12. molding and casting¶
At the begining, we have done the group assignement here. I help with the resin part, mold comparison and material comparison.
Hero shot¶
1. mould design¶
In my project, I have a very angular part, and it’s very difficult to make except in 3D printing, but at a high cost. This part needs to be liquid-tight, and needs to contain a sponge that will dissipate a flow of air into the water. I used FreeCad to make this 3D model:
This part is actually in two parts, and will be fixed with screws and a thoric joint. As this method takes a lot of time, I’m only going to do the small part for now, and I’ll do the rest later. So the part looks like this:
And after inverting the part, the mould looks like this:
As you can see, I’ve got a major problem: I need to make a tube. But with the angles, I couldn’t cut the part any other way, so my parts are interlocked. This is going to cause me problems, because the finer the drill bits, the shorter they are.
2. Machining¶
During this phase, I use the same CNC as before, as well as a 6mm drill and a 3mm drill. I also use a blue machinable wax (parafine). But first, I use wood, to make sure my files are correct. The reason I use wood is that it’s much cheaper than wax.
For the cuts, I did them in 2 phases: the thinning phase with a 6mm cutter and a precision phase with a 3mm cutter. The reason I do it this way is that the 6mm cutter is much faster than the 3mm cutter, and the result is the same. So I only use the 3mm cutter where the 6mm cutter can’t reach. For the first block, I use a 0.2mm pitch, which gives me this result:
It’s clear, I’ve got the right shape but the cone lacks precision, I can see the lines. During cutting, I can also see a thin layer of uncut wax. This may be due to temperature and material expansion, and it’s very easy to remove.
For the second cut, I decided to use a 0.1mm pitch.
The result is much better, I can’t see the steps any more.
3. The mould¶
To make the mould, I chose a silicone material the Smooth-Sil™ 940. My plan was to make an easily removable mould to make an epoxy part. I chose epoxy because it can be transparent, is water resistant and has good mechanical properties. To make the silicon, I have to prepare a mixture in a ratio of 10:1 by weight with the silicone and a reagent.
To find out how much silicone I need, I first fill my mould with water and pour the water back into a plastic glass. I then made a mark. Then I filled the glass to the mark and mixed the two phases. This phase was a bit tricky because of the sticky nature of the paste. I couldn’t do it without getting a lot of air inside. To eliminate this air, I have to put my dough in a pressurised box, so that the air is forced out of the dough.
But I got so much air that it overflowed and I almost didn’t have enough batter to make the whole silicone mould. Next time, I’ll try to remove the air directly from the glass first, then put the contents on the mould, and put the whole thing back in the chamber. After one night, the result is really good, with almost no visible bubbles.
And the result with the more precise mould:
4. The parts¶
To use clear 1070 epoxy, I first read the MSDS file. To determine the volume I need, I use the same technique as above. Then I do the 10:4 ratio by weight. This must be done outdoors or in a well-ventilated area, and nitrile gloves and goggles must be worn.
Once again, I can’t make a clear paste, but this time it’s much more difficult to eliminate the bubbles because the viscosity is higher. It gave me this result:
4. The files¶
Here you can find the FreeCad file