One extra note: if you use a thin, slow-curing silicone for casting, be sure that the lower half of the mould is absolutely water-tight. Silicone slowly creeps into the tiniest openings, even only microns wide, and would leak away. With thick, fast curing silicones, this is less of a problem, as they are already cured before they have time to leak away. If there are non-watertight seams, you can close them with plasticine or wax.
Another option is to make the whole mould out of plasticine. Be sure to use non-sulphur plasticine, as sulphur inhibits curing of silicone.
I have used both 3D-printed moulds, and plasticine moulds, and combinations.
Obviously, you need non-stick silicones for mould-making, not the sticky sanitary or construction silicones.
It took me a while to figure out what a backing tray was. The ice cube tray mention actually somehow helped. I fixed your spelling above.
2) I've never tried this but I'd consider something more flexible like TPLA (tough PLA) or anything to the left of PLA and ABS in this diagram. Maybe even TPU or "ninjatech cheetah" (which I think is TPU):
I'm not sure how brittle ABS, PLA, etc get at freezing temperatures. Maybe someone else knows.
1) Pretty much there aren't any materials like this. I mean PEEK can do baking trays but it's very expensive and no Ultimaker printer can print it without pretty much replacing or relocating most of the parts (servos need to be outside the printer, enclosed printer, heated bed needs to have another 200W heater added, print head needs to be higher temp, firmware needs changing).
HOWEVER, I recommend making a mold out of PLA and pouring silicone into the mold. Silicone comes in 2 part mixtures - look on amazon, ebay or other places. Check how hot the silicone can withstand. For example they talk about "tin" silicone which is designed for molten tin (450F - hotter than most baking). Look around for temperature limits for various silicone mixtures. Many baking products are made out of soft silicone. Like muffin trays.
Silicone mold making is a very rewarding experience I have found! Also once you make a single mold you can make many baking sheets and sell them on etsy if you want. They can be custom shapes like if you want to make minion baking trays to create minion shaped cup cakes! Yay for minions!
For first try it's not a big deal but if you wan't professional quality molds with no air bubble defects, you need to buy a vacuum chamber and put the silicone mixture in that to remove all the gasses before pouring into your PLA mold. There are TONS of videos about working with 2 part silicone.
Check out these 2 videos:
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