Heat Sink synthesis requires manufactured materials, which means you can't gather those materials while you are out in deep space.
But, mats have no weight, and having just a few reloads worth in stock before you leave on your trip means you only need one heat sink launcher and the odds are that you won't need it very often anyway.
My DWE Asp is equipped with 2 Heat Sink Launchers and I'm down the last Heat Sink - with only ever having fired one. The other were lost to malfunctioning HSL while overheating in critical situations.
So yeah, I had my hopes up a little to be able to re-stock via synthesis and was a little disappointed when it became clear, that materials are required that I naturally don't have aboard and can't gather out here. But then again, I never expected/hoped the game would make our life out here any easier, so I'm ok with that and will keep my last Heat Sink close.