strength and stiffness needed for a long skinny mold.

General Composites Discussions Engineering Talk
#1

I could use some suggestions on layups and support for a large mold. It is 35 feet long and about 18" wide and shaped similar to a rowing shell. I estimate about 10 square yards of surface area for the entire mold including flange. When not in use the mold will be hung from a ceiling at two points. When in use it will be set on saw horses or tables. I would like the mold to be stiff enough that it holds its shape when supported at 2 points. I would like to use tube bag when I make my parts so the external skeleton cant be too large and the mold can’t be too heavy. Are these goals reasonable? I have found a few pictures online of similar molds and they have an exoskeleton of truss that extends to the floor. I have also seen some that appear very flexible but are set in a cradle.

I can use an unlimited amount of:
epoxy resin
24 ounce biax fiberglass

For a very low cost I could use 19.7 ounce carbon fiber.

I am not opposed to purchasing other material if needed.

I am having a hard time deciding the material and strategy for a core. I have considered:
Western red cedar - heavy.
XPS pink or blue foam - dents easy. With enough cloth this doesn’t seem to matter.
PVC tube cut in half length wise - The weight adds up quick.
divinycell or core cell - expensive and hard to ship where I am.

I have built a very similar mold that was 24’ long. It used 4 layers of 24 ounce fiberglass with epoxy resin. There was a 5" wide stringer made of 1/4" thick by 1" wood strips down the middle sandwiched between the fiberglass. This was adequate for the 24’ but would need to be significantly stiffer for 35’.

If i used the pink foam I would use 1" thick and cut strips 3" wide with a taper on each edge. The taper makes it easier to lay the cloth.

My current plan for the mold is:
2 complete layers of 24 ounce over the entire mold.
Over the middle 25 feet add a stringer of pink foam down the middle and ribs between the flange and the stringer every 2-3 feet or so.
Two more layers of 24 ounce on top of the middle 25 feet of the mold.

Thanks

#2

I might be a little bit out of my scope here given I’ve never made anything this large, but the way I support the larger (anything 1sqm and over) moulds I make is to use 6" PVC tube cut in half lengthways as you suggested, but then I spray the insides with PVA and pour in a high density 2-part urethane foam. The stuff I use has a free-rise density of 96kg/m3 and is intended for use as a core material. I then trim these flush with the PVC tube before demoulding them, and glassing them onto the back of my mould. This gives me a very stiff lightweight support structure, however I’m not sure if something like that would be too heavy/not really feasible for something your size.

No doubt one of the other guys on here will have a better solution :slight_smile:

#3

My first half of my Composite career I made rowing shells, up to 60’ long. The moulds were carbon, and the supporting structure was 1" balsa core with 19oz carbon laminated on both sides. We made the structure height the same width of the balsa, 24". Its very effective and light. The taller the structure you make, the stiffer it will be, not sure what your limit is in this case since you are suspending it from your ceiling. Good luck!

#4

The taller the structure you make, the stiffer it will be

Right. I try to make a make a box shape whenever I need stiff moulds. I’m not a big fan of core materials in (heated)moulds( print etc.), so I make a box shaped mould whith an open underside. costs some materials and extra time. A lot less risk of warping, etc.