I'm glad to dedicate my very first instructable to my very first casting experience and to share it with you.
To get started, let me quickly introduce the subject.
The part we are talking about was originally made of zamak and used to pimch a rubber tube inside an iron tube, so that they can not move each. Both of the tubes form the gasoline inlet of an old car tank.
The problem is that this specific part is known for its weakness and often collapse over the years.
I have recently been asked to reproduce some pieces of this part. Obviously, I could have machined it with standard lathe and milling machine but the inside shape was somehow challenging.
Then I decided to try my hand at casting it in zamak in silicone mold with my humble knowledge and noexistant expérience on casting.
For the materials, I used no exothic products. I found high temperature silicone in a composit shop, and zamak in an e-shop that retails raw material for jewellery industry.
Special thanks goes to my friend from english space (please go and like their page) that helped me revising this instructable.
I would also like to thank my divers sources of inspiration (they will recognize themselves)
Step 1: 3D Model of the Model
First of all, I had to draw the part according to the one I have been confided.
The main difficulty came from that part was broken and glued again so that it looks like what it should. But that made it tricky to measure it precisely. Then I did some averages and drew it so that I was sure that was at least enough of material to machine important surfaces after casting (a way for saving my ass, avoiding to do all the work again in case of a mistake)
Given I was considering to 3D print the model I'll use for the mold, I decided to draw the blowing hole and casting channels at the same time.this would definitly cost me some more printable resin but also make the mold execution straighter .
At the end, I finally added the removable part I'll use as a core for the inside shape.
Step 2: Preparing the Mold Container
I started to cut 4 pieces of wood that I assembled together to form a frame.
Then, I added another plate to close one side of the frame.
Finally I adjusted these 2 little black pieces of plastic in order to save some silicone, lowering the volume I'll have to fill in.
When the container was ready, I placed the 3Dprinted model in it. I simply used double side adhesive tape to avoid it moving in the container.
Step 3: 1st Molding Phase
At this stage, everything was ready for the first molding phase. Then, I carefully mixed the silicone and its catalyser, and filled it out in the container untill the level reaches the middle of the casting channel (which is also the symmetry plane of the model).
I must confess that I didn't have preoare enough silicone... This is the unofficial reason why I added this green piece and plastic.
The official one, being I wanted an additional centering shape for the two main parts of the mold. (-;
Step 4: 2nd Molding Phase
Once the first part was polimerised, i had to shorten the green magic sheet of plastic. I did this on the lathe to get the surface as smooth as possible. This allowed me to confirm silicone was not adhering on plastic.
Then, I placed it again in its hole.
Before processing the second phase, I applied a thin and regular layer of talc on it for the second part not to adhere to the first one.
Once again, I mixed some silicone and catalyser and filled the container out with it. Once again, I didn't prepared engouth silicone,so I quickly prepared a piece of wood to "eat" some volume.
Step 5: 3rd Molding Phase
For the next step, I didn't want to separate thefirts two parts of the mold. However, I had to take the inner part of the model out of the mold so that I can make the last silicone part. Then, I had no choice and finally separate the first two parts to make it easier to remove the inner model core.
Then, I prepared a pvc tube and another piece of green plastic because I was really running out of silicone and I really needed to save it.
One more time (and hopefully last) time, I prepared some silicone and filled out the cavity after applying some vaseline oil.
There is no real reason why I used once talc and once vaseline oil. I only wanted to see if both of them make the separation process easier. And they actually did!
Step 6: Casting
Serious things start here! Exciting and scary at the same time. Be really careful if you try some similar experiments... So far, we were using liquid silicone: this can blemish your cloth and is maybe not very good for health but melted metal is way more dangerous...
My CAD software helped me to determine which amount of material I'll need to melt. From this information, I calculated the ingot length I had to cut.
Then, I tried to prepare the workbench so that everything was in proper place. I disposed the mold, the camping stove, and the melting pot where I placed some metal cutting wastes to start melting operation more quickly. I also placed a temperature sensor (a K type thermocouple connected to its display unit) to checkhow long I'll have to wait before I pour the melt metal in the mold. I knew the melting temperature was about 390°C then I waited until it reached 430°C just to be sure it will have enough time to flow in every single nook.
Step 7: Opening the Mold
Well, this is when we see if we worked properly at previous steps.
Every single default, mistake or neglected detail will come back to you on the final part.
From my side, I was so pleased with the result I obtained. My observations are as follow, begining with worth one:
- the final part was not as round as expected.this is due to silicone elasticity. Hovever, this was not a real problem because I anticipated this problem by increasing outer diameters when I drew the model.
- surface quality was really nice except some little holes from here to here. Maybe due to the fact my melting pot was not perfectly clean and I neglected to clean it better because I was too eager to see the result.
- details reproduction were amazingly precise. On the caste part, we can still observe the little faces produces during 3D printing even despite the 3 laps of black adhesive tape added on the outer diameter to increase it.
As a conclusion, I'm so satisfied of this first experience. In hindsight, I think I could have included the threads directly on the model to avoid another machining operation after casting. I'll keep this in mind for next time!