12. Molding and Casting¶
Introduction
This week I learned how to use various types of silicon and different molding techniques to cast a mold from a milled block of wax that I milled on one of our lab’s CNC machines. I used my mold to create a contiguous plastic component. While many of the assignments that I have completed thus far have comprised “deeper dives” into processes involving machinery and softwares that I was familiar with prior to beginning Fab, this week’s material was entirely new to me. After navigating the molding and casting workflow throughout the course of this week, I created a relatively simple final product. As the possibilities of the molding and casting techniques that I learned throughout this week seem limitless, I am excited to continue my exploration of different molding and casting techniques in my future engineering endeavors, as molding and casting allows for the fabrication of contiguous plastic pieces, whereas 3D printers create layered objects which reduce the durability and the professional appearance of a print.
Assignment
This week’s individual assignment was to “design a mold around the stock and tooling that you'll be using, mill it (rough cut + (at least) three-axis finish cut), and use it to cast parts, review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them. The group assignment for this week was to “review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them.”
Week 12 Individual Assignment
I began the design process by opening my CAD software of choice, Fusion 360, and commencing the design process of the shape that I would mill out of wax to create my mold that I would eventually cast my final silicon piece out of. As the mold is made with the CNC’d wax, I would need to invert the extrusions of my design to allow for an accurate final mold. As I would be designing my mold based off of the YUUY logo, which is a modified logo that pays homage to the almighty Yugoslovian Yugo. I first needed to create a trace of a cropped version of the logo image that I downloaded that omitted the text and borders from around the main logo shape, as I did not want to include the borders nor the text in my final mold. To accomplish this, I decided to use Inkscape to trace the geometry of my logo that I would then import into Fusion360 to extrude the design.
After tracing the logo, I exported a .DXF file of the trace from Inkscape. By making the file format a .DXF, I would be able to extrude certain lines in Fusion without needing to manually trace any of the lines on the image.
In Fusion360, I created a new sketch and imported my .DXF file into the XY sketch plane.
As I would be milling out of a wax, I created an extruded rectangle that mirrored the dimensions of the wax that I would be milling out of. The wax is 1.5” thick, and as I wanted my logo to be around 4” in length and width, I made this piece 5.5” x 5.5” and then added a 0.2” extrusion to the bottom of the shape so the machine would not mill through the wax which would harm the machine and destroy my mold.
After creating a body of the rough shape of the wax that I would be milling out of, I added the silhouette of my logo onto a plane that I situated atop the base of the structure that I made.
After situating the logo on the shape of my design, I extruded the different portions of the design that would ultimately allow me to create my mold. I extruded the geometry inversely relative to what I wanted the final design to look like, as I would be creating the mold out of a milled version of this file and creating my final cast in that mold. After finishing my extrusions, I decided to fillet the edges of the rectangles on the sides of the design to allow for a cleaner cut on the CNC, since the bits in our lab are sometimes unable to accurately cut the sharp angles of certain geometry, making filleted squares far easier to mill than squares.
Toolpaths
After completing the basic design of my file, I was ready to begin creating toolpaths that I would use on the CNC machine to mill my mold out of a block of wax. As I am familiar with both Aspire and Fusion360, I decided to use Aspire to generate the toolpaths for my design as its interface for designing toolpaths is significantly more intuitive than that of Fusion360. Since Aspire does not import .DXF geometry accurately, I needed to export my file from Fusion as a .DXF and use an online converter in order to convert the file into a .svg, which makes the extrusion process in Aspire significantly more efficient.
I selected the entirety of my design and reduced the size to 4.5” x 4.5”, and made the material 5” x 5” x 1.5” to reflect the accurate dimensions of the wax that I would be milling into.
After joining the individual lines of each shape using the ‘join vectors’ tool with, I began extruding the individual components of my design to the necessary heights.
After extruding the components of my design, I was left with a model of what a successfully milled version of my file would look like. After verifying that each portion of the design was accurate, I began designing the toolpaths that I would use to mill the design on our lab’s ShopBot Desktop CNC machine. At this point, I began working on other Fab Academy assignments and would not return this assignment until several months after the writing of the above section. At the time when I returned to this assignment, both of our ShopBots were temporarily out of commmission due to various problems that they were experiencing. As such, I had to use our BanTam Tools Desktop Milling Machines instead. The process for using these machines is documented in extensive detail below.
To begin the next portion of my file preparation process, I re-opened the original model of my YUUY logo in Fusion.
As my model was already ready for toolpath generation, I tabbed over to the 'manufacturing' section of Fusion 360 to begin creating toolpaths for the BanTam Desktop CNC Mill to mill the mold for my project.
As the base installation of Fusion 360 only includes several bits and does not feature the 1/16" flat end mill bit or 1/8" flat end mill bit that I planned on using for the milling of my mold, I would need to add these tools myself, specifying various parameters so Fusion would be able to generate accurate toolpaths for my GCODE file that would be sent to the milling machine.
I began by selecting the 'new tool' option within the Fusion 360 bit library.
I started the process of creating my bit by providing it with a descriptive name so I would be able to find it again for future molding and casting or other tasks that require the implementation of the 1/16" end mill bit on our BanTam mills.
Next, I filled out the various fields within the 'add bit' menu to specify the dimensions of different sections of my bit. Fusion requires many fields to ensure accurate generation of gcode during post-processing.
Next, I populated the 'cutting data' fields for my bit. These fields determine the feeds and speeds of a bit and various other important details regarding its operation. For molding and casting, it is important to have a relatively low rpm on your bit, as the bit spinning too fast can potentially cause the wax to melt due to the extreme heat that the bit creates when spinning quickly. 12,000 RPM for my bit seemed to maximize the milling speed and mitigate (maybe eliminate?) the likelihood of error due to melting wax when milling my mold.
After I defined the cutting settings for my bit, I confirmed all of the settings were accurate and added the bit to my local tools library. The image above shows what the tool looks like in my Fusion 360 tools library.
After generating my 1/16" flat end mill bit, I was ready to begin generating toolpaths for the CNC. I started the toolpath generation process by selecting an "Adaptive Clearing" cut in the "3D" toolpaths section. While a rough cut, the adaptive clearing toolpath runs incredibly quickly, allowing for large areas of material to be removed relatively quickly compared to other processes. Fortunately, this was the only toolpath that I would need to generate, as the entire model could be milled with just this single process.
After generating my toolpath, the above model shows what the completed cut looked like after using the "simulate" tool in the manufacturing section of Fusion to display what my finished model would look like.
After generating my single toolpath, I was ready to export the gcode from Fusion 360 to mill the mold on one of the Latin lab's desktop CNC mills. This required navigating to the "post processing" window of the manufacturing section and filling out several required fields that would then allow me to export the gcode from Fusion. In the post processing section, I had to select the proper post processor for my machine, which is the OtherMill (OtherMachine) post processor, as I am using a machine manufactured by OtherMill. This was the only field that I had to modify, as the default values for every other field were already correct.
I exported my gcode from Fusion to my Desktop, and then loaded the file into the BanTam Tools software on a PC connected via USB to the machine.
Milling my Wax Mold
After exporting the gcode to the BanTam machine, I was ready to prepare the wax and the machine for the milling of my mold.
I began by selecting the wax that I wanted, which is 3.1" x 3.1", and applied Nitto Tape to the bottom of the wax to ensure that it would stick to the bed of the machine during milling.
After applying the Nitto Tape, I stuck the wax to the base of the machine. I placed it in the corner to allow for easy setup of the file in the software.
I imported my file into BanTam and selected the 1/8" flat end mill bit that I planned on using as the tool for the job. I also changed the settings from a copper blank PCB to a generic material with the proper dimensions of the wax that I would be milling into to allow BanTam to generate accurate toolpaths for my design. After doing this, the software revealed a modest milling time of 23 minutes. I started the machine and wanted for the file to finish millng.
After the file wax finished running, my finished mold was revealed. I immediately noticed a slight problem with the design. On each side of the logo, a small amount of wax had not been removed by the machine, meaning that I would not be able to generate a proper mold from this piece of milled wax. As such, I needed to return to Fusion and either modify my design or choose an entirely new file to generate a mold from.
I decided to restart entirely, choosing an svg of the Bitcoin logo that I had on my desktop as the source for my mold instead. I began by creating a 2.5" x 2.5" box with offset extrusions to visualize the wax that I would be milling into.
Next, I imported the BTC logo onto the face of the box, taking time to place it in the precise center of the box. I increased the size of the logo slightly, but I made sure to leave extra room so the same problem that emerged during my first attempt at milling a mold would not happen again.
Next, I extruded the logo. These extrusions would allow me to pour the EcoFlex rubber into the mold and generate a mold that I could then cast parts out of. My file was now ready for the generation of toolpaths.
I generated a new setup for the logo. Setups in Fusion are essentially locations where toolpaths can be generated. Exporting a setup will export all of the toolpaths that are contained in the setup's file location.
I generated a new stock, making sure that the dimensions of the stock were less than those of the wax that I would be milling into in order to ensure that my part milled out correctly on the first try.
I began by generating an adaptive clearing cut for the outer edges of my design. This pass allows for all of the excess material to be removed from my design with stunning efficiency compared to more precise toolpaths.
I was able to generate this toolpath with a 1/8" end mill bit, maximizing the effiicency of the milling process of my design.
The generated toolpath was incredibly inaccurate in the most detailed areas of the cut, so I needed to generate a second finishing toolpath that would clean up these areas of my design.
I decided to generate a 2D contour toolpath with a 1/16" flat end mill bit that would be used to mill out the most precise areas of my file. The gcode files would need to be exported separately and the tool on the BanTam mill would need to be changed before the finishing path, but I was fine with this change for the sake of the quality of my mold.
I post-processed each of my files and exported them with the names 'roughing' and 'finishing' to help me distinguish between the unique toolpaths contained in each gcode file.
I opened both my finishing and roughing passes on a PC connected to one of our lab's BanTam milling machines.
I first loaded the file for my roughing pass into the BanTam Tools software.
I ran the 39 minute file, leaving me with a block of wax with all of the least precise areas removed. Now I simply needed to switch the bit on the machine and run my finishing pass to complete my mold.
Next, I loaded the finishing file onto the BanTam software and ran the 16 minute detailing pass with a 1/16" flat end mill bit to ensure the most detail possible from my design.
After the milling of the finishing pass finished, I removed the machined wax from the machine and was left with the above finished mold.
Molding and Casting
with my mold finished, I was ready to begin casting a rubber mold that could be used to cast plastic parts out of. I decided to utilize ecoFlex for the rubber portion of my mold, as it is a durable rubber material that features an incredibly fast cure time. After reading through the instructions on the bottle of the material, I discovered that I needed to mix the A and B solutions with a 1:1 ratio in order to achieve the correct concentration of each liquid for the two to react together and create the rubber that my final mold would consist of.
I retrieved the two bottles of EcoFlex A and EcoFlex B from our lab. The products are very clearly labeled and color-coded to avoid confusion.
I used two measuring cups to measure out a sufficient amount of EcoFlex into each of the two measuring cups. I made sure to not use too much material, but I also did not want to run out whilst pouring my rubber mold or I would have to restart the mixing process so I poured slightly more than I would actually end up using into the mold.
After measuring out equivalent amounts of EcoFlex and pouring them together to begin the reaction, I poured the combined solution into my mold and waited for the duration of the 5 minute cure time to elapse.
After waiting roughly 7 minutes, I removed the finished rubber mold from the milled wax and was left with the above mold that I could use to cast plastic parts out of.
After casting my rubber mold, I was ready to use SmoothCast to generate a plastic part out of this mold. SmoothCast is also concentrated in a 1:1 ratio between its A and B solutions, so mixing it is a relatively simple process.
Again, I made sure to pour excess SmoothCast into both the A and B bottles in order to ensure that I would have enough to fill the entirety of my mold.
SmoothCast features a slightly longer cure time of 15 minutes, so instead of watching it like I did for the curing of the rubber mold, I decided to record a time lapse of the curing process of the mold.
After the SmoothCast that I poured into my mold finished curing, I was left with the above final cast.
Group site
This week's group assignment was to read over the safety datasheets for various molding and casting materials and to test some of these materials in our lab and compare the results. My contributions to this week's group work included writing a large portion of the group site and helping with the testing of some of the materials. Our group's work features extensive documentation on our group site page for molding and casting. Enjoy!
For my personal work for this week, I read over the datasheets for the two molding and casting materials that I used, ecoFlex and Smoothcast 300 in addition to various others that are documented on the group site.
EcoFlex Datasheet SmoothCast 300 DatasheetConclusion
Throughout the course of this week's assignment, I learned a lot about the molding and casting process, which I found super interesting and to be incredibly applicable in many of my futrue endeavors. I like how molding and casting occupies a space in the prototyping process that bridges the gap between very rough 3D-printing and final product states, as it allows for the creation of contiguous, more stable bodies as opposed to the layered bodies that are created from 3D-printing parts. Overall, this week's assignment taught me a lot about the molding adn casting process and how to remain safe when practicing thse tasks.
Download this week's files! (35 kB)