Sandwich schedule

[massimilianoporta]

Hi,
I am planning a pretty big hatch cover (150 cm x 90 cm - 4.9 ft x 2.9 ft).
I will use a 3 cm - 1.2" thick PVC foam core.
Reinforcement will be glass with epoxy, infused.
Would an asymmetric layup be appropriate as the load will be exclusively from top to bottom because of people standing on it?
If this is the case, which layer should be thicker, the upper one or the bottom one?
Thanks!

[fallguy1000]

The glass choice is important, but the glass does its work in tension, so the bottom.

What layup is planned?

[massimilianoporta]

Thanks fallguy.
My idea of the layup, from top to bottom, would be:
- gelcoat
- 1 layer of 100 grams/3 oz twill
- 1 layer of 800 grams/28 oz quadriaxial
- 1 layer of Lantor Soric 2 mm
- 1 layer of 800 grams/28 oz quadriaxial
- PVC foam core
- 4 layers of 800 grams/28 oz quadriaxial

What do you think?

[fallguy1000]

I don't know about the twill prep for the gelcoat enough to critique, but the layup looks good, make sure the long tows are out always, so the biax is to the foam always.

[massimilianoporta]

Thank you, I will!

[fallguy1000]

It might be a little soft in the middle, just so you know.

A 50-60" span on 3cm of core is going to move.

A lot depends on attachments. Given this is a hatch lid; you may also consider stiffeners, because no attachments. Tophats are ideal, but you can use a piece of wood and make a stiffener after the lamination.


Put the hatch on 2x4s flat to the floor and see how you feel about the flex. If you are disappointed, do not add another layer. Just put a 3/4" or 1" hardwood board maybe 2" wide flat on the long distance in the middle and rout the edges and bond it to the hatch with 2 layers of 1708 tapes, csm in, allowing for clearances.

The board should be vertically grained and not flat sawn.

[massimilianoporta]

Perfect, I will follow your advice
Thanks again

[Fuzz]

For what you are doing a thicker core would make things much stiffer and add very little weight. If you do not have access to thicker core material then adding braces below like Dan says will help a lot.

[massimilianoporta]

Thanks