Crowned panel - predicting shrinkage

[burke]

Making a 3 by 7 foot roof composite panel section from 1" end grained balsa. Will make a simple plywood convex mold ( crown up) that the scored balsa will lie on to conform to the desired 4" crown over 7'. Plan to lay up the top laminate, let it cure, then check to see if the crown approximately matches the boat. If the crown flattened from shrinkage, I can adjust it, before turning it over and doing the underside/concave surface.

If the crown is within 1/2", it can be joined adequately with clamps to its mating surface on the boat.

My question: should I adjust the mold crown to account for the predicted first/top laminate polyester shrinkage? Or will the shrinkage of the cured top laminate be corrected back to the desired crown by the shrinkage when the bottom is laminated?

Thought of doing both laminates together on a concave (crown doen) surface. But I want gravity to help resin fill in the scrim-hinged slit separations created by bending the core .

Thanks again

 

[ondarvr]

There are far too many variables to say exactly what the change may be, if any at all.

It will depend on the exact resin, type of catalyst, catalyst %, laminate schedule, the gaps in the balsa, ambient temp, resin/glass ratio, how long you leave it on the mold, plus other things.

 

[burke]

To some of your questions: orthothalic laminating resin from LBI, 60F (inside), MEKP catalyzed for 30 minutes pot life, one 18 oz biax, two 3/4oz mats per laminate. The balsa slit gaps would be zero on the bottom & approx. 1/64" on top.

 

[endarve]

I'll skip a safe non committing politcal answer and say you are splitting hairs about shrinkage. That is a thick core on a small span. Lay it up at the exact crown you need and don't count on bending it once one side is laminated.

 

[DCockey]

Perhaps an test run with a smaller panel would be useful.

 

[ondarvr]

Double post

 

[ondarvr]

ORTHO resins tend to shrink more than an ORTHO/DCPD blend (most common type of resin) but is a bit more flexible.

Is this a summer resin with a long gel time, or are you trying to stretch out the gel time by adding less catalyst? A slower curing summer resin will have a more reliable cure than just cutting back on the catalyst of a faster resin. A long gel time resin will shrink less due to normally having a lower peak exotherm. LBI doesn't make resin, they repackage someone else's, so frequently it may be difficult to get an actual data sheet to know what you're using.

There are catalysts available that can reduce the peak temperatures and extend the gel time, as a result they help reduce the shrink a great deal, but finding a small amount of it could be difficult. (One gallon would be the minimum container size, MCP 75)

The resin content of the laminate will influence the shrink a great deal, biax holds less resin than CSM, plus how much resin you apply. You can make a fairly dry laminate that has little extra resin, or a sloppy wet one, bagging helps reduce the excess though.

On a flat panel with a core on one side there can be huge amount of shrink on the laminate side, so it will warp as it cures, the longer you leave it on the mold the less curve you may see, but it will still happen. With a thicker core it will be reduced slightly, then with a crown and gaps in the balsa being filled with resin, you're increasing the resin content and shrink of the non laminate side, but the laminate side still tends to overpower it.

On yours, if a read it correctly, the gaps and the laminate are on one side, so you're compounding the shrink of the initial cure, this can cause the finished product to look like a potato chip sometimes.

If at all possible you try to apply the laminate to both sides of the core at the same time, next best is to do it while it's still on the mold and as soon as possible before the first laminate fully cures over the next couple of days.

De-molding, flipping it and bagging the other side can create all kinds of odd stresses that result in more warp.

 

[burke]

Thanks to all for your cautions about uneven warping if I do one laminate and flip after curing to do the other side.

After a small trial, here's Plan B;
* Convex 4" crowned mold (arched up), release wax or wax paper.
* Resin on mold, resin on 1808 0/90 biaxial fabric in biax side, lay on mold, mat up, resin to saturate.
* Beforehand, cut balsa completely through balsa into strips 1.5" wide (as the balsa is drying and no longer dead flat).
* Resin/Cabosil with 1/8" notched trowel on balsa strips (scrim side) and edges so no voids.
* Press balsa strips on fabric and squeeze out edge mixture so no voids.
* Resin to balsa top side (non scrim). 1808 biax, mat down, saturate, roll out.
* Cloth, 6 oz, resin, roll out.
* CURE. Leave on crowned form as long as possible until I join it to the boat.
* Final fill resin coat if necessary, 3M poly hand-sandable filler to fair, paint or gelcoat.

How does that sound?

 

[burke]

Comments on last post?

Did a small trial. There was advice on leaving it in the mold as long as practical to minimize distortion as it cures. My mold is just a symmetrically crowned 3' x 7' piece of hardboard. It is non-constraining ( it's not 3 dimensional). How would that help? Nothing will prevent a portion from lifting, unless I weighted it down with something after it was tack free.

Thanks

 

[ondarvr]

The mold will help the part hold it's shape, there's a bond between the part and mold, while not strong, it does help to prevent warp and shrink. If it pops loose while curing it can warp significantly more. Depending on specific details of the resin, laminate schedule, catalyst ambient temp, etc, it may or may not pop loose (pre-release). Being bagged (I think you said it was bagged), the part will be held even tighter to the mold, so if the mold is well supported, and it should be for bagging, it shouldn't pre-release, and hopefully limit the warp.

 

[burke]

No, not bagged.

Thanks

 

[dirtydiego41]

If you have .5 tolerance add that to your crown. If it shrinks you have 1" tolerance, if it doesn't you're still good.