Casting: Casting the Miniatures

Finally we come to the actual casting of the miniatures. Everything has arrived. Nothing left to chance. I am finally in a position to start turning out the miniatures that will mean the difference between a Polish 10th Motorised Brigade comprising anything but the equipment that they actually used and a Polish 10th Motorised Brigade that is historically accurate portraying the gear that they were tooled with. All I had to do was take the plunge!

...

Just me and my tools! (You know what they say about bad workmen and their tools right?)

All the moulds are made and I was ready to rock and roll... and I just so happened to have a day off from my job which is taking altogether far too much time from my hobbies these days. I tucked in my bottom lip and struck out.

Now, I have read a LOT about casting and as such I felt that I was pretty well informed. I thought I would know how to overcome the problems before they occurred and bought into my own bullshit that I would be able to produce work of an outstanding quality

How wrong I was!.. but more on that later.

First thing that I decided to do was to lay out my work-space in such a way that I would have everything to hand quickly. 

When you are mixing polyurethane resin, the pot life (i.e how long it is usable in its mixing vessel) is only a couple of minutes long so you need to be quick. By a couple of minutes I mean that 2-3 minutes is considered standard. I opted to have a Long Part B which would give me a pot life of between 5-7 minutes. The is very misleading however. The pot life includes the time that its pourable consistency is akin to thick treacle which for what we are doing would prove useless.

The beginning consistency of the Polyurethane Resin that I've chosen (TOMPS Fast Cast Polyurethane Resin) is more like milk which is perfect for miniature casting because you can start by slush casting the inside of your mould, thereby ensuring that the resin touches and sticks to all of the undercuts and fine detailing on your miniature.

If the resin thickens too much this will become impossible and you will be left with gaping holes in your miniature when you extract it from your mould.

The first thing that I did was place my disposable cup onto my electronic scales and turn the scales on, which gives a beginning reading of 0g

The set up of the casting workspace. Moulds to the left, resin at top and mixing pot and scales in the middle... complete with disposable chopstick stirring rod.

Its a well known fact that Polyurethane Resin is sensitive to its mixing ratios which if you get it wrong can lead to vehicles that either never cure or sweat once cured. Neither of which is a result that you want. When I received my Polyurethane Resin Part A and Part B I was quick to notice that there seemed to be a lot more of Part B than there was of Part A and this is because the instructions for use are very specific about using a 1:1 ratio of Part A to Part B by weight... which is why I have electronic scales in the picture. 

34g of Part B in the disposable mixing container.

When I first started I decided on mixing up a big batch that I would use for all of my moulds and duly when about measuring a large amount (about 70g worth) of Polyurethane Resin by adding Part A to Part B which was already in the cup. I proceeded to give it a thorough stir and it was indeed the consistency of milk which I was happy about. Then it went into the degasser to degas it.

Pointless exercise! The milky consistency means that air bubbles are not generally caught in the cup so this was effectively wasted time.

However after about 30 seconds in the degasser I whipped it out and started to pour it into my moulds quickly. Once full the moulds were put back into the degasser so that it could do its work on the rapidly thickening resin.

A very bad picture of the moulds being degassed. Lots of net curtain reflections :D

This was worth the time! There were bags of air bubbles being pulled out of the mould and after a couple of minutes I pulled out the moulds from the Vacuum Chamber and left them to cure on the desk.

The first moulds sitting on the desk waiting for the resin to cure.

The demould time for this resin is supposed to be 30 minutes. Bear in mind however that the smaller and thinner the piece to be removed from the mould the longer that the curing process will take. This is something that I had failed to take into consideration... to my cost!

After a half hour or so had elapsed I started to remove the castings from their moulds. The largest of the moulded vehicles were solid enough, although if pressed and twisted hard enough would deform. This is called being in the Green Stage which is where a casting is solid but still retains some slight flexibility. With vehicles that have intricate parts such as fenders and like this flexibility post demoulding becomes extremely valuable as you are able to move things around slightly.

The smaller vehicles such as the Polski Fiat 508's were still far too soft to be removed and so had to be reput back into shape and left. As it happens, with the correct mixing of the Polyurethane Resin these smaller vehicles take about an hour to cure hard enough to remove from the moulds without being destroyed. At about an hour the vehicles are still in their Green Stage so retain some limited flexibility.

The first vehicles being released from their moulds

Now, it has to be said that on removing the vehicles from their moulds I was mighty pleased with what I had achieved, and that pleasure over rode all of the flaws which jump out at you. It turns out that despite having a de-gasser there are things that I must have not done effectively enough because as you can see from examining the C7P and the Polski Fiat 508 Furgon there are small cavities all over the miniature meaning that there were air bubbles trapped all over the place

My first C7P  still on its resin funnel which needs to be cut away, and showing all of the cavities.

The first C7P had its flaws. There were plenty of cavities all over it BUT the issues were small enough and localised enough (predominantly in the area of the tracks) that I am rather of the belief that most of them can be fixed with putty filler and failing that when painting I can weather the vehicle and plaster it with MIG's mud to disguise the casting errors. 

The first Polski Fiat 508 Furgon showing the fender cavities

Another view of the Furgon from above showing the fender cavities

I've spent the whole day casting now and I've had plenty of opportunity to try different things to see what works and what doesn't.

I have to say that I feel that I have it nailed now. The vehicles no longer have any air bubble cavities, cure well and look good.

I decided soon after my first attempt that I would need to mix up smaller amounts of Polyurethane Resin and pour just into one or two moulds at a time. This meant that I would be able to make use of the milky consistency of the resin to get into all of the mould details and wouldn't have to wrestle with a rapidly thickening sludge!

I no longer pour the mould full straight away. There are parts of each mould where air collects and in order to get rid of these air bubbles I only initially put a small amount of resin in and massage the resin into the mould extremities by twisting and turning, pushing and pulling the mould. This isn't fool proof but it does make a massive difference. This is especially useful when doing stuff like turrets and you need to get the resin down the gun barrels.

Once the resin has invaded all nooks and crannies I will then pour the mould full and place into the vacuum chamber and switch the pump on. I make sure that I leave it a good while now on. I don't mind the resin curing inside the chamber for as long as its in there unless there are a mass of air bubbles that are pulling the resin over the lip of the mould. In this instance I keep thumbing the air tap on the vacuum pump on and off to tease the air out.

Again this isn't fool proof but the vacuum pump does 90% of the work, with me providing a bit of manual vibration giving the last 10%!

...

And this ladies is the quality of what I am getting now:

A brand spanking new Vickers E tank. No defects, no air bubbles... just lots of Polish goodness! 

This is a casting of a Vickers E that I have done towards the end of my current learning curve. There are no air bubbles on the casting and the detailing is accurate and high, Now I just need to do another 9 of these!

I'll do an update post when I have cast all of the resin vehicles for the Black Brigade so that you can see whats going on OK?

Whats that?

Fix BAYONETS!!!!!

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!!!!