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Old Feb 07, 2014, 12:29 PM
If it floats....sail it!
FoamCrusher's Avatar
Elk Grove, CA
Joined Sep 2002
4,073 Posts
In general about how thick does an Ultracal mold reinforced with burlap need to be to survive a few pulls? Think of a 1 meter sailboat hull here.

In making gelcoat surfaced FG molds, generally 1/4" (~6mm) seems to work well.

And...for bare Ultracal molds, is waxing enough or do you also need PVA?
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Old Feb 07, 2014, 03:12 PM
The Junk Man
Jacksonville, Florida
Joined Jul 2006
1,169 Posts
Quote:
Originally Posted by FoamCrusher View Post
In general about how thick does an Ultracal mold reinforced with burlap need to be to survive a few pulls? Think of a 1 meter sailboat hull here.

In making gelcoat surfaced FG molds, generally 1/4" (~6mm) seems to work well.

And...for bare Ultracal molds, is waxing enough or do you also need PVA?
You do not have to reinforce Ultracal 30 molds at all. I have made dozens, probably scores, and never reinforced a single one.

The only one I ever broke was pushed off a shelf 5 feet high onto a concrete floor. And that one just broke in two and was epoxied back together.

What do you mean by "bare"? I always coat mine with thin coats of epoxy. And I always use PVA. It is just easier. 5 coats of wax and 3 coats of sprayed PVA. A light mist, then just a tad heavier than a mist, then a flow coat. Stuff just pops right out.

Tom
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Old Feb 08, 2014, 12:54 PM
If it floats....sail it!
FoamCrusher's Avatar
Elk Grove, CA
Joined Sep 2002
4,073 Posts
Quote:
Originally Posted by T_om View Post
You do not have to reinforce Ultracal 30 molds at all. I have made dozens, probably scores, and never reinforced a single one.

The only one I ever broke was pushed off a shelf 5 feet high onto a concrete floor. And that one just broke in two and was epoxied back together.

What do you mean by "bare"? I always coat mine with thin coats of epoxy. And I always use PVA. It is just easier. 5 coats of wax and 3 coats of sprayed PVA. A light mist, then just a tad heavier than a mist, then a flow coat. Stuff just pops right out.

Tom
Thanks.
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Old Feb 09, 2014, 01:39 PM
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USA, CA, Long Beach
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You do not need re-enforcement on the Gypsum molds. It helps make them thinner, hence lighter, by strengthening the material. The Gypsum is surprisingly strong by itself. My Gypsum Mold Presenter used hemp as backing that is my impetus to continue using it. I tested thin layups, with and without hemp. The hemp backed were way more than twice the strength.

I have put many surface finishes on Hydrocal and UltraCal, from resins/epoxys, wax (soft and hard), PVAs/PVCs and an oil mix of mineral oil and petroleum jelly. I have had enough success with just wax, that it is all I do now. I layer soft and paste (two different) wax about 7 times on the Gypsum surface, parts come out great. I do not pull many parts out of the molds as I am using it for rapid prototyping so not usually more than 5. I am presently using it for a project that I will be using the same mold for hand layup (maybe), vacuum bagging and pressure bladder molding. First processes to make sure I get something to put away, on the shelf, for later... just in case.

You would have to work, not to have success with gypsum. If you try it you will find that it is really forgiving. If you happen to get bubble problems, you can wait for the mold to dry and use plastic filler or even the same gypsum to fix it. You can sand it, scrap it, and polish it. The largest draw back is that it is not as durable as aluminum or Epoxy for large production runs. I do not think that many here are looking to go into manufacturing but rather want to know how to get consistent results from easy to use, and clean up, materials. This is definitely one to consider.

Dan
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