From a medical-oriented site:
In general, each type of resin has its own distinct advantages. Epoxies are ideal for high temperatures (up to 450°F), or for highly corrosive applications. For example, formulations of epoxy are used when steam sterilization is essential. Polyurethanes are an excellent general-purpose material for both soft-rubber and hard-plastic applications where exceptional toughness and wear resistance are important. They are used routinely in devices where blood and patient contact is expected. Silicones are best for product applications that require rubber that is soft or of medium hardness over a broad temperature range.
- strong sensitizer
- moderately toxic when curing
- once cured (properly), basically safe
- usually in ratios like 1:1 or 2:1
- think this is the same stuff as epoxy glue
- not aware of any rubbery epoxies
- clear domes over LEDs
- medical implants (but they have very long and controlled curing)
- cast figurines
- gives off that classic "fiberglass" smell
- highly toxic when curing
- you use polyester resin and a few drops of MEKP catalyst
- MEKP is bad news -- a few drops in your eye and you will go blind
- not 100% sure on toxicity when cured
- highly toxic when curing
- when cured, basically safe
- available in variety of rubberiness
- goes off quickly when stored in presence of air
- condoms (Avanti)
- basically safe when cured (in general also might depend on any additives)
- flexible molds
- places where there is repetitive flexing
- sex toys
Hot melt thermoplastics
The usually come as sheets or pellets. You heat them to about 350F (177C), depending on the plastic, and they turn liquid. You then pour them into your mold and as they cool they solidify. PVC is one of these. One advantage seems to be that you can get it very soft (e.g. Shore A 5). I believe this is what is typically used in fishing lures and in some sex toys.
When doing the SMT toaster oven reflow thing, you typically take your board to about 450F (232C), but that's only for 60-90 seconds. Trying to encase an AVR board in hot melt might be problematic... ATTiny26 max storage temperature is 150C (302F) Might be worth experimenting with though. SMT tantalum caps might be alright but I could see electrolytic being an issue.
Seems like the major danger here is from burning yourself (ouch). Or overheating the liquid and creating noxious gas (e.g. hydrogen chloride).
One potential advantage is that the plastic can be reused simply by remelting. There are some types that when heated undergo a reaction that is not reversible (these are used e.g. for the plastic coating on tool handles).