Most epoxies don't play well with heat.
About a decade ago I did approach an industrial epoxy manufacturer when I was inquiring about an epoxy to use as a filler for sleeving barrels, but dropped the project.
Never got that far with it. Most epoxies tend to experience heat distortion at under 200 degrees.
Curing additives like quartz (used in JB Weld's "Extreme Heat") and carbon fiber are used in high-temp applications.
Those that I'd seen were paste/putty- and I was looking for a "pourable", heat-conductive epoxy that could be used after the sleeve was in place -thin viscosity injected into the gap between the sleeve and barrel.
No idea what you're molding- but the JB Weld product might work for you, I'm sure there's others on the market as well.