|The Vacuum Pump|
The Pump that I bought must have been an end of line piece of gear as they don't manufacture it any more and it is absolutely impossible to find instructions on how to use it online. All I have is a German language instruction booklet that was produced in China and was delivered with it!
|Any excuse to get some exposure! She made me do it!!! :D|
... and that's where everything became touch and go!
So having finally received my two giant syringes (which I believe in a former life may have been used to impregnate Diplodocus' in Jurassic Park!) I allocated one syringe for the Catalyst and one for the base of the Silicon. So long as they are only used on what they are labelled for the liquid will never cure and so the syringes will be reusable.
Now this silicon is supposed to be extremely fluid, which is the main reason that I bought it BUT when I drew it up into the syringes (same for both parts) it was as liquid as Golden Treacle! That's thicker than Maple Syrup to all of you Yankees and Cannucks out there ;) This worried me a bit as with all of the undercuts on these models I thought that this may present a serious problem as I attempted to make these moulds.
I drew up 80ml of each of the two liquids and deposited them into a mixing pot. This Silicon is addition cured which means, amongst other things, that you can measure it by either weight or volume so long as its a 1:1 mix. Happy days! I gave it a good stir in its mixing pot and then put it into my vacuum chamber to be degassed. Thankfully as I stirred it around it also became more fluid as well so it may not come with the casting issues that I was stressing about.
Now, this is the first time I have ever used a vacuum pump in anger so the results were perhaps predictable. I sealed the chamber and switched on the pump...then sat there watching the pretty air bubbles rising to the surface of the silicon. I bled the air out periodically and then when I decided I had done enough degassing I did a final opening of the air valve... unfortunately I opened it immediately and completely not knowing that it would also bleed air into the cylinder straight down into the silicon... which exploded all over the vacuum cylinder! Doh!!!
|Silicon Explosion after catastrophic re-gas!|
Thankfully I had made enough to still complete my task so I took it out and poured it into the mould wall containing the prototype. Still pretty viscous but I angled the mould and poured slowly into the bottom corner allowing the silicon level to rise under the undercuts of the prototype levelling the mould to the horizontal only when the silicon was in danger of spilling over the top.
|Mould after the initial pour and before degassing.|
This time instead of degassing and re-gassing cyclically I decided to leave the pump on for a good thirty seconds or so until all of the air bubbles had been drawn to the surface only bleeding air into the chamber slowly and only when it looked like the silicon would pour over the surface of the bricks.
|Degassing the mould. You can see the remains of the silicon explosion to the right.|
I also realised that I probably had enough silicon left in the pot to do another small mould and so I knocked up a mould for my Polski Fiat 508 Lazik Staff Car as well, and went through the same process with one exception. I stopped feathering the gas tap and just let the chamber do its work for longer periods. The results were much MUCH better!
|Both moulds being left to cure after degassing.|
|The state of the moulds after an hour.|
On the silicon tins it says that the de-mould time is 2.5 - 3 hours although I will probably play it safe and just leave the moulds until tomorrow before I remove them.
Well after three hours the silicon was nowhere near cured, in fact it was only just reaching the 'tacky' stage. This did cause me some concern as I left for work wondering whether or not the moulds would cure at all.
Flash forwards to 6am and I arrive back home to find some beautifully cured moulds with absolutely no air bubbles inside the silicon at all.
|The two finished and cured moulds, labelled, with the prototypes next to them|
|A look at the detail inside of the mould|
I'll do another blog on the actual casting of the products soon but for now...
Lessons and Observations
i) When using the vacuum pump don't keep removing and adding gas into the chamber. Switch it on and leave it on to allow it to do its work, only adding gas back into the chamber IF the silicon looks like it will flood over the top of the mould retaining wall.
ii) The Lego retaining walls should be twice the height of the pour level of the silicon if degassing is to be attempted to prevent the silicon flooding over the top of the walls.
iii) Even though the retaining walls are glued to the base and the bricks fit tightly against each other the vacuum chamber will pull silicon through the joints between bricks and base. It isn't a problem as its such a small amount but it does still happen. This is one of the reasons why you still need your border around the wall.
Now, when all is said and done... FIX BAYONETS!!!!