Heated Syringe Extrusion for Soft Gripper Fabrication in Additive Manufacturing

Kwang Yeol Yu; Woo Jin Jeong; In Hwan Lee

doi:10.57062/ijpem-st.2024.00206

Int. J. Precis. Eng. Manuf.-Smart Tech. > Volume 3(1); 2025 > Article

Yu, Jeong, and Lee: Heated Syringe Extrusion for Soft Gripper Fabrication in Additive Manufacturing

Regular Paper

International Journal of Precision Engineering and Manufacturing-Smart Technology 2025;3(1):59-64.

Published online: January 1, 2025

DOI: https://doi.org/10.57062/ijpem-st.2024.00206

Heated Syringe Extrusion for Soft Gripper Fabrication in Additive Manufacturing

Kwang Yeol Yu¹ · Woo Jin Jeong¹ · In Hwan Lee¹

¹Department of Mechanical Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea

In Hwan Lee: anxanx@chungbuk.ac.kr

Received October 4, 2024 Revised November 25, 2024 Accepted November 26, 2024

Abstract

This study explores the feasibility of creating a pneumatically driven soft gripper using Thermoplastic Polyurethane (TPU) through Direct Ink Writing (DIW) technology via a heated syringe extrusion process. Unlike traditional DIW methods that do not involve heating, this approach allows for the extrusion of heat-sensitive materials like TPU, thereby broadening the potential applications of additive manufacturing. The soft gripper was designed based on prior research, and key fabrication parameters—such as temperature, pneumatic pressure, and nozzle diameter—were optimized. The resulting soft gripper was assessed for its mass, surface quality, and bending performance. The findings indicated that the soft gripper produced through heated syringe extrusion exhibited surface characteristics and mass comparable to those made using conventional Fused Filament Fabrication (FFF) techniques. Additionally, bending performance tests demonstrated that the gripper could be predictably controlled through pneumatic pressure, achieving bending angles consistent with previous studies. These results suggest that heated syringe extrusion is a viable method for producing high-quality TPU-based soft robotic devices, providing a cost-effective and versatile solution within flexible manufacturing systems.

Keywords: Additive manufacturing · Material extrusion · Heated syringe · Soft gripper · Machine vision

1 Introduction

Direct Ink Writing (DIW) technology is a type of Material Extrusion (MEX) process in Additive Manufacturing (AM) that creates three-dimensional structures by loading liquid materials, such as ink or precursors, into a syringe and extruding them under pressure [1,2]. This technology operates witha a mechanism similar to Fused Filament Fabrication (FFF), which forms products by supplying solid filament to a heated extrusion head. However, a key characteristic of DIW is that heating system is not necessary. DIW technology offers advantages over other additive manufacturing techniques including lower process costs, simpler structure, and the ability to use a wide range of materials [1]. Additionally, achieving higher control precision is easier due to the lightweight extrusion head, allowing the use of fine nozzles to produce intricate structures [1,2]. However, for materials that require melting through heat, issues such as temperature non-uniformity and overheating in heating systems can negatively impact extrusion quality. In particular, thermally sensitive materials like Thermoplastic Polyurethane (TPU) are prone to degradation if temperature control is inadequate, leading to a decline in the material’s physical properties. For this reason, previous studies have implemented processes using heat-resistant syringes equipped with heating elements, focusing on materials that are relatively stable under thermal conditions [3–6]. To date, no research has been conducted on utilizing flexible materials like TPU to fabricate products with specific functions using such processes [7,8]. This study aims to fabricate a pneumatically driven soft gripper by extruding TPU with the DIW technology using heated syringes. Additionally, the quality and operational performance of the fabricated soft gripper will be assessed to demonstrate that the material extrusion process employing a heated syringe is suitable for producing high-quality products. This approach surpasses previous study [9], where soft pneumatic grippers were successfully fabricated using other DIW techniques, by achieving the formation of higher-quality pneumatic soft grippers. The findings of this research are expected to expand the applicability of DIW processes, which are characterized by high design freedom and cost-efficiency.

2 Modeling and Fabrication

2.1 Design for Additive Manufacturing (DfAM) of Soft-gripper

Soft grippers are robotic devices made from flexible and stretchable materials, such as silicone polymers, and are widely utilized in various fields, including agriculture, food processing, the medical industry, and manufacturing [10]. Research aimed at enhancing the versatility of soft grippers has included studies on finger-like shapes that mimic human hand functions [11]. Additionally, studies have been conducted on soft grippers that operate through a mechanism in which pneumatic pressure is applied to channels, inducing bending [12–14]. Kim et al. proposed a structure of soft gripper specialized for additive manufacturing that operates with this mechanism [15]. Jeong et al. evaluated the performance of this soft gripper, confirming that the structure proposed by Kim et al. is well-suited for additive manufacturing; therefore, the DfAM was successfully implemented [16]. Meanwhile, TPU-based soft grippers offer the advantage of providing appropriate flexibility and strength, preventing sagging during actuator overhang. These characteristics become even more evident when comparing studies on TPU-based soft grippers [15,17,18] with those on soft grippers made from other flexible materials [14,19]. This is attributed to the fact that TPU structures fabricated through additive manufacturing typically exhibit Shore hardness values ranging from 65 to 85A [20], whereas silicone materials generally have Shore hardness values between 10 and 50A [21], indicating relatively lower hardness. In this study, the soft gripper was designed based on previous researches [15,16], as shown in Fig. 1. The air channel gap in the designed soft gripper has been validated through previous studies, which analyzed its impact on printing quality. Specifically, these studies confirmed that the absence of internal supports within the air channels does not result in printing defects, such as air leakage, and does not compromise the gripper’s functionality or control performance. These findings support the suitability of the air channel structure in the soft gripper designed in this study in terms of both printing and performance.

2.2 Fabricating Process and Parameters

The designed soft gripper was fabricated with a DIW technology using heated syringe. To achieve a result similar to the CAD model, parameters were adjusted to deposit TPU material in lines 0.4 mm wide and in 0.15 mm layers.

The material extrusion process using a heated syringe employed in this study is configured as shown in Fig. 2. Specifically, a heat-resistant syringe made of stainless steel 304 is equipped with a precision nozzle, which is secured to a head containing a heating element. The material is extruded by applying pneumatic pressure, while the nozzle is moved to shape the product. To minimize the thermal deformation of the material, the heating element is placed exclusively at the lower section of the heat-resistant syringe. This design prevents heat distortion across the material, which could occur if heating were applied uniformly. By localizing the heating element, potential defects are mitigated, and the stability of the extrusion process is ensured.

The material loaded into the heated syringe for molding the soft gripper consists of a cylindrical TPU block with a height of 32.5 and a diameter of 24 mm, topped with a PTFE plunger measuring 6.5 in height and 24.5 mm in diameter, as shown in Fig. 3. The Teflon (PTFE) plunger, with its low coefficient of friction, enables smooth material extrusion within the syringe. The tight fit between the plunger and the syringe is designed to maintain effective sealing during extrusion, ensuring stable operation in the material extrusion process using a heated syringe. The parameters for the fabrication of the soft gripper are provided in Table 1. Using the heated syringe material extrusion system, the temperature of the heating element (as shown in Fig. 2) was set to 207°C, and pneumatic pressure of 550 kPa was applied above the Teflon plunger to extrude TPU for additive manufacturing of the soft gripper. The heating temperature of 207°C was experimentally optimized to maintain the thermal stability of the TPU material and prevent thermal degradation caused by excessive heat. Although the recommended extrusion temperature for TPU filament by the manufacturer is 210–230°C [22], this study utilized a lower temperature of 207°C to ensure the quality of material deposition and the functional performance of the final product.

3 Performance Test

To evaluate the quality of the soft gripper fabricated through the DIW technology using heated syringes, its mass and surface conditions, including vertical and horizontal resolution, were assessed. Subsequently, the bending performance of the fabricated soft gripper was measured to verify that the DIW technology using heated syringes can successfully extrude TPU and form the product properly.

3.1 Quality Inspection

The soft gripper fabricated through the DIW technology using heated syringes had a mass of 5.24 g, as shown in Fig. 4(a). In previous research, the soft gripper shown in Fig. 4(b) was fabricated using FFF technology [15,16], resulting in a weight of 5.46 g, which closely matched the weight predicted by the slicer program used in the FFF technology. Considering this, the soft gripper produced using the heated syringe showed only a minimal difference in mass (5.24 g), indicating that it was properly formed.

Additionally, the surface of the soft gripper fabricated through the heated syringe extrusion technology was observed using an optical microscope. Figs. 5(a) shows a photograph of the fabricated soft gripper, while 5(b) displays optical microscope images of the top and side surfaces of the soft gripper, captured at 20x and 40x magnification. Analysis of these images revealed no occurrence of internal bubble formation or unstable material extrusion, which are common issues in the heated syringe extrusion process [7,8]. This confirms that the soft gripper was successfully fabricated.

3.2 Bendable Angles Inspection

After confirming the quality of the fabricated soft gripper, its proper operation was verified using the shape-change measurement system configured as shown in Fig. 6. This measurement system involves controlling the soft gripper with a pneumatic regulator (SMC Co., ITV2050-RC28L), capturing its movement with a machine vision camera (KIKROBOT CO., MV-CA050-20GM), and using custom-developed measurement software to calculate the soft gripper’s operational coordinates, thereby enabling the observation and measurement of its range of motion [16]. The custom software used in this study is designed to recognize color differences at various positions in the images captured by the machine vision camera, enabling it to specify coordinates. Following the method depicted in Fig. 7, four points (A, B, C, and D) were marked on the soft gripper, and the coordinates of these points were obtained. The vectors V₁ (A→B direction) and V₂ (C→D direction) were then calculated, and the bending angle (θ) was determined using Eq. (1).

(1)

θ=cos-1(v1·v2‖v1‖‖v2‖)

After preliminary experiments identified the pressure range of 0 to 400 kPa as safe for the soft gripper without causing damage, the bending angle was measured in increments of 50 kPa. Figure 8 shows overlapping images of the soft gripper at different pressure levels, revealing that the bending angle increases as the pneumatic pressure is raised. Fig. 9 presents a graph depicting the average bending angles measured three times for each pressure level. The graph indicates that the bending angle of the soft gripper increases linearly with the applied pressure, demonstrating that the bending angle can be predicted based on the magnitude of the applied pressure. This finding aligns with previous research results [15]. Thus, it has been confirmed that the soft gripper fabricated with the DIW technology using heated syringes and TPU material is feasible.

4 Conclusion

This study successfully demonstrates the feasibility of using a heated syringe extrusion technology in AM by fabricating the pneumatically driven TPU soft gripper. By controlling the extrusion parameters, including temperature, pneumatic pressure, and nozzle diameter, a high-quality soft gripper was produced with surface characteristics and mass comparable to those created using conventional FFF methods. The detailed inspection of the gripper’s surface under an optical microscope showed no signs of internal defects. Furthermore, the bending performance tests confirmed that the soft gripper fabricated using this heated syringe method could be predictably controlled through pneumatic pressure, achieving bending angles in line with previous research. Overall, the findings of this research indicate that the heated syringe extrusion technology can effectively be applied to extrude thermopolymer materials like TPU, thereby expanding the application range of DIW technology. This approach offers a cost-effective, versatile solution for manufacturing functional devices, paving the way for further innovation in the field of soft robotics and flexible manufacturing systems.

Acknowledgements

This research was supported by the National Foundation of Korea (NRF), funded by the Ministry of Science and ICT (No. 2022R1A2C1091587).

Fig. 1

CAD model of the soft gripper

Fig. 2

Schematic of the DIW technology using a heated syringe system

Fig. 3

Photograph of TPU material (left) and PTFE plunger (right) supplied into the plunger

Fig. 4

Photograph showing the weight of the soft gripper fabricated using (a) the heated syringe DIW technology and (b) the FFF technology

Fig. 5

Photograph of (a) the fabricated soft gripper and (b) optical microscope views of the soft gripper

Fig. 6

Experimental setup for deformation inspection under applied pneumatic pressure

Fig. 7

Definition of the deformation angle of the soft gripper

Fig. 8

Overlaid photograph of the soft gripper deformed by applied pneumatic pressure

Fig. 9

Measured results of the soft gripper’s deformed angle under applied pneumatic pressure. Error bars indicate the maximum, minimum, and number indicate standard deviation of the measured values

Table 1

Fabricating parameters of the soft gripper

Parameter	Feed rate (mm/min)	Temperature (°C)	Extrusion method	Layer height (mm)	Nozzle
Material	Feed rate (mm/min)	Temperature (°C)	Extrusion method	Layer height (mm)	Type	Diameter (mm)
TPU 95A	500	207	Pneumatic 550(kPa)	0.15	Precision Nozzle	0.5

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Biography

Kwang Yeol Yu is a Master’s candidate in Chungbuk National University. His research interests focus on multi-material additive manufacturing, aiming to advance the development of innovative and versatile manufacturing techniques. With a strong academic foundation and a commitment to exploring cutting-edge solutions, Kwang Yeol is poised to make significant contributions to the field of additive manufacturing.

Biography

Woo Jin Jeong is a Master’s candidate in Chungbuk National University. His research interests focus on flexible strain sensors. His work aims to improve the performance and reliability of these sensors, which are crucial for applications in advanced monitoring systems and wearable technologies. With a passion for innovation and hands-on expertise, Woo Jin is committed to contributing to the development of next-generation sensor technologies.

Biography

In Hwan Lee is a seasoned researcher in mechanical engineering, specializing in additive manufacturing and precision engineering. He earned his Ph.D. in Mechanical Engineering from POSTECH and has since built a robust career in both academia and industry. With extensive research experience, Dr. Lee has contributed significantly to the advancement of 3D printing technologies, serving as the Head of the Additive Manufacturing Committee within the Korean Society for Precision Engineering. He has also participated in high-impact national projects as an expert in 3D printing and additive manufacturing, contributing to policy development and technological advancements. Recognized with prestigious awards, including the Minister of Science and ICT Commendation, Dr. Lee continues to drive innovation and knowledge in his field, positioning himself as a leader in cutting-edge engineering research.