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jason str

Veneered face plywood and horn subs

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Been asked why not to use veneered plywood a many times in the past and a few times lately and figured maybe i should post about it.

 

This is just a subwoofer problem, not bass bins.

 

Most of these subwoofers including all BFM horn designs, F-20 and probably some tapped horns as well.

 

Pressures run very high in these designs.

 

This high pressure will cause the veneer to separate from the thicker layered ply's underneath.

 

It may be a month or a year later but it will happen, no doubt about it.

 

If you are looking for a nice finish for your project use the proper plywood and veneer afterwards, paint, stain or whatever you have planned.

 

Feel free to post any questions and i will do my best to answer them best i can.

 

Edit:

 

Just wanted to be absolutely sure everybody was clear in what i was talking about here.

 

This is not an exterior cabinet finish that is the problem it is the exterior plywood veneer on the plywood stock and glue joints in construction phase that delaminate.

Edited by jason str

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I would think that you could use one grade thicker plywood sheet for the box/horn, then veneer over the top, then use epoxy or polyester-vinyl as the capping layer: one is hard and the other more flexible. Think surf boards.  The issue is flexing and fatigue.   If it doesn't flex, it doesn't fatigue...in such thin sections. If it isn't brittle/high strength, it doesn't form delamination cracks.

 

Same thing is true for aircraft skins.

 

Chris

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Not sure i catch your idea, if I'm off let me know.

 

Veneer is fine for the cabinet once it is completed, it is building the box using plywood with veneer face that is the issue.

 

I have never experienced any flexing in any cabinets i have built but i would say if you leave braces out it would be more prone to resonating and flex.

 

Speaking of aircraft type construction the Jack Lite series cabinets are made from 1/4" stock but have not constructed any as of yet, lots of bracing in the plans.

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If the veneer in prelaminated plywood is delaminating in service (as you seem to be talking about), then either the glue itself is just aging and becoming too brittle, or the walls of the plywood are flexing, leading to delamination crack growth in the glue layer at the veneer/base plywood interface.  In either case, the canned veneer-plywood glue sucks.

 

The solution is to not use preveneered plywood--do it yourself.  Use thicker face sheets of plywood to reduce the micro-flexing, then apply a better veneer bonding polymer to stick the external veneer down, then use a capping layer of either a very hard or a very flexible/adhesive polymer to seal the veneer on.

 

That will last until the polymer bonding layers age and harden - in about 20-30 years, when it can be rebonded if necessary.  I think that you'll get much more than 30 years life out of that, especially if you use UV-stabilized resin - like the marine stuff used for surfboards.

 

All this is very similar to boat construction methods that last a long time under extremely high UV, salt water, impact, and flexing fatigue loads (boat battering through waves/surf, etc.).  This problem is well within the state of the art, and is solvable using a variety of materials and methods.

 

Chris

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I would not think they would use different glue just for the outer skin but i guess it could be possible.

 

If you are familiar with different plywood or even have some laying around try peeling off the veneer face, it is much easier to pull off than say a layer of  cheapie 3 or more ply even layers of wood.

 

Even the higher quality hardwood veneered plywood has this issue, quite sure its just the veneer dilapidation itself causing the problem.

 

If flexing was the cause i would think the thinner layer would hold up better than thicker ply's as thinner stock will bend very nicely before breaking in comparison.

 

Maybe the glue sinks in to the thinner ply and causes it to be more brittle.

 

Being from Chicago i know absolutely nothing about surfboards.

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As someone that is getting ready to build a sub or two, I find this thread very interesting.

Edited by Ceptorman
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