Building a hard case: internals from expanding foam or polystyrene sheet?

[Jughead6]

Not sure if this post is in the right place. If it isn't, then I trust the good moderators to move it to an appropriate forum . I am building a hard case for one of my basses, and my problem may be more a question of general carpentry/upholstery, rather than luthiery.

I have built the external hard case, and I am pleased to say it is going quite well considering it is my first time. But I am struggling with the internals. I want a nice snug, form-fitting cavity for the body of the bass to sit in, and I figure my choices are (1) build a mold and create the form-fitting internals using expanding foam (the stuff that comes in a can) or (2) buy a piece of a polystyrene sheet and carve the form-fitting internals.

Does anyone have experience with this, and can you share some pointers/tips?

 

[Bruce Johnson]

First, the spray-in expanding foam isn't what you really want for a case. That's a rigid foam. Once it's cured, if you crush it down, it doesn't spring back. It's permanently damaged. The same with rigid polystyrene foam board. Once it takes a load and squishes down, it stays squished.

You want foam rubber, like what's in upholstery and pillows. Foam rubber comes in different densities, rated in lbs/cu. ft. The real light squishy stuff is 2-3 lbs./cu. ft. The heavier foam that you want for a case surround is like 8 lbs./cu. ft. The light stuff is too squishy, and won't absorb the shock of the weight of the bass, when the case is dropped. You need the heavy stuff. And it isn't cheap. Most towns have a foam shop that supplies the upholstery shops.

Most case companies build up the foam inside of the case in two layers. First, they put down a layer of thin sheet foam rubber, like 1/2"-1" thick, covering the whole inside bottom of the case. Then, they take a layer of 2" thick foam rubber. Lay the bass on top of it, and trace around it with a Sharpie. A bandsaw is used to cut out the outline. A bandsaw cuts thick foam rubber sheet very easily. Then the cutout thick sheet is glued down to the thin sheet with spray-on contact cement. That's the foam structure. From there, the plush fabric is traced and cut out with scissors, sewn together on a sewing machine, and glued down to the foam with contact cement.

Now, to save money and weight, you don't have to do a complete foam surround of the instrument. That's actually kind of a waste expensive foam. You really just need to hold the instrument at several good places. For example, make a padded neck yoke with a Velcro strap that holds the neck at about the 12th fret. Let the neck and headstock float in free air. That's actually safer for the bass. Likewise, you can put padded blocks at several places around the body, to hold in place securely, without completely surrounding. Overall, you want the bass held inside the wooden outer shell, so that a shock load in any direction will make the bass squish at least 1/2" of heavy foam rubber. 1" of foam is even better. That's the goal.

 

[Jughead6]

Thanks Bruce, you're a champ. I really appreciate your comprehensive response.

 

[Will_White]

First, the spray-in expanding foam isn't what you really want for a case. That's a rigid foam. Once it's cured, if you crush it down, it doesn't spring back. It's permanently damaged. The same with rigid polystyrene foam board. Once it takes a load and squishes down, it stays squished.

You want foam rubber, like what's in upholstery and pillows. Foam rubber comes in different densities, rated in lbs/cu. ft. The real light squishy stuff is 2-3 lbs./cu. ft. The heavier foam that you want for a case surround is like 8 lbs./cu. ft. The light stuff is too squishy, and won't absorb the shock of the weight of the bass, when the case is dropped. You need the heavy stuff. And it isn't cheap. Most towns have a foam shop that supplies the upholstery shops.

Most case companies build up the foam inside of the case in two layers. First, they put down a layer of thin sheet foam rubber, like 1/2"-1" thick, covering the whole inside bottom of the case. Then, they take a layer of 2" thick foam rubber. Lay the bass on top of it, and trace around it with a Sharpie. A bandsaw is used to cut out the outline. A bandsaw cuts thick foam rubber sheet very easily. Then the cutout thick sheet is glued down to the thin sheet with spray-on contact cement. That's the foam structure. From there, the plush fabric is traced and cut out with scissors, sewn together on a sewing machine, and glued down to the foam with contact cement.

Now, to save money and weight, you don't have to do a complete foam surround of the instrument. That's actually kind of a waste expensive foam. You really just need to hold the instrument at several good places. For example, make a padded neck yoke with a Velcro strap that holds the neck at about the 12th fret. Let the neck and headstock float in free air. That's actually safer for the bass. Likewise, you can put padded blocks at several places around the body, to hold in place securely, without completely surrounding. Overall, you want the bass held inside the wooden outer shell, so that a shock load in any direction will make the bass squish at least 1/2" of heavy foam rubber. 1" of foam is even better. That's the goal.

What about using a rigid foam with a layer of foam rubber over the top of it?

 

[Jughead6]

What about using a rigid foam with a layer of foam rubber over the top of it?

Yes, I was thinking the same thing. Wonder if anyone has tried this?

 

[mapleglo]

Although I never finished it, I used a "pad" for a sleeping bag, cut it up into layers, and spray glued them together. I used regular foam for the top portion. Seems to work fairly well.

 

[Jughead6]

Although I never finished it, I used a "pad" for a sleeping bag, cut it up into layers, and spray glued them together. I used regular foam for the top portion. Seems to work fairly well.

OMG, that case looks suspiciously like the one I am building. I like your idea. My cat was sitting on my wife's yoga mat this morning, and it got me thinking of using something similar.

Turns out that timber is cheap compared to foam rubber!

 

[mapleglo]

OMG, that case looks suspiciously like the one I am building. I like your idea. My cat was sitting on my wife's yoga mat this morning, and it got me thinking of using something similar.

Turns out that timber is cheap compared to foam rubber!

The pad was something like $15.00. I seem to remember getting it on sale at Cabela's.

 

[Bruce Johnson]

What about using a rigid foam with a layer of foam rubber over the top of it?

But why do it? You'd just be wasting space. If you have 1" of space between the back of the body and the inside of the wood shell, the best thing is to fill it with 1" of 8 lb foam rubber. The idea is that, when the case gets dropped, the weight of the bass will squish the foam rubber down maybe 3/4" and then spring back up. That motion absorbs the shock and prevents applying it to the bass. If the foam is too soft (like 2 lb foam), the bass will squish the foam down to nothing and bang on the wood case shell, defeating the whole purpose of the foam. If the foam is too stiff, then the bass gets hit with more of the shock load.

Again, the purpose of the foam rubber is to hold the bass suspended in all directions within the case shell. When the case is dropped, you want the bass to gently move 1/2" or so and spring back. That motion is what absorbs the shock and prevents damage to the bass. Here's a guideline: If you set the bass on top of the foam rubber and press down on it with, like, twice the bass's weight (20 lbs), the foam should compress to about half its thickness. That's about the right stiffness for the foam, for it to be able to absorb the shock loads.

As a side note, that's why most classic Fender-style hard shell cases are actually terrible at absorbing shock. The interior of a classic hard shell case is basically wooden blocks covered with plush fabric. The plush fabric has virtually no shock absorbing capability. Drop that case and the bass slams right against the wooden blocks.

At the opposite end, flight cases have excellent shock absorbing capability. The shells are larger, and they have at least 2" of 8 lb. foam in all directions. They can take a heavy impact from any angle.

Anyway, there are two problems with the rigid foam. First, when it crushes, it stays crushed and doesn't spring back. It's only good for one impact. Second, most of it is way too stiff for this application. If you were to lay your bass on a sheet of the rigid foam, think of how hard you'd have to press down on it to crush the foam down to half of its thickness. Probably enough to damage the bass!

 

[Bruce Johnson]

Yeah, MapleGlo's got the the right idea. That's an excellent design for a case. Those dense foam rubber sleeping bag pads are a good inexpensive source for the foam. My only comment on the design is that you could have left off the neck yoke at the 1st fret. Tighten up the padding at the yoke at the 12th fret, and just let the rest of the neck hang freely inside the case. It's safer that way.

By the way, to trim the foam layers neater, rough cut them and glue up the stack first. Then cut the final instrument outline in the stack in your bandsaw.

 

[Jughead6]

I'm definitely sold on the 8lb rubber. I just need to find a cheap source (here in Australia).

At the end of the day, this case is likely to be more of a decorative and semi-protective storage case for use at home rather than an actual roadcase. I'm currently between jobs, so I am trying to keep busy in the workshop and this is my little time killing project.

Anyway, thanks for all the good advice and suggestions.

 

[mapleglo]

Thanks for the tip about cutting the foam Bruce, I'll have to give that a try next time.

 

[sissy kathy]

An electric knife works wonders with foam too, and doesn't make any 'snow'.