the increased viscoelasticity, rigidity, and structural recovery capacity. Additionally, CP treatment facilitated the migration of free water into immobilized states within the starch ink and shortened relaxation times, indicating an enhanced water-binding ability of the CP-treated starch. Correlation analysis showed that rheological properties, firmness and water distribution of starch ink were significantly linked with the printability. The improvement of starch 3D printing performance after CP treatment may be attributed to the higher amylose content and more hydrophilic groups (especially carboxyl) generated by CP depolymerization and oxidation effects, which could lead to a denser network structure with greater mechanical strength of starch gel. Overall, this study not only reveals the mechanism that CP treatment enhances 3D printing performance of starch, but also provides a green and effective way to improve the 3D printing performance of starch.

Flexible actuation with intrinsic sensing for ram extrusion 3D printing

Conventional actuation mechanisms used in liquid deposition modeling (LDM) technology limit the flow handling capability and the visibility of the printing materials’ properties. This work presents a flexible actuation system with intrinsic sensing for a ram extrusion printhead used in LDM technology. A mathematical model is used to design and simulate the printhead. The experimental results demonstrate the system’s ability to control the extrusion flow in two modes. The pressure control mode is favorable for the flow in a transient state such as flushing. The volumetric control mode provides a constant printed line width with less than 0.1-mm deviation between each tested material (biogel, chocolate fudge, and silicone sealant). A lower standard deviation for printed lines in the volumetric control mode indicates more precise line printing. The system also incorporates real-time monitoring of extrusion pressure and flow rate. The intrinsic capability to detect printing material properties is validated in both simulation and experiment. It provides valuable insights for further optimization of the printing process. The proposed system offers the advantage of improved pressure and flow control as well as the ability to monitor and respond to the properties of printing material.

3D printing dielectric elastomers for advanced functional structures: A mini‐review

AbstractThree‐dimensional (3D) printed bionic products play an important role in intelligent robotics, microelectronics, and polymers. The printing and manufacturing process of 3D printers is conducive to obtaining soft structures that meet specific requirements, and saves time and cost. Soft intelligent robotics, an emerging research field, has always been developed based on soft materials and actuators with their biological properties. This article reviews the current understanding of 3D bioprinting technologies for dielectric elastomers (DEs), DE actuators (DEAs) and soft robots, such as inkjet, extrusion, laser‐induced and stereolithography bioprinting. 3D printers for fabricating soft materials are presented and classified. The approaches to exploit 3D bioprinters for DEs/DEAs are as follows: (1) 3D printing DEAs utilize ionic hydrogel–elastomer hybrids that are analogous to human muscles, and the DEAs usually have flexible structures and large deformations with multiple functionalities. (2) An electrohydrodynamic (EHD) 3D printer confers high printing resolution and high production efficiency, which offers advantages such as full automation and flexible design. The optimal printing conditions are mainly determined by the effects of printing voltages and ink properties, which are related to the formation of the liquid cone and the printed line width. Furthermore, the advantages of 3D bioprinting technologies have accelerated their development and applications.

Thixotropy and printability study of intense pulsed light sinterable copper paste formulations for flexible substrates

At present, most of the reported articles on IPL sintering of flexible copper paste focus mainly on the effects of IPL sintering parameters and copper powder modification methods on conductive copper patterns, which ignores the influence of thixotropy, printing and other properties that are of great concern in industrial production on the conductive copper pattern. In this article, four resin materials with distinctive mechanical properties, heat resistance, and stability are selected to develop a flexible copper paste formulation for IPL sintering. By focusing on the thixotropy and printing properties of different flexible copper pastes formulations, we found the relationship between these two properties and the performance of conductive copper patterns. The results show that thixotropy property affects the printing property, and the printing property affects the density and boundary definition of the conductive copper pattern film layer after sintering, thus affecting the conductivity. Finally, we obtained a flexible copper paste formulation suitable for IPL sintering with a printed line width of 35.47 μm, a line space of 31.55 μm, and a square resistance of 15.7 mΩ/◻.

Analysis and Modeling of Line Width of Aerosol Printing based on Image Processing

This article proposes a method for calculating the line width of aerosol printing based on image processing and analyzes the line width results obtained using this method, aiming to establish an accurate mathematical model between parameters and line width. This article conducts experiments using carrier gas and sheath gas as parameters in aerosol printing and uses an industrial camera to characterize the printed lines. The line images are processed in MATLAB to obtain high-precision printed line widths. Analyzing the line width data after image processing, it was found that the line width increased with the increase of carrier gas and decreased with the increase of sheath gas. Then, using the discovered regularity, mathematical relationships between sheath gas and line width, and carrier gas and line width are established. The calculation method of print line width used in this article is highly accurate and efficient and establishes a functional relationship analysis model that can accurately describe the relationship between variables and line width, guiding printing-specific line widths.

Effects of starch–fatty acid complexes with different fatty acid chain lengths and degrees of saturation on the rheological and 3D printing properties of corn starch

The effect of corn starch-fatty (CS-FA) complexes from varying carbon chain length and degree of unsaturation on the rheological and 3D printing properties of corn CS–FA complex gels. The CS-FA complexes with longer carbon chain lengths and lower saturation enhanced the ability of gels to bind water, promoting the formation of intermolecular hydrogen bonds. The CS-FA complexes inhibit retrogradation and increase the amount of bound water, thereby reducing the structural integrity and transforming the original skeleton structure into a flake-like structure. These changes in gel structure led to lower flow stress and storage modulus for CS-FA gels containing FAs with shorter carbon chain lengths and lower saturation, resulting in reduced “extrusion swelling” of the material and facilitating its extrusion. The decreased “extrusion swelling” of gel improved print line width and printing performance. The CS-FA complex gel-printed product with a 12-carbon chain FA has the greatest printing accuracy, thanks to its moderate G', flow stress, and viscosity. This study provides important information for the CS-FA complexes for the preparation of starch-based 3D printing materials.

Improving the energy absorption capacity of bending-dominated additively manufactured polylactic acid (PLA) lattices

This study aims to investigate the energy absorption performance of three bending-dominated lattice structures including quad-diametral (QD), quad-diametral-line (QDL), and dodecahedron (DO)). Compression test specimens with these three lattice types were manufactured using the material extrusion additive manufacturing technique with PLA material. The specimens were manufactured with different sets of values of the print temperature, layer thickness, and line width to identify their effect on the energy absorption performance. Taguchi’s method was utilized for this purpose which revealed that the temperature is the most effective and its optimized value is 225 °C. It was also found that the energy absorption performance of the DO lattice is superior to that of QD and QDL lattices. In the following set of experiments, the print temperature was fixed to its optimized value (i.e. 225 °C), and another process parameter (i.e. print speed) along with layer thickness and line width were investigated among eight different groups of DO lattices. Numerical investigations were also performed and validated based on the experimental results. Parametric studies were also done for the DO lattice to improve its energy absorption performance by changing the strut diameters and introducing grading to the struts at each lattice cell.

Tension and bending behaviors of 3D printed PLA+ at different printing parameters

3D-printed polylactic acid (PLA+) is one of the newest biodegradable materials seeking entrance into the industrial field. The effect of 3D printing process parameters (infill pattern (IP), infill density (ID), layer height (LH), line width (LW), shell thickness (ST), printing temperature (PT), printing speed (PS), and raster angle) on mechanical properties has been studied. In addition, this paper aims to investigate the effect of changing both the IP and raster angle on the mechanical properties of 3D-printed PLA+. The results revealed that the highest value of tensile strength can be obtained at an ID of 100%, concentric IP, LH of 0.1[Formula: see text]mm, LW of 0.8[Formula: see text]mm, ST of 0.8[Formula: see text]mm, PT of 215∘C, and PS of 60[Formula: see text]mm/s.

Accelerated and inexpensive Machine Learning for manufacturing processes with incomplete mechanistic knowledge

Machine Learning (ML) enables deployable modeling of parametric effects in manufacturing processes. But this paradigm is largely limited to established processes, since the state-of-the-art ignores the significant cost of creating qualitatively accurate physics-based models for new processes. We propose a transfer learning based method that addresses this issue by pushing the boundaries of the qualitative accuracy demanded of the physics-based model. Our approach is evaluated for modeling the printed line width in Fused Filament Fabrication and shows reduction in the model development cost by multiple human-years, experimental cost by 56–76%, computational cost by orders of magnitude, and error by 16–24%.

Curing performance and print accuracy of oxidized SiC ceramic via vat photopolymerization

To improve the print quality of SiC ceramic, an amorphous SiO2 layer is prepared on the surface of SiC by oxidation (SiC@SiO2). The changes in curing depth and width direction of SiC@SiO2 curing sheets with different degrees of oxidation were investigated. The results showed that curing depth increased while curing width decreased with an increasing degree of oxidation. The edge morphologies of SiC@SiO2 curing sheets changed from serration to flat shape with increasing degree of oxidation, and uneven overcuring regions appear in the width direction. Additionally, 12% SiC@SiO2 ceramic was fabricated via vat photopolymerization combined with reaction sintering. The dimensional deviation ratio of printed green bodies increased with decreasing printed line width and pore diameter. The minimum printed line width of 329 μm and pore diameter of 483 μm with good print quality were obtained. Furthermore, the flexural strength of 225.4 MPa and bulk density of 2.77 g cm−3 for sintered 12% SiC@SiO2 parts were obtained.

Study on the forming quality of double-base gun propellants based on extrusion 3D printing technology

In order to improve the forming quality of 3D printed gun propellants, rosette-shaped 19-perf double-base gun propellants with different geometric dimensions (inner diameter and web thickness) were printed by extrusion 3D printing technology based on conventional double-base propellant formulations. The printed gun propellants were characterized in terms of surface structure, inner diameter and web thickness dimensional uniformity, and density. The effects of geometrical dimensions (inner diameter and web thickness) and printing process parameters (line width and flow compensation) on the forming quality of gun propellant were studied. The results showed that printed rosette-shaped 19-perf double-base gun propellants have a good surface structure with a clear outer profile and inner diameter from 1.5 mm to 3.0 mm. When the printing process parameters were the same, the larger the geometry, the higher the dimensional uniformity of the printed gun propellant. When the printing model was the same, the decrease in printing line width and the increase of printing flow compensation in the printing process parameters had the same effect on the dimensional uniformity of the printed gun propellant. The rosette-shaped 19-perf gun propellant with a model inner diameter and web thickness of 1.5 mm had the best dimensional uniformity at the printed line width and flux compensation of 0.45 mm and 1.0, respectively, with relative standard deviations of 2.699%, 1.986%, and 2.458% for inner diameter, inner and outer web thickness, respectively. The density of the printed propellant ranged from 1.565 to 1.606 g·cm−3. This study has a guiding significance for the application of extrusion 3D printing technology in the field of gun propellant.

Fabricating 1D stretchable fiber-shaped electronics based on inkjet printing technology for wearable applications

Advanced microfabrication on small and curved fiber surfaces remains a critical challenge and an urgent need to develop high-performance fiber-shaped electronics and advance the next generation of wearable electronics technology. In this study, we propose the preparation of 1D stretchable fiber-shaped electronics via inkjet printing technology for wearable applications. Utilizing self-built precision rotary inkjet printing equipment and a surface chemical modification process, we achieve high-precision and customizable microfabrication onto ultra-low diameter fiber surfaces (minimum printing line width of 133 µm and a printable fiber diameter as low as 500 µm with a large curvature of 4000 m−1). More importantly, this fabricating method is non-destructive and can prepare 1D stretchable conductors by printing conductive inks with stretchable structures and optimizing various synthetic fibers, which exhibit remarkable conductivity, mechanical stability, and strain-insensitive properties in practical applications. Furthermore, we demonstrate the performance of several 1D stretchable electronics applications, including a fiber-shaped electrothermal device, triboelectric strain sensor, and supercapacitor. Our work will greatly promote the development of 1D fiber-shaped electronics and smart textiles with wearability, high performance, functional diversification, and low cost.

Experimental Investigation of High-Viscosity Conductive Pastes and the Optimization of 3D Printing Parameters

Traditional contact printing technology is primarily controlled by the shape of the mask to form the size, while for the more popular non-contact printing technologies, in recent years, adjusting the print parameters has become a direct way to control the result of the printing. High-viscosity conductive pastes are generally processed by screen printing, but this method has limited accuracy and wastes material. Direct-write printing is a more material-efficient method, but the printing of high-viscosity pastes has extrusion difficulties, which affects the printed line width. In this paper, we addressed these problems by studying the method of printing high-viscosity conductive paste with a self-made glass nozzle. Then, by parameter optimization, we achieved the minimum line width printing. The results showed that the substrate moving speed, the print height, and the feed pressure were the key factors affecting the line width and stability. The combination of the printing parameters of 0.6 MPa feed pressure, 200 mm/s substrate moving speed, and 150 μm print height can achieve a line width of approximately 30 μm. In addition, a mathematical model of the line width and parameters was established, and the prediction accuracy was within 5%. The results and the prediction model of the parameters provide an important reference for the printing of high-viscosity pastes, which have immense potential applications in electronics manufacturing and bioprinting.

Effect of Surface Treatment on Performance and Internal Stacking Mode of Electrohydrodynamic Printed Graphene and Its Microsupercapacitor.

Microelectronic devices are developing rapidly in portability, wearability, and implantability. This puts forward an urgent requirement for the delicate deposition process of materials. Electrohydrodynamic printing has attracted academic and industrial attention in preparing ultrahigh-density microelectronic devices as a new noncontact, direct graphic, and low-loss thin film deposition process. In this work, a printed graphene with narrow line width is realized by combining the electrohydrodynamic printing and surface treatment. The line width of printed graphene on the hydrophobic treatment surface reduced from 80 to 28 μm. The resistivity decreased from 0.949 to 0.263 Ω·mm. Unexpectedly, hydrophobic treatment can effectively induce random stacking of electrohydrodynamic printed graphene, which avoids parallel stacking and agglomeration of graphene sheets. The performance of printed graphene is thus effectively improved. After optimization, a graphene planar supercapacitor with a printed line width of 28 μm is successfully obtained. Its capacitance can reach 5.39 mF/cm2 at 50 mV/s, which is twice higher than that of the untreated devices. The device maintains 84.7% capacitance after 5000 cycles. This work provides a reference for preparing microelectronic devices by ultrahigh precision printing and a new direction for optimizing two-dimensional material properties through stacking adjustment.

High-efficiency and high-resolution patterned quantum dot light emitting diodes by electrohydrodynamic printing.

The development of quantum dot light-emitting diode (QLED) fabrication technologies for high-definition and low-cost displays is an important research topic. However, commercially available piezoelectric inkjet printing has reached its limit in reducing pixel sizes, which restricts its potential use in high-resolution displays. Here, we exhibit an electrohydrodynamic (EHD) printing method for manufacturing QLEDs with a high resolution of 500 ppi that remarkably surpasses the resolution of conventional inkjet printing displays. By optimizing the EHD printing process, a high-resolution pixelated bottom-emitting passive matrix QLED with a maximal current efficiency of 14.4 cd A-1 in a pixel size of 5 μm × 39 μm was achieved, indicating the capability of the EHD method in superfine printing and high efficiency QLED. Moreover, a top-emitting device is designed using a capping layer; the maximal current efficiency of top-emission passive matrix QLED devices can reach up to 16.5 cd A-1. Finally, a two-color (red and green) bottom-emission QLED device with 500 ppi was fabricated. The successful fabrication of these high-efficiency QLEDs with 500 ppi demonstrated that the EHD printing strategy has numerous potential applications in high-resolution and high-performance QLEDs for a range of applications, such as mobile or wearable devices.

AI-Aided Printed Line Smearing Analysis of the Roll-to-Roll Screen Printing Process for Printed Electronics

Screen printing has been adopted for fabricating a wide variety of electronic devices. However, the printing defects and reliability have been an obstacle for industrialization of printed electronics. In this research, the artificial intelligence (AI) model was developed and integrated with the in-house roll-to-roll screen printing system to detect smearing defect, which is one of the main defects of screen printing. The U-Net architecture was adopted, and a total of 19 models were designed with model sizes ranging from 8E + 3 to 3E + 7 number of parameters. Their performances as validation mean Intersection over Union (IoU) were analyzed, and the optimal model was chosen with a validation mean IoU of 95.1% and a number of parameters of 8E + 6. The printed line images were evaluated by the AI model for various printing conditions, such as printed line widths, printing paste premixing, printing speeds, and printed line directions, which showed that the model could effectively detect the smearing defects. Also, the AI model capabilities were investigated for repeated printing, which demonstrated that it can be used for the reliability assessment of the screen printing process.

Stability Bounds for Micron Scale Ag Conductor Lines Produced by Electrohydrodynamic Inkjet Printing.

Continuous conducting lines of width 5–20 μm have been printed with a Ag nanoparticle ink using drop-on-demand (DOD) electrohydrodynamic (EHD) inkjet printing on Si and PDMS substrates, with advancing contact angles of 11° and 35°, respectively, and a zero receding contact angle. It is only possible to achieve stable parallel sided lines within a limited range of drop spacings, and this limiting range for stable line printing decreases as the contact angle of the ink on the substrate increases. The upper bound drop spacing for stable line formation is determined by a minimum drop overlap required to prevent contact line retraction, and the lower bound is governed by competing flows for drop spreading onto an unwetted substrate and a return flow driven by a Laplace pressure difference between the newly deposited drops and the fluid some distance from the growing tip. The upper and lower bounds are shown to be consistent with those predicted using existing models for the stability of inkjet printed lines produced using piezoelectric droplet generators. A comparison with literature data for EHD printed lines finds that these limiting bounds apply with printed line widths as small as 200 nm using subfemtoliter drop volumes. When a fine grid pattern is printed, local differences in Laplace pressure lead to the line width retracting to the minimum stable width and excess ink being transported to the nodes of the grid. After printing and sintering, the printed tracks have a conductivity of about 15%–20% of bulk Ag on the Si substrate, which correlates with a porosity of about 60%.