Step 8: Parts That Don't Lend Themselves to Vacuum-Forming
When this happens, it's time to invest in some silicone RTV moldmaking rubber and learn a whole new set of skills. I'll refer you once again to Thurston James' excellent opus: the Prop Builder's Molding and Casting Handbook. I own three copies of it because they keep getting abused and the pages are all stuck together.* If you're going to tackle a project like this, I'd recommend getting at least one.
For this particular project, I used methods detailed in that book and made one-sided silicone rubber block molds for pieces like the charging handles on the bolters, and the teeth for the chainsword. I made two-part molds for the pistol grip and some of the other small widgets. But the most interesting molds were the rotocasting molds for the helmets and the ball vents on the backpack.
The process of making a block mold is pretty simple. You build a watertight box big enough to fit your original into with at least 1/2" clearance all around. Then you pour silicone in to fill it up and cover the original. When the silicone cures, you remove the original and it leaves a hole with the same shape behind. Now you fill that hole with a 2-part urethane resin which will cure and become an exact copy of the original piece.
For larger pieces, this starts to get expensive. Instead, you can create a "glove mold" by brushing the liquid silicone onto the original, then adding layers until it's thick enough to hold up to being pulled and pried at. Once that sets up, you create a rigid "mother mold" to help the flexible rubber retain its shape when the original is removed.
After the mold is made, you can make hollow "rotocast" or "slush cast" copies by pouring a bit of mixed resin into the mold, then rolling the mold around so the resin coats the interior. When the resin cures, it forms a hard shell in the same shape as the original. By adding more and more coats to the inside, you can make the shell as thick/strong/heavy as you like.
There are countless tutorials available online that will tell you everything you need to make silicone molds. I've read dozens of them and even written a few. Here's one of them in my blog: http://protagonist4hire.blogspot.com/2011/07/star-wars-republic-commando-helmet-part_22.html If that's not your speed, here's another one: http://protagonist4hire.blogspot.com/2011/02/warhammer-40000-space-marine-chaplain.html
My favorite video detailing the rotocasting or slush casting process is this one from Volpin Props: