vacuum plenum

i am gonna make a new plenum fot the router, last one was 1.5" squares .5 wide and .375 deep in four zones.



are smaller channels better, is there a formula, we use high cfm blower are there advantages between aluminum and phenolic (grade) as the last one i made from mdf and sealed well.

Aluminum that is thick enough seems a bit excessive but the 3 main advantages of Phenolic are the stiffness, dimensional stability, and non-porousity. It's also not cheap. If you are OK with the extra bucks you may want to compare it to PVC. Whichever is less, they would work equally well.
That being said, I use mdf with a laminate back over 3/4 ply. Also make sure it is well sealed (been using Shellac for years) Never had problems. I have always used a interconnected channel pattern rather than squares. The last replacement time I made the channels deeper and closer together than they had been previously and also increased the size of my lines. Suction is quite good.