Part Shrinkage

[CLM65]

Before Capt Bob replies with an inappropriate comment   ...I'm talking about parts cast in a mold.  I will be making an MDF mold for my transom enclosure in the near future, and the enclosure will basically span from one side of the boat to the other, with the lip on each side where the rub rail mounts.  I've read some articles that talk about shrinkage of the resin, particularly polyester resin (which I am using), during curing.  Most seem to be talking about unreinforced resin, but I'm not sure how much of a difference the fiberglass cloth makes.  I really don't want to make an enclosure only to find out that it is an inch too small when I try to install it.  Does anyone have any experience they can share on this subject?

I may end up doing a controlled test with a known length of fiberglass and see how much it shrinks during curing, but I've also read that it may continue to shrink for weeks or more after the initial cure.  I don't know if I can wait that long....

[dburr]

From:  
http://www.eng-tips.com/viewthread.cfm?qid=121190

Polyester resin has about a 6% volume shrinkage on cure compared to less than 3% for epoxy. How the shrinkage shows up in a part can be pretty complex. It may go unnoticed in a wet-laid or vacuum-bagged part but can cause insurmountable headaches in a closed mold where shrinkage results in separation from the mold surface and pitting. Fiber orientation, part thickness, fiber volume and cure temperature are all important factors.

The cooler/slower the cook off the less shrinkage there will be.  When caps,consoles and decks are molded you can see which ones were shot then cooked off quickly because there will be a bunch of spiderwebbing in the gelcoat and especially in and on the corners.  For some reason, mixing at less then 1.5% comes to mind but I have no idea why, this time of year you have to be careful to go just hot enough for the lowest projected temp during the curing period to keep the temp down..  

[Capt. Bob]

Before Capt Bob replies with an inappropriate comment   ...

Not fiberglass but still shrinkage......
http://www.youtube.com/watch?v=8DoARSlv-HU

[wingtime]

"like a frightened turtle"         ha ha!

[CLM65]

Thanks Dave, that's pretty much along the lines of what I have read.  It think I will do my test with a 8' piece and see what happens.  I typically add MEKP at about 1%, so hopefully that will be good.

Bob - excellent clip, probably one of the most memorable Seinfeld episodes.  Gotta love it!

[gran398]

According to Seamark...1/16th shrinkage max.

[CLM65]

According to Seamark...1/16th shrinkage max.

Thanks Scotty, but for clarification, is that 1/16 of an inch per inch, or per foot, or per 10 feet, or the width of the boat, or some other distance?  Inches per inch is a common way to express linear shrinkage or expansion, but that would equate to about 5" for the width of the boat , so I don't think that is what they mean.

I appreciate your help, buddy!

[Georgie]

Craig,

1/16 of the overall lamination = 6% (which coincidently is equivalent to the shrinkage factor provided in Dave's response earlier).  

Two different sources, and two matching answers.  

[CLM65]

Ryan,

If you read Dave's posted quote, it says the shrinkage "may go unnoticed in a wet-laid or vacuum-bagged part...".  I might believe that a part casted completely of resin, without any reinforcement, may shrink 6%, but I'm not buying that a fiberglass reinforced lamination will shrink that much.

As an example, a couple of months ago I made a fiberglass shelf to support the poly tank I got for my rebuild.  The tank is about 68" long IIRC, so I made the shelf about 1.5" longer than the tank to account for the expected 2% expansion of the tank.  If the shelf shrank by 6%, it would now be about 65" long.  Since the tank still fits with room to spare, I can safely say the shrinkage is much less than 6%.  I wish I had taken better measurements when I made the shelf - then I could answer my own question .  Keep in mind that I don't want to over-estimate the shrinkage, as that would lead to an enclosure that is too wide.  I'm hoping Scotty comes back and says the 1/16" is for the width of the boat.

[Capt. Bob]

As an example, a couple of months ago I made a fiberglass shelf to support the poly tank I got for my rebuild.  The tank is about 68" long IIRC, so I made the shelf about 1.5" longer than the tank to account for the expected 2% expansion of the tank.  If the shelf shrank by 6%, it would now be about 65" long.  

Is 6% the entire surface area or one direction only?

Would it not be similar to the tank expansion but in reverse. I would think the percent gain /loss would be expressed based on the surface area. I'm visualizing the mold example. When laid up it has a surface area of A and when cured, the new surface area B equates to A-(.06 x A).

[gran398]

Bob, your math is correct.

As initially stated it was a part cast from a mold....so the casting (ie part) in it's entirety would be subject to shrinkage.

Totally different method of casting, but in my industry it is centrifugal lost wax casting in precious metal.

In 14K gold, we plan on 1/8ths max shrinkage for the part.

[CLM65]

Capt. Bob,

I agree with Scott that your math sounds correct.  But the way a part shrinks I believe is a function of how it is made.  In other words, a part made of 100% resin (or gold in Scotty's case) would probably shrink essentially equally in all planes, much like a poly tank is expected to expand.  But a part made of resin and a reinforcement such as fiberglass gets a bit more complicated.  The reinforcement probably doesn't shrink much, so I'm guessing the overall size should stay close to it's as-cast size.  In this case, I think the resin shrinkage shows up as an exposure of the weave of the fiberglass.  In other words, the weave is more pronounced as the resin cures.

I'm not sure if this makes sense.  I certainly don't know if it is right.  I do know that I have not seen 6% shrinkage in any reinforced parts I have made (although my experience is admittedly limited).  And videos and pictures of fiberglass-reinforced parts, hulls, etc., on the web do not show parts popping out of their molds as they cure.  I think I will do my little test (I need to make my stringer tops anyway, so this will be a good test case) and see what the data shows.