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Author Topic: Resin casting problem  (Read 6102 times)
Buzzard Bait
Swab

United States United States


« on: March 12, 2009, 12:54:36 pm »

Hello to all,
I am very new at casting resins (polyester). I am having a problem with the finished casting retaining a sticky outer film after it has cured. I am using a RTV silicone mold and I am pre-heating the mold. I vacuum the mold after the resin is poured and then cover any exposed area. Even after 72 hours it retains a sticky film after it is removed. Is there something I am doing wrong?
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Robo Von Bismark
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Australia Australia



« Reply #1 on: March 12, 2009, 02:00:28 pm »

Are you using the right type of polyester resin?

It comes as either casting or laminating resin. Casting resin has added wax content so that it can cure within the mold. If it the wrong type it will only cure on exposure to air (ie opening the mold)
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Rev. Marx
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« Reply #2 on: March 12, 2009, 03:42:28 pm »

I occasionally get a similar problem with casting polyurethane. Usually it is because my mix ratio of resin to catalyst was off. I've never tried polyester, but perhaps you could be experiencing a similar situation.
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Titus Wells
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« Reply #3 on: March 12, 2009, 04:38:03 pm »

I find Polyester resin is usually quite forgiving, depending on the exact resin you are using. It can happen with casting resins that if you're working in a not-entirely-dry workshop (ie. a damp shed) the moisture will prevent proper cure and the resin will be crumbly and sticky.
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groomporter
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« Reply #4 on: March 12, 2009, 04:48:32 pm »

Yeah some resins can be hydroscopic in that they absorb moisture from the air.

I used to use a Smooth-on urethane resin that when it got old would continue to "weep" fluid for a while after hardening, and never lost the annoying moth ball smell.
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Buzzard Bait
Swab

United States United States


« Reply #5 on: March 12, 2009, 06:25:05 pm »

Thanks for all the suggestions,
I am using a casting resin and mixing according to the label, so many drops of catalyst per ounce of resin and having the same results. I have even added more and less than the label notes. The mold gets as hard as it should, but even exposure to air will not cure the sticky film. The only other thing mentioned would be high humidity causing this. I would not think the product could soak up enough moisture in the 10 minutes from mixing to pouring, but I suppose it could.
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Titus Wells
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« Reply #6 on: March 12, 2009, 06:50:10 pm »

10 minutes from mixing to pouring? That seems a long time. Most of my casting resins would be cured in that time. Do you know the make?
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Buzzard Bait
Swab

United States United States


« Reply #7 on: March 12, 2009, 07:50:26 pm »

The brand is Castin Craft. It is a clear polyester casting resin. It only takes a minute or so from mixing to actually pouring the mold. I over stated the time just as a matter of chance that long could cause the humidity absorption.
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Titus Wells
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« Reply #8 on: March 12, 2009, 10:53:41 pm »

Ooh yuck... Clear casting resins are something I avoid unless I absolutely have to use them, they're temperamental buggers! Maybe you got a duff batch, how's their customer service? Don't know that make, are they US based? I generally use Ultrakast/Por-a-kast resins or smooth on, and smooth-on's customer support is brilliant.
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Gryphon
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« Reply #9 on: March 13, 2009, 12:11:49 am »

Try post-curing it by setting it out in a warm, sunny spot on a dark surface.  It needs to come up to between 90 to 110 degrees F and stay there for as long as you can keep it there, and it will SMELL.  It may take a few days, but it will eventually firm up.  If the problem is moisture, the casting will have a cloudy whitish discoloration wherever it's still sticky; if your problem was your catalyst or some retarding agent in the mold, the surface will be sticky without discoloration.  Some retarded-cure resins post-cure faster if you seal them with a coat of wax or paint, but don't do this if you're trying to cook out water.  Use the sun - don't do this in an oven or with a heatgun, as the fumes are flammable.
« Last Edit: March 13, 2009, 12:18:49 am by Gryphon » Logged

Titus Wells
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« Reply #10 on: March 13, 2009, 02:01:27 am »

Thanks for all your posts Gryphon... some very useful tips you are posting!  Grin
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Gryphon
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« Reply #11 on: March 13, 2009, 04:50:55 am »

I once had an entire 16-foot sailboat hull refuse to surface cure just like this, because I had undercatalysed the gelcoat layer on a humid day.  It took a solid three weeks in the August sun to set up, but eventually it did.  I just hope that, with my tips, a few others can be spared from the horrible sinking feeling I had that day as I put a finger to that shiny, shiny surface on my first homemade boat and that finger stuck....
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Titus Wells
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United Kingdom United Kingdom


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« Reply #12 on: March 13, 2009, 10:18:05 am »

I once had an entire 16-foot sailboat hull refuse to surface cure just like this, because I had undercatalysed the gelcoat layer on a humid day.  It took a solid three weeks in the August sun to set up, but eventually it did.  I just hope that, with my tips, a few others can be spared from the horrible sinking feeling I had that day as I put a finger to that shiny, shiny surface on my first homemade boat and that finger stuck....

Ouch... feeling for you sir! I hope it worked okay in the end... and that you managed to remove your finger! You could claim the fingerprint was an identifier or signature! Smiley
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