Monday, 27 March 2017

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?


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