Originally Posted by
JMCS
Just a thought,you have a lot of wood products right now, (base, plenum, spoil) that's a lot room for movement. Instead of using 8020, why not use a sheet of 3/4" 6061 Aluminum plate? With the 8020 you have a lot of smaller pieces that have to be aligned and secured to each other or they can move independent of each other. A sheet of 6061 weighs around 514 lbs. A bit heaver than 8020, yes, but a lot more stable. A few extra cross members will handle the weight. A heaver machine is a more stable machine. The expansion coefficient is minimal. You could machine your grid right in to it and use gasketing or use a structural bonder and attach another plenum board.
With regards to the MDF shrinking, could it be with a 13 hour run, the moisture content is going down in the board with that much air flow going through it? That time of the year, dry, dry,dry. With the big guys running heavy iron, there changing there spoil boards so frequently they don't have a problem with it shrinking.
Double refined MDF. Try Ranger Board by West Fraser. Awesome product, finishes out after machining like it has been sanded to 150. Also no extra edge sealing, the edges absorb the same as the flats.