Thursday, 23 March 2017

Casting: Making the mould

Well I promised you guys that I would walk you through my learning process as I attempt casting models to a level of quality that I would be proud to sell to my fellow modellers and gamers.

Nothing professional here, but more a desire to have decent looking models as opposed to whats out there and available for the Black Brigade. 

So we have already had a look at the gear list that I have had to put together in order to be able to start knocking out all of this stuff that I need. Its now time to start using it.

First stop was the vacuum pump, and I have to say that this was a real ball ache.

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!

Thankfully my significant other is fluent in German

Any excuse to get some exposure! She made me do it!!! :D
However, the diagrams do not point out vital elements on the schematics and the German descriptions turn out to have a generous spattering of Polish in them as well.

Poor show!

After wrestling with it for an hour or so I FINALLY worked out where to put the oil into... and had to spend an hour and a half pouring oil into it! I ended up resorting to a micro funnel with a cocktail stick stuck down the middle of it to get the oil into the pump without air bubbles causing constant spillage! 

WHAT A BITCH!!!! No wonder they don't make it any longer! Incidentally all of the other ones that you see now online all have Oil Intake Ports on the top of the pump so my problem will no longer be anybody elses!

After testing the pump to make sure it did what it says on the tin (so to speak) the next step was to prepare the prototype for making a mould around it.

I've chosen to do the soft back for the Polski Fiat 621 truck. The reason for this choice is that its a simple exercise in learning a process. Very little undercutting and not much surface detail that isn't bass relief. Should be something easy to pull off... and as I need so many of them anyway I thought it would be a good place to start.

This process is called Drop Moulding and is probably the most simple of Moulding processes. Simply put you just drop liquid Silicon over the prototype in order to create the mould. 

The very first thing that needs to be done is to measure up a base board and trial fit a Lego retaining wall to the board which will create the receptacle for pouring the silicon into which will actually create the mould.

Once the board is measured up cut it with a healthy border surrounding the Lego.  Its important that the mould base board can fit inside the Vacuum cylinder otherwise you wont be able to degas the mould.

Once everything is measured up the first thing to do is to glue the Lego bricks to the base board so that there is no chance of any of the Silicon bleeding through under them and ruining the mould. To make it easier for the Glue Gun to reach the bottom of the bricks only use two layers of bricks initially.

The next step is to but a Blue Tack 'funnel' onto the bottom of the piece to be moulded. This Blue Tack fills three functions. Firstly, it creates a tight seal of the item to its baseboard so the mould integrity is not compromised by creating flashes and invasions of silicon where you do not want them to be. Secondly this Blue Tack actually creates the 'funnel' through which you will ultimately pour your resin when you are casting new copies. Thirdly this 'funnel' creates a receptacle for extra polyurethane resin which, when you degas each new casting, will provide enough extra resin to ensure a perfect casting when the elimination of the gas bubbles within the casting depletes the extra resin as it takes the place of the bubbles. 

Depending on the viscosity of the resin this may be a negligible amount BUT personally I would rather have more than not enough to cover this possibility.

The next thing to do is to press this Blue Tack funnel to the Moulding Base Board to ensure that there is a tight seal all the way around to prevent silicon leakage.

The final step in the preparation of the mould is to build up the Lego retaining wall to a level that is higher than the piece to be moulded ensuring that it is high enough that the mould will have a solid and sturdy base once the silicon is cured.

...and there you have it. The Mould is now ready for its silicon to be poured in.

... 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.
Once the mould was filled I could see that there were an incredible amount of air bubbles still in the silicon so I decided to put the whole mould into the vacuum chamber.

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.
When I finished degassing I took the mould out and noticed that the majority of the air bubbles (of which there were millions!) that were left were laying on and just under the surface of the mould and so wouldn't affect the integrity of the mould detailing so I was happy about this at least.

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.
Something else that became obvious after I sat there watching the moulds cure was that despite the silicon being extremely viscous the air bubbles that are initially trapped in it will still slowly rise to the surface and burst. I wouldn't want to bet my mould detail integrity on this though because any excuse at all to trap an air bubble will be taken I think BUT it is a help.

The state of the moulds after an hour.
Looking at the photo above we can see that whilst the initial mould still has plenty of gas bubbles the smaller of two moulds, the second mould has almost none, and those bubbles that are there are all tiny and near the surface.

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
The prototypes were surprisingly easy to release from the mould. Looking inside the mould I couldn't find any evidence of air bubbles spoiling any of the detail either so I can say at this point that I would consider this to be a successful first venture into the heady world of casting miniatures.

A look at the detail inside of the mould
So, the next thing to do would be the actual casting of these miniatures BUT instead of that I'm going to spend a couple of days making all of the moulds that I will need for all of the vehicles that I'm going to be casting up so that if I am over ambitious with the amount of resin I'm mixing up I will have extra moulds in which I can pour it so that it doesn't go to waste.

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!!!!


  1. Awesome bro and well done.... lots of hard work but looks like its paid off .... looking forward to the next installment .... ps cant stop singing "im forever blowing bubbles" for some reason lol

    1. Cheers Johnnie. Who would have thought it eh? Hard work in the United Kingdom can actually pay off! I'm just gonna run off and blow some bubbles to myself :D