Gewa Violin case question
Has anyone seen the Gewa Air Prestige cases? I'm in the market for a case and I'vee seen a lot of BAM cases but have seen no reviews at all about these relatively new Gewa cases. It seems like the Gewa might have better thermal protection than the BAM contour and slim line cases and at a slightly lower price.Tweet
Replies (55)
Posted under my real name in accordance with Vcom's rules.
Cheers Carlo
In the thread Roger mentioned there was a comparison done of a Gewa Air vs. a Negri plywood case for cold protection, and the Gewa was marginally worse (something like 2* C cooler than the Negri after 20 minutes).
I'll only vouch for or against cases I've actually owned and used, instead if projecting my experiences with one brand of case onto the whole lot of them.
I have an oblong Gewa Air. I weigh over 200 lbs, and I can stand on either the lid or the back with minimal deflection (<1 cm). There is 2 cm clearance between the bridge and the lid, and 2 cm clearance between the bottom and the back of the violin. There is also about 2 cm clearance around the perimeter of the instrument and the sidewalls.
I also have the Bobelock 1017 that is widely recommended around here. It can only take 90 lbs on the lid before the arch flattens to the point that the instrument would be crushed.
There's other complaints I have about the Gewa but structurally it is far more rigid than the Bobelock and thermally it is competitive with most plywood cases. It's not as simple as "plywood good, plastic bad" as some other posters and other threads claim.
Beware of people regurgitating one-liners (often inaccurately and with very little understanding of the underlying mechanics) from that Plywood vs. Plastics/Composities article that Dimitri Musafia wrote some time ago, which also has a few basic factual inaccuracies that can easily be checked against material spec documents.
Posted under my real name, according to Vcom's rules.
Cheers Carlo
The one that stuck out to me was the claim (per your "9 Reasons not to get a carbon fiber case" post) that CF is more flexible or less rigid than plywood, when in fact it isn't. Typical flexural modulus of 50% CF by volume CFRP is between 40 to 50 GPa, whereas most plywood used for structural purposes has a flexural modulus of 10 to 12 GPa. I've made a post somewhere in one of the previous case topics detailing how to reconcile this with the seemingly contradictory observation that the existing CF cases on the market perform rather poorly.
The next point was the issue of melting. Thermoplastic cases can melt - this much is obvious. However what can be misleading is that your original "Wood Laminate vs High Tech Plastics" article groups thermoplastic shells with fiber-reinforced thermoset shells, which can't melt. Thermosets such as most epoxies (including the kinds used to laminate plywood) will gradually soften and become slightly rubbery as they exceed their glass transition temperature, but will not melt. With high enough temperatures however they will thermally degrade and break down. The actual fibers themselves, be it glass or carbon, are largely unaffected by the temperature ranges they'll usually be put through (even something catastrophic like a housefire).
I also found your point about an overly-flexible shell bouncing "subjecting the instrument to an even greater shock" to be a bit puzzling. I think you meant that it would subject the instrument to secondary impacts - it's not possible for the rebound to have a greater acceleration and peak force than the initial impact.
While this isn't a statement that you made, I do think it is a bit misleading to group both unfilled plastics (e.g. thermoformed ABS) with composites (GRFP, CRFP), as well as solid panel and sandwich layer all together. Each of these can provide orders of magnitude difference in the mechanical properties of the shell, as well as significant thermal insulation differences.
After all, the sandwiched ABS/PU foam/ABS shell of the Gewa air is what gives it the rigidity to support a 200+ lb person, while a solid-panel CF case can collapse with less than 100 lbs despite the fact that intrinsically CF is almost 20 times as stiff as ABS.
In addition, the manufacturing methods are vastly different yet you characterize them as "having a machine stamp them from a mold." If I remember correctly, you've worked with or are associated with boatbuilders that use composite layup processes, so you would know that the various thermoset layup processes (Be it vacuum bagging, light RTM or RTM) are actually pretty time-consuming and can be quite labor intensive.
Concerning your acid test at the end of the article - the reverse can be said for a much wider market for guitar cases. If fiberglass and carbon fiber are such poor candidates for case shells, why is it that the the most well-known top guitar case makers (Karura, Hoffee, Calton) pick those materials (albeit sandwiched with a foam core)? Keep in mind these cases have much wider unsupported spans with relatively flat tops and are treated much less carefully (i.e. thrown in aircraft luggage holds) than violin cases, and seem to be quite successful in protecting their contents.
Still, the momentum transfer (force x time) can be twice as big in the case of an elastic rebound and that quantity is typically more important for damage than the peak force. There is a reason that car crumple zones and motorcycle helmets are designed to crumple without elastic rebound. I'd argue that the scientifically undefined term "shock" should be interpreted as "damage potential".
@“The one that stuck out to me was the claim … that CF is more flexible or less rigid than plywood, when in fact it isn't.” - Well, since we seem to be talking Gewa here, take a carbon-fiber Gewa Idea 1.8, place it on a table, then open it up and see how flexible it is. Then take a comparative weight wood laminate Maurizio Riboni Unoeotto and do the same test. In the real world, the difference in rigidity is enormously in favor of a well-made wood laminate case.
@“The next point was the issue of melting. Thermoplastic cases can melt - this much is obvious. However what can be misleading is that your original "Wood Laminate vs High Tech Plastics" article groups thermoplastic shells with fiber-reinforced thermoset shells, which can't melt.” - BAM on their own website says not to place their “High Tech” model next to heat sources because it can melt; that was my point.
@”I also found your point about an overly-flexible shell bouncing "subjecting the instrument to an even greater shock" to be a bit puzzling.” - If you’ve ever seen an automotive crash test in slow motion, you’ll see how the car body absorbing the crash instead of bouncing off the barrier makes the occupants fare better. The same goes for violin cases. I've been studying this for years.
@”While this isn't a statement that you made…” - then why are you bringing it up?
@”In addition, the manufacturing methods are vastly different … so you would know that the various thermoset layup processes … are actually pretty time-consuming and can be quite labor intensive.” - That’s entirely true. However a properly-built wood laminate shell starting from pre-cut parts takes about 90 minutes to assemble, and it then requires a cover, which takes another 90 minutes to cut, sew, and install. That’s three full hours total highly specialized labor, Jeff, far more than it takes to make any plastic shell.
@“Concerning your acid test at the end of the article - the reverse can be said for a much wider market for guitar cases.” - Absolutely true, but we are talking about VIOLIN cases. Guitarists don’t seem to care about how much their cases weigh, while many violinists consider lightness to be of primary importance. It follows that manufacturers of guitar cases can use other materials. When guitar case manufacturers build violin cases with the same method (Hiscox for example) the result, while indestructible, is far too heavy for many violinists (8 lbs and more). (Disclaimer: my company sells Hiscox cases in Italy)
The purpose of my posts on v.com is to better inform violinists in the field of instrument safety, so they can make more educated choices regarding their purchase of a protective case and not fall for dishonest advertising ploys. In every article I’ve posted here about cases I have never touted my own products; on the contrary I’ve frequently endorsed my own competition.
If you look at my v.com profile you’ll see that, among other things, I am a researcher with the Milan Polytechnic University; our research group (which includes among others the Stradivari Violin Making School of Cremona and the University of Pavia) is currently testing the comparative safety of violin cases in their FAA-approved Transport Safety Laboratory (La.S.T.). The results will be published and provide scientifically obtained information beyond any reasonable reproach.
Would you mind sharing more details about it? What are you measuring and how? Will you be publishing the results? I hope you don't mind sharing - that is intriguing research.
Especially in recent times, with the sky-high values of some violins on the one side, and the trend to light-weight, often flimsy cases on the other, you see an obvious collision of interest.
Another factor is the demise of most of the small case manufacturers, leaving the lion's share of the market to deep-pocket multinationals which purchase advertising space by the square foot and sway musicians through hyperbole.
Hence the usefulness of our project. I cannot publicly share any of the details yet, as I am only a member of the group and not heading it. But I can broadly say that first we crush-pressure tested, via computer-controlled hydraulic press, a number of cases made of different materials to see their maximum resistance before instrument damage, also in relation to their material of construction and weight.
Next testing will involve impact resistance (crash-testing) in which the La.S.T. is highly specialized. It is the only FAA-approved laboratory outside of the U.S.A., which obviously guarantees competence and objectivity.
The final report will be published by the Milan Polytechnic in their scientific journal, after which I will have permission to republish the findings.
I don't know if you saw my post before or after the edit, but I had previously mentioned that I wouldn't mind a heavier case for absolute protection. I do consider your cases exceptionally beautiful, however. There is no replacement for good aesthetic or craftsmanship.
I don't really understand the obsession with ultra-light cases. More weight (such as the hiscox example) can provide better protection. It still weights less than a load of students books. I appear to always be in the minority though - but I don't mind. A little extra weight to me is negligible, probably due to size and being used to hauling a guitar case everywhere and not having to fly with my case.
I think that regardless of material choice, there are going to be trade offs in any decision made. I like this study because it's going to quantify some of them with hard numbers. We can all theory craft for hours or throw math around, but at the end of the day it's hard to disagree with a hydraulic press on what can take the most crushing, and even harder to disagree with peer reviewed results without recreating the study with different results.
I'm curious: If the results showed a different material to be more suitable, would you change your products to that material? I'm not taking a side in the matter, but rather being curious about how the data will affect you either way. If it shows in favour then you have a great new tool for selling products with.
Also, is the project also conducting thermal tests? I notice that this has been a point of contention in previous threads. Temperature can affect performance. I'd like to see testing done on both the internal temperature of the cases on a spectrum of, say, -30 to 30C, then the cases also pressured under the same conditions.
As a manufacturer, over the past 15 years we were forced (by "market forces") to reduce the weight of our cases in order to keep selling them. We lobbed 30% off the average case weight, but of course there are trade-offs. Not only have they become more complicated and expensive to build in order to maintain the same safety characteristics, but some lighter-weight materials, especially for the covers, simply won't last as long.
If people didn't care about how light violin cases are (and some competitors didn't understate the weights of their products by as much as 20%) I'd be a lot happier. That said, if our research leads to the discovery of more suitable materials for the task, without significant trade-offs, I'll be more than happy to use them. I've already learned a lot from these tests and am making modifications to our production. Only through continued testing and research can any progress be made, and continue to stay on the market!
RE thermal tests, I have many of my own to share, but I don't want to post them here because products are named in the graphs. If you'd like to see them, please email me privately.
I do remember your anecdote about the girl and the case and remember thinking at the time that it was madness - how much lighter can you actually make a case and expect it to survive anything at all? 5lbs is nothing to carry - the next step down in weight is cardboard! My teaching bag (laptop, books, theory reference, pens, etc) weights more than that.
I would rather have a well designed case that offers superior protection that weights 20lbs instead of a 2lb cardboard box that will crush if I set a book on the lid, and the fact that that sentiment isn't more common where there are ten's of thousands of dollars on the line astounds me.
Unfortunately there isn't much to be done I guess - the market is what the market is. I sincerely thank you for engaging with the community so readily and hope you continue to make high quality cases, no matter how silly consumers are!
The next case I buy, I will pay for extra reinforcement, AND extra weight. Protection then style come as high priorities for me. I carry an extra bag for everything else (Italian leather of course).
Cheers Carlo
Posted under my own full name in accordance with Vcom's rules.
You must always be fashionable - do not forget that the case cover must also be Italian leather to match the bag!
For example, positioning a case standing up on the sidewalk in direct sunlight at 35°C for a half hour reproduces waiting for a tour bus to show up on a summer afternoon a lot better than placing the case into an industrial oven and setting a thermostat.
The most important thing when doing comparison testing is that the two (or three) cases receive the exact same exposure simultaneously, be it to sunlight in high ambient temperature, or to cold in wintertime.
Yes, there is a margin of error, but usually the differences between cases are great enough to root them out, and in addition the tests are always repeated at least twice, looking for the same results.
Doing the repetition, we even used to switch probes to rule out measuring hardware influence, but don't do it any more as the results didn't change.
That is the argument my wife uses and after all, "what is sauce for the goose is sauce for the gander".
@Dimitri. What colours are available in leather for your cases?
Cheers Carlo
Posted under my own full name in accordance with Vcom's rules.
Cheers!
Cheers Carlo
Posted under my own full name in accordance with Vcom's rules
For this reason, the "Case Safety in Transport" project, as well as my own research, focuses only on new products made to original specification. It's then up to the manufacturer to choose materials which degrade slowly.
This is one reason why styrofoam, even when reinforced, is a poor material for case shell construction, although it is used by many (we used it ourselves as well between 2000 and 2003 on three pilot models).
On paper, it has the lowest thermal conductivity of any material used to make case shells; it is lightweight and exceptional at absorbing impact. The real world situation is that it suffers from fatigue: continued mechanical solicitation (such as opening and closing actions over the long term) causes it to lose its original strength, and once it's broken it is difficult to repair.
Styrofoam remains great for beer coolers though ;-)
I get that your goal is to inform violinists of the current market status of various cases. Mine is to dive into the details of the material specs and construction methods to provide a framework with which to better explain or understand said status.
I stated that CF is a more rigid material than plywood, and you rejected it with the example that the Riboni Unoeotto case is more rigid than the Gewa Idea 1.8. The two are not necessarily contradictory - I am making a statement concerning the materials themselves, measured through three point bending tests of flat samples that are then normalized per dimensions. The flexural modulus of these materials is not something that eggheads in a lab dream up and stick on a sheet - they are the actual results of "real world" tests, albeit controlled to reduce as much as possible confounding variables. You are comparing an example of a CF case with a Plywood case, with the only common link being that they are roughly the same weight. However your example still has the uncontrolled variables of geometry, shell thickness, shell construction, etc. and thus your example can only support the claim that Gewa's CF cases are less structurally sound than competing plywood cases.
That said, an equal-weight solid laminate CF panel with the same width and length as a plywood panel will be significantly less rigid due to the fact that CF is denser than plywood -> therefore it must be made thinner -> negatively impacts rigidity. The math on this is in one of the previous case threads, but it essentially boils down to how the actual rigidity of a panel scales cubically with panel thickness and only linearly with material stiffness. However, most competent designers or engineers would know not to use a solid CF laminate when rigidity is critical, and instead sandwich it to reduce overall panel density. When sandwiched, optimized CF panels will be stiffer than either optimized solid or sandwiched plywood panels.
Concerning Bam and thermoplastics- not much else to say here, in fact the Gewa and Hiscox cases (as well as any other molded ABS case) will have the same issues. Bam may be especially sensitive to it as the foam core they use (Airex, and presumably one of the vinyl-foam grades of Airex) actually degrades and softens at an even lower temperature than the ABS outer shell.
Concerning the bouncing shell: I am discussing the scenario in which a shell impacts against a surface, undergoes elastic deformation, and then rebounds from said deformation. My point was that the rebound cannot have a higher peak force and acceleration than the initial impact - this would be like saying the ball bounces higher than the point from which it was dropped. I am not saying that the rebound is in any way beneficial. Your example concerning automotive crash tests does not necessarily contradict this either - typically the vehicles that deform plastically (i.e. stay deformed instead of bouncing) have crumple zones that are "soft" enough that they even deform at all, which reduces the peak force during impact. The vehicles that bounce are the ones that have "crumple zones" that are rigid enough that they don't undergo as much plastic deformation, and this rigidity is also what drives up the peak force during impact.
Concerning cycle times of manufacturing methods:
These are just for comparison to the time you provided for plywood construction, though it's not entirely compatible because some processes don't need human interaction/supervision throughout the entire time.
Structural foam RIM: 2-3 minutes
Vacuum-forming: ~3 minutes for a single-layer shell, so ~6 minutes for both halves
Vacuum-forming with sandwich construction: ~12 minutes for the plastic shells, ~15 minutes to inject PU foam and cure so 27 minutes total
Hand layup with vacuum bagging: varies wildly by shop and process, can take half an hour or several hours depending on operator skill, and then some more if autoclaved (I don't think any composite casemakers are at that level)
RTM or Light RTM - 30 to 45 minutes
As for the time it takes to make the cover - that's a design choice on the part of the casemaker, though it may be more integral for wood-shell cases. Hiscox offers their violin case both with and without a cover
While guitarists seem to be more tolerant of case weights than violinists, they do face some pressure to go lightweight. Another guitar casemaker, Visesnut, also uses the ABS/PU Foam/ABS shell construction, similar to the Gewa Air. Their guitar cases weigh just a little over 7 lbs (albeit without a cover), but have been demonstrated to be able to support at least 180 lbs standing on the center, and around 500 lbs total when distributed over the entire lid. Despite the fact that ABS has mediocre innate rigidity, the sandwiched construction significantly bolsters its structural properties. These are used as flight cases as well. A point I'd like to make here is that the structure is just as important as, if not more important, than even the materials that are used.
Also Dimitri, it was one of your wood vs. plastic case comparisons that actually spurred my interest in material science & engineering almost 10 years ago. I was shopping for cases and became intensely curious why some were terrible and some weren't despite outward similarities. And to quote something from your site, "God is in the details."
Adrian asked: "Do all plastics split easily if not reinforced with fibres?"
No, but this gets complicated. Plastics that have relatively large %elongation to break values such as LLDPE and PC don't split very easily because the edge of any beginning crack will tend to stretch out and form a fillet, relieving the stress concentration and preventing further splitting. Most unfilled plastics become much stiffer and slightly stronger at low temperatures, but they also become more brittle, and the increased stiffness exacerbates the stress concentrations at any existing damage or cracks.
Fiber reinforcement (typically glass or carbon) has the result of increasing stiffness significantly. However, depending on the type of reinforcement (meaning short fiber, long fiber, or continuous fiber), the impact resistance of the plastic could actually become worse. Continuous fiber generally improves the impact resistance as the fiber must either break (which takes a lot of force) or it must be pulled out of its surrounding matrix (which absorbs a huge amount of energy). Short fibers often have deleterious effects on impact resistance as they introduce many discontinuities within the plastic itself, and the increased stiffness again exacerbates existing stress concentrations.
Many plastics are also susceptible to UV degradation if not coated properly, and some such as ABS and Polycarbonate are especially weak against certain chemicals like acetone. Others like thermoplastic polyethylene are fairly chemically inert, but are still rather susceptible to environmental stress cracking due to VOC's, but when cross-linked become extremely resistant.
Weight matters when you're walking with the case a lot. When I was a university student, I used to have a routine 20-block walk between my home and rehearsals. I was toting a Bein & Fushi case (a Hill-style case that was relatively heavy, manufacturer unknown, I'm still curious who made those). I'm sure that case contributed significantly to the fatigue of those walks.
Most people aren't just carrying the case by itself, either. The accessory pockets are crammed full, the music pocket is bulging, and they may be toting a backpack or the like as well. So it's a whole lot heavier than the baseline. No one seems to have designed a really comfortable backpack configuration for a violin case, as far as I know.
If you never walk more than a couple of blocks with a case (or for many players, a greater distance than the parking lot to the building), weight doesn't matter much, and you can indulge in other protections that add weight. If you take public transit, for instance, thermal protection matters -- the 20 minutes you can spend waiting in the hot sun or freezing cold will have an impact. Waterproofing matters when you're trekking through a downpour, even for a handful of blocks. Impact protection matters if you walk through snow and ice, and you take a tumble -- slip on ice (or anything else) and you can smash your case into the ground with amazing force, and add the impact of landing hard on top of it. (I have done all of these things before, and have been grateful to my case's protections each time.)
My new Musafia Enigma isn't that much heavier than my old Aeternum, but the minor difference is still enough to notice. The additional protections are worth the weight, but I'm glad I'm not slogging 20 blocks, rain, shine, 100-degree or freezing-icy temperatures, with it.
The only thing that I would object to is your statement about the rigidity of CF cases vs. that of well-made wood laminate cases in the same weight and price category (although actually the Riboni is much cheaper than the Gewa). Let’s take this into the practical world. Do you know of *any* pure CF case, currently on the market, which can compare favorably to the Unoeotto in terms of both weight and rigidity?
Also, you have quoted production time numbers. Just to be sure we’re on the same page here, are these numbers relative to making violin case shells? If so, I would assume it’s proprietary information – may I kindly ask your source?
- In the U.S. heavier cases are often accepted because people drive their cars everywhere; many people don't use the back pack system at all and for years we provided it but uninstalled
- in Holland people ride their bikes everywhere, hence the need for a lighter case with bike-optimized back-pack system, i.e. a lower position of the ring(s) on the bottom panel
- in Japan people use public transportation walking to-and-from it, and have a smaller average stature. This requires a lighter case with back-pack system with a different optimization, i.e. a higher position of said ring(s)
...etc.
Over time these requirements evolve. In my private collection I have two 30-year-old cases from Hill and from Jaeger, that not only weigh a ton but don't have any strap attachments, let alone back-pack capability. Yet, they were the gold standard for violin cases not even all that long ago.
While I don't care about weight I do care about my options to handle it. I frequently wear my case with a single strap cross chested, since these days I do drive a lot (I used to take public transit and have decent foot commutes daily, though) and just need to walk 2-300 feet.
There are even days when I need to carry a viola case and a guitar case and a laptop bag. Something needs to give!
I think without the straps to allow easier multi tasking I might care more about weight; but with them I'm much more flexible.
The Gewa Air oblong measures 6.6cm in depth in the lower part, and 5.9cm depth in the lid (considering the 6mm overlap of the tongue-and-groove closure). So the total vertical space inside the Gewa Air oblong is 12.5cm.
A typical new professional-quality violin measures an average of 9.6cm from back to bridge when placed on a flat surface. Assuming the suspension system is correctly designed, and that the clearance is evenly distributed between the back of the instrument and the bridge, this leaves 2.9cm total space (1.45 for the back and 1.45 for the bridge).
These are actually quite good numbers but inferior to the 4.0cm total space (2.0 + 2.0) quoted by the poster.
Just for the record.
Cheers Carlo
Why do we even waste our breath discussing plastic cases made by robots when masterpieces like these are available to purchase.
Now I'm thinking, do I NEED another case...
Cheers Carlo
Posted under my own full name in accordance with Vcom's rules.
The case is a trivial percentage of the cost of its contents, which made it much easier to decide to spend the money, especially since I am accident-prone.
I think the decision is much harder when the contents aren't as expensive.
You asked, "Let’s take this into the practical world. Do you know of *any* pure CF case, currently on the market, which can compare favorably to the Unoeotto in terms of both weight and rigidity?"
I've not claimed that a pure CF case will ever be more rigid than a well-made plywood case. From one of my paragraphs of too many on this subject:
"That said, an equal-weight solid laminate CF panel with the same width and length as a plywood panel will be significantly less rigid due to the fact that CF is denser than plywood -> therefore it must be made thinner -> negatively impacts rigidity. The math on this is in one of the previous case threads, but it essentially boils down to how the actual rigidity of a panel scales cubically with panel thickness and only linearly with material stiffness. However, most competent designers or engineers would know not to use a solid CF laminate when rigidity is critical, and instead sandwich it to reduce overall panel density. When sandwiched, optimized CF panels will be stiffer than either optimized solid or sandwiched plywood panels."
Basically what the above plate mechanics boils down to:
- Pure CF vs Plywood: Plywood will always be more rigid at the same weight, or lighter for the same rigidity. I can't name a pure CF case that can match a plywood case weight for weight at the same rigidity. If someone told me there was, I'd be highly skeptical.
- Sandwiched CF (e.g. something akin to Karura's guitar cases) vs Plywood: Depending on the scale of the panel, properly sandwiched CF panels (e.g. CF/Nomex/CF) can be more rigid than quality plywood panels. Larger structures tend to benefit more from sandwich structures than do smaller ones due to space and minimum thickness constraints.
- Sandwiched CF vs Sandwiched Plywood: When optimizing for rigidity (and nothing else), sandwiched CF panels at the same weight will be slightly more rigid than sandwiched plywood panels.
The reason that I keep bringing up these distinctions is this model more accurately describes real-world scenarios beyond just the scope of violin cases, and applies equally as well within violin cases. Otherwise we're left with a "sorta rule-of-thumb but with a ton of exceptions."
The carbon fiber Musilia P2 case -claims- a weight of 1.8 kg, and a crush resistance of > 600 lbs. Since I neither own one nor do I have easy access to something that can apply and control 600 lbs of force, I can't support or deny this claim. If they are being truthful, then I am 95% positive they must be employing a sandwiched construction.
Food for thought: If you took a 6-layer plywood panel, and replaced the middle two layers with a typical sandwich core foam that's half the density of plywood but twice as thick, you would find that the overall weight of the panel is more or less the same but it has become significantly stiffer in bending. Intuitively, the outermost layers of the laminate bear the brunt of the compressive and tensile stresses. As you approach the center of the laminate, each layer bears less compressive or tensile stress, and carries only shear stress (in pure bending scenarios). Of course there are trade-offs to doing this.
Concerning the production times: none of the times I quoted are specific to violin cases. However, with the exceptions of vacuum-bagging and RTM, none of the other processes are significantly affected by the size or shape of the object being molded.
Vacuum-forming times are quoted from an interview with Brynn Hiscox (http://www.acousticmagazine.com/interviews/brynn-hiscox-case-history/), where he states it takes 3 minutes to form a shell. Cycle times for vacuum forming depend mostly on the material and material thickness, which affect how long it takes to heat and cool.
Concerning the foam-fill time: http://blog.fibreglast.com/fibre-glast-products/a-practical-look-at-mix-and-pour-foams/
"foam can be poured in layers, waiting 15–20 minutes between layers"
It takes longer to fully cure but the part doesn't have to remain fixtured once it has reached full expansion.
Structural Foam RIM (reaction injection molding):
http://www.reactioninjectionmolding.com/rim-process/
The only case I know of that uses this process is the Gewa Maestro/Venetian and the modern Jaeger cases. This is in their advertising material, where they explicitly state they "replaced the wooden shell of the old case with a 2K foam." 2K foam refers to two-component foam.
I'll get back to you when I find my reference again concerning the vacuum-bagging times. If I don't find it then I'll retract my statements for it.
Concerning the clearances within the Gewa case - while I appreciate that you didn't name me specifically, I'll take this chance to own it and say I bungled that measurement - sorry.
But now that you have an Air case on hand, I'm curious what your thoughts are concerning the attachment of the feet and the hinges on the hinge-side of the case. To me, it seems a bit risky to have them sharing the same rivet.
I’ll just say at this point that if we talk about all kinds of thermoplastics, fiberglass, carbon fiber, Airex, Nomex, other foams and their sandwiching in relation to “plywood”, then we should take into account the many varieties of “plywood” as well, such as softwood, hardwood, marine grade, aircraft grade, and others.
Properties include type of wood, thickness, number of plies, orientation of the plies (i.e. not necessarily 90°), and types of glues used; there are about a dozen quality grades, basically from “defect-free” to “all defects permitted”. Lastly there are multiple grading standards and Lloyd’s of London even has their own certification for marine grade laminate (BS1088).
I’ve been using the term “well-made laminate shell” to define not just a generic “plywood shell” but one in which the manufacturer actually put some effort into choosing the laminate with the right characteristics for the application, and not simply the cheapest (which a lot of low-end manufacturers do for obvious reasons).
Some high-end manufacturers actually have their own laminate specially made. In the 1990s, when I had some cases produced on license in China, I had to ship the laminate over there from Europe because the locally-sourced material was really terrible.
You’ve piqued my curiosity though about an inner foam lining, which I think I am going to test. I’m not sure what the actual result will be, but there is only one way to find out.
Lastly, regarding your query about the attachment of the feet and hinges of the Gewa Air, it’s obviously a labor saving solution (one rivet does double duty) and only three feet (again labor-saving) means that it will always sit in plane and not rock when set down on an uneven surface. The only real drawback in my opinion is that it’s a lot harder to repair if broken, than if a screw or nut-and-bolt assembly were used, but at the same time it looks pretty strong.
Cheers!
Now, I just happen to have a copy of the test report carried out by TUV Rheinland (a testing-certification agency, like the American National Standards Institute) of the Musilia P2 crush pressure test, which took place on Jan. 14, 2015 in TUV's Leipzig, Germany facility.
Yes, the case indeed was subjected to a pressure of 3200 Newtons (326 kg/ 717 lbs.) but at the expense of a inwards collapse of the lid measuring 32mm. Our experience at the lab at the Milan Polytechnic showed that 15mm inwards collapse was enough on many cases to cause damage to the violin. (The graph shows that at 900 Newtons of pressure the Musilia P2 had already flexed inwards 15mm)
In addition, the pressure was spread out over a large surface, the extent of which was not specified, nor was the pressure point location (over the bridge of the violin? In the middle of the case? Toward the end, where it's strongest?). Lastly, during the test the case bottom was not even fully supported. A scientifically-conducted test? Yes... for those with a sense of humor.
This clearly explains why TUV Rheinland declares, on the document, “This test report does not entitle to carry any test mark”. In other words, it proves nothing at all.
Although that doesn't stop our friends at Musilia from claiming so anyway.
Lydia, although I do think $195 is cheap for a case that is billed to be of professional quality, you're are certainly right about Musilia cases. Their prices have gone up considerably since I last looked and the Musilia P2 is indeed quite costly for a Chinese-made industrial product with only a 1-year warranty.
-Stay away from polystyrene foams not only because they have poor mechanical properties compared to more typical structural sandwich core foams (e.g. Vinyl foams), but also because they react very poorly with certain resins like vinyl ester.
-Beware of Polyurethane foams as a core material - these are also lower in mechanical properties at the same density compared to vinyl foam. If you ask a composite boat builder about PU foam cores for hull elements, they might just ask you to leave their shop! Also, the polyiso versions are quite brittle and can delaminate without warning (think of spray-on insulation foam)
-Alternatively you can also use end-grain balsa as a core. It comes in densites as low as 8 pounds/ft3, which is a little heavier than the typical 4 to 5 pcf density foams. The downside is it's much harder to get balsa (even the scored kind) to conform to a radius, while it's fairly easy with a vinyl foam core.
I've been scouring more spec sheets, and the highest flexural modulus for any grade of plywood I've seen is 16 GPa, which is between 6 and 7 times as stiff as ABS (2.5 GPa). If a clever arrangement of ABS/PU foam/ABS can produce a very rigid structure at a reasonable weight, then by logic a similarly clever arrangement of Plywood/low density core/plywood should produce an extremely rigid structure also at a reasonable weight. In fact during WW2, the British de Havilland Mosquito bomber used a 11 mm thick plywood/balsa/plywood sandwich for most of its fuselage and wings, and was remarkably successful.
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