Written by Dimitri Musafia
Published: March 13, 2015 at 1:00 PM [UTC]
Case manufacturers choose the materials they use for their own reasons, but many claim that their cases give good heat/cold insulation even when they don’t. Well, hype and advertising aside, we can discover the truth if we look at the materials themselves, and their heat conductivity properties expressed in Watts per meter per degree Kelvin (W x m-1 x K-1)
Average heat conductivity of case shell materials
Wood laminate 0.13
Composite plastic (FRP) 0.23 – 1.06
Carbon fiber (molded) 5
Steel 16 - 24
Wood laminate is clearly more protective in this sense compared to composite plastics by a factor ranging between 1.8 to 8.1; to fiberglass by a factor of 4.6; to molded carbon fiber by a factor of 38.5; all the way up to aluminum (factor of 1,615!).
This is unbiased proof that wood laminate remains a superior material for violin case shells, despite newer materials being available.
(source for thermal conductivity: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html)
Wikipedia is a good starting source for these kinds of data
Note that this is about a third of that of laminated wood (0.13).
One would never be able to move medium-size and large devices packed in it.
Someone misinterpreted the density of polystyrene, a very hard, clear and brittle aromatic polymer, BEFORE it is foamed and turned into STYROFOAM.
Note that Styrofoam has a relatively low melting point. It does not look very promising as a substance for the frame of a violin case.
Wood 1, Styrofoam 0.
First off, styrofoam is much much much less dense (for typical) than your sg=0.3
Secondly, "compressed wood" is much much denser than that (sg = 1.1 to 1.4) but nobody wastes money using compressed wood!
Wood other than balsa ranges from sg= .35 to sg = .85 depending on species.
Styrofoam weighs next to nothing. sg = 0.02 to 0.06.
But are any cases made completely of aluminum? Or is a typical aluminum case lined with something that is sufficiently thermally insulating? I could easily envision a thin aluminum or fiberglass shell surrounding a styrofoam core. I have no idea how those types of hybrid constructions could compare to plywood ("wood laminate") for impact resistance.
In any event, since a case is a complete system, better to test whole cases than just one component thereof. The easy way to test a violin case for its insulating capability is to feed a copper-constantan thermocouple into the case, put the case into your chest freezer, and see how fast the internal temperature goes down.
Styrofoam is definitely the best material to make case shells from the standpoint of thermal insulation, having a conductivity of about 0.015 in the standard 25kg/m3 density.
The problem is that it is an inferior material to make case shells out of in terms of durability, stress fatigue, resistance to traction, and a whole lot of other issues. Having built over 2,000 cases with this material, I know, believe me! Which is why I didn't include styrofoam in this list.
I included aluminum because, yes, years ago there was such a case on the market made by an important manufacturer and I assume these products are still in circulation. Flight cases usually make extensive use of aluminum too.
Lastly, Paul, you are absolutely correct that a case is a system. I singled out case shell materials in this post because there are case manufacturers in the real world that do the same in their advertising.
Wood is cellular and therefore has that thermal advantage of foam, yet unlike foam, it has considerable strength. Also unlike foam, it is anisotropic in strength which is actually advantageous in terms of toughness--and especially energy absorption under limit loading. Wood will produce robust cases at near optimized weight.
To outdo wood, you need to go to a epoxy matrix kevlar-carbon skin with high density structural foam core. That can be built at about 30% less weight but the same thermal properties (approx) and roughly the same limit load. But the cost is ridiculous. The downside is that to keep that weight less than wood, you have very thin skins subject to puncture. Again, here is where wood is nearly impossible to better.
It is interesting to note that the density of cellulose is about 1.55. Carbon fibers are about 1.75, and kevlar about 1.44. But wood is "foamed" by an evolved sophisticated cellular structure whereas all composites are crude laminated epoxy saturated systems. The lignin in wood serves the job of epoxy but it doesn't fill the pores. As such, wood has both local resistance to buckling as well as global buckling stiffness. High performance compoxites are much poorer in buckling. The foam core is required to stabilise but must be lighter density--and inherently weak--compared to wood. It is too complicated to explain why the isotropic nature of foam is deleterious to toughness but just take my word on this (wood has a "weak" direction which actually makes it tougher in ultimate behavior).
Steel would be the toughest, except for one problem--it is much too heavy. It makes optimum tough structures for cars, but not for small things.
In terms of limit loading, I'm thinking of car driving over case.
To keep a stable microclimate within the case you need hygroscopic materials, which absorb and release moisture as necessary to slow changes of relative humidity, protecting the instrument.
Latex is a form of rubber, which does the opposite: it is waterproof to the point that medical gloves and condoms are made with this material.
For instance, does a velvet or velveteen move moisture in/out of the air faster than an 8 harness satin? Is rayon different from cotton for the same denier and weave? Is there a way to measure or design the amount of cellulosic material in the system relative to rates of humidity change?
The more the interior lining is hygroscopic, the more it flattens out the dips and peaks of relative humidity changes by absorbing and releasing humidity depending on the current and previous conditions.
For example, if the case is moved from a humid environment to a dry one, the lining will release the humidity it had absorbed previously, helping protect the instrument from the effects of dryness.
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