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Copy sample using rubber mold


#1

Hi there,

I am new in jewelry casting, a customer of mine asked me to copy
several pieces from him pendants, my question is that how can I cast
it use rubber mold for same size or have to make it one by one by
hand ?

Thanks in advance!
Peter Deng


#2

You are going to get at least some shrink duplicating pieces. BUT,
“can” you legally reproduce the pieces? Be very careful here as you
may well be impinging on another artists copyrights. If not (and I
doubt it) using silicon RTV or silicon vulcanized mold materials
will give you the least shrink in the mold. Keeping the wax as cool
as possible when making the waxes will again, keep the shrink to the
minimum. Lastly, when casting, you will get a bit of shrink, period.
3 to 7% shrink is to be expected in the “new” piece compared to the
original. But I MUST emphasize, casting someone else’s work is
TOTALLY illegal (unless you have their permission).

John Dach


#3

Thanks for your reply, John, the original piece is sent by a
designer, so it is legal I think. But I never use RTV before, is it
the only way to do it?


#4

There are 2 different materials. one comes as sheets of “soft” rubber
that is cut and formed around the item to be molded and all is in a
metal frame and once “packed” it is compressed and heated in a
heating unit for what ever time period for the thickness of the mold.
The second way is using a catalyzed rubber (silicon or others)
material. Here the piece to be molded is again placed into some sort
of “frame” or containment unit, but since no heat is used with this
type of mold material, the frame does not need to be something that
withstands the high temperatures used in the first instance.

Then, with either material, there are a number of ways that the mold
can be developed. One way is to just pack the frame (vulcanized mold)
or pour the containment unit full of the catalyzed rubber. Going this
route, the resulting mold will have to be cut in such a way as to get
the original out of the mold and to allow a wax to be injected into
the mold and also easily removed.

With both types of mold materials, one can also “make” a separation
barrier where one wants the mold to separate. This method works
better on some forms than others but without seeing what is being
molded, I cannot say whether this would be a proper method for you
to use.

If the molds are filled with rubber, using either material type
discussed above, the mold will have to be cut to get the master out
and to get waxes out. Really good mold cutters are amazing to watch
when they can cut molds in ways you would NEVER believe, to get very
fragile waxes out of molds. Just writing this makes me recall some of
the molds I have seen that were truly something to see being cut. So
cutting molds of relatively simple pieces is “relatively” easy to
learn, but practice sure plays a huge role to do it “right”.

RTV (Room Temperature Vulcanizing) materials do not take heat to
catalyze/vulcanize so molds can be made of waxes and most anything.
Generally, but not always, RTV molds materials are softer than
vulcanized molds, and this can be an advantage and/or disadvantage,
just depending the piece and what you want and are used to using.

Hope this is a bit of help. If more questions please feel free to ask
and I will try to answer to the best of my ability. I am surprised
there have been no other comments on this, we shall see what others
think (hopefully).

John Dach


#5

Presuming the sample is metal then you can use vulcanising rubber.
This is less flexible and will allow you to produce waxes with less
distortion and less wear on the mould.

Nick Royall


#6
Presuming the sample is metal then you can use vulcanising rubber.
This is less flexible and will allow you to produce waxes with
less distortion and less wear on the mould. 

And much greater shrinkage than RTV

James Binnion
James Binnion Metal Arts


#7

Thank you, John, I really appreciate your help.

I wonder if I have method to extend the wax mold to approx 5-8% ? is
it possible to do it by a heat air ? the original pendant has a
centre hole for setting stone. I find it reduced after casting and
could not fit the stone you see.

Peter


#8

There is no way to make it bigger as far as I know. Re working the
wax before casting would be about the only way other than remaking
the whole piece again. I know there are products available to reduce
sizes (Reducit is one) but I know of nothing to really expand/make
bigger of pieces save remaking the item.

What wax are you using for injection?? There are 100’s of different
formulations with all sorts of different qualities. We generally use
Serria Red in our wax injector as it is hard when cold but flexible
when warm. A few moments in hot water gets the wax warm enough to do
a lot of moving and blowing on it cools it enough to get the movement
to stay. Also with ANY wax, do not have it hotter than necessary as
the hotter it is the more shrinkage you will get upon cooling. Some
of the big, pressurized units actually inject a wax paste rather than
a liquid wax. There are also some “filled” waxes that shrink less but
we have never found one that has all of the qualities of Serria Red
that we like.

Also, not heating the casting metal more than needed to get it to
cast will give a bit less shrinkage, but you need to have it hot
enough to catch the detail and all of the fine points and the like.
“The right temperature” is something you figure out after a LOT of
casting. I don’t always get it right but getting better all the time.
Life is a continuous learning experience you know!

John Dach


#9
There is no way to make it bigger as far as I know. 

Well, John and others, there is a way, but it’s unpredictable…
Electroforming, in essence. There was once a dedicated machine on
the market for boosting the bulk of jewelry models specifically.
I’ve never seen it since, and I doubt it took off in any real way.

Which is to agree with John - technically and theoretically, there
is a wayto boost the bulk of models. Practically speaking, make a
whole new one, it will be easier in the long run.


#10

I use Freeman Aqua wax for injection, how much temperature of the
warm water do you advise to expand the wax pattern ?


#11
There is no way to make it bigger as far as I know. Re working the
wax before casting would be about the only way other than remaking
the whole piece again. I know there are products available to
reduce sizes (Reducit is one) but I know of nothing to really
expand/make bigger of pieces save remaking the item. 

http://www.industrialpolymers.com/water_abs_expanding_urethanes.html

The above may not work too well in this application, but they may
have another solution to the problem. They sent me a business card
made from Hydrospan, throw it into a bucket of water and it gets
bigger.

I can’t find it now but I recall a student expanding a silicon
rubber mould by soaking the mould in something, then re-casting in
resin, then making another mould and expanding that. The objective
was to make a really large from the initial toy car. I’ll post a link
if I ever find it again.

Regards Charles


#12
There is no way to make it bigger as far as I know. Re working the
wax before casting would be about the only way other than remaking
the whole piece again. I know there are products available to
reduce sizes (Reducit is one) but I know of nothing to really
expand/make bigger of pieces save remaking the item. 

I know of one, soak a regular rubber mold in naptha (coleman stove
fuel) and it will swell. Surface quality on injected waxes was not
great or maybe I didn’t try hard enough. And before safety people
jump all over me work outside as much as possible.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand