How to fill a hollow object to vulcanize a rubber mold

I am wanting to vulcanize a tiny sea urchin. It is hollow, so I
would like to fill it with something to give it more support and to
also make it solid.

I have thought of filling it with fimo clay, or epoxy glue. But both
seem inadequate to me. I only have 1 chance with each of these. I do
not want to make it collapse. They are about 1/2" in size. I have
two, and one that is about 1/4" in size. So I want to be careful so I
don’t ruin them. Any ideas? I am sure there must be a way to do this
correctly. Of course there is some pressure when in the vulcanizing
machine. Thank you for any help! I appreciate it a lot.


Laura, what if you did a silicone mould? That way you’re pouring in
liquid instead of vulcanizing the shell.

I would use a pour in liquid mold rubber. No pressure and no need to
fill. That said. if you want to fill it and then vulcanize it as
usual I would use JB Weld two part epoxi as it will take the heat of
the Vulcanizer and will not affect the rubber as some epoxies and
glues will. Have been using this technique for years to build up
items to mold including the underside of leafs.

Laura- The sea urchins will not survive the heat and pressure of a
Vulcanizer. You will need to make an RTV mold. It’s a cold cure
silicone moulding material. It takes 24 hrs at room temp to cure. You
can speed the cure time up a bit by putting it under a warm lamp. I’m
sure that Rio and Stuller both carry it.

Have fun and make lots of jewelry.

Jo Haemer

It sounds like these are fragile and will not withstand the heat and
pressure of vulcanization. I would suggest making the molds from RTV
which produce neither pressure or heat and the support can be clay
or epoxy.

Waves at the girls Teri

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Hi Laura,

I would use RTV moulding rather than vulcanising for a rather
fragile object such as a sea urchin skeleton. in this case any sort
of filler would do, plaster, clay, epoxy. I would use an injection
wax, largely because I do wax modelling and have the wax available.
You would need to seal the surface of the sea urchin as well. I use a
spray fixative that is used by artists for charcoal and pastel
sketches. Hair spray would also work reasonably well.

If you are working with RTV you can make the sprue with wax or
plastic rod.

Again I use wax because I have it to hand.

I have attached a photo of a piece I made from an RTV mould from a
sea shell

All the best
Jennifer Gow

Hi Laura,

I’ve used 2hr epoxy to fill stuff in for vulcanizing with some
success. It breaks loose after the vulcanizing, but you generally
don’t care at that point.

The problem is that the urchin spines will almost certainly crush
and snap, no matter what you fill the body with. I’d rate your
chances at getting a usable mold down in the single digits. Like
maybe 5%. You’re putting tons of pressure on the rubber to get it to
flow, and that pressure loads up long before the rubber gets
fluid. The rubber will smash the spines like toothpicks.

I’d be looking at RTV molding compounds. Much more likely to give a
good mold of something that fragile. I’ve had good luck with the
castaldo clear stuff. Makes cutting intricate molds much easier,
since you can see the bloody thing. (more-or-less)

Sorry to have bad news.


Plaster might be your best bet for this; at least it’s not affected
by heat and is liquid enough to go into that very small hole. You
might have to use an eye-dropper or syringe to get it in there. (Sea
urchin shells have a lot of tiny perforations in them, so don’t mix
the plaster too thin or it might leak out through them.) But once
you do that and make your mold, you’ll still have an object that’s
hard to cast, because it’s a solid lump.

For things like that, it’s usually better to make an RTV rubber
mold, and slush-cast the wax into them, pouring it in and then out,
so you get a hollow casting. But the other problem I’ve had with sea
urchins is that all those little points tend to catch air in the
mold, which become little voids just where you don’t want them. So
in this case, the injection mold might work better. If you hollow
out the injected wax before investing it, there will be fewer
shrinkage problems in the casting.

Andrew Werby

I am wanting to vulcanize a tiny sea urchin. It is hollow, so I
would like to fill it with something to give it more support and
to also make it solid. 

Instead of vulcanizing, use one of the liquid RTV (room temperature
vulcanizing) silicone rubbers. There are several on the market. I
like the clear one from Akron. Rio carries small kits. These are easy
to use, easy to cut, and put no strain on the model. If the model
absorbs or gets some of the rubber inside, no problem, just don’t
worry about it when you cut the mold, leaving inside whatever got in
there that you don’t need to remove. These rubbers have the
additional feature of curing with no shrinkage to the rubber, so
waxes are closer to the same dimensions as the model (was itself
still shrinks a little though, and so does metal during casing,
unless you use of the high expansion crystobalite investments meant
for the dental industry that are designed to compensate for metal
shrinkage…) You do need to use one of the mold frames designed for
these rubbers, with an outer rim and glass or plexiglass mold plates.


Epoxy putty has worked for exactly a super small sea urchin for me
in the past

The answer to your problem Laura, is an RTV (Room Temperature
Vulcanized) mold. No vulcanizer pressure, no high temperature, no
damage, very little shrinkage. You will not hurt your master by
making a mold of it unless you cut it or something, so you will have
more than one shot at it if you somehow mess it up. If you want the
finished castings to be solid, you can fill the sea urchin with clay
or wax or anything else you might have around and then make the mold.
Cut it just like a vulcanized mold. The only downside to an RTV mold
is that they often don’t last as long as a traditional vulcanized
mold, although Castaldo has a product called LiquaCast Long-Life,
that it supposed to fix that issue. I’ve not tried it yet, so I can’t
comment on it.

Any of the larger tool companies will have everything you will need.
Castaldo is my preferred brand of all mold making supplies, their
website has a lot of and will probably answer any
questions you might have.

Dave Phelps


Why do you only use heat vulcanized rubber material? With this item,
RTV (Room Temperature Vulcanizing) mold materials would be easy to
mold this sort of object. Then you could fill the piece with any of
the materials you mentioned. If you were to use an RTV rubber or
silicon (silicon would be my choice as it is usually softer and does
not present the sticking to the object that some of the other RTV’s
do and platinum cure verses tin cure as the mold will last for near
ever while tin cured lasts about 20 years) you could use any of the
filler material you are thinking about. I would be VERY worried
about crushing the urchin skeletons with the pressures in a heat

OR you could invest the shells and try to burn them out but that is
pretty iffy with the high calcium content.

If this was happening in our operation, I would do a very light
spraying of the outer surface with a sealer, I would use a good thin
varnish as it dries pretty fast, fill the piece with your choice of
materials, here probably either a soft clay or a soft wax, and
either do a 2 part mold (so no cutting) or do a single pour and cut
the mold, which ever you are more comfortable with doing. Depending
on hoe thick you pour the rubber, you might meet a hard mother to
keep the mold in shape but for these small pieces I think not. If
you decided a mother was best, plaster is fast and cheap. We are set
up here to most often make mothers with fiberglass. I always have
it, it is fast, it is light weight and if is very strong and

As I always say, there are a LOT of ways to skin the cat… none
wrong just different.

Best of luck,
John Dach

Laura, be prepared to lose the model in the process of cutting the
silicone RTV mold. The spines will break off and then you have to
figure out to pick them out of the mold.

I think the only way to do this is invest the sea urchin in casting
investment and then burn it out. I’ve seen thistles done this way,
but they tend to be more sturdy than a dried sea urchin.

Good luck,

Bill Mull
Zero-D Products, Inc.

If the form is not filled first, the RTV will be inside the piece
and connected to the mold material on the outside of the piece. Fill
the piece with something, then use RTV and you got it.

John Dach

Use cold pour silicone rubber

Hi Thank you for the input, the reason why I only could vulcanize this was that was all I had to use at the time. I am not in the USA and it is hard to get certain items here in Mexico. But I was sucessful and it did work well, I filled it with plastic using a syringe and then vulcanized it, Of course the urchin was destroyed in the process but I did actually get a good mold. I now have the RTV stuff! I also am going to have Delft clay here which would have worked well I think. But all these hints and ideas are just wonderful and I appreciate them so much, Thank you!

This is Michael Knight at Castaldo. We have a vulcanizing silicone molding rubber that would be perfect for this application. It’s so stretchy that you can use it to fill in sea shells,sea urchins,etc and keep the inside plug as part of the mold. No need to find weird things to stuff the cavity with.
Super Stretch SMR-pulling-rubber

You can see more about our SuperStretch rubber on our website at :

If you - or anyone else-- would like a small free sample, please let me know.I will need your shipping address, of course.

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