E29 Spring 2023 - HW3 - Additive processes

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1 University of California, Berkeley Department of Mechanical Engineering E 29: Manufacturing and Design Communication Spring 2023 Homework 3 Additive processes Due: Friday February 24, 11.59pm on Gradescope (This week, you can have a penalty-free extension of up to five days past this date without needing to request one; if you find you need a longer extension, please inform your GSI.) Important note: uploads to Gradescope must be in image (e.g. .png) or .pdf format, and not .docx format (which is very cumbersome to download and grade). Also, we require that you use the Gradescope interface to identify the location of each part of your solutions in the file(s) you upload, so the reader does not have to scroll through a large document to find each of your answers. The instructions on how to do this are here . Total points: 40 1. Applications of additive manufacturing [12 points] Below are listed four objects that are regularly manufactured: 1.1. The body of a space rocket 1.2. Food eaten by astronauts on the Space Station 1.3. Transparent dental aligner 1.4. Heat exchanger for an air conditioning system For each item, do the following: (a) Ask ChatGPT whether or not additive manufacturing is suitable for manufacturing the object, and why. Abridge ChatGPT’s answer to not more than 100 words and quote it at the start of your response. Make it clear which part of your answer came from ChatGPT. [1 point per object] (b) In your own words, discuss whether or not you agree with the response you obtained above. Include in your answer [2 points per object]: • A brief discussion of the relative merits of using an additive process for this product, as opposed to using a “traditional” process such as machining, injection molding, casting, stamping, etc. • If you think additive manufacturing could be appropriate, briefly explain why (as inspiration, you can use the list of possible motivations for using additive processing in the lecture notes).

2 • If you believe additive manufacturing could be appropriate, suggest which additive process(es) of the ones we have studied in lecture would be most appropriate, and why. • If you think an additive process would not be warranted, briefly say why not. 2. Carbon fiber-reinforced FDM extrusion path planning [12 points] The Markforged X7 printer combines an FDM thermoplastic extrusion printer with the ability to incorporate continuous fiber reinforcement, producing extremely stiff, strong, and lightweight composite components. 2.1. Study the Jacobs online training materials for the Markforged X7 printer ( here ), then take the online quiz at the end of the training materials and upload a screenshot showing that you have successfully completed it with a score of 7/7. [3 points] Notes: • The section of the Jacobs training entitled “Eiger File Setup Process” asks you to “Log in to the Jacobs Institute’s Eiger Organization”. For this homework, you will not be printing directly to the Institute’s physical printer, so, instead, simply create a free online Markforged account at https://eiger.io to enable you to use their proprietary cloud-based slicer software, Eiger. You will be invited to activate a free trial of the simulation features within the Markforged software. Simulation is not necessary for this homework, and you may want to wait to activate the simulation trial in case you end up wanting to use it for your project. • You do not need to schedule any in-person/hands-on training for this homework, though are welcome to do so if interested. Next, we will load, slice, and analyze an example object in the interface. • Import the RepRap extruder mount .stl file from Lab 3. Accept the default orientation of the part as imported. • Go to Part View. • In Part Settings (right hand of screen), select Onyx FR material (the chopped carbon fiber-reinforced Nylon material), and Carbon Fiber reinforcement material (the material for the continuous fibers to be woven into the print). The printer Type should then appear as “Industrial Series (X3, X5, X7)”. • Click Save, and the software will slice the object and generate print paths (this will take a couple of minutes). • Click on Internal View (bottom right), then select 2D mode.

3 2.2. Scan through the layers using the slider at the bottom of the screen. At which layer numbers (as displayed above the slider) does continuous reinforcement fiber appear? [1 point] 2.3. Upload a screenshot of one of the layers containing continuous fibers. [1 point] 2.4. Describe the pattern traced out by the continuous fibers in these layers. How are fibers near the edges of the part laid down, compared to those in the center of the part? [1 point] 2.5. Why do you think the slicer has placed continuous fibers at these layers and not others? [2 points] 2.6. Record the projected material cost from the left hand of the screen. [0.5 point] 2.7. Suppose that the component is anchored at the two locations shown by the yellow arrows in the diagram below, and that it is loaded in the direction and location shown by the red arrow. If maximizing the flexural rigidity of this component in this loading state is important, suggest, in words, one modification to the placement of continuous fiber that could be made, within the physical constraints of the printer’s capabilities, to increase the rigidity. [2 points] 2.8. Use the “Create new override” function to implement your proposed change. (If you want to add fibers to multiple layers at once, you will need to switch into 3D view, and click and drag on the layer bar to select the range of layers to which fibers are to be added.) Include a 3D screenshot of your modified toolpath plan in Internal View. [1 point] 2.9. Report the change in projected cost associated with this modification. [0.5 point]

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4 Stereolithography [12 points] As introduced in class, some stereolithography printers use an oxygen-permeable window at the bottom of the resin tray to create a ‘dead zone’ of resin in which photocrosslinking is inhibited by the presence of oxygen. The purpose is to prevent the printed part from sticking to the tray. As the part being printed is drawn upwards out of the tray, liquid resin is sucked into the space vacated by the part. In this question we consider how the thickness of the dead zone and the design being printed affects the vertical force that needs to be exerted on the component. Below is a figure taken from the Science paper that introduced the Continuous Liquid Interface Production (CLIP) process (Tumbleston et al., Science vol. 347, no. 6228, p 1349):

5 Consider the following simple model for the pressures in a thin disk of liquid between two rigid surfaces that is being made thinner or thicker at a specified rate. The printed feature illustrated is a cylindrical post with radius a : The pressure distribution across the fluid disk is parabolic, falling to zero at the edges and peaking at the center. The average suction pressure across the disk is given by: 𝑝𝑝 0 = 3 𝜂𝜂𝑎𝑎 2 2ℎ 3 � d ℎ d 𝑡𝑡 � Where 𝜂𝜂 is the viscosity of the resin, h is the thickness of the dead zone, and t is time. The vertical force associated with the suction pressures within this resin disk is 𝐹𝐹 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 = π a 2 p 0 . Note the inverse cubic dependence of pressure on dead zone thickness for a given printing speed. In the printer, the lower rigid plate corresponds to the oxygen-permeable window, and the upper plate, with radius a , is a single feature that is being printed. A printed object will contain many features distributed across the window. 2.10. Suppose that the viscosity of the resin is 0.35 Pa.s (Pascal-seconds), which is typical for a stereolithography resin. Take a = 1 mm, and the dead zone thickness to be h = 50 µm. Also suppose that 100 of these features are arrayed across the part being printed. The print speed, d h /d t , is 100 mm/hour. Showing your working, compute the total force that would need to be applied to the part during printing to overcome viscous forces in the resin. [6 points] 2.11. Now suppose that instead of printing 100, 1 mm-radius pillars, a single pillar with radius 10 mm is printed. It has the same area as the 100 smaller pillars. Find the printing force for this single, larger, pillar. [4 points]

6 2.12. Compare your value from 3.2 with the value you found in part 3.1, and comment on your findings and any implications for the application of dead zones and geometric freedom in stereolithography printing. [2 points] 3. Gathering your opinions on computer programming in ME [4 points] In the ME Department we are currently reviewing how we teach computer programming at the undergraduate level. We would like to give you an opportunity to provide your perspective and influence the development of our curriculum in this area. Please complete the following short survey as part of this homework: https://forms.gle/iaXS8GAZsTgitQL66 We will award 4 points when your response is logged by the Google Form. There is no need to upload anything specific to Gradescope for this part of the homework.

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