Female Fiberglass Mold - Thoughts?

[Captain Bill]

The bow rail seats on my cat have delaminated on the bottom. I have decided to build my own replacements. As the top of the seats are in pretty good shape i want to use them as a plug to make a female mold for the new seats. Since I intend this to be a one off project, I don't wish to expend a lot of money on a fiberglass female mold. I was thinking of using plaster of paris to make the female mold rather than fiberglass. Once I pulled the plug I would seal and wax the surface before beginning the layup. When finished I would simply break away the mold. Has anyone ever heard of anyone doing this? I've found some references on the net to people doing this with small parts but never on anything this size(20x33x1 inches).

[Cheechako]

you might need some reinforcement on a piece of plaster that large. POP works very fast so you will need to be quick, yo may not be able to work that fast. There must be something that will give you a little more time....? I dont see why it wont work though.... I have an old lath and plaster house and have used POP to repair some areas, It hardens in the container faster than bondo!

[Captain Bill]

My plan was to build the molds on a piece of 3/4 ply with a 1x2 frame. I figure I'll wax up the plug really well and pull it once the plaster sets pretty well.

[bwalsh]

Are the original seats made of wood? Or are they cored fiberglass? Id suggest a rebuild of the core if thats the construction and if that wont work, why not buy some polyester resin, fiberglass cloth, pva mold release and make some nice molds that you know wont break apart (plaster of paris) before you get the seats built... Sometimes i have to remind myself not to skimp because my time is worth alot more than what the materials cost for most projects. And of course when you use accepted methods (such as building a fiberglass mold) there is already a wealth of knowledge you can dip into here and on other forums. My .02 And then there is building with teak... but thats another ball of worms (and money too)

[James S]

I made a really thin FG shroud for a flying machine using plaster as a mold...worked OK but even with a very thorough waxing I still had quite a bit of sticking....came off no problem buy I had lots of scraping and sanding to remove the plaster from the final pieces.
In the picture you can see the difference from the one section that’s been cleaned compared to the other two.
I think if you click on the picture you'll get a better view.

[James S]

Picture of the mold

[Allan S]

Dang nabbit, these hi falutin' flying machines are going to be ruin of us all!!!!

[Lodesman]

Have you considered silicone rubber? You can get it at plastic supply stores or hobby stores. It's usually used for casting smaller items, but if you made a plywood frame that's slightly larger than the seats, you wouldn't use too much rubber.

[Christian Van H]

Originally Posted by James S

I made a really thin FG shroud for a flying machine using plaster as a mold

There is NOTHING on this earth that James cant build! First class work!

[James S]

Thanks Chris

[Blue Stocking]

CVH,

It maaaayyyy nnnooottt bbbeee fffooorrrr aaaannn eeeeaaaarrrttthhh mmmmaaaccchhhiiinnneee.
We have not yet seen all of James' talents

[James S]

Funny guy:-)

[sandy daugherty]

Assuming there is no non-skid texture on the seats, your best bet is to spray several thin coats of parting compound on the best one, then build the mold directly over it in glass. It you are going to pull more than a couple seats from this mold you should consider using a very hard gel coat. Since the design isn't changing much, its likely the new seats will crack too, so make a few spares. The parting compound is water soluable, so clean up and prep are pretty easy. If the part is small enough, you can bag it in a food vacuum bagger and get very good control of the resin/glass ratio.

It's a fun way to learn about glass work.

[Captain Bill]

I'd like to thank everyone for their input, I'll try to address each one below.

Originally Posted by bwalsh

Are the original seats made of wood? Or are they cored fiberglass? Id suggest a rebuild of the core if thats the construction and if that wont work, why not buy some polyester resin, fiberglass cloth, pva mold release and make some nice molds that you know wont break apart (plaster of paris) before you get the seats built... Sometimes i have to remind myself not to skimp because my time is worth alot more than what the materials cost for most projects. And of course when you use accepted methods (such as building a fiberglass mold) there is already a wealth of knowledge you can dip into here and on other forums. My .02 And then there is building with teak... but thats another ball of worms (and money too)

The original seats are fiberglass cored with plywood. I believe that the top and sides of the seats were made in a mold, the plywood placed in the molded top and then a layer of chop or mat was laid up/poured on top of the plywood. Unfortunately this meant that the bottom of the seats was not adequately bonded to the sides and eventually a crack occurred between the sides and bottom allowing water to enter. The plywood swelled and separated the bottom from the sides. Note that it was the plywood that delaminated. The poured on bottom is still firmly attached to the first ply of the plywood. I thought about simply removing the bottom replacing the core and pouring a new bottom. This however would leave me subject to the same failure mode that I have now. I just don't see a decent way of bonding the bottom to the 1/8 inch of exposed side.

The fiberglass mold might be the way to go, in fact I know that if I was going to have to do this more than once I would go there. It is my backup plan if the POP doesn't work.

Originally Posted by James S

I made a really thin FG shroud for a flying machine using plaster as a mold...worked OK but even with a very thorough waxing I still had quite a bit of sticking....came off no problem buy I had lots of scraping and sanding to remove the plaster from the final pieces.

Originally Posted by James S

In the picture you can see the difference from the one section that’s been cleaned compared to the other two.
I think if you click on the picture you'll get a better view.

I'm a bit encouraged to see that someone has used a POP mold for something even larger than my pieces.

Originally Posted by Lodesman

Have you considered silicone rubber? You can get it at plastic supply stores or hobby stores. It's usually used for casting smaller items, but if you made a plywood frame that's slightly larger than the seats, you wouldn't use too much rubber.

I have considered Silicone rubber, but the quantities I have been able to find locally seem to be for pretty small parts and to buy enough kits to make a large mold would be very expensive. I've used this before for small stuff and it works very well. I'll just like the mold to cost less than the seats.

For my first attempt I think I'll stick the POP. I plan to use divinycell foam for the core instead of plywood. I've made things with PVC foam before and they hold up very well. I plan to use gelcoat, two layers of mat on the top and two layers of biaxial cloth on the bottom. I have some chopped glass to mix with resin and use as a filler in the corners. If things go as planned I will wrap the edges of the foam with at least 4 inches of mat before putting on the biaxial cloth. This should encapsulate the foam and produce a stronger bond between the sides and bottom, thus eliminating the current failure mode. Even if I don't get a perfect seal the PVC foam should be more resistant than plywood to moisture. I can't imagine with the layup schedule I plan that I will not get a perfect seal. The biaxial cloth on the bottom may be overkill but it is better in tension than mat and the bottom will be under the most tension. Me and a couple of friends are relatively big people and we can apply a lot of tension.

[motion30]

PVA works well as a parting agent and washes away with water