Vacuum Plenum - MDF problems

[gerald_martin]

Hello Shopbotters:

I cut my first vacuum plenum board (4x8) from MDF Saturday, and sealed it with a waterproofing sealer for wood/masonry. I was very disappointed with the result - it seemed that the MDF soaked up enormous quantities of the thin liquid and swelled up...

In retrospect I think maybe I should have used a urethane sanding sealer type product? But, after this experience...

Is it really sensible to use MDF? Is there some type of plastic or PVC material that would be air tight, require no sealing, and be more rigid than MDF? Gary Campbell you did something like this, right?

I would rather abandon the MDF and do something else now if that is what needs to be done to make a reasonably permanent plenum board. But, if I just used faulty technique, I'm open to advice.

Gerald

[thewoodcrafter]

Sorry to here the problem.
Why don't you just throw a second sheet of plywood on the table and cut that. Some Baltic birch.

[gerald_martin]

Interesting idea Roger. I used Baltic Birch for the support board on my table - the stuff machines pretty nicely but I had trouble with long sections of the surface ply coming loose where I drilled by carriage bolt holes thru. Would I have to seal the Baltic Birch too?

Gerald

[beacon14]

If you use Baltic Birch I don't believe you would have to seal it. Very dense wood and many layers of glue make it all but non-porous. I'd still seal around the outside edges just for good measure but I don't think you would loose any air through the bottom of the plenum with two layers of BB ply glued together.

PVC is also a good choice if you can figure out how to bond it to the plywood base. A few folks have used a single layer of PVC instead of the plywood and MDF layers. If so I'd want to use 1" minimum thickness for stiffness.

[knight_toolworks]

I tired a poly finish to seal the mdf big mistake smelled for months. the fastest thing to sue is some thinned yellow glue brush it on and it is sealed in one coat and dries pretty fast too.

[gerald_martin]

Thanks for the feedback everyone...sounds like I made a mistake. Other ideas, anyone?

Gerald

[sawkerf]

Gerald,

I've had mymdf plenum for 4 years with no problems at all. two or three good coats of polyurethane and your set. Kip

[cip]

Gerald, I used a product call Extira, it is water proof MDF. It's made with resin glue instead of Formeldahide glue. It works just great and it isn't much more then regular MDF. It needs no sealing and is air tight.

Mike

[cabnet636]

i used an acrylic clear on mdf and it worked well,

i was considering using phenolic on my next one as i found some in fla for under 500.00 per sheet but i did not think of exteria, thanks!!

jim

[gerald_martin]

I like the Extira idea, is there any downside to this? No sealing would be a real advantage.

Gerald

[cip]

I have found no downside to Extira. It machines just like MDF and requires no sealing and it does not expand with humidity like MDF.

Mike

[Gary Campbell]

Mike, Gerald, et al...
We are in the process of cutting a project spec'ed with Extira.(our first use) I have some notes on the product.

1) It is not a sealed product. Even tho much less porous than ULDF or MDF, even the 1" material allows enough air to pass thru that you can feel it while moving a zero plate on top of the material. 3/4" and 1/2" flow even more. If used for a plenum, sealing is, I believe, mandatory.

2)I submerged a 3/4" piece in water and overnight it was thoroughly saturated and swelled to almost 7/8". It did not seem to want to break apart any easier than the dry part. It was more flexible tho. I would say the 3.2% 24 hour swell myth on the Extira website is BUSTED!

3)This product may have some exterior uses if completely sealed to water intrusion, but care should be made to seal all sides.

It is a less costly alternative to some of the plastics we normally use, but as is the case with most lesser cost products, this cost savings may come with a higher replacement cost.

Gary

[jeffreymcgrew]

We used a product called Medex. It's like Extira, an exterior-grade MDF, with a resin-based glue. It worked great! We didn't even seal it, and we're not getting any air leaking. I was going to seal it, but after cutting the plenum and then testing it I found it worked just fine without. We've got a sheet of Europly under it that it's glued and fastened to, and tape on the sides.

[Gary Campbell]

An update...
We also have experienced humidity swelling of our machined parts. many of our flat panels have increased in thickness .010 to .015 and dado widths seem to have decreased also. This is after a week in the shop and priming. It seems that both have affected the dimensional stability, but dont know which is worse. Either way, this is our last Extira job. Thanks Jeffrey, I will try Medex MR next time.
Gary

[porscheman]

Jeffrey & Gary,

You may already be aware of this, but Medex is the substrate of choice for very high-end veneer marquetry projects, where dimensional stability across a wide range of humidity conditions is paramount.

I don't know how resilient it might be in an outdoor application, but it's the best form of MDF that I've ever used - it's also formaldehyde free. It is, however, considerably more expensive than Extira and somewhat harder to find.

I hadn't previously considered it as a vacuum plenum, but will be curious to see how air tight it turns out to be in that application.

Regards.

John

[Gary Campbell]

John...
Thank you for the information. This was the 1st fibrous panel job we've done in 30 years. Being a "solid wood" kind of guy, I am at best very sceptical of any of this type of product.

Even tho the customer seems very happy with how the job has turned out, I dont feel that I will use it again in the future.

Dave Rosenbleeth told me about the Medex, but Extira was available from our prefered supplier. The application is not really exterior, but then again, it is not in a conditioned space either. All the humidity of exterior, but not the rain or sun.

I wont say that the cost isnt a factor, but if it is a superior product, and I can beleive in it, it is much easier to sell. I will pick some up in the near future.

I will use one of the leftover sheets to make a new plenum, but it will be thoroughly saturated with epoxy on all 6 sides to make it more stable.
Thanks again, Gary

[gerald_martin]

Hello again, everyone:

Thank you for the helpful feedback. I decided against extira due to availability/time. I tried again with regular MDF. This time I used a spray on 24% solids vinyl sealer which did not soften the mdf like the other sealer I used. After 2 coats of this to the surface, I coated my entire support board (baltic birch) with wood glue, laid the plenum on top, put a couple of sheets of mdf/plywood on top of the plenum, taped the edges and turned on the vacuum. Wood glue squeezed out all over the place including into my vacuum manifold. Left the vacuum run for an hour. This seems to have glued the plenum securely to the table and sealed the back side of the plenum.

Then I surfaced the grid of the plenum with my new Amana 2.5" spoilboard cutter. This of course "unsealed" the top, so I brushed a coat of the vinyl sealer back on.

So, I think I'm about ready to lay down some Trupan and surface it, and see how well it flows.

By the way Gary the Ametek 220v motors you recommended to me do indeed pull a good 9 inches of vacuum with no leakage.

Gerald

[donchapman]

I never sealed my MDF plenum with anything and it has worked just fine with one small Fein for three years and hundreds of projects.

[pfulghum]

I agree with Don...my plenum was cut out of 1.125" MDF 4 years ago. I did seal 1/2 of the sheet with 1qt of shellac, after it soaked it up I decided to just leave the other half alone.

I run 4 zone with one Fein.

I did put one coat of $1/can mis-tinted primer on the edges.

I pull 5" here in Boise at 3000'. Works great for me. I can cut Melamine, MDF, and Plywood so long as the part is about 40 sq in.

-- Pat

[beacon14]

If the plenum is glued down to a base layer such that no air can leak in through the bottom, and you seal the edges with sealer or tape, then I don't see any need to seal the grid of the plenum, except maybe to protect it from ambient humidity.

[Gary Campbell]

David...
That is exactly why I would do it. A partial sheet of MDF left over from a speaker box grew mold sitting in the rack. We are going to a glued down spoilboard due to the fact that when sitting, ambient moisture curls the spoilboards up on the corners, run the vacs for an hour and they will bow up in the center. Being 40+ miles offshore and very seldom having temp or humidity numbers under 80 we have more humidity to deal with than most.

Fall is here tho.... we just had our 3rd morning since April 26 that the morning temps were under 80. (78) LOL
Gary

[harryball]

I use shellac with great success. I seal the entire surface, plywood, plenum... everything to protect against moisture. I seal the edges of my trupan bleeder board with shellac and then surface.

/RB

[Brady Watson]

Guys - In the vacuum game, there's no 'wrong' way to seal or setup your plenum. Personally, I like to use a removable setup like the BradyVac-I with a 2.25" hole in the spoilboard so that I can still clamp, screw or otherwise hold parts that are not conducive to being held with vacuum.

If all you do are sheet goods day in & day out, then a permanent setup is probably a good choice. A sheet of expanded PVC (Komatex,Sintra etc) or sealed sheet of MDF with the grid machined in it, is just the ticket. I prefer to keep things flexible in terms of not being married to any particular hold-down solution to keep things easy on me if I have to setup for a job that doesn't use vacuum. I like to use a plastic grid on my BT48, along with a Trupan bleeder that is aluminum foil-taped around the entire perimeter. This seals very well and has enough holding power to keep the bleeder from moving on the grid when switching out material. Last time I ordered, it was about $100 for a 4X8X0.5" sheet of expanded PVC...personally, I find the $100 price tag a lot cheaper than my time coating or sealing an MDF sheet. For my larger 5X16' Alpha, I use a BradyVac-I or II depending on the job, using a single sheet of Trupan and nothing else. It really boils down to what YOUR exact needs are with your tool.

-B