Certainteed PVC 1"-4x8 Sheet for Support Board/Plenum

[mrgadget]

Do you guys think this will be okay for the support board for a 4x8 PRT Alpha? I was wanting to get the 1" and cut the plenum on the top along with t-track slots. I want to use the t-tracks to hold sacrificial boards used to hold whatever I'm cutting. This was my idea to get started cutting until I can get my vacuum system put in at a later date. The humidity is really an issue in my shop as it's not climate controlled and I really have to stay on top of all my equipment table tops to prevent them from rusting.

[Brady Watson]

Jay - been there, and from my experience, it would be a resounding 'No' for several reasons. First, it is not a structural material and when you lay that sheet across the crossmembers, it has a substantial amount of sag between them. Second, it is very difficult to glue or mechanically fasten anything to it with any degree of reliability. Third, you really don't need PVC to 'seal' the vacuum plenum, as polyurethane sealed MDF will hlod vacuum in there just as well. Fourth, it does not machine well when you take a skim coat on it to make sure it is flat and true to the machine.

My recommendation from setting up at least 50 vacuum systems on these tools would be to get a nice sheet of 3/4 or 1" MDO plywood. It has all the right properties for a support board. Then, get a better than 'big box' quality MDF if you can (Plum Creek brand comes to mind, or MedEx from Sierra Pine) and glue it to the support board with wood glue. Crown the bottom face down, clamp and weight the center & let it sit overnight. Then machine your vacuum grid & seal it. Then top it off with Ultralight MDF or Trupan Ultralight as your bleeder and you're good to go. If you didn't seal the entire sheet that is OK - if you just seal the edges (with vac on) with 50/50 wood glue and water, that would be fine.

-B

[mrgadget]

Thanks for the reply Brady. I knew from reading the forum that was what you recommended for a table setup. Just throwing around some ideas. I went ahead this morning and ordered the MDO for the support board, Plum Creek MDF for the plenum, and a sheet of ULMDF (Masisa brand??) for the bleeder. Can't get Trupan anywhere right now around here. Going to be fun and interesting getting this together!!