That would be very nice. A good sized 4x10 cab with some heavy duty neo speaks all at a weight that is easy to carry. The dream that may be a reality.
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DIY Lightweight Composite Cab Build
- Thread starter zac2944
- Start date
That would be very nice. A good sized 4x10 cab with some heavy duty neo speaks all at a weight that is easy to carry. The dream that may be a reality.
I just read your post here and have some observations.... My back ground is in boat manufacturing / Molding R&D. I believe the tone would be more wood like and the strength would be enhanced by using nidacore polyvinyl foam rather that EPS Foam... your foam serves no structural purpose other than being a mold for the glass. A polyvinyl of ¼ inch would be at least 10 times stronger... if you where to puncture the outer skin of your lamination you have no real structural middle... I know that this was a prototype and I would have done the same thing I am just making some suggestions if you are contemplating producing some.. The stiffness added by nidacore would allow you to stand on the thing no need for additional bracing! Second you could cut your material waste and labor significantly by molding and using polyesters. (I am not the biggest fan of polyesters but they do work well in molding) you are looking at a 3 piece mold. Good Idea that seems to have a lot of following... Good execution as well.
Um.. Are you sure of those statements? With all due respect to your background and experience, I'm not convinced your ideas work out. A 10 times (1000%) increase in stiffness (not to be confused with strength) just by changing core materials and even going from a 1" to a 0.25" thickness? Can you provide more info? Tell me more about this awesome NidaCore PVC material. It really sounds great.
Ok not 10% I actually do not know what the comparison would be because xps the pink stuff is not used in building just mockup. composite core foams are as light as your pink xps home insulation but will not break by hand... they are extremely rigid. this is what gives the strength to hang outboards off the back of boats. you can literally take a 8foot piece across two chairs stand in the middle and no flex with a 1" thickness no glass skins. most boat decks (the part you walk on) have a sheet of 1/4 Divinycell laminated to the skin to keep the deck from flexing. This would be the best material due to cost.
Nidacore is a honeycomb that is very strong but $$$...
I am not trying to nit pick but help you... I would do this myself but I know nothing about the design aspect for cabinets....
I'm with you. I know you're not nit picking. I appreciate your input and sharing your experience.
I'll look into the foam you're describing and take a look at the math. Sounds very promising.
A ~30 pound 2x15 would make my pants get tighter. Only by a little bit, though.
Yeah, i had some pants like that too
Ok, I did a little research. PVC foam looks like nice stuff. But as far as I can tell Nida-Core only makes PET, PU, and Poly-ISO foams along with their honeycomb and balsa products, not PVC. PVC does seem to be the standard in boat building though. I'm sure it would be nicer than EPS. The leader in PVC foam cores is DIAB, and thier two biggest types are called Klegecell and Divinycell.
For my project I used XPS (eXtruded polystyrene), not EPS. EPS (Exspanded polystyrene) is the white "beadboard" type of foam that breaks off into tiny white ball. The XPS that I used is also known at Styrofoam (DOW, the blue stuff), or Foamular (OwensCorning, the pink stuff). XPS has much better physical properties than EPS.
Here's the quick answer to why I didn't use PVC (or other high end core material) and went with XPS. Price vs. structural benifit. Have you ever priced out PVC and compared it to XPS. I can get a 4'x8' sheet of XPS for $5. The same sheet of PVC can cost more than $100! Availability is another thing. I can get the XPS at Home Depot. Where do you go to get a sheet of PVC?
Mmmm.. sort of. Because of its low density, the material properties of the foam core do not contribute directly to the stiffness of the composite panel. This is true for any low density (< 5lbs/ft^3) core materils. It is the thickness of the core material that adds to the siffness. The XPS I used it about 2lbs/ft^3. This is why you see XPS used in low weight, low cost products like surf and sail boards.
As far as I can tell you are very wrong about that. A 1/4" thick composite panel made with even the the most dense PVC foam DIAB makes will still not be as stiff and rigid as my 1" panel made with XPS. This assumes all other variables are the same except the core material and core thickness. Remember, I'm talking about a finished panel with thin (0.010") fiberglass/epoxy skins on either side, not just the foam core.
Your post got me thinking last night so I decided to do the calculations to predict what the change in stiffness and change in weight would be when switching from XPS foam to PVC, and then comparing the PVC at 1/4" thickness to my XPS core at 1".
Here is a summary of my results:
So, as you can see, your 1/4" thick PVC core panel is much less stiff than my 1" XPS panel. Actually, my 1" thick XPS panel should be 15 times stiffer than a 1/4" thick Klegecell R45 PVC panel, and 14 times stiffer than a 1/4" thick Divinycell H250 PVC panel.
If the panels were all the same 1" thickness, the Klegecell PVC panel would be 3% stiffer than my XPS panel, but would be 1.6lbs heavier. 1.6lbs isn't much, but consider that the finished cab is only 23lbs. That extra 1.6lbs is a 7% increase in total weight of the cabinet. So here's the trade off. I'm getting an extra 3% gain in stiffness for a 7% gain in weight. This doesn't even consider the extra cost involved.
This situation only gets worse when you look at the Divinycell H250. I gain 21% in stiffness, but now I add 21.4lbs to the total weight of the cab! My 23lb cab now weighs 44.4lbs.
Here's the kicker. You want your 1" XPS panel to be 21% stiffer? You don't do this by making the core material stiffer. You do it by making the core thicker.
If my XPS core was 1.25" instead of 1", my new stiffness would be 56% higher, but would only add 0.8lbs to the total weight of the cab. This is way stiffer than the 1" PVC cores and with hardly any weight increase.
Please understand that I'm presenting this here as an exercise for my own learning, and not trying to be a know-it-all engineer geek. I hadn't actually gone through all this math until know and it was very interesting to see.
I will present all my math and sources for material data in another post to follow.
Now to comment on a few other things in your post.
I can actually stand on my cab without any issues, and I don't have any extra bracing in the cab. I'm about 175lbs or so and wouldn't hesitate to jump up on the cab. I'll have to post a pic of me standing on it.
I hear you about the molding process. I'm seriously looking into this at the moment. There are some serious hurdles to overcome though.
As far as polyester is concerned I didn't use it for two reasons. 1. It melts XPS. 2. Much lower physical properties compared to epoxy. The strength, stiffness, and impact resistance of the fiberglass skins are very critical. Using polyester would have saved a few bucks, but would have lowered the physical propertied of the finished cab by too much.
Thanks man. I love to hear from folks who know a thing or two about this technology. I'm fascinated by it and think it has a lot of promise.
Here's some info about my calculations.
The general term for bending stiffness is flexural rigidity (D), which is the product of the material(s) elastic modulus, and the cross section moment of inertia (I). For a symmetric sandwich beam (both skins have the same thickness and material properties), the formula for flexural rigidity is:
source
Where:
Ef = Elastic Modulus of the Facings (Skins)
Ec = Elastic Modulus of the Core
b = Width of the Beam
d = Distance Between Facing Centroids
t = Thickness of a Facing
c = Core Thickness
Ef = 3390000 lb/in^2 [Invalid or Expired Link Removed] see page 6-35
Ec_XPS = 2030 lb/in^2 [Invalid or Expired Link Removed]
Ec_Klegecell R45 = 7977 lb/in^2 [Invalid or Expired Link Removed]
Ec_Divinycell H250 = 46412 lb/in^2 [Invalid or Expired Link Removed]
b = 1"
d = 1.01" or 0.26"
t = 0.010"
c = 1" or 0.25"
Density of different foam cores:
Styrofoam Core 40 = 2.0lb/ft^3
Klegecell R 45 = 3.0lb/ft^3
Divinycell H250 = 15.6lb/ft^3
all density data comes from sources above
Volume of foam used to build cab
When core is 1.25" thick = 1.96 ft^3
When core is 1.0" thick = 1.57 ft^3
When core is 0.25" thick = 0.39 ft^3
I'm pretty sure all the info you need to reproduce my calculations is above. If I'm missing anything just let me know.
The general term for bending stiffness is flexural rigidity (D), which is the product of the material(s) elastic modulus, and the cross section moment of inertia (I). For a symmetric sandwich beam (both skins have the same thickness and material properties), the formula for flexural rigidity is:
source
Where:
Ef = Elastic Modulus of the Facings (Skins)
Ec = Elastic Modulus of the Core
b = Width of the Beam
d = Distance Between Facing Centroids
t = Thickness of a Facing
c = Core Thickness
Ef = 3390000 lb/in^2 [Invalid or Expired Link Removed] see page 6-35
Ec_XPS = 2030 lb/in^2 [Invalid or Expired Link Removed]
Ec_Klegecell R45 = 7977 lb/in^2 [Invalid or Expired Link Removed]
Ec_Divinycell H250 = 46412 lb/in^2 [Invalid or Expired Link Removed]
b = 1"
d = 1.01" or 0.26"
t = 0.010"
c = 1" or 0.25"
Density of different foam cores:
Styrofoam Core 40 = 2.0lb/ft^3
Klegecell R 45 = 3.0lb/ft^3
Divinycell H250 = 15.6lb/ft^3
all density data comes from sources above
Volume of foam used to build cab
When core is 1.25" thick = 1.96 ft^3
When core is 1.0" thick = 1.57 ft^3
When core is 0.25" thick = 0.39 ft^3
I'm pretty sure all the info you need to reproduce my calculations is above. If I'm missing anything just let me know.
A ~30 pound 2x15 would make my pants get tighter. Only by a little bit, though.
I assume you're joking. Right?
I did some quick math and i would estimate I could build a 2x15 using two Eminence Kappalite 3015LF and one mid driver like the 18sound, and have it down to about 45lbs. If I used the non-LF version on the 3015 and no mid driver I could get the weight down to just under 40lbs. Considering the size of these cabs that is extremely light, but 30 lbs just ain't going to happen using the methods I used on the 210.
With something like a 2x15, will the larger size of the cabinet make balance a problem? The cab will probably be deeper, so will the larger size of the 15" speaker keep the center of balance or will it get front heavy?
You could probably fix that with a slanted front baffle or something... not saying that I know there would be a problem, just a possible fix to a hypothetical problem.
I hear you. It wouldn't be much different than the balance of the 210. For a more stable cab, you ideally want the center of gravity of the cab to be in the middle of the cab and closer to the ground. The drivers center of gravity is back closer to the magnet, so that helps a bit. One way to help move weight toward the center is to set the baffle deeper into the cabinet. This allows you to get the cab feet out in front of the drivers.
On the 210 I located the top handle above the COG, and a little to the left of it (looking down) so that the handle was in the center of cab, but closer to the baffle. It is actually only 3-4 inches from the front of the cab. The drivers on the 210 are on the right side, so the COG is shifted a bit to the right. I put the handle in the center though for aesthetics.
Bottom line; a 215 would be very stable, but weighted a little toward the baffle.
Here's a nice pic of Fred (TB: Freddles) holding the ZacLite 210. This gives you an idea of where the COG is. See how the handle is in the front, but centered about the middle (side to side). Also note how it tips a bit since the COG closer to the drivers but the handle is centered (side to side).
Also note his posture. This thing is really light. It doesn't look like he's struggling.
Also note his posture. This thing is really light. It doesn't look like he's struggling.
Also note his posture. This thing is really light. It doesn't look like he's struggling.
Umm yeah, I'm actually really, really strong. I'm also known for my great posture.
It's really a very light cab. I'd love to try out a 2x15 or 4x10 or 6x10 with my SVT-II.
We'll be having crafting classes in fiberglass at Zac's house.
Also look at the guy in the back- that look of astonishment is PRICELESS!Here's a nice pic of Fred (TB: Freddles) holding the ZacLite 210. This gives you an idea of where the COG is. See how the handle is in the front, but centered about the middle (side to side). Also note how it tips a bit since the COG closer to the drivers but the handle is centered (side to side).
Also note his posture. This thing is really light. It doesn't look like he's struggling.
The check is in the mail.
Seriously though, a 45 lb. 2x15 would be amazing! I like the idea of angling back the top section by a few degrees -- and of course the midrange driver would go on the angled section. The only downside is, while I could lift it with one hand, I still couldn't fit it in my trunk...
I thought I remembered in some of your earlier posts that you used some timber, covered in packaging tape, to mould out some fibreglass bracing. Here I was thinking that the cab was braced, but it turns out you didn't even use those braces. That's awesome.
And for what it's worth, your calcs on stiffness Vs Weight Vs Cost look ok to me. Having physically handled a piece of XPS for the first time recently, I was initially very surprised at how stiff it felt, even without the fibreglass skins on either side. Yes there is "better" stuff out there, but "better" is relative to the design, the final product's intended use, and of course cost.
The way I look at it, there's probably "better" timber out there than the typical birch ply you find on most standard cabs, and the reason they don't use it is similar to why Zac doesn't use fancy foam.
I assume you're joking. Right?
I did some quick math and i would estimate I could build a 2x15 using two Eminence Kappalite 3015LF and one mid driver like the 18sound, and have it down to about 45lbs. If I used the non-LF version on the 3015 and no mid driver I could get the weight down to just under 40lbs. Considering the size of these cabs that is extremely light, but 30 lbs just ain't going to happen using the methods I used on the 210.
Yes, only kidding. Kinda. More like a joke mixed with an uneducated guess.
So to fix it:
A ~45 pound 2x15 would make my pants get tighter. Only by a little bit, though.
Ok, that's an understatement. It would give me a full-on chubby.
