I agree, Gavin.
This is what I've got so far:
Sand casting:
€3650 for tooling
€145 per manifold for casting
will need machining
CNC aluminium
400 euro for the runners
+/- 100 euro for the flanges (stil waiting on quotes)
will still need welding and machining
3D printing in high temp thermoplastic
I can do the printing at home
about 100€ worth of materials for printing
will then need infiltrating with epoxy for strength and fuel resistance.
Sandcasting isn't too expensive,
but would need some commitment for some manifold from others,
would be too much to take the hit on, if I am left with 10 manifolds.
But I am leaning more and more towards the thermoplastic manifold. Cylinderhead temperatures will almost never reach 200°C (400°F), because the head and block wil act as a giant heat sink. If the head would reach that temperature, even the aluminium head would distort. If I had a running car, I would try it without hesistation, but am still a long way from that. A simple test would prove or disprove the idea. But you would need to pay attention to it, I wouldn't test it driving at first. If it shouldn't work, you'll have fuel spraying on the exhaust manifold.
Serge