I thought I'd make this post a little different, and it's likely to be a long one, so bear with me. I thought some might find it interesting to see how I do my part casting and what products I use. As most know, there are several parts of the Tamiya Centurion MKIII kit used in various places throughout the build. As these kits are about $200 CAD after shipping, it makes sense to make copies of the parts required rather than buying a bunch of kits. You could absolutely 3D model and print these parts, there's nothing particularly difficult about them, but the real parts have nuances, like sink and ejector pin marks, that would be pointless to 3D model. So casting duplicates will capture all that.
For my purposes I use all Smooth-On products.
OoMoo 30 2 part rubber for molds, and
Smooth Cast 300 and
310 for the casting resin. None of these products require a vacuum system to degass them, although if you do this for the rubber it couldn't hurt. The Smooth Cast 3xx series of casting resins do not have enough working time to make it doable, and they already have an extremely low viscosity, so bubbles are very rare. The 300 has a pot life of 3 minutes and a cure time of 20 minutes, which is really only suitable for single part molds that you are pouring into a cavity. For two part molds, you want to use 310. It has a pot life of 20 minutes and a cure time of 4 hours. This lets you work with 2 part molds easier without need to rush everything together.
As these parts will be "2 part" molds, I need to build up a temporary bottom. To do that, I use off the shelf kids playdoh. This is very soft modelling clay that doesn't harden, but I find it is slightly too soft, and sticks to everything, even with mold release. If someone has a better suggestion for the clay, please let me know. I use LEGO for mold walls because they provide even sides that give me uniform mold thicknesses and, more importantly, you can make molds any size you need, then reuse all the parts.
Once the temp layer is complete, and the parts placed, I add the fill and overflow runners. I usually use solder for these as I can bend it around corners and easily cut it to any length I need. A little bit of Elmer's white glue is used to create larger pour ports and seal the edges. The LEGO bits in the middle are just there for alignment purposes.
Once that's all done, the first layer of molding rubber is poured in. Smooth-On OoMoo 30 takes about 6 hours to cure, but I leave it over night.
Once the top half of the mold is completed, I take the LEGO walls off, and peel off the playdoh from the bottom. Remainder playdoh, of which there's a lot, gets scrapped off with toothpicks. The LEGO walls are put back and the runners resealed on this side with more white glue.
At this point you must use mold release. The Smooth-On casting rubber will never stick to any of the parts, but it will stick to itself. If you don't use mold release, you'll end up with a single part mold with plastic stuck in the middle. For this I use
Mann Release Ease 200. When selecting mold release, be sure to NOT use one that is silicone based. Otherwise you're just adding casting rubber in between to layers of casting rubber.
Once the second half of the mold is cured, you should have something like this;
Pull out all the parts and runners, do any minor clean up as required, and put the two halves back together. I use rubber bands and tongue depressors to hold the two halves together, but I'm sure there are better ways.
From here, mix your resin and pour into the fill holes. The other holes you added allow air to be pushed out while you fill. If you don't have these extra holes, air can't escape and you end up with cavities at the bottom of your parts. If large enough, you can pour the resin directly, but I put the casting resin into a
disposable 1ml syringe and 'inject it' into the mold. This pushes the casting resin into all the little nooks and crannies. The casting resin I use is very close to water in terms of its viscosity, so when it starts to squirt out of the air relief holes, you can stop the injection process. Now just wait for the curing to take effect. With the Smooth-On brand, the resin will change from clear to white (and become quite warm).
After the cure, you can pull the two mold halves apart.
At least in my case, with how I do things, there is some clean up of the parts required, but you still capture all the fine details of the original, and once in place and painted, there will be extremely few people that will know they're castings and not original kit parts. Also, you end up with more, completely usable, LEGO parts. So there's a bonus there.
For those that want to give this a try, Smooth-On sells a 'starter set', of the products I use on Amazon -
Smooth-On Starter Set
I'm not sure if this is useful information or not, I think most people tackling studio scale models probably already know this, but maybe it helps someone.
