Week 13 : Casting & Molding
Summary
This week I designed a 3D mold on Fusion 360 from an STL of a chess piece found on cults3d. I then produced the mold in PLA using a Prusa printer. In the end, I cast a Smooth on polymer, as rigid as possible.
Assignments
Group Assignment
- Review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them compare printing vs machining molds.
Individual Assignments
- Design a mold around the process you’ll be using, produce it with a smooth surface finish, and use it to cast parts.
Materials Review & Casting Comparison
This part was done in group and is accessible on the group page.
Mold Design
To design the mold, I had two options:
- 3D design the mold and produce it.
- Design the negative of the mold, which will make a first mold that will be used to create the mold for the part.
I decided to go for the simpler option as I’m a neofite in practice, if I have time I’ll try the other method!
For this week, I’ve decided to produce a chess piece, the queen to be precise.
To do this, I first need a 3D model of the part. I find a design that suits me on the cults3d website.
I import the part into Fusion 360, which will be the starting point for the mold design.
The idea then is to create a solid around the part and use the part and combine functions between it and the solid. The result is a solid hollow with the negative of the chess queen.
I now need to work on the mold to make it easy to use: to be able to open it to remove the part after casting, and to have notches to align the two parts.
I use the “split body” function to have two pieces.
I then work on one of the two halves by extruding polygons (better for alignment). I then make a new “combination” to remove the extrusions from the first half on the second.
Finally, I chamfered the edges for a smoother finish and I add a 0.75-mm offset in the holes so that the two pieces can fit together.
My mold is ready to be made! I need to export the STLs for import into the Prusa software.
Mold Production
I import the STLs into Prusa to generate a gcode for 3D printing. In the parameterization section, I set the size of my mold according to the size of the part I want. I also set the precision along the z axis. Better precision could result in smoother rendering, but 3D printing takes much longer. I decide to go for an in-between precision of a tenth of a millimetre for 8h of printing.
The printing process went smoothly, and the result can be seen below!
Now I’m ready for the final step, which is casting the polymer into my mold to create my chess queen!
Casting
For the choice of polymer, there was a huge choice available between the FabLab and my research laboratory, so I decided to use a material that my colleagues had already used, to have a little confidence in the result. It’s called Smooth-Sil 960 and has a hardness of 60 A, the equivalent of a car tire.
I start by calculating the volume needed to make my part. I’ll need about 10 ml, which isn’t much. So I decide to make more to make it easier to measure and to make sure I’ve got enough (even if I have to make another sample on the side). As the ratio of A to B is 10:1, I need to be precise. In total, I’m aiming for 25 grams of polymer.
I start by taring my container and adding element A.
I then add element B in a 10:1 ratio, so here I need to add 2.25g of element B.
Then mix gently to prevent bubbles from being encapsulated in the polymer. I’ve got 45 minutes (pot life) to mix, vacuum and cast, so I’m not worried.
You can see that despite my precautions there are bubbles. It should be noted that the polymer is not very fluid, which favors the encapsulation of bubbles.
To get rid of as many bubbles as possible before casting, I pass my container through the vacuum chamber to create a vacuum outside the polymer, which will stretch the air outwards. I’m careful to protect the outside because there’s a risk of overflow, although I don’t have much volume here.
Below is a short video showing the bubbles escaping from the polymer.
After about twenty minutes under vacuum, I can take my container out of the vacuum chamber and fill my mould. The mould is fixed beforehand so that it doesn’t move. I try not to create a new bubble by pouring the polymer gently.
Now that the mould is full, I have to wait 16 hours for the polymer to cure.
After the curing time I can open the mould. I use a small scalpel to separate the two parts of the mold. The result is quite satisfying! I still need to clean up the excess bits.
After removing the excess material, the result is really good, quite smooth and precise.
A little bonus: the piece is very non-slip, so you can play a game of chess vertically.
