Creating a structure/frame in a structureless backpack

[vast nova]

Hello, I've wondered how to improve cheaper backpacks which have no frame, so to improve the support for carrying things or keep rigidity of the backpack when oddly formed components are inside.

I need help on some advice, since I have three different approaches in mind.

1. take cardboard and cut it to shape, then layer it together using glue to create something composite like. On the glue, which would be the best for sandwiching between the cardboard? Or something else?
   a. Polyurethane wood glue
   b. Naphtha based no-nails type glue
   c. some type of "contact" cement glue

2. Take a steel sheet, cut it to size, and sandwich it between cardboard using glue and silicone on the edges to seal it in, and prevent accidental tears and cuts.

3. Making a skeleton frame using small aluminum flat bars, and rivets/bolts or a good two component epoxy between the flat surfaces of the alu bars.

Feedback for the approach would be much appreciated!

[nocturne bane]

aluminium bars is what proper backpacks use
a steel sheet sounds very heavy and not that comfortable

plenty of lighter backpacks also use a foam or plastic sheet instead of a proper frame
I would look into that direction

[worldly hemlock]

how about plastic mesh with alu outer frame. keeping the weight down with a rigid outside frame. also maybe cross struts for additional rigidity?
alu tubes in place of flat bars to reduce weight as well? heck why not go carbonfibre tubes

[vast nova]

@nocturne bane Yeah, was thinking I could make things lighter by using cardboard and making a composite, I think plastic sheets make more sense and are easier overall. Looking at materials, maybe PVC hard foam or ABS sheets could be cut to shape and inserted on the whole backside, fixed with glue or rivets to create rigidity.

@worldly hemlock the frame would totally make sense, but would be the most time and design intensive. Carbon tubes would be awesome, but fixing them together, drilling them, thinking about shattering them and the load direction compared to the weave of the tubes already makes my head hurt 😄

[worldly hemlock]

i was thinking just carbon tube rods as a replacement for the alu tubes for the outer rigidity while keeping simple plastic mesh for the inner support. like a screen door design.

except instead of trying to drill into carbon tubes just glue the mesh around the tubes as a mounting method maybe with epoxy

[vast nova]

hmm, the strength would come from joining the tubes together to form a complete shape, like a triangle or square, but I could use them as extra support around the plastic mesh like you suggested. Then the epoxy would hold the rods together, and I could create an outline of as long as possible rods around the sheet, which would keep the load perpendicular to the rods hopefully, and maybe do some cross-struts like you mentioned. I'm only afraid that the rods would act as a lever, and get unbonded from the sheet over time, especially on the edges? Or would that simply not happen with epoxy?

[worldly hemlock]

overlapping the meshes or wrap around the meshes so they form a sleeve on the rods should increase the strength to mitigate that hopefully?

think rod pockets for shower curtains?

[vast nova]

yeah, makes sense, but what type of plastic mesh would you use?

[worldly hemlock]

depends what you're looking more for breathability or rigidity

finer mesh would allow for less stiff material to be used. while a coarser mesh would require a stiffer material. but still needs to be able to be rolled around the carbon tubes.

[oblique crane]

You could use a heat form plastic and then make a composite out of it...

And make it curvy.

Curvy is stronger

[queen tapir]

Do you have a 3d printer? you could make the base out of something like carbon reinforced nylon and have just straight carbon tubes for the frame with 3d printed angles/connectors

[crystal tulip]

Are you looking for a solid shape more like a luggage case or are you looking for a soft shell that can still conform.

Most backpacks are sewn with an eva foam between the fabric layers. Plastic sheeting is also used in combination typically at the bottom so the backpack maintains the base bottom shape easily.
You could cut 1/4"-3/8" eva foam sheets and hand stitch them into your backpack depending on how much effort you want to put in.

If you want more effort, make an inner shell made of lining or similar outside material, and tack it together in primary points putting the foam between. You can fully join the two layers by fully sewing the the two layers at the zipper

[vast nova]

@crystal tulip Makes sense, I looked at what other backpacks did, and ended up deciding on the foam and cardboard fiber approach. I think your suggestion with EVA foam and stitching is the way to go, and I've already taken some thick cardboard and applied polyurethane to it to create a composite, it worked quite well, which I will use as the inner hard liner to retain the shape and add eva foam where it makes sense. I just need to be careful about the stiching method I think, since I do not want to ruin the weave of the backpack or create stress points for further tear.

[crystal tulip]

If you need less complex shapes, a good option is to get some thin sheeting and adhere it to one side of the eva foam. I prefer something with some give so its not too ridged.
For the lining, just make the inner lining a little bigger than the outside shell to help give yourself more wiggle room. Linings are typically bigger than the outside anyways. I'm sure it will take a bit of effort, but I'm sure you'll get it
Hope your project has turned out the way you wanted.