Week | 10
Molding and Casting
How I’m designing a 3D part, machining a mould and casting it.
26 March 2018 11:55:
This week’s assignment is around learning how to mould and cast. So, we are designing a part in 3D, machining (milling) it in a block of wax (a positive part) then, creating a mould with a silicone material (a negative part) and, finally, casting it to create a part (positive).
I had other ideas but, once, again, decided to do something quick design-wise, that would not consume a lot of design time.
Designing the part
Based on a nice bottle opener I found on the web (which I’ve used as a reference to sketch over), I have created a model on Fusion 360:
Then, created a box around it, that would serve as a container for the silicone. The box had to have angled walls, that would ease the access of the milling tool:
After modelling both, I’ve combined them into a single body and exported this.stl file.
Machining the part
I’ve imported the .stl file on Modela Player 4 (a CAM software application from Roland), a standard here at Fab Lab BCN when creating toolpaths for the SRM-20 milling machine.
The milling was done in two “passes” one for the “roughing” which would remove material from my wax block quickly, but, roughly and the other a “finishing” pass, that would fine-mill the wax block.
Here are the parameters I have used for the Roughing and Finishing;
I have opened the G code files on the VPanel software and followed the procedures for milling on the SRM-20, with very similar steps to the ones already documented here. After the milling, I realised some of the parameters could have been different in order to improve overall machining time and process, so, from the above parameters, I would decrease the cutting-in amount chosen (3mm) for the Roughing, since it seemed too deep for the tool X wax block, which was likely the reason for an error generated “Spindle motor experience an excessive load” that stopped the job twice. Also, on the finishing parameters, I would also change the Cutting-in Amount (0.2mm) to a higher value, because it took too long for the piece to be machined and my part is rather flat (with not many curved angles that would require such a small stepdown).
I have used a freeman wax block.
Here are some images from the milling process:
Molding the part
My part was meant to be casted in metal, but I decided to do a test with plastic material before, so, I have used a regular silicone to do the mould, instead of a high-temperature tolerant one, which would allow me to cast metal.
We have used gloves and protective glasses to handle the silicone.
We have used “Formsil 25”” silicone and catalyst. The ratio between them is 5%, meaning, 5% of catalyst to 100% of silicone. The steps were:
1 - Mix well the Catalyst (5%) and Silicone (100%);
2 - Put the mixture inside the vacuum ring, so that it pulls most of the bubbles away;
3 - Pour the mixture into your milled mould, slowly, to avoid the creation of bubbles;
4 - Put the mould with the mixture inside again in the vacuum ring, to remove remaining bubbles;
4 - Let it cure for at least one day.
The process has to be done rather quickly, otherwise the material starts to harden.
We have used gloves and protective glasses to handle the silicone.
Here are the results of my mould that turned out relatively ok, apart from 3 bubbles:
Casting the part
After the mould is ready, you can use a range of materials to cast it, I have made 3 tests, with different materials:
A) Marble Cast
It did not work out well, the piece broke when I removed it, probably due to any (or a combination) of the following reasons: the expiry date of the mixture (which is unknown since I’ve used a leftover here at the lab from god-knows-when), not enough curing time or/and incorrect mixture ratio.
B) Smooth-on Liquid Plastic - Smooth Plastic 327 EU
My second attempt didn’t turn out as expected as well, the part ended up full of bubbles (though I have put it in the Vacuum ring which I’m suspecting have created more bubbles than reduced them, due to the overall vibration on the table with the motor running) and the curing time was too long, the piece took 3 times the recommended wait of 4 hours. Here’s the technical bulletin for this product.
C) Smooth-on Liquid Plastic - TASK 2
This one was actually OK. Curing time was as expected and overall fidelity with the mould was also fine. You can find info about the product here and its technical bulletin here
When I have the time, I will finally create a mould with a different type of silicone (a heat resistant silicone rubber compound from Smooth-On: Mold Max 60) and cast a metal (probably tin) part.
For the time being, that’s it!