13. Molding and Casting¶
Group Assignment:¶
- Review the safety data sheets for each of your molding and casting materials
- Make and compare test casts with each of them
- Compare printing vs milling molds
Individual Assignment:¶
- Design a mold around the process you’ll be using, produce it with a smooth surface finish, and use it to cast parts
3D Printed Mold¶
I began this week by CADding and 3D printing a mold using the Prusa Mini. I wanted to make a pusheen mold by basing it on a picture, so, before creating the CAD file, I first downloaded the picture I wanted, converted it into a bitmap in Inkscape, and exported it as a plain SVG file. The picture I used is shown below.
Then, I created a new sketch in Fusion and went to Insert > Insert SVG and chose my file. I then extruded this into a 2 in x 2 in x 1 inch box I created. I made sure to extrude the eyes and tail segments of the picture at a different depth than the rest of the image so that they would be visible on the final product.
After finishing the design, I exported it as an STL file and brought it into PrusaSlicer to convert it into gcode. After printing, this is what my 3D printed mold looked like.
It is important to note that I will use the same Fusion file later on to mill this same design in a mold made out of wax, but, for now, I will continue onto creating my resin printed mold.
Resin Mold¶
To create the resin mold, I used the same STL file as what I used for the Prusa Mini print, except, instead of exporting the file into PrusaSlicer, I exported it into the FormLabs software (the resin printer we have in the lab). Here, there were a variety of options, but the only ones I really needed to use were the support options and the print orientation. Because the resin printer struggles when printing objects with flat faces completely on the bed, I needed to reorient my print a bit. To do this, I simply pressed the auto-orient button, which flipped my mold onto one of its corners. Then, I generated adaptive supports for my design and sent the file stright to the printer.
Before I actually began printing, I made sure that the bed was completely clean by wiping it down with isopropyl alcohol before securing it to the handle at the top. Then, I pressed the “start print” button.
This printed ended up taking around 7 hours to finish, not including the roughly 30 minutes of preparation the printer took to refill its resin. The picture below shows what the finished print looks like.
Wax Milling (Pain)¶
Preparing the Wax¶
After printing my resin mold, I moved onto making the last kind of mold I needed for this week’s assignment: the mold milled out of wax. To prepare for this, my classmate Collin Kanofsky and I first took out several long pieces of wax and cut them down. Then, we put them in non-stick trays and inserted them into the oven at around 280 degrees Fahrenheit. We eventually raised the temperature to 300 degrees because the wax was melting so slowly. After the wax melted fully, we took the trays out of the oven and poured the melted wax into pre-prepared containers and waited overnight for them to cool down. The picture below shows what they looked like right after being poured back into the containers.
Creating Fusion Toolpaths¶
Once we returned on Monday, we cut the condensed wax blocks into smaller pieces for each person to use. I took a smaller wax piece which was around 2 in x 2 in x 2 in and began making toolpaths for my design in Fusion 360. To do this, I began by going to the “Manufacturing” tab in Fusion and clicking on adaptive cleaning. Here, I struggled to make a correct toolpath for several hours (and suffered because of it). The main issue I encountered during this process was the errors that popped up whenever I would run a simulation of my adaptive toolpath telling me that the stock and bit were colliding. After trying to fix these issues for several hours, I eventually realized that this wasn’t actually a problem because my 1/16th bit that I was using was longer than the one in the simulation and therefore wouldn’t collide with anything. The next problem I encountered was with choosing the size of my bits. I wanted to use a 1/8th bit for roughing and finish with a 1/32 inch bit, but the 1/8 bit ended up being too big to fit in certain parts of my design and the 1/32 bit wasn’t long enough to avoid collisions with the stock. Because of this, I eventually had to settle on the use of a 1/16th bit for both the roughing and finishing. This process was done with a lot of help by Collin Kanofsky and I probably would have spent a lot longer on this step if not for his help (THANK YOU COLLIN!). The 3 toolpaths I ended up creating are shown below.
Now that I was done with toolpath creation, I could move onto exporting them as a gcode file and actually milling my board.
Design Considerations when Creating the Molds¶
Before I explain how I actually milled my molds, I wanted to discuss a little about the design considerations when creating them, especially how it pertains to the limitations of 3D printing/CNC milling them. There were a few very important aspects that I took into account when creating these molds. First, I wanted to avoid milling or printing very small indents/holes in my mold. As you can see from my CAD example, in order to avoid having to mill out or print these very small pieces, I indented the eyes/nose of my Pusheen mold upwards in the CAD (inwards into the eventual cast). Secondly, if I had been creating a larger mold, I would have more heavily considered the inclinations of walls in my design, as wall inclinations are very important when it comes to CNC milling molds (not as important when 3D printing molds though). Finally, the last aspect when creating molds is to consider the differences between 3D printing them and CNC milling them. I used the same design for both 3D printing and CNC milling my mold here, but if I had been creating a bigger or more complicated mold, I would definitely consider the inclinations of the walls of the CNC milled mold more. On the other hand, with the 3D printed mold, I would probably feel more comfortable adding small details to my mold, as it is easier to create those small details when 3D printing than when you are CNC milling (at least with most bits).
Milling using Bantam¶
After exporting the gcode file, I uploaded it to google drive and downloaded on the computer connected to the Bantam desktop milling machine in my school’s lab. From here, I uploaded that file into the Bantam software and grabbed my piece of wax which I would be milling with to measure its dimensions. The wax piece I was using was about 50 mm x 60 mm in length/width and 40 mm tall, so those were the dimensions I put into Bantam before putting nitto tape on the bottom of the wax and securing it onto the bed. Now, because I was only using a 1/16 inch bit for the entire milling job, I inserted it into the milling machine (I didn’t insert it fully because it was a small bit, so I left around a half inch gap). After offsetting the position of my mill appropriately and checking to see if the other settings were correct, I began milling. This entire job took around 1 hour and 10 minutes. The picture below shows the complete mold.
Casting!¶
Now that I had all 3 of the molds I wanted to have this week (3D printed, resin printed, and milled with wax), I began casting. Most of my classmates, including myself, used Mold Star 15 fast to cast this week. The safety datasheet for this product is linked here. You can find my notes on this datasheet in my group’s documentation for this week, which will be linked at the end of this documentation.
In order to cast, I first grabbed both part A and part B of the product, put them on cardboard sheets, and measured how much resin I would need. After checking my dimensions in Fusion, I calculated that I would only need around 30 mL to fully cast my 3 molds. To meet this requirement, I poured 15-20 mL of parts A and B, respectively, into separate cups to make sure I would have enough. I then stirred each cup with a plastic spoon for about 1 minute before pouring them together. Now, I sprayed the mold release onto each of my molds before slowly pouring the product. I made sure to pour slightly more than what was required in order to make sure that each mold had enough. I then scraped off any excess liquid from the pour with a small sheet of cardboard. The picture below shows what my casts looked right after finishing the pour.
Why the Resin Mold Didn’t Work¶
After finishing this week’s assigment, it was brought to my attention that the resin I used for the resin 3D printed mold may not have been compatible for casting silicone. Because of this, I decided to do more research and found this article on Formlab’s website. After reading this article, however, I came to the conclusion that the resin I used, Formlabs Clear Resin, was in fact compatible with silicone casting, as this is the resin that is referenced and used in images in the above Formlabs article. Because of this, I believe that the most plausible explanation for why the resin mold didn’t work with silicone castins is that it wasn’t dried/cured properly, leading to issues when casting. The reason I came to this conclusion because, after taking the silicone cast out of the resin mold, it stayed wet/sticky for several days and never dried.
Final Product¶
I waited about a day before going back to the lab to check on my casts, and the picture below shows what they looked like right after taking them out!
Degassing Methods?¶
I did not use any special degassing methods when pouring the mix into the mold. However, I did spray the mold with mold release before pouring in the mixture (using something very similar to this). I believe that this both helped to make the mold easier to remove at the end of the process and also helped a bit to prevent air bubbles. In addition to spraying the mold with mold release, I made sure to pour slowly and gently, covering the entire area of the mold evenly, which I also believe helped to reduce the number of air bubbles.
Problems¶
Both the 3D printed cast and the wax milled cast looked good, but, unfortunately, the cast I did using the resin printed mold still hadn’t dried for some reason. I even waited another day after taking it out of the mold and it was still sticky. I wasn’t too sure why this was the case, but I eventually decided that it was most likely the result of some excess moisture from resin that hadn’t fully dried at the time of pouring.
Group Documentation¶
This week, I worked with classmate Kabir Nawaz to review the safety datesheet for our casting materials, compare casting with different materials, and compare printing/milling molds. We chose to compare my 3D printed mold to Kabir’s milled mold. Our documentation can be found here.
Reflection¶
I feel like I learned quite a bit this week, mostly in Fusion’s manufacture tab and with casting. If I wanted to cast this design again with the same kinds of molds, the biggest change I would make would be to check how dry the resin mold is before actually casting. Overall though, I didn’t encounter too many problems this week (other than when creating the milling toolpath in Fusion…). My files for this week can be found here.