Latex is perfectly suitable for armor molds. It only shrinks noticeably if you use cheap (i.e. high water, low solids) latex. I posted these in the greatest mold-making site thread.
A note on latex... When you have the patience to work with it, it's perfectly suitable for making larger i.e. armor molds. I use it for almost all my bigger pieces, helmets, predator armor, shields & set panels... On average, a well made mold takes me 2 to 3 days to make. I've done them in considerably less time, but usualy only for one-offs or emergency castings. I angle a 20" fan over my molding tables, & it dries each layer of latex in about 20-45 minutes, depending on temp & latex thickness.
An average mold is 10 to 15 layers... 2 to 4 'beauty' coats, directly on the model, then I use thickened latex for faster buildup & strength You use wood or pecan flour for this, I usualy use 2 to 4 cups per 2-quarts of latex depending on how 'stiff' I want it...
I do a layer of cheesecloth with the first thick layer, & let that dry for a good hour, then a couple more coats of thick, then a layer of burlap & thick, then 2 final finish coats of thick. Once it's done, it has to sit for a day, maby 2 depending on temp.... Leave the fan on it the entire time to draw of moisture.
Once the latex part is done, I do a mothermold or shell in either fiberglass or plaster & burlap. There is also a super-thick 2-part resin that you can brush or trowel on for a mothermold as well.
A well made latex mold will last for years of use, & was indeed the industry standard untill silicone became more readily available, & more affordable. I have cast both fiberglass & casting resin in latex molds, as well as wax, concrete, & plaster.
The main difference in casting between silicone & latex is that you HAVE to use a release agent to seal the latex when casting. For stones & plasters, I use liquid soap (dish soap), & for resins, I've used cooking spray, vasiline, WD40, silicone spray, etc... Keep in mind that any petrolium product will degrade latex, shortening it's useful life.
I mainly use Synlube release agents, they come in spraycans or jugs... I buy by the gallon & use a spray bottle to coat my molds. They have both silicone & non-silicone formulas, depending on what you're casting... I buy that locally, but ask John if he has or can get it or something similar...
I do want to add another note about latex. The very best place to order from price-wise is here-
Testworth Laboratories Inc.
401 S. Main St., PO Box 91
Columbia City, IN , 46725-2143
$95 shipping included for a 5 gallon pail. Gallon jugs are around $25 shipped.
They have many different formulas, from mold making, mask casting, to sprayable... Their's is the absolute highest quality of all I've used, & I've tried just about every brand out there over the past 20 years.
They don't have a real website ( old-school company, they've been in the latex biz since 1941), so call & ask for their current pricelist and catalog. The main 2 types I use from them are the 'slushing' for mask making, & the mold builder, wich is a thicker super-high solids lattex with additional additives for durability.
Another option is mold-making urathane, basicly the rubber version of casting resin. Personaly I hate the stuff, but it does work, & is very durable. My main problem with it is you have to soak it in release agent or it'll bond to your casting...
& sometimes it does it anyways. I usualy use about 6 coats of silicone release when I work with it. The plus side is that it's about $80 for 2 gallons. Twice the amount for the same price as a gallon of silicone.