External Carbon Frame Backpack from Scratch

tcg144 · Jan 26, 2026

What is the design thickness of the finished piece? How does that relate to you mold depth? What did you use to cut the carbon out?

I’m interested to see how you finish the edges and cut the holes.

Traskie · Jan 27, 2026

Do you have the stls for the mold available anywhere?

Trumpkin The Dwarf · Wednesday at 5:59 AM

This is wicked cool! Now what I would like to know is if the foam core makes an appreciable strength to weight difference over a thin marine grade plywood core. The old kifaru ultralight stays were built like bow limbs. Just a curious idea.

nighthawkf-117 · 2026-02-03T06:30:49-0700

tcg144 said:
What is the design thickness of the finished piece? How does that relate to you mold depth? What did you use to cut the carbon out?

I’m interested to see how you finish the edges and cut the holes.

I am for a little over 4mm finished thickness. There isn't too much engineering that goes into this, its plenty strong. I just want enough frame thickness that all the edges can be rounded over nicely and look right. The mold depth is also right around 4mm. When a part is made I like the layup to essentially end up flat with the flanges of the mold. This just helps the vacuum bag easily conform to the layup without having to sink into a recess. The carbon fabric is cut with a rotary cutter over a self healing cutting mat. The dry fabric is very fragile and frays easily.

I should note here I'm not an expert on any of this. This is all what Ive learned over years of making composites as a hobby. Its just what works for me.

nighthawkf-117 · 2026-02-03T06:31:10-0700

Traskie said:
Do you have the stls for the mold available anywhere?

I would share STLs on a case by case basis

nighthawkf-117 · 2026-02-03T06:32:34-0700

Trumpkin The Dwarf said:
This is wicked cool! Now what I would like to know is if the foam core makes an appreciable strength to weight difference over a thin marine grade plywood core. The old kifaru ultralight stays were built like bow limbs. Just a curious idea.

I just use the foam because its easily available to me and easy to cut out with an knife. I'm sure a wood core would work just fine and be a similar weight and strength.

nighthawkf-117 · 2026-02-03T06:49:42-0700

Work on the frame continues. I pulled the part from the mold. Overall this part turned out great and will work just fine. You might see in the pictures there are some small voids in the surface where the carbon didn't conform perfectly to the surface of the mold. I sanded these areas and spot filled them with some epoxy.

The flashing around the outside of the frame was trimmed off with a band saw. I then went on to cutting the slots. As mentioned I used a dremel with a 3d printed guide. I start by drilling a starter hole in each slot with a drill press. The guide is then located to the slot and clamped. The center section of the guide is then removed. The slot is then cut with a dremel and a 1/8" endmill. The guide is sized so the dremel can just follow the inside perimeter of it and creates the size slot I desire. Each slot is then cleaned up with sandpaper and a nice radius formed on all edges.