How can I stop the heat traveling back up the head?

I'm trying to print a large flat base and by the time it has printed 80% of the first layer the filament has got to hot at the top of the head, causing the filament to go soft.
I've tried increasing the speed but it had no effect.

Thanks

Logically, making it 100/80 times faster would mean it would complete the first layer and then the fan should come on and cool it.

A lower nozzle temperature helps.

Perhaps more silicone tape on the hot end would help.

Failing that a tiny fan that blows on the top of the hot end and is always on could be a solution.

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Thanks for the reply. I've managed to sort it out by increasing the preasure on the filament.