CONVERSION OF ECONOMY 3D PRINTER TO E-JET PRINTER

Transcription

1 CONVERSION OF ECONOMY 3D PRINTER TO E-JET PRINTER Ryan J Tepper University of Michigan Department of Mechanical Engineering Professor Kira Barton University of Michigan Department of Mechanical Engineering ABSTRACT Micro-fabrication is an amazing manufacturing process because you can make very complicated systems extremely tiny. 3D printing can improve this process because it is a fast, low-cost, versatile alternative for product prototyping. However, there are currently many barriers preventing the advancement of 3D printing at the micro-scale. The purpose of this research is to address these barriers through the characterization of key design criteria, process parameters, and environmental conditions. We will first choose an off-the-shelf 3D printer to modify into an e-jet printer and install it a Caron Environmental Chamber. An e-jet printer uses electricity to pull fluid through a microscopic hole. Due to the extremely fine scale and high resolution needed for this process, very fine motion control is needed. We will design our experiments to fill any gaps in knowledge behind necessary design criteria and execute several g-code applications to run experiments that will help determine these criteria. We will execute these experiments with varying environmental conditions, printing materials, and other parameters in order to determine the parameters to which micro-scale printing is most sensitive. As another aspect of this project, we want to educate students, teachers, and parents on the field of advanced manufacturing. We plan on doing this by developing interactive outreach activities and workshops tailored for families and K-12 students and publishing experiment results and activities on micromake.wordpress.com. We expect the results of our research to have a direct impact in the manufacturing of electronics, biosensors and optics. By exposing students to cutting-edge technology at a young age, we hope to inspire them to pursue careers in advanced manufacturing. The research results will be disseminated through publications aimed at reaching local, national, and international audiences.

2 INTRODUCTION Electrohydrodynamic jet (E-Jet) printing is a form of additive manufacturing that relies on electrohydrodynamically induced flows to print at extremely high-resolution. This resolution can be extended to below 100nm, which provides the unique opportunity for the manufacturing of a broad range of micro-devices with large-scale implications in the biomedical device industry. As shown in Fig.1, a polarizable fluid is input into the fixed E-Jet nozzle. A large voltage is then applied to the grounded printing substrate, which causes the polarizable fluid to be pulled through a fine nozzle tip. At such a fine resolution it is actually easier to pull the fluid through the nozzle tip than to push it through due to the large amount of pressure required to overcome capillary forces [1]. This differs from standard ink-jet technology where the fluid is pushed though a comparatively much larger nozzle Fig. 1: Basic diagram of E-Jet printer and axis of motion Although e-jet has been shown to have tremendous potential, we are currently still lacking key criteria for the environmental parameters needed to achieve optimal printing conditions. Some of these conditions may be destructive, due to humidity and temperature, for the nano-positioners on which the printing substrate is fixed to. Environmental testing of current e-jet printers could prove to be costly if these nano-positioners are damaged. To remedy this issue, we intend to modify an economy grade fused deposition modeling (FDM) printer for e-jet use. The first step of this process, which this paper will cover, is the characterization of an off-the-shelf printer and the determination of whether or not the current control system is sufficient for e-jet. 2

3 OBJECTIVE Our objective for the research project is to characterize the X/Y motion of various off-the-shelf economy grade 3D printers. We have identified 100µm as the minimal accuracy needed, in either direction, to achieve successful e-jet prints. METHODOLOGY We acquired three economy grade FDM printers, shown in Fig. 2, to begin our characterization. The Polar 3D and Velleman K8200 Kit were both of interest since they both operate with a fixed nozzle and moving build plate. To test if this was the most accurate method we also acquired the FlashForge printer, which has a static build plate and moveable nozzle. We also wanted to see the effect that the coordinate system has on the accuracy of positioning. We were able to test this since the Polar 3D operates in polar (r,θ) coordinates while the other two printers operate in Cartesian (x,y) coordinates. Fig. 2: Economy grade FDM printers; (Left to Right) Velleman K8200, FlashForge, Polar 3D To find the resolution of these printers, we developed the characterization process below. 1. Set a fixed point to track 2. Use a high resolution digital camera to measure displacement in pixels and convert back to microns 3. Verify a consistent homing command 4. Take photos before and after the jog commands 5. Analyze the photos using ImageJ So far, only the Polar 3D has been characterized; however this process will be the same for the remaining two printers. 3

4 Setting a fixed point In order to determine the displacement of the build plate when g-code jog commands were sent to the printer, we had to have a point to track. This point needed to be extremely fine in order to reduce error in choosing the same point for analysis between the two photos. We also wanted the point to be reflective so it would be easier to see under the high-resolution camera, since the camera had a very fine focal length (~300µm). Given these criteria, we decided to use a sewing needle as our point to track. We then fixed the sewing needle to the build plate using tape, as shown in Fig. 3. Fig. 3: Needle fixed to build plate Converting pixels to microns To determine the displacement of the needle, we needed to know how many pixels equated to 1 micron or vice versa. We did this by using the Infinity 2 high-resolution digital camera to take a picture of a standardized scale that had marks at 1000, 500, 100, and 20µm, as shown in Fig. 4 Fig. 4: Found 1.91µm = 1 pixel 4

5 Having a consistent homing command In order to aid in eliminating lash and hysteresis from our system, Tom wrote a g-code script to home the needle to same spot after each test was run. When we analyzed the accuracy of the homing command, we found the position to be nearly identical (~2µm) before and after the command was run. I modified the script for one of our tests to lower the feed rate in order to alleviate the system of some chatter that was occurring. Full experiment set-up We taped an LED strip, that completely surrounded our needle, to the Polar 3D in order to have optimal lighting, as shown in Fig. 5. Fig. 5: Experiment set-up for Polar 3D Procedure Using the software Reptier Host, I was able to send g-code commands to the Polar 3D, specifying desired jog length and direction. A photo was taken before and after each jog command was sent. In order to find the difference in pixel values, I compared before and after photos and tracked the same point of the needle on each photo. I estimate the error of choosing the point to be 1-2 pixels per photo resulting in ±4 pixel error total. This equates to ±8µm error overall. Before we could begin testing, minimal jog values needed to achieve motion in either direction had to be determined for each radii that the Polar was tested at. 5

6 RESULTS Threshold Values We expected our threshold values to be relatively consistent in the x direction, since motion in the x direction is controlled via rack and pinion on the Polar 3D. We also expected the threshold values in y to scale linearly since y motion is controlled via rotation of the build plate using a gear train. As shown in Table 1, the threshold values line up with our previous assumptions. It is important to note that these were the minimal values sent to the printer, not the actual distance the needle moved. Using these threshold values, we were able to determine starting our starting jog values. From there we chose 5 data points at each radius to measure. Table 1. Threshold assumptions hold true for Polar 3D X threshold Y threshold X-jogs (um) Y-jogs (um) R = 0mm 103 n/a 110, 150, 200, 250, 300 n/a R = 30mm , 140, 180, 220, , 125, 175, 225, 275 R = 60mm , 140, 180, 220, , 175, 225, 275, 325 R = 90mm , 140, 180, 220, , 225, 275, 325, 375 Error in x direction As shown in Fig. 6 and Fig. 7, pg.7, we saw a significant and consistent negative error in the X direction, with errors as large as 90% at R=0mm and with the minimal error equal to 40% at R=90mm. Because of this significant error it is not recommended to use the x direction as the primary jogging axis for e-jet printing with a stock Polar 3D printer. 6

7 Fig. 6: X direction shows significant and consistent negative error for R=0mm and R=30mm Fig 7: X direction shows significant and consistent negative error for R=60mm and R=90mm Error in y direction As shown in Fig. 8, pg.8, the error in the y direction is consistently less than that of the x direction, with errors as low as 2% at R=90 and as large as 14% at R=60. This is a stark contrast from the 40% and 90% respectively from the x direction. Based on these results, I would recommend using the y direction as the primary jogging axis for e-jet printing on the Polar 3D printer. 7

8 Fig. 8: Y direction shows very minimal error at all radii CONCLUSIONS So far, we have only characterized the stock Polar 3D FDM printer. Through our experiments, we were able to determine that the Polar 3D would be somewhat effective as an e-jet printer. Currently the Polar 3D is only suitable to be used in the y direction (Fig.8) as the error in the x direction (Fig.6, Fig.7, pg.7) is too large to be reliable. Future modifications, such as buying a new motor with more steps, could improve our resolution. Since the Polar 3D prints in arcs and since resolution only meets desired requirements one direction, this makes the printer only partially effective. We will hold on to this printer for e-jet consideration, and a decision will be made once the Velleman K8200 and Flash Forge are characterized as well. 8

9 REFERENCES 1. O. A. Basaran, AIChE J. 2002, 48,