After a silicone food mold has been used to prepare soap using very caustic materials (such as sodium hydroxide), is such a mold safe for use with food again? Is a regular dish washer cycle enough to clean away such materials? Or is some of the sodium hydroxide potentially absorbed into the silicone?
I think it sounds fine.
Strong acids and bases do their thing by dissolving into water. They are highly soluble. Once diluted through rinsing, they’re not really a problem. (A dishwasher would be sufficient for this; detergent would not be necessary.)
Now, there’s still a worry that the lye could have survived the rinsing by seeping into porous cookware. But pure* silicone is not very porous. The last pretzel you ate probably had more sodium hydroxide clinging to it than those moulds do.
* Silicone with fillers may be porous. It’s not common in cookware. If you suspect fillers, try bending the cookware hard. A color change in the bent area indicates fillers.
I use my own homemade soap to clean dishes on occasion. In particular I usually let my mixer bowl, measuring cups etc. stand overnight after a batch of soap, with the residue still in them, and clean them the next day along with any other undone dishes. I also use the soapmaking lye as a cleaning agent every now and then. As one commenter pointed out, sodium hydroxide is used in dishwasher detergents.
Generally, I'd say dishwasher safe stuff is safe to use in soapmaking. As for cleaning with lye, I have only had issues when I forgot I had an aluminum pot in the sink when I dumped the solution. Oh and when I tried to dry NaOH grains in the oven it ate through the glaze on the ceramic bowl I used.
Finally I'd point out that soap batter is at maximum about 15% NaOH by mass, some of which is neutralized by the fatty acids before pouring into molds. You would expect the saponification process to neutralize the rest as well, with oil left over. Any residue that infiltrates surface pores will be more problematic as an off flavor and actual lye will be in homeopathic dilutions, and will probably just saponify some of the oils in whatever food is cooked in the mold next.
Incidentally, sodium hydroxide itself is not toxic, and only harms living tissue by corrosion. In foodstuffs like pretzels, it is neutralized by the Hydrochloric Acid, naturally produced by your stomach, into water and table salt.
Frankly, I wouldn't risk it. My understanding is that strong alkalis are sometimes used to clean up part-set silicones (RTVs etc.), and as well as the risk of chemical combination there's the risk of something nasty being absorbed (into the structure) or adsorbed (onto the surface) and only released slowly.
Now quite frankly I tend to be pretty blase about things like use-by dates. But in the current case and considering the probable price of replacement molds: it's just not worth it.