Calibration of a Piezo-Electric Printhead in the Selective Inhibition Sintering (SIS) Process for Fabrication of High Quality Metallic Parts

Payman Torabi*, Matthew Petros*, Behrokh Khoshnevis*

*Daniel J. Epstein Department of Industrial and Systems Engineering, Los Angeles, CA 90089

Abstract Selective Inhibition Sintering (SIS) is a disruptive Additive Manufacturing process capable of printing parts from polymers, metals and ceramics. In this paper the application of a commercial piezo-electric printhead in SIS-metal is studied. This replaces the single-nozzle solenoid valve previously used in the process and allows the fabrication of high quality metallic parts due to smaller droplet sizes as well as high resolution printing mechanisms. A Design of Experiments (DoE) approach has been utilized to study the effects of important factors in printing the inhibitor. These factors include: composition of the inhibitor, quality of the print, and amount of fluid deposited for each layer. Based on the results of these experiments, parameters have been identified for the creation of highly accurate three-dimensional parts.

Introduction

Additive manufacturing Additive manufacturing (AM) processes fabricate physical objects directly from 3D models and other digital data sources through a layer by layer manufacturing process. There are many different processes with different methods for the fabrication of each layer of the part, but all share four common steps:

i) The part is first modeled in a CAD software in computer ii) The 3D model is sliced digitally into different layers with specific thicknesses iii) The physical part is built layer by layer in the AM process iv) The part is post-processed to achieve the desired aesthetics and/or mechanical

properties

Industrial processes are looking for ways to fabricate parts with higher qualities without sacrificing the production speed. With this end in mind, inkjet technologies present a tremendous opportunity to AM processes due to their high resolution and high speed print abilities. Therefore, integrating inkjet printing into a potential process can vastly increase its capabilities.

Use of inkjet printheads in AM Inkjet printing involves depositing droplets of ink as small as 10 microns in diameter under precise digital control. There are many different methods for generating and ejecting the droplets out of the print-head nozzles. Most of these methods can be classified into two main categories; continuous inkjet (CIJ) and drop-on-demand (DoD). This classification can be seen in Figure 1. Both methods print liquid through small orifices called nozzles. In CIJ the flow of the liquid is continuous. Conversely, DoD systems are impulsive, meaning small droplets are formed which can be positioned separately as needed [1].

323

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324

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327

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328

Before calibrating the printer settings, the powder layer thickness needed to be adjusted. The metal powder used in this research was a fully alloyed bronze with chemical composition presented in Table 3. The powder particle size has a distribution of 98.7% -325mesh, 1.3% -200/+325mesh as reported by the manufacturer. Thus, the layer thickness cannot theoretically be less than the largest particle size, approximately 80 µm. For calibration of the layer thickness, the current spreading mechanism in the machine was used to spread layers with differing thicknesses. The minimum layer thickness that could be spread consistently was 120 microns. This layer thickness was used in the determining the print settings. It should be noted that the SIS process is applicable to smaller layer thicknesses. The limiting factor in the current study was the particle size.

Table 3: Chemical Composition of the used bronze powder

Copper 90% Tin 10.00% Lead 0.025% Zinc 0.04% Iron 0.058% Phosphorous 0.085%

A design of experiments approach was utilized here to investigate the significant and optimum values of the print parameters. The goal was to choose the best combination of factors that gives a depth of penetration of slightly more than 120 microns, an acceptable surface quality, and a satisfactory print speed. Each DoE consisted of a set of small squares printed on a thick layer of loose powder. Each square was printed with a unique combination of the controllable factors. The factors included: printed color which determines the number of nozzles engaged, print quality which determines the droplet sizes, and number of passes which directly affects the penetration depth as well as the surface quality. Three different levels were considered for each factor. The factors and their corresponding levels can be seen in Table 4.

Table 4: Important factors and their levels used in the DOE

Levels

Factor Description 0 1 2 1 Color Cyan Magenta Both (Blue) 2 # of Passes 4 7 10 3 Print Quality Text/Image Photo Best Photo

The experimental design is shown in Figure7a. Table 5 shows the print factors for the first 5 squares of the experiment. After printing the squares with their assigned factors, they were bulk sintered in the furnace. The inhibited powder in the printed areas was then removed (Figure7b). As can be seen in the figure, the final part consists of small squares with different depths of penetration. By measuring the depth of each square and visually comparing the surface qualities, the settings that give the most penetration depths as well as the best surface qualities were determined. The experiment was repeated a three times to ensure repeatability.

329

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4

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Figure 7:(

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330

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331

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332

References

[1] Hutchings, Ian M., and Graham D. Martin, eds. Inkjet technology for digital fabrication. John Wiley & Sons, 2012.

[2] Gibson, Ian, David W. Rosen, and Brent Stucker. Additive manufacturing technologies. New York: Springer, 2010.

[3] Khoshnevis, Behrokh, et al. "SIS–a new SFF method based on powder sintering." Rapid Prototyping Journal 9.1 (2003): 30-36.

[4] Asiabanpour, Bahram, et al. "Advancements in the SIS process." Proceedings from the 14th SFF Symposium, Austin, Texas. 2003.

[5] Behrokh Khoshnevis, Mahdi Yoozbashizadeh, Yong Chen, (2012) "Metallic part fabrication using selective inhibition sintering (SIS)", Rapid Prototyping Journal, Vol. 18 Iss: 2, pp.144 – 153

[6] Yoozbashizadeh, Mahdi. Metallic part fabrication with selective inhibition sintering (SIS) based on microscopic mechanical inhibition. University of Southern California, 2012.

[7] Ink Jet Printers Technical Brief. (2007, May 1). Epson

[8] Epson Micro Piezo Print Head Technology. (2010, January 1). Epson Exceed Your Vision.

[9] Kwan, Joyce G. Design of electronics for a high-resolution, multi-material, and modular 3D printer. Diss. Massachusetts Institute of Technology, 2013.

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