Resin Casting

No worries Mark

The difference between vacu-forming and resin casting is basically, that you get a solid part from resin casting, and a hollow part using vacu-forming.

Resin casting is an art, which is very easy to learn. At least if you start with a one-piece mould, which is what I use. In any case, this seems to be a sufficient technique to make most parts I need for my ship modelling.

When to use resin casting:
Resin casting is an excellent way to reproduce identical solid parts. Not only if you need many of them. Also if you only need a few, but they are difficult and time consuming to construct one by one.

The master:
You can use more or less any material when you make your master - timber, plastic, brass, clay, plaster, etc.
The only real danger here, is if you make too big undercuts, but in most cases you may just turn the master upside down to fix that.
Just remember that no copy will be better than the master, so make it as perfect as you can.

The box/container:
When your part is ready to be reproduced, you have to find or make a box that fits around it.
I have made a box from 6mm MDF, held together with small 13mm wood screws, so it is easy to put together and take apart.
The size should be as small as possible to limit the use of the mould material (only a question of money), and large enough to leave a minimum of 10mm space around the master.
The same goes for height, and of course in between masters if you want to cast multiple items.
My box is 5x5x5cm, which seems to be a nice universal size for what I am doing.

The mould:
There are numerous materials available in the market for mould making.
I use SRTV-2 silicone which is a 2-component white silicone using 5% catalyst (comes as a set) and mixed by weight 20:1.
The reason I use SRTV-2 silicone is that it is very flexible, has a pot life of 30 minutes, and doesn't need a release agent when you do the casting.
My box needs 60g of RTV-2 and 3g of catalyst, so I use a letter scale when I do the mixing.
I use a styrene cup directly on the scales for the SRTV-2 and a 5cc cup measure (bought at the chemist) for the catalyst. Then I pour the catalyst into the SRTV-2 and mix.
In the supermarket, I bought a "length" of cheap styrene cups for the mixing.
1kg has lasted me for 18 months.

Making the mould:
I attach my master(s) to the bottom of the box with either model clay or, if the master is very small, with a drop of CA glue (doesn't seem to harm the MDF surface), and then attach the sides to the box.
I am now ready to pour the silicone
So what can go wrong here? Two things:
1) You are dealing with a liquid - thick, but still a liquid - so make sure the box is as tight as possible. If in doubt, you can use model clay on the outside to seal for any leaks. And always use a sheet of baking paper underneath.
2) After you have mixed for a couple of minutes (remember to scrape the sides of the cup), you have induced a lot of air bubbles into the silicone. They can harm the result only if they attach themselves to the surface of the master, so I always use a toothpick to "paint" the surface of the master before pouring in the bulk of the silicone. Any bubbles left in the compound can now only be attached to the outside of the "paint" on the master and will do no harm.
Next day you can safely remove the mould from the box and remove the master(s).
The mould is now ready for "serial production".
One final word; A few different variations of SRTV-2 silicone is available, and I always buy the most "liquid" of them, 1) to best reproduce the details, and 2) the better to get rid of the air bubbles. Where I buy it, it is called SRTV-2-24, but I am not sure if that is an international name for it.

The Resin:
Again the choice of material is overwhelming.
I decided on using polyurethane because it is very easy to mix (1:1), doesn't smell, and doesn't shrink. The one I use is called TC-80, and I still have 30% left of the 2x500ml bottles I bought 18 months ago, producing heaps of castings during those months.
Because of the small amount I use each time, a 5cc measuring cup is the ideal choice for me to use for mixing. I then clean it with a paper towel immediately after use, before the resin sets, which makes it reusable.
The only disadvantage: Only 6 minutes pot life, so you have to be quick.

The casting:
As with the mould, the enemy here is air bubbles. Not from the mixing, but trapped when pouring the resin into the mould.
And in the same way as before, I use a toothpick (or in some cases a needle) to scrape the inside of the surface in the mould and loosen any bubbles before the resin sets.
After a couple of minutes when I am happy that everything is alright, I scrape the excess resin off the top of the mould - one other reason that I always use a sheet of baking paper underneath.
An hour later, I demould a perfect copy of the master. Only the top of the part (where the opening in the mould is) has to be sanded flat to measure.

Happy casting