Composite 2x12 cabinet DIY

[subdude67]

Every time I look at a commercial composite design, I keep coming to the same conclusion that to do it well (as robust as a traditional cabinet) is very expensive, and the weight savings isn’t all that much.

Trade off robustness and weight/cost shrink somewhat.

I first looked at composite cabinets when we were doing the Genz Benz thing and we were owned by KMC. KMC had a composite division in house with Ovation guitars, and the parent company Kaman Corp. was a leader in composite technologies with their Kaman Aircraft division (composite helicopter rotor blades and components. Even with (really) deep resources, it didn’t make sense.

Let me be clear, this is just an exercise! Even though I think this cab could be built to a reasonable price, as it is, it would never stand up to regular loading/unloading without looking shockinly beaten up within weeks But to me that is not so important as I don't tour or gig regularly.

 

[subdude67]

Nice... always a method of interest to me. I think you are (by necessity) the first to post a build with EPS instead of XPS. Since panel rigidity is related to the distance between the skins, you probably didn't lose much.

I'm reminded of two old composite build threads, by zac2944 (XPS/fiberglass/epoxy). They were well conceived, executed, and documented. But in digging up those old links, I found that they were older than I thought, and sadly, the pics are dead links now. Still some good info for someone wanting to go this route.

DIY Lightweight Composite Cab Build (May 2008)

DIY Lightweight Composite Cab Build II - ZL1263, 3-way (Jun 2009)

If I had access to XPS, I would have used 25mm thick sheets instead. I have easy access to polyester resin, so it would have been cheaper and quicker to build it, it took me a while to find the slow curing epoxy the EPS requires. While the XPS cabinet volume would be a bit smaller, due to the higher density of XPS, the weight would be about the same as the one I now made. But the shell would be much stronger, which would be a great thing.

 

[1n3]

If I had access to XPS, I would have used 25mm thick sheets instead. I have easy access to polyester resin, so it would have been cheaper and quicker to build it, it took me a while to find the slow curing epoxy the EPS requires. While the XPS cabinet volume would be a bit smaller, due to the higher density of XPS, the weight would be about the same as the one I now made. But the shell would be much stronger, which would be a great thing.

I believe that in your case, the strength of the core material is less important than it's thickness.

From Wikipedia: Sandwich-structured composite:

If the sandwich is supported on both sides, and then stressed by means of a downward force in the middle of the beam, then the bending moment will introduce shear forces in the material. The shear forces put the bottom skin in tension and the top skin in compression. The core material spaces these two skins apart. The thicker the core material the stronger the composite. This principle works in much the same way as an I-beam does.

So your 50mm thick EPS construction should be significantly stiffer than 25mm XPS would have been, with little difference in weight.

Also, I don't believe you can use polyester resin with either EPS or XPS; it dissolves both. Rigid Urethane foam... yes.

 

[agedhorse]

The problem with thick cross-sections of core material is that the internal volume of the cabinet decreases which reduces performance.

One thing I looked at was a composite with a balsa fiber core, something that was suggested by the Ovation engineers as a compromise, but the cost was pretty substantial.

Here's one source: DragonPlate | Engineered Carbon Fiber | Made in USA

Balsa core

Premium balsa core materials and panels with high strength-to-weight ratio, ideal for marine and wind turbine applications.

sky-composites.com

 

[CochinoBass]

I know nothing about speaker enclosures, but I worked with fiberglass a lot and I could easily make that enclosure so it wouldnt dent with very little extra weight. All you would have to do is what I already mentioned, skin the outside of the foam core with epoxy saturated plain old moving box cardboard, then glass mat/weave over the top of that. Epoxy saturated and cured cardboard is incredibly strong, it creates a cheap honeycomb core structure like they use in aerospace construction, but in a DIY form. This cardboard skin would also allow you to cut the foam core size in half.

 

[1n3]

The problem with thick cross-sections of core material is that the internal volume of the cabinet decreases...

... or external dimensions increase. I assume that subdude67 worked from internal dimensions.

Also, points taken re: commercial viability. But DIY is another matter.

 

[agedhorse]

... or external dimensions increase. I assume that subdude67 worked from internal dimensions.

Also, points taken re: commercial viability. But DIY is another matter.

The other side of the same "problem coin".