Thermal paste is a must. When I assembled my first e3d v6 many years ago, this wasn’t part of the instructions, so I had never put any on when changing heat breaks since.
Started using rigid.ink PLA and found that the filament was swelling in the heatsink and jamming where the reamed part of heatbreak starts. After changing heatbreak, and fan, and trying a bunch of other things, I decided to check the assembly instructions, to find they now include thermal paste at the top of the heatbreak. Stripped the hotend back down and applied the paste. Straightaway the performance was noticeably different, even feeding by hand.
Presumably the aluminium heatsink expands more than the stainless heatbreak, and reduces thermal efficiency, resulting in a hotter heatbreak.
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