[PSUBS-MAILIST] fiberglass shell

Wed Aug 12 09:45:29 EDT 2020

Hi Brian,
I've done a lot of working with (and sculpting with) plaster of paris - I
love the stuff, but think you'll find it just makes a mess in this case
because the surface will be near impossible to finish properly. Partly
because of the size of area you need to work really, really fast just as
it's setting up. How big does the cone need to be? And can it be spherical
or does it have to be cone shaped?
If I may suggest, a good old dollar store beachball or sports ball makes an
excellent mold. You can inflate it to the right size, and then plaster that
and put plastered burlap on the outside of the plaster to hold its shape.
Simply deflate the ball to remove it, and now you have a nice, even, smooth
finish plaster mold which will be much more easy to release your fiberglass
from. And your fiberglass winds up with a super nice, smooth finish.
Further to that, but along the same idea, you can get tons of cheap,
inflatable pool toys that may have a cone shape you can use as well if you
need a cone shape.
Just be aware that pressure changes will occur in the inflatable because of
temperature variations. Namely, plaster will both heat up the ball and the
air in the ball, and so the ball will expand. This was a fatal problem for
me because I was trying to make a huge globe, so I either had to make it in
two halves (and the halves would invariably wind up different sizes because
of the variable expansion of the ball as a mold) or make one sealed sphere,
which both trapped in the heat and the outward pressure of the air
expanding inside the ball would stress the plaster mold and crack it. I had
to have tolerances that were too tight so the two halves would match to
make a sphere. But in your case, I suspect you have some tolerance and room
for error on the size of the cone/sphere? So that shouldn't be a problem
for you then.
Do you have access to a wood lathe? Can you turn a stryrofoam blank and
produce a mold from that? Or apply the fiberglass straight to the
styrofoam? Sculpting fiberglas and getting a decent finish is a pain.
You've got the right idea to make a mold in which to cast the cone - that's
the best way.
For what it's worth, here's one short article on sculpting a fiberglass
nose cone:
https://www.apogeerockets.com/education/downloads/Newsletter413.pdf
But by far and away, the best thing is to start with something cone-shaped
and the right size already.
Oh - if you can get away with a sphere, you can blow-mold a sphere as well.

Ian

On Wed, Aug 12, 2020 at 12:32 AM Brian Cox via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Alan,   I could coat the plaster of paris with resin and then polish that
> really well , then apply mold release to that , then shoot gel coat over
> the mold release , then fiberglass and resin over the gel coat.  I would
> bury some sort of lever underneath the cone so I could pry it out of the
> ground.
>
> Brian
>
>
>
> --- personal_submersibles at psubs.org wrote:
>
> From: Alan via Personal_Submersibles <personal_submersibles at psubs.org>
> To: Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] fiberglass shell
> Date: Wed, 12 Aug 2020 16:24:28 +1200
>
> Hi Brian,
> I am hearing alarm bells. The plaster of Paris will have to be pretty dry
> & any
> resin from your fibreglassing would suck in to it unless you gave it
> several coats
> of shellac first then put on a mould release wax. This is from memory from
> a long
> time ago. Also I have vague memories of coating polystyrene with pva glue
> which
> doesn't dissolve the polystyrene, & fibreglassing on to that.
> Will take a big block of polystyrene.
> Alan
>
> On 12/08/2020, at 3:14 PM, Brian Cox via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> Hi All,
>                I"m thinking of doing a fiberglass nose cone for the front
> of my sub which will have flotation foam in the top part of it.  This time
> rather than make a whole mold assembly I was thinking I would dig a large
> hole in the shape of the nose cone, use plaster to get it smooth and then
> just put fiberglass against the inside of the plaster.  Then lift the thing
> out and break off the plaster.
>
> Brian
>
>
> _______________________________________________
> Personal_Submersibles mailing list
> Personal_Submersibles at psubs.org
> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
> _______________________________________________ Personal_Submersibles
> mailing list Personal_Submersibles at psubs.org
> <http:///eonapps/ft/wm/page/compose?send_to=Personal_Submersibles%40psubs.org>
> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
> _______________________________________________
> Personal_Submersibles mailing list
> Personal_Submersibles at psubs.org
> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20200812/c92a5df0/attachment-0001.html>