For my clear ice project, I wanted to use silicone trays which produce ice cubes that are reasonably, well, cubic, instead of one of the various elliptical/ovoid shapes you can get.
The problem is that silicone is sorta “self-healing” when you poke a hole in it with a nail or needle; yes, you have a puncture that slowly leaks water, but it’s not a good gap like we want for letting the freezing process purify the ice. We need to actually cut a small circle in the silicone.
Here’s what I did. Start with a small, narrow brass tube from a hardware store.
Now the ends of the tube, while you don’t want someone stabbing them with you full-force, are not too sharp, and won’t cut the silicone well. We can fix that. Just take a file, and bevel the outside edge so that the end is a much sharper circle.
When you’ve done that, simply put the sharp end down in your ice cube tray, press down, and twist back and forth. If you’ve filed the tube enough (took me less than a minute), hand pressure alone should be enough to cut the holes. Make sure you are resting the tray on something, like the brick wall I’m using here, that you don’t mind scraping with a sharp brass tube!
The only remaining annoyance is that every few punches you’ll want to pry the cut disks out of the tube, because eventually they “clog” the end of the tube and greatly increase how much pressure you need to apply. I’m using the needle end of a file here, but an actual sewing needle would have worked better.
And just repeat until you’re done. I cut two holes in each cell to help improve the flow during the freezing process, but I didn’t experimentally verify that this matters.