Most of the kits are constructed of resin; it's a two-part, room-temperature-curing urethane. Molds are made of room-temperature-curing silicone RTV poured over a master; since neither resin nor molds require tooling or an oven, the process lends itself to small-scale garage manufacturers such as yours truly.
Balancing resin's advantage of easy manufacture are some disadvantages the buyer should be aware of: it's expensive, it's not especially strong, it's heavy, and it's not always easy to work with.
The expense is a result of both the materials cost and the small-scale nature of the model maker. Production runs are small, and a set of molds will deteriorate over the course of about 50 pulls anyway.
Resin's strength/weight ratio makes it a poor choice for hollow parts such as fairings, interstages, and the like, as you may be used to with styrene kits from major manufacturers. I'm experimenting in this area, and have a number of kits with detatchable parts...but nothing that approaches, say, the Airfix Saturn V in its ability to be assembled, disassembled, etc.
Resin is easy to sand...sometimes too easy! Be careful not to sand too much.
Also, resin doesn't work with my favorite adhesive, Plastic-Weld. Cyanoacrylate (super glue) or epoxy are the adhesives of choice. My one experience with "for resin only" adhesives didn't impress me; your mileage may vary.
When you get a resin kit home, take inventory of your parts. Compare them with the kit instructions, and familiarize yourself with both. Now would be a good time to trim flash, etc., then wash all resin parts in soap and lukewarm water, as this will wash off any Â“weepingÂ” that may have come off the resin during shipping. Serious cases of "weeping" means that yours truly failed to properly mix the resin, and such parts should never have been shipped to you. But SOME separation at the mold surface is almost inevitable. Paint won't bond well to these areas, so wash them off!
Many parts now use micro bubbles to make them lighter. This does nothing for strength/weight, though, and micro-filled resin has more trouble getting into the crevices of the molds, so it can't be used on parts with fine details.
A very few castings have also used a balsa dowel core to lighten them. This worked quite well, but now that I've left Japan I haven't yet found a source of balsa dowels in useful sizes.
In a few instances (Ariane V engine bay), I've started casting parts with styrene rods inserted in them. This fixes the strength/weight problem, eliminating the common broken-strut problem. It also prevents one misplaced air bubble from ruining the casting, which used to be the case. In this one instance, plastic-weld is the best way to rejoin the styrene in these parts, should they break.
Currently, I don't do photo-etching. This limits my choice of subject matter more than I'd like, so I'm going to have to learn soon.
Decals are laser-printed to a hobbyists' decal sheet, available from Bare-Metal Foil. I don't own an ALPS printer, so I can print to a white sheet, or to a clear sheet (which works best for aluminum-colored launchers), but putting white AND aluminum on the same sheet is basically impossible.
For the curious, resin material is Por-A-Kast 2, mold material is generally Polytek Platsil 71. Both are available at the Bare-Metal Foil website.