Ulsan National Institute of Science and Technology

This research presents the verification of the newly introduced AM process through a pillow manufacturing case study. In the conventional pillow manufacturing process, there are inefficient processes, such as requiring many types of foam and relying on a manual assembly process that relies on the skills of the individual worker. In this study, a selective laser sintering (SLS) process is suggested for improving the damping process.

Through orthogonal and regression analysis, the relationship between three SLS process parameters (laser power, scanning speed, and hatching distance) and cushion comfort factors (compressive load, sag factor, and hysteresis loss rate) were derived by polynomial fitting to replace conventional cushion foams with an additively manufactured mesh structure. In addition, the results of the comparison between the AM process and the conventional manufacturing (CM) process in the two aspects of energy use and environmental impact were obtained through a life cycle assessment (LCA) of the pillow manufacturing process. The results show that AM has clear advantages in terms of energy consumption and environmental impact in less than 75 production volumes, and the derived regression equation shows that the desired cushion properties can be introduced with only one process and one material.

３８ Figure 4.7 Energy consumption per part according to the volume of production of the functional unit in AM and CM.

List of Tables

Introduction

Background (Additive manufacturing)
Background (Life cycle assessment)
Research objective and direction
Outline

The hybrid method aims to reduce the number of manufacturing processes by combining two independent techniques into just one process without post-processing. As shown in the two studies above, there are continuous reports on improving the limitations of the existing process through the AM process. In LCA, the pollution to the environment from all life cycle stages of the product production phase can be calculated from the very first life cycle stage to the very last or to any life cycle stage in between.

In particular, the additive manufacturing process is generally considered stronger in manufacturing sustainability compared to traditional manufacturing processes due to near-net shape printing that reduces tooling time and material and chemical waste. In addition, a comparison of the environmental impacts of four different AM methods was also carried out [10]. The purpose of this study is to improve the inefficiency of the pillow manufacturing process and to propose an additive manufacturing process that has been verified for its environmental friendliness.

To confirm the manufacturing sustainability of a pillow made with additives in terms of Life Cycle Assessment (LCA).

Figure 1.1 life cycle stages including all elements of the life cycle [8]

Research background

Additive manufacturing

SLS process
Cushion factors

In this study, we selected independent variables such as laser power, the scanning speed of the laser and hatching distance which are influential process parameters of SLS [18]. Relative density (Figure 2.3) is expressed by the ratio of the box volume where the lattice structure exists and the volume of the lattice [20] and is described by the equation. The relative density (RD) of the lattice was controlled to measure changes in compressive mechanical properties.

The unit cell dimension and relative density of the gyroid were controlled to optimize its mechanical properties. However, most studies have focused on the maximum performance of the grill, not on quality factors such as comfort. In this simplifying assumption, the most important thing is to minimize the distortion between the real-world results and the analysis.

For example, less important components that contribute little to the overall footprint may be excluded from the scope of the study.

Figure 2.2 Process parameter of selective laser sintering method

Parametric study of lattice structure

Lattice Foam design and fabrication

Design of Kelvin lattice structure

Design of experiment

Orthogonal analysis
Single factor analysis

Quasi-static compression test
Result and analysis
Fatigue behavior of lattice structure

DOE is important in the AM field due to the increase in production time, along with the limited bed size and powder material cost. The experimental results calculated with OA analyze the interaction between the process parameters, which are the laser-related values of the SLS equipment, and the compressive behavior of the additively manufactured mesh structure using a main effect analysis. Main effects analysis compares the influence of different independent parameters from the result of the experimental response.

The purpose of the compression test is to measure the compression behavior of the fabricated lattice structure and determine the interaction between the state of the laser-related parameters of SLS equipment and the cushion properties. For each parameter, Analysis of Variance (ANOVA) was performed to verify the statistical significance of the three process parameters. The level (Table 3.11) of each parameter was set to 7 and the sample repetition was 2.

The results of the single factor analysis (Figure 3.10) showed the same results as the orthogonal analysis. The purpose of the regression curve fitting is to formulate the relationship between the process parameters and the compressive load to present the optimal process parameters for realizing the required damping properties. The relationship between the process parameter and the two-cushion properties can be confirmed in the main effect plot (Figure 3.13).

The derived regression model is shown in Table 3.15 and the R-sq values for the case factor and hysteresis loss rate are 78.22% and 45.89% respectively. A solution can be derived by scoring two or more responses according to the movement of the independent variables. In this session, repeated compression tests were performed to confirm the preliminary insight into the durability of the lattice structure pad.

The main weakness of the proposed structure is identified through the change of the compressive behavior of the sample according to the cyclic fatigue and the initial fracture pattern. The values at and 10000 cycles were derived from the quasi-static compression test result data in Table 3.4 after the compressive fatigue test (table 3.17). As a result of the cyclic compression test, all three specimens showed similar fatigue behavior (Figure 3.15).

This led to a compressive behavior of folding along the layers of the lattice as the struts of the additively manufactured lattice structure broke.

Figure 3.3 Selective Laser Sintering (Farsoon, Flight HT403P), Thermoplastic polyurethane powder (AM polymer, TPU01GR)

Life Cycle Assessment (LCA)

Framework setup
Goal and scope (Boundary setup)
Life cycle inventory

Product manufacturing processes

Life cycle impact assessment

Energy use
Environmental impact

Interpretation

In the LCI stage, the human work in the manufacturing process, including the residual dust elimination process of the AM process and the manual sewing process of the CM, was ignored for the LCI evaluation. For the SEC of the nitrogen venting process, values from the Ecoinvent library were used. For the functional unit of the AM process, the time of each stage was calculated through the logging software of Build star (Farsoon, China).

The total energy of the SLS process calculated as the sum of energy consumption in preheating, building and nitrogen ventilation is 254.02 MJ. The total energy consumed in the CM process consists of the sum of energy used in the molding process, polymer foaming process, and artificial weight manufacturing process. All SECs of the three processes of CM were obtained from the Ecoinvent database (Table 4.3).

The total energy consumption of the CM process, calculated as the sum of the energy consumption of the mold, foam and artificial leather manufacturing processes, is 25,411 MJ. The categories listed at the bottom of the graph are in the same order as the 17 environmental impact categories mentioned in the previous paragraph. In 13 categories there was crossover in the defined range of 10 to 100 parts, and in one category the environmental impact of the AM process was high across all production volumes.

In contrast, the environmental impact of the CM process in the 4 categories was high for all production volumes. Therefore, the environmental impact crossover did not occur at less than 100 parts, and the crossover occurred at the production volume of 340 parts. The environmental impact of the 'Redmud from bauxite decomposition' process is 81.16% of the total, accounting for the majority of the environmental impact in the 'Human carcinogenic toxicity' category of CM.

In addition, most of the environmental impact contribution of the AM process occurs in the electricity generation process. Given the advantage of being able to replace the manual sewing process in the CM process with an automatic process called additive manufacturing, the functional improvement of the AM process will be further emphasized.

Figure 4.2 Cushion manufacturing process (AM)

Conclusion

However, the sample size was smaller than the actual pad application, so further experiments with larger sample sizes are needed. In particular, the width length and width were relatively large compared to the height of the sample, which caused the sample to bend during the compression process. This means that the results of the current study do not accurately describe the full-scale compression behavior of the cushions, as this would not occur at the actual dimensional level of the cushion.

Therefore, it is necessary to perform compression tests on specimens with larger dimensions for the optimal process parameters derived in this study. For the actual application of the grid structure, it is necessary to undergo a test from the side of the pillow sheet. For further studies, the scope of the LCA analysis should be extended to the distribution, use and disposal phases, as well as the raw material and production phases considered in this study.

In addition, further research into the economics of the process, i.e. costs and manufacturing time, is essential as an accurate decision support.

Study on optimization of process and parameters of selective laser sintering of walnut shell powder. Statistical evaluation of the effect of laser energy density on the mechanical properties of polyamide parts produced by selective laser sintering. Effect of processing conditions on pillar structure and compressive properties of cellular mesh structures fabricated by selective laser melting.

Investigations on mechanical properties of lattice structures with different values of relative density made of 316L by selective laser melting (SLM). 3D soft auxetic lattice structures fabricated by selective laser sintering: TPU powder evaluation and process optimization. Effect of powder size and shape on the SLS processability and mechanical properties of a TPU elastomer.

Quasi-static and dynamic compressive properties and deformation mechanisms of Kelvin cell 3D printed polymeric cellular structures. Additively manufactured polymeric foams as templates for custom ceramic foams - Comparison of SLS and FFF techniques. Identification of key factors of capacity decay in lithium-ion cells using orthogonal design of experiments.

Retrieved November 25, 2022, from https://ecoinvent.org/the-ecoinvent-database/. FLIGHT® 403P Series from https://www.farsoon-gl.com/products/flight-403p-series/. Design of cooling channels in a non-rectangular flat injection mold. 2007), Injection Mold Design Engineering, Hanser Gardner Publications, Cincinnati, OH. Prescription 2016: Harmonized Method of Midpoint and Endpoint Life Cycle Impact Assessment Report I: Characterization 2016-0104.

Appendix

Appendix A: Raw data of Kelvin lattice structure’s cushion properties in the orthogonal analysis

Appendix B. Raw data of Kelvin lattice structure’s cushion properties in single factor analysis

Acknowledgement