Mix Quick is just a solid polymer clay with a lot more of the oily and plasticizer stuff in it than regular solid polymer clays. It would be similar to SuperSculpey (which is tinted a pinkish-flesh color) but even softer. After baking, it wouldn't cure up flexible though, and would probably capture a bit less detail than some of the other brands/lines of solid polymer clay.
If you want flexible molds, generally you'd just need to use a two part silicone putty (best), or you could something like Bake and Bend which will be somewhat flexible even if thicker. Silicone putty will make very detailed molds, where Bake and Bend will do less fine detail. You could also try making a brush-on latex mold (and then perhaps use it with a stiff "mother mold" behind it). And you could also try things like PVA glue molds, and silicone caulk molds, etc.
(Sculpey's Mold Maker isn't all that great, especially for durability and some roughness of surface, and is more expensive.)
I guess you could also try to make very thin flexible molds by coating an item with liquid polymer clay, then baking and separating. You'd have to use a good release between the liquid clay and some materials though.
You could also just use a pretty thin sheet of solid polymer clay to make a mold since most polymer clays will be flexible after baking if they're thin. Don't use a brand/line of polymer clay that's brittle after baking when it's thin though (Sculpey, SuperSculpey, Sculpey III, Craftsmart/Bakeshop). Both of the Fimo lines, Cernit, and most other polymer clays will be very strong after baking when they're thin. You might need a "mother mold" behind them if they're too flexible though which could even be a hunk of raw clay the liquid clay mold is pushed into.
Why do you need even semi-flexible molds though? Are you just having trouble removing solid clay from other molds? If so, there are ways to deal with that. Or are you trying to create molds with actual undercuts? For those you'd need a flexible clay mold or a very flexible clay mold, or perhaps a two-part mold.
Their edges don't look very sharp though, especially since they aren't metal (but fine for pasta dough), so if you used these to cut polymer clay the outer edges wouldn't look really neat and sharply-defined. You could perhaps cool the clay sheet first though then press the cutter down and kind of wiggle it around before lifting off which could pull in and incorporate some of the ragged edges.
...You could also use them like regular ravioli cutters--through two layers of clay after folding over or layering.
...Or you could the cutters only as stamps for all kinds of polymer techniques.
I'd suggest for where you are and what you find too soft/sticky, try Kato Polyclay. It's probably the highest quality polymer clay out there and will not be too soft/sticky. You'll have to condition it though (easy to do if you know how to do it easily**) and perhaps more than you're used to, but the results then will be excellent.
Fimo Classic is currently the other "best" and firmest polymer clay, but its formulation will be changing the U.S. and it will probably be "dumbed down" (so, softer at least) even though I think it will be renamed Fimo "Professional."
As for Kato Polyclay, it's currently available in local stores only in Hobby Lobby, but you can easily order it online. It also comes only in pure spectral colors so you'd have to mix any colors you didn't get from those 8. (It also comes in white, ivory, black, brown, translucent, and mica-containing metallic colors, as well as 4 concentrated "colors".) http://www.katopolyclay.com one place to buy: http://prairiecraft.com
You can also read more about the different brands, etc, how to stiffen too-soft polymer clays in some ways, etc, in my previous answer about 6 answers below, from August 2010.
**slice off slabs from a block, then run through a pasta machine repeatedly, or without a pasta machine beat with a hammer (in a plastic bag, etc) or roll over firmly with a roller of some kind, then stretch, roll into a ball, roll into a log....repeat
Really nice custom figures! Love those kinds of personalized things, and sounds like your friend did too.
If you're worried their about weight (on a cake, etc) if she uses them later as cake toppers, you could always attach them to some kind of wide base at the feet. The base could be visible like a painted wood plaque, or hidden under the frosting like some of the plastic disk bases used for adding items to cake tops), or it could be made from polymer clay too if you want to take the super precaution of protecting the frosting from the clay at least where it would touch, or put everything up on some kind of riser. (some examples: https://www.google.com/images?q=polymer+clay+cake+toppers )
Haven't been back to Craftster in a while and just noticed another post with your clay and freestanding resin aquarium link. Really nice job, and thanks too for the exact time and temp you've found to work for baking (thin) polymer clay on cured epoxy resin. I subscribed btw.
(written before I noticed that the poster is the original tutorial maker...lol)
Nice job! Everything looks great and quite a good conversation piece too.
Thanks for the link to the tutorial too. That's the first time I've seen an actual specific time and temperature given for curing raw polymer clay on the outside of resin (220 F for 10 min, she says) though I suspect clay any thicker than that would require longer heating and therefore wouldn't work (unless cured separately and later glued on). Helpful.
I was also surprised to see the hot glue used for the sand since I'd have assumed PVA/permanent-white glue would be used which does work fine inside resin. Since resins heat up while curing, didn't know if the hot glue would be adversely affected, and at what temperature. Hot glue is certainly quicker than waiting for PVA glue to dry though. She doesn't heat the hot glue/sand base when curing the polymer clay reef in another video though (bakes separately then glues onto the sand base).
Nice! And isn't it fun to make bowls (and boxes, trays, etc) with polymer clay!? Plus you combined this one with stamping into the clay, a double whammy. Really great to see at least a few of the many other uses besides plain sculpting for polymer clay being done here at Craftster.
As mentioned, polymer clay is used on glass ball ornaments (even the still colored ones) all the time and no special baking considerations are needed (and especially if the clay will be completely covering the glass since it would be a buffer between the clay and glass).
You might need to use epoxy resin as a coating on your clay items (I assume you meant polymer clay items) if the others are absorbing too much moisture (and turning cloudy or milky over time).
Various kinds of clear-drying finishes work well as sealants as long as they aren't exposed to too much moisture and/or over too much time (and all will be more moisture resistant if allowed to "cure" over a week rather than only "drying"). So various thinned-down PVA glues (including Mod Podge) can work fine if conditions don't become too moist or for too long.
But tougher clear finishes are usually necessary (or can be added on top of other finishes) if there will be more moisture. In general, of that type I'd say that acrylic fingernail polish should normally work okay, and certainly cyanoacrylate floor polishes like Mop 'N Glo, Future (or Pledge with Future Shine, Klear, etc). Clear polyurethanes are thicker than floor polishes and perhaps even tougher. You'd generally buy those in the wood finishes aisle of hardware stores, and the brand Varathane has been a favorite of polymer clayers a long time because of some extra characteristics it has (the water-based gloss version). The same thing may now being bottled as one of the "Sculpey finishes" nowadays, though even if it's exactly the same it'll be lots more expensive by volume than buying at hardware stores. Polyurethanes also come in "marine" versions which are even more resistant to constant moisture but since they can't be applied directly to polymer clay, you'd need to put a water-based sealer on the clay first then put the marine version over that.
You can also just put cured polymer clay items directly in your terrarium since polymer clay is waterproof for all intents and purposes. If you were to put them underwater for long periods of time, you might eventually see what looked like a whitish coating on the darker colors since polymer clay can absorb a tiny bit of moisture under those conditions (especially the less-dense brands like the main Sculpeys, where the densest brand is Kato Polyclay). You could just remove them awhile to let the surface dry out though if you wanted. (It's not recommended to put polymer clay inside aquariums though just because it's possible that small fish could be affected but that's not understood well.) You can read more about using polymer clay around water on this page at my site, if you're interested: http://glassattic.com/polymer/outdoor_snowglobes_fountains.htm
**http://glassattic.com/polymer/other_materials.htm (under the category Epoxy Resins, click on Brands)