Miniature Ordnance Review looks at the world of historical and fantasy miniatures wargaming and model building. From 15mm Flames of War, to Warhammer 40K, to 1/35th scale tanks, with some potential surprises on the horizon - you'll find them here!

Wednesday, January 14, 2015

Castaway - Progress on the Polish Vickers Tank Front!

Cast resin models and miniatures have become common because they're fairly easy to do with a minimum of space and capital outlay. At the very least you'll need a master, RTV, and a casting resin. From the master you'll make a mold out of RTV - room temperature vulcanizing rubber. Most RTV's are silicones that use a tin or platinum based catalyst for the vulcanization process. The RTV is poured over the master in some sort of mold box. Once the rubber mold has cured the master is removed and a negative of the master is now captured in the rubber mold. Then a casting resin, typically a two part system, is poured into the mold, allowed to harden and multiple duplicates can be made from a single mold.

For the Polish Vickers tank project, I've been having difficulty with mold stability - most of my early molds began to break down almost immediately or the RTV failed to cure properly. I've recently picked up some new resin and RTV from MPK Enterprises, and so far its working well, though I've had to modify my technique a bit. In order get all of the air bubbles out of both the mold and the resin, I use a vacuum chamber (shown below).  


The RTV I chose was a medium strength rubber (the purple mold in the photo below), and it is far less flexible once cured than the previous RTV I've used (the pink molds). Both the original mold type and the new type should take roughly 24 hours to completely cure. Once the molds are complete, you can start casting with resin. Most resins cure completely within a few minutes, but I purposely chose a resin this time with a longer pot life (10 minute initial gel time, though full hardness takes much longer). 


Once the resin is put into the molds (the molds above are sitting on the base plate of the vacuum system, the brick colored ring is a gasket), the rest of the vacuum chamber is assembled with a top plate and gasket. The photos below show the molds ready to have the vacuum applied.


Once you apply vacuum to resin or RTV, tiny bubbles suddenly grow and the mixture will have a tendency to foam and expand. When designing your molds and mold boxes a good rule of thumb is to allow 2X the rubber or resin volume for expansion. I've found this new silicone does expand the expected amount, but the resin expansion is only about 25% (shown below)


Because of the catalytic process, most hobbyist resins will continue to foam for some time. For short pot-life resins, that means the resin will harden while it is still foaming if left under vacuum, which will ruin the cast. With the new, longer pot life resin, I'm able to pull a nearly full vacuum, let it sit at low pressure for 30 seconds or so, and then slowly bleed air back into the system. The majority of trapped gases should be out of the body of the cast resulting in good quality replicas. As you can see from the cast below, using vacuum techniques it is possible to get even very small features to cast well (the mold includes the very thin Vickers dual turret variant hull top plate, a couple of wheels, and some Mauser rifles for another part of the project).


As I indicated earlier, the new resin and RTV seems to be casting quite well overall. So well that I've managed to get the first of my dual turret Vickers tanks together. 



I've gone ahead and cast a particularly nice example of the 7TP dual turrets to save myself the trouble of cleaning up the metal ones with their inconvenient mold line. The new hull casts are coming out well. Final assembly of the dual turret version does take a fair amount of "fitting" in the end, though!


As you can see from the photo above, the end result looks pretty nice!


Finally, there had been some questions around the relative scale of the Battlefront Polish figures versus other lines, and how they stack up to the True North and Forged in Battle figures. I grabbed a stick of "in progress" Gebirgsj├Ąger and based on their height (and the fact that they're a little foreshortened because of the camera angle), it looks like the Battlefront and True North figures generally match up well. The Forged in Battle look a little shorter, but not enough to make a difference once everything is based. 

2 comments:

  1. Those casts look very crisp. The detail you added looks great. Thanks for posting up the tutorial.
    -Narcissus from WWPD

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    1. Thanks! I think the project is finally starting to come together - if my dang halftracks would just come in so I can get that part going! :D

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