Warpage, Baltic Birch ply over particleboard?

[ro9397]

This subject is critical because subs (1cf internal, about 24x15x11) may build in ultra-dry climate then move later to very high-humidity climate. A local pro cabinet shop and member of the AWI (Architectural Millwork Institute) emphatically said that hardwood (term includes Baltic Birch ply my favorite) absorbs moisture at a higher rate (he estimates twice) vs. MDF and/or particleboard.

IIRC our esteemed member Joachim Gerhard currently selects two different panel materials to distribute resonance: some panels are plywood, other panels are MDF, both covered by finish veneer.

The above AWI cabinet shop once built 3/4" solid hardwood over 3/4" MDF for a celebrated local pro speaker designer I know. The speakers shipped from this dry climate to a super moist climate and split (cosmetic only) at the joints.

I have superb audible results (don't tell anyone ) with 5/8" Baltic Birch ply directly laminated over 5/8" particleboard (preferred for its improved damping via irregular sizes of wood chips vs. MDF).

The shop suggested the following to minimize warpage/joint cracks:

1. Post-build, seal (lacquer, etc.) all internal cabinet seams and surfaces to minimize moisture absorption.
2. Slightly under-cut all particleboard and separately laminate each panel. At all corner joints glue/join only the Baltic Birch ply, maintaining a small seam between the particleboard panels, allowing its volume to grow slower than the plywood to maintain integrity.

How likely is my laminate recipe to crack/warp? (No external veneer, finish is lacquer over the Baltic Birch ply.)

How effective are the proposed fixes?



TIA!

[jcx]

sealer generally only slows moisture

have you tried balanced construction? ply : mdf : ply - as long as the glue holds, miositure fairly uniformly affects all layers there shouldn't be warpage

Veneernet (plenty more in Google search of balanced plywood)

wood expands/contracts unequally with/across the grain

[Saturnus]

I recommend construction grade plywood instead of furniture grade plywood especially for (professional) sub cabinets. It's stronger, stiffer, "deader" (even than MDF), cheaper, and completely impervious to moisture changes. It's basically baltic birch plywood but with the layers glued with epoxy glue. It's also surface sealed with the same glue.

Here a PDF of what's readily available in my area. In your area availability may vary but construction grade plywood are used extensively around the world so it shouldn't too hard to find it, and cheap, at least compared to furniture grade baltic birch.

http://w3.upm-kymmene.com/upm/intern...m_birch_en.pdf

Please note that you have to use boat epoxy primer for glass/carbon fiber if you want to paint on top of these boards as it's the only paint that will stick. Not even car/industrial primers are strong enough.

If you want laminates. A construction grade birch plywood laminated with HDF (most common type is masonite) but using roofing tar (aka bitumen) instead of glue yields a far superior result to the standard baltic birch/MDF laminate. It's also yields better results with different material thicknesses for the laminate materials. For example a 2/5" plywood over 3/5" HDF gives better result than 1/2" plywood over 1/2" HDF.

[ro9397]

Son of a biscuit eater, you guys got lots o'help! Thanks.

[evanc]

Wood grows and shrinks across the grain but not in length with changes in humidity. Since plywood has each layer running perpendicular to the layers above and below it the finished sheet does not change size with humidity. I believe particleboard is also stable this way. All bets are off if either material gets wet enough to loosen the glue. All this is saying I don't believe you should have a problem with your lamination. The example of hardwood laminated to mdf was certain to fail.....

[ro9397]

Originally Posted by evanc

Wood grows and shrinks across the grain but not in length with changes in humidity. Since plywood has each layer running perpendicular to the layers above and below it the finished sheet does not change size with humidity. I believe particleboard is also stable this way. All bets are off if either material gets wet enough to loosen the glue. All this is saying I don't believe you should have a problem with your lamination. The example of hardwood laminated to mdf was certain to fail.....

Makes sense...The cabinet maker listed BB ply under the heading "hardwood", same as the solid wood panels. Do you agree with his listing? (I understand that in my application the results are different between BB ply and a real solid wood panel.)

So you'd thoroughly join and glue the particleboard at the 90-degree joints, not just the BB ply? Would you bother sealing internal surfaces? Again, it's going from dry N. Utah to FL.

Thanks!

[evanc]

baltic birch ply wood is made from hardwood plies, but is still plywood. I would glue everything. I also like kreg screws on butt joints. Also the glue you use matters. unibond minimizes creep between layers.
Vacuum Pressing Systems -- Veneer Glue
Kreg Tool Company | Kreg Jigs, Deck Jig, Precision Routing Systems, Klamp Components, and more
I would seal the box in and out. Shellac is excellent sealer against moisture transfer. It also dries fast and is not too toxic.

[jcx]

read several of the Google search results for balanced & plywood, add moisture, warpage search terms - this stuff is well known - seasonal humidity variations are an issue in any precise dimension woodworking project

you want balanced construction, all surfaces treated the same so that moisture diffuses in at the same rate from both sides (you shouldn't build true "sealed boxes" even if you could - you should always have air leakage path with at most minutes time constants to allow weather barometric pressure changes to equalize)

thin varnishes may only add months to the moisture diffusion time constant, heavy polymer finishes like epoxy or urethanes can be better – but water will eventually diffuse through most polar polymers, varnishes

[Keriwena]

FWIW, Formica recommends against using plywood underlays because the dimensional stability of plywood is too great. MDF is preferred. (Formica is also a grainless wood product, under all that pressure, and it will swell with humidity.)


You cabinet maker was wrong to list it with hardwoods.

[chrisb]

Originally Posted by Keriwena

FWIW, Formica recommends against using plywood underlays because the dimensional stability of plywood is too great. MDF is preferred. (Formica is also a grainless wood product, under all that pressure, and it will swell with humidity.)


You cabinet maker was wrong to list it with hardwoods.

I've worked in the commercial millwork cabinet trade for over 18 years - a couple of notes:

You probably meant to type "dimensional instability"

Formica & all plastic laminates are fabricated from layers of heat/pressure treated paper and (still usually) phenolic resin - it's the underlying paper backer that is subject to swelling, and once properly applied to any substrate, it will generally take more than even very high atmospheric humidity to cause that (i.e. standing water penetrating poorly caulked seams at plant-on back splashes, around sink cutouts or at horizontal butt joints), and generally that will cause more grief with the substrate core (if you really want to see swelling, get the MDF wet) otherwise it (P-lam) couldn't be use for kitchen / bathroom vanity counter-tops.

Architects will frequently specify and we often use plywood for countertop cores - it's much lighter than MDF, and makes installation much easier - but either material needs a humidity resistant balancing backer, usually a thin phenolic paper.