Building a mold

[sailorboy31]

Hello
I am trying to find some info on Pro's and Con's to building a mould using plaster, and do you need to lay down the gel-coat before laying up your epoxy?
Thank so much!
Casey

It is very porous, but thats all. The brittleness can be handled by using old potatosacks as reinforcement. (might be a bit difficult to find them nowadays).

You do NOT use gel-coat crap when using Epoxy!!!
Just let the first layer of Ep. go gelled then lay up a very light fabric (less print through).
Do the complete layup in one go, do´nt let the layers cure thoroughly before applying the next!!!
Paint the hull with a UV blocker paint, otherwise your boat will disappear after a while.

Regards
Richard

[Steve W]

Not sure what you are trying to achieve,are you wanting to build the male plug or the female mold out of plaster? It is not uncommon to use a framework,lath,plaster,Burlap as Apex mentioned etc,whatever it takes to make the plug but make the female mold out of conventional moldmaking materials such as polyester tooling gelcoat,glass and polyester resin and some kind of supporting structure be it steel or wood depending on how big the mold and how many pulls you are expecting to do.You could use epoxy at greater expense but im not sure of any advantage,id save the epoxy for the actual product,i think you need to tell us what you want to make,mold or plug,what is the item you want to make,is this for a one off or production etc.
Steve.

[wardd]

years ago when i made plaster plugs and molds, we mixed sisal in the plaster then a layer of plaster slurry

and as a parting coat we used simonize car wax, many coats

[SamSam]

I've made small 1 and 2 ft square molds of plaster and sealed them with cheap spray paint in a can. A coat or two of wax and some pva and they work good. They can capture an incredible amount of detail.

Fiberglass window screen works good for reinforcement. For that matter, I've heard of window screen being used instead of glass in layups. I suppose metal window screen would add an interesting sort of strength to a "fiberglass" layup. Anyone ever tried it?

What please is window screen Fiberglass???

Do´nt let me die dumb please.

[wardd]

window screen is a wire mesh to keep bugs out

and the post was to add it to the fiberglass or instead of fiberglass

i dont see the advantage unless fiberglass irritates you

[Landlubber]

sailorboy,

It would depend entirely on the number of items you wish to pull of the mould....or do you really mean the plug.

The plug is the thing that makes the mould, though you can of course go direct, but that is rare for production work

Originally Posted by wardd

window screen is a wire mesh to keep bugs out

Wardd, I know what that is. But Sam said "Fiberglass window screen" and I have no clue!

[Howard N]

Originally Posted by apex1

Wardd, I know what that is. But Sam said "Fiberglass window screen" and I have no clue!

It's hard to find metal window screen in lots of places. Up here, what you find is made out of a plastic they call fiberglass.

Thank you Howard!

[SamSam]

I've never used it but if resin stuck to it it might offer some structural goodies like light weight, flexible, stretchable, who knows what else. Sort of cheap also.

Fiberglass Window Screen 96"x100' Full Roll 2.4 x 30.4 meters
$129.95
Standard fiberglass insect screen used on most windows and doors. Available in charcoal color. Easy to install in window screens. Mesh is 18x16, .011 diameter.

[sailorboy31]

Thanks everyone for the responses
We are going to attempt to build 3, KHD 19 foot Trimarans, and at first we were just going to cold mold them, then there was a suggestion made about using plaster and forming a female mold then laying the glass up in side the mold and popping the shell out and lay in the bulk heads and stringers in afterwards. What do you think of this idea?

Thanks so much!
Casey

Originally Posted by Steve W

Not sure what you are trying to achieve,are you wanting to build the male plug or the female mold out of plaster? It is not uncommon to use a framework,lath,plaster,Burlap as Apex mentioned etc,whatever it takes to make the plug but make the female mold out of conventional moldmaking materials such as polyester tooling gelcoat,glass and polyester resin and some kind of supporting structure be it steel or wood depending on how big the mold and how many pulls you are expecting to do.You could use epoxy at greater expense but im not sure of any advantage,id save the epoxy for the actual product,i think you need to tell us what you want to make,mold or plug,what is the item you want to make,is this for a one off or production etc.
Steve.

[SamSam]

Originally Posted by sailorboy31

Thanks everyone for the responses
We are going to attempt to build 3, KHD 19 foot Trimarans, and at first we were just going to cold mold them, then there was a suggestion made about using plaster and forming a female mold then laying the glass up in side the mold and popping the shell out and lay in the bulk heads and stringers in afterwards. What do you think of this idea?

Thanks so much!
Casey

If the shape is something a sheet of ply can conform to, there are cheap sheet materials that will work. It's hard to fair a female mold with materials like plaster.

It's way better to put the bulkheads and structure in while the shell is in the mold where it is supported in the correct shape. Once you take it out it is usually too flimsy to keep the shape.

[PAR]

Richard, you can get aluminum, steel, painted steel, galvanized steel and stainless steel (really hard to find), bug screen in this country. The 'glass materials are easily the most common now, but only in the last few decades. The metals are still used in tougher environments, like animal enclosures. Elongation issues would cause problems if used in a resin matrix for most marine applications. The coated screens would have peel strength issues as would the aluminum, unless etched just as you applied the resin.