Tuesday, 27 February 2007

Resin Casting Tutorial: Part I

(versión en Español)


Well... Yesterday you saw a custom resin tank. But, the original model has to come from somewhere, isn't it?

The model used to make the silicone molds is called a Master Model. I did it using scratchbuilding techniques (that will be discussed here later on), divided in some basic parts and cast them separately.

Now I going to show you how to cast one of those parts, in particullar the hull piece of my KV-II like futuristic tank.

First, we must be completely sure that the piece we are going to cast is completely airtight. That mean that you must close all little gaps or cracks before starting. Once everything is ready we'll start making a mold from our original piece.

I am going to do a two-part mold, so I start making some masking with plasticine (modelling clay). I use rubbing alcohol (isopropilic alcohol) and a soft brush to clean and seal the joint betwen the model and the plasticine.

I put the plasticine covering the lower hull and making a gentle slope (you'll see why later). The mold line will be at the same level of the plasticine, so be careful when where you put it.

also add four small stumps using more plastecine, this will serve to make the two halves fit once the molds is ready.

Then I build a box around the model and seal it with more plasticine (those are casting blocks that I got from www.hlj.com, but you could use Lego blocks, plasticard, wood, etc...).

This is the Room Temperature Vulcanizing (RTV) that I use. I get it hera, at Spain, from www.feroca.com, but you can buy similar products locally. RTV means that the silicone cures without need of heat (wich would be fatal for our litte plastic model). The small flask is the catalizer, you add it at 5% of the silicone weight (that means, you add 5 g of catalizer for each 95 g of silicone). I recommend you to have a spare bottle of catalizer around, as you normally use more than you really needed.

You can also see that there are some silicone chunks lying around. those are old molds that I am going to recycle. I will use also a handy disposable brush (those little thingies are very useful, come in great quantities and are dead cheap. What can I say?)

I weight the silicone and mix the catalizer.

Now I do some evil things to the brush. I bend the point and cut the bristles. This is to fix the bristles in place and make it easier to access to difficult places.

Then I add a little silicone to the mold and reserve the rest.

I use the brush to force the silicone into difficult places and remove superficial bubbles.

It's a bit like painting...

Now I start recycling all those old silicone chunks.

I simply push them carefully in the mold, to be later covered with more silicone. Once the silicone vulcanizes it will become a solid piece... So, just don't throw away old molds. Recicle them!

You can see all the bubbles raising to the top. Now, it's time to put it in an even surface and let it cure during 24 hours.

As this is real-time, we'll have to wait until tomorrow (or later, as I simply run out of silicone) to continue our little tutorial. Next day we'll learn some things about demolding agents and how to produce the second half.

See you...



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