I don't know how much glue, but I used a wide putty knife to spread an thin even layer to both pieces and scrape off the excess. A few finishing nails to hold it in place and parked my jeep on it for pressure. Some squeezed out of the sides with pressure. Afterward, I had to cut 2-3 inches off the edges to get to wet glue. I always use wood glue, but for the large panels I wondered how it would dry in the middle. Sure enough, it didn't, just like it doesn't in the bottle.
LOL! You ran over your sub box?!?!? THat's a hilarious mental image right there. SO it seems that noraml PVA glue (wood glue, white/yellow glue) isn't good since it won't dry in the middle. I know that 2-part epoxy WILL work, but that'd be mildly pricey but not too bad. I'm not sure about liquid nails, polyeurathane (ultimate glue, I think gorilla glue is poly, too), or "GOOP" (or similar glue with that strong smell - I don't remember the generic name for it).
Also worth a mention is the fact that some glues outgas gasses that can damage driver cones / surrounds / spiders etc ... so if you use any glue that outgasses you'll probably want to leave the drivers out of the box for a considerable period of time (a few days upto a month depending on the glue) just to be safe.
Dr. Photon said:
No, just a pair of scraps of plywood glued together as a test. It was the simplest way I knew to apply significant pressure, but still nowhere near the recommended 100 lb/sq in.
johninCR said:
?!?
Where have you seen that recommended? That's like 7 times the pressure used by commercial cabinet makers for ply bonding, veneering, etc. Hell, even composites consolidation in industry. Maybe they use something like 100psi generated from hydraulic presses for plywood bonding or MDF consolidation, but I really don't think you need anything like that kind of pressure to assure a good bond between two pieces of typical ply or MDF.
If you've seen otherwise on glues or woodworking instructions/texts etc., I'd really like to know. Maybe I've been doing something wrong? 😀 I've always held vacuum pressing to be my "golden standard" for typical woodworking purposes, and if I can achieve fairly even force distribution using standard clamping that even approaches that (10-15psi) I'm tickled pink. I usually settle for much less and haven't seen any problems.
This may be too simple for some, but epoxy works vey well for MDF and cures in a predictable manner. Epoxy also does sport quite fine adhesive properties for most plastics (excluding PP, PE and some types of PVC), so it is perfect for mounting binding posts, ports and the like.
Magura 🙂
Magura 🙂
A good vacuum press pulled to 25HG will produce approximately 1800 - 1900LB / sqft of pressure. Although I'm sure I messed up the math but to me that would be around 13LB/sq in of pressure.
As for adhesive, MDF is made by coating small wood fibers with epoxy and then pressing them into a sheet. Since epoxy is how it is originally bonded, it is in my view the best candidate to make the two layers act as one. Epoxy can be difficult to work with, and takes some experience to mix properly. If you do not get a proper mix you will most likely not get a long lasting bond. If you are looking for something with a longer working time and a bit easier to handle try a urea formaldehyde glue like Cascamite. And for the simplest approach use a quality yellow wood glue like TiteBond. Make sure that whatever you use is evenly spread across the entire gluing surfaces of both sheets that you are gluing.
As for adhesive, MDF is made by coating small wood fibers with epoxy and then pressing them into a sheet. Since epoxy is how it is originally bonded, it is in my view the best candidate to make the two layers act as one. Epoxy can be difficult to work with, and takes some experience to mix properly. If you do not get a proper mix you will most likely not get a long lasting bond. If you are looking for something with a longer working time and a bit easier to handle try a urea formaldehyde glue like Cascamite. And for the simplest approach use a quality yellow wood glue like TiteBond. Make sure that whatever you use is evenly spread across the entire gluing surfaces of both sheets that you are gluing.
RHosch said:
?!?
Where have you seen that recommended? That's like 7 times the pressure used by commercial cabinet makers for ply bonding, veneering, etc. Hell, even composites consolidation in industry. Maybe they use something like 100psi generated from hydraulic presses for plywood bonding or MDF consolidation, but I really don't think you need anything like that kind of pressure to assure a good bond between two pieces of typical ply or MDF.
If you've seen otherwise on glues or woodworking instructions/texts etc., I'd really like to know. Maybe I've been doing something wrong? 😀 I've always held vacuum pressing to be my "golden standard" for typical woodworking purposes, and if I can achieve fairly even force distribution using standard clamping that even approaches that (10-15psi) I'm tickled pink. I usually settle for much less and haven't seen any problems.
The 100 lb/sqin is right on the glue bottle. Lanco is the only yellow PVA glue available down here. I don't understand why because the white stuff has 0 moisture resistance and during rainy season we go months with almost 100% humidity. Who said anything about veneering, pressing plywood, etc, I'm sure none of that requires high pressure ? I too doubt that I've ever put 100lb/sqin of pressure on anything I've built and have never had anything come apart.
Magura said:
Yes, my grandfather showed me how epoxy and fiberglass can fix almost anything.
Very interesting thread.
It brings up a question I have regarding laminating hardwood(red oak) to MDF. I have read that one should never laminate hardwoods to MDF because the physical properties of the two are so different, that it can cause things to go FUBAR.
I am currently building a sonotube based subwoofer using RythmikAudio’s direct servo kit. I used 1.25” MDF for the baffle insert and .75” for the top insert. I had planed on using red oak endcaps, top and bottom for decorative purposes only.
My plan is to use a constrained layer of P.E.’s dynamat between the MDF and oak, with no glue between the dynamat and oak, just wood screws to hold the two together.
Is this an accident waiting to happen?
Is there some other way, that will be more effective?
TIA
😕
It brings up a question I have regarding laminating hardwood(red oak) to MDF. I have read that one should never laminate hardwoods to MDF because the physical properties of the two are so different, that it can cause things to go FUBAR.
I am currently building a sonotube based subwoofer using RythmikAudio’s direct servo kit. I used 1.25” MDF for the baffle insert and .75” for the top insert. I had planed on using red oak endcaps, top and bottom for decorative purposes only.
My plan is to use a constrained layer of P.E.’s dynamat between the MDF and oak, with no glue between the dynamat and oak, just wood screws to hold the two together.
Is this an accident waiting to happen?
Is there some other way, that will be more effective?
TIA
😕
TNT said:
Could someone expand on this issue please. Why would it be a problem using the same thickness on the two bords? Or is it just a problem when doing "constrained layer damping" enclosures?
You say this as if either you have evidence to prove this or you have theory so convincing that it doesn't need anything more to back it up ... but I'm not sure you are completely correct.
First let's be clear on what is happening with a subwoofer box.
1. The driver vibrates and this vibration is transferred to the enclosure. The box then acts as a secondary transducer, effectively adding harmonic distortion that is not present in the original.
2. The rear wave of the driver is trapped within the box and pressurises air inside the box. This pressure causes the walls to flex. Now we are dealing primarily with transmission of the sound through the enclosure walls.
So how do we best make a dead box?
I'll suggest two approaches which would work well.
1. Make the box as thick and as dead as you can. Brace it with a matrix bracing system.
2. Attach the driver to an inner layer which is braced with a matrix bracing system. Then add another box around this box with a damping layer in between.
Which is better? Given the same thickness (36mm each) and bracing, I'd expect them to be very similar. 1 will have a little more stiffness but 2 will reduce vibration transferred from the driver.
I think measurements are required to determine which is better.
I have seen measurements of wall systems and their sound transmission performance, where a single material was compared to two half thickness layers with an air gap. In the bass range their performance was the same. However, the vibration aspect may favour the constrained layer damping method.
I agree that the CLD method is likely generally more effective in the midrange.
Still, none of this gives much inspiration to put the extra effort into building a box within a box!
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