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Casting the Karl shell and wasting a lot of rubber and resin
My first idea was to cast the shell in multiple parts, and to use Vinamold for the molds. Vinamold is a polyvinyl plastic that have a great advantage to be remelted again and again. As it is hotter than boiling water when poured, of course your master must resist the heat. It was perfect for my little aluminium shell.
But the results where poor. First, the Vinamold texture is like a soft rubber. But it don't resist very well and tear easily on small details. So the little plug formed by the holes around the base where tear out, both when removing the master, and also when removing the casts.
Another problem seem to be an chemical incompatibility with the resin. I only tried once because the fact the details where tear out implied I needed to think about some other way, but the inner surface of the molds looked funny, and so was the cast part.
Okay, I may have strange ideas, but I'm not stubborn. I bought a more "classical" rubber.
The first I could find was the MBS RTV. A very soft rubber, but extremely thick. I was incredibly difficult to pour. The result was some molds with missing details, and uneven surfaces.
Some friends advised me that this rubber was too thick, and that Alumilite rubber was much liquid. They where right, liquid it is that's for sure, it is a joy to pour. But also much harder, and a lower resistance to tear. A small cut will tear out very easily. The result was again some little plugs tear out, and a very hard time trying to unmold the pieces. The molds where partially destroyed as the opening I left couldn't open enough.
At this time, I almost throw away all the stuff, rubber, resin and returned to my dollhouse miniatures (here and here). But I tried again, thinking that maybe the way I wanted it was not adequate. After I asked on missing-lynx Axis WWII AFV forum, everybody answered they wanted a single part cast to avoid alignment problem. Well ok, alignment was not an issue in the way I imagined my previous cast, but a single part made with a two parts mold should worth a try, as it seem to be the more obvious way. After learning a few techniques grabbed here and there on the web, I made a 2 parts mold using the Alumilite rubber, that seem very fine for this purpose. I made a box out of Lego bricks (30 years without playing with this stuff, and I still remember... there's things you can't forget...) and pour the rubber, using Klean Klay to support the master.
I didn't like the process. Lots of cleaning involved, my hands where red from the clay, the Lego bricks are a mess, the clay stick everywhere. Still, I was able to finish the mold, and tried it.
As you can see, a few opening where needed to avoid bubble trap. Still, there is a few bubble here and there. I manage to avoid the seal to pass through the details in the base, this cast need a careful sanding work to be nice. It surely can be done, but in my view it's just not good enough, too much sanding work involved on rounded parts.
So I tried again ! Returning to my primary idea, I used again the MBS RTV rubber, this time carefully applying some rubber it the details with a little stick before pouring the rubber around. After the limited past experience I had pouring resin (I use Alumilite resin BTW), I figured I would never had time to pour at once the 3 parts, plus inserting a bolt in each opening. Yes, a bolt. I discovered this bolt trick by myself, but surely it is a well known technique. The bolt help me to remove the cast part. Instead of deforming the mold to be able to grip the cast part, I do it the opposite : I grip the bolt dipped in the cast part in a vise, and remove the mold around the cast while stretching and expanding it a little. This is why I *love* soft rubber ! No way I could do that with Alumilite. After that, I simply unscrew the bolt from the part :-)
And it was machining time again. I built a little box with a few L-shape aluminium lengths, that could easily disassembled, grip the 3 surrounding box for the molds, and be able to dip 3 bolts in a blink. So, here it is, I proudly present the Nuts'n'Bits molding machine :
And it work. This weirdo thingy is used both to pour the rubber around the master to make the RTV molds (left), and also to pour the resin inside the molds (middle) with a dispositive that can put 3 bolts in the uncure resin in a blink (right) before it harden. Believe me, I don't have 3 seconds left to complete the whole operation !
I now have enough time to pour resin inside the 3 molds in a single step, and then insert the bolts on the top. It disassemble manually very easily, and as it is all made from metal I can heat the molds. Yes, I heat the molds. Each time I didn't do it, I had some funny looking surface on the resin, like it was not cured evenly. When the mold is hot, the resin is always perfectly and evenly colored. As it is said in the Alumilite resin documentation that it is better to do it, I guess there's some truth in that.
Here's the result:
The two ends just plug inside the long middle part that is hollow. As you can see, a few bubbles are still there sometimes. But they can be easily filled with putty. Anyway when a bubble is present on a detail, I just trash the bad cast.
It a very soft rubber, and in the meantime extremely tear resistant. I was able to cast from a single base mold (photograph below, smallest mold) 27 perfect casts. After that, one of the little plugs to make the holes (diameter and length 3/64") partially tear out. The other molds with less undercut are capable I'm sure of many more casts.
For comparison, I was not able to do more than 10 casts with the MBS rubber for the same work, and the original plugs where not as well defined, as it is a thicker rubber more difficult to pour.