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Shrinking casting problem

I am trying to reproduce cell phone cases using silver/gold by
casting. First I tried making a rubber mold from the plastic case,
it didn’t work. The rubber stuck to the plastic and it was a mess.

Then I tried by sprueing the plastic cases and burn them out in the
oven (treating them as wax). It was somewhat successful. However,
using this method comes with several problems.

First is that the plastic won’t burn out completely as wax does, so
it leaves the surface rough (by impression of the unburned plastic).

Second is the problem of shrinking by the metal. It won’t fit into
the original cell phone, it’s too small.

I need to solve these two problems. I was wondering if any of you
have any previous experience doing this sort of thing and if so what
would be the best solution.

Thanks for your time and thoughts. I greatly appreciate it. :slight_smile:

Welly Tandoko G.J.G.

I am trying to reproduce cell phone cases using silver/gold by
casting. First I tried making a rubber mold from the plastic case,
it didn't work. The rubber stuck to the plastic and it was a mess. 

Are those cell phone cases not the property of the Mfg.? i.e.: the
modeling and such?



our Invest C/P not only allows 2 hours casting but to adjust metal
shrinkage compensation for a wide variety of metals from Sterling to
Pt. If this is what you are looking for please contact me offline to
direct you to one of our suppliers in the U.S.



There are investments and methods for minimal shrinkage during
casting which, hopefully, others will provide on.

On the moldmaking, however, I would suggest first coating your phone
case with either Krylon matte fixative (the original not the
"oderless") or with Teflon spray. There is clearly something in the
plastic which is inhibiting the curing process in the rubber. The
spray fixative will seal the case and allow the proper curing.
Personally I found the Krylon fixative much more effective than the
spray teflon but you might want to test the Krylon on your plastic to
be sure it won’t have a solvent effect. Cad Blue in NY, 212-481-8700,
markets the Teflon product.

You also might contact Circle K Products, a vendor of silicon rubber
mold compounds. They have one platinum based group which, while
having practically no shrinkage, will not cure against any epoxy
based resin - hence the need to seal the material. They also have,
however, a tin based series which has no trouble with epoxy resins
but I’m not sure about shrinkage rates. Their phone number is

Les Brown
L.F.Brown Goldwork
17 2nd St. East, Ste. 101
Kalispell, MT 59901

Welly, You’ve stepped over the line. Go to the library, or somewhere
on line. You need about foundry work, (casting bronze
sculptures). Possibly you might want to look into sprueing,
different molding materials, hot and longer burn out, there are a
million things to try. but first you need to resolve your problems
with the model, it’s dimensions don’t compensate for directional
shrinkage. You are incurring shrinkage a little here and there all
through the casting process. I think it’s a cool idea, I hope you
get it working.

Regards, Craig

Hello Welly

There are a number of things to look at, but since it seems that the
cast plastic cases are the shrunken ones, we can skip talking about
Castaldo’s no shrink pink rubber. Take a look at your investment. You
may need to use a bit less water and stir more (use a cheap hand
mixer). Also watch your burn out temps. I think I had a similar
problem and it looked like I was cranking the burnout temp up too
fast. My biggest shrinkage problem was from not treating the rubber
molds right. I think I was cooling them down too quick from the
vulcanizer. Good luck – sounds like a neat project try to put up a
picture when you’re done.

A&M Jewelers
Baltimore, MD

BTW- I just thought of this-- I wonder how these places that make
grilles (fronts?) handle shrinkage.

You cannot do it by direct burnout or molding. You will always end
up with a casting that will be off in one or more dimensions.
Virtually all casting and molding processes have some shrinkage
involved. The shrinkage is not uniform in all directions and almost
impossible to calculate. Your plastic case is a precision fit in
the .010" range. Your best bet is to have the case laser scanned have
a model grown or milled and then cast with an experienced model-maker
to estimate the shrinkage correction factors.

James Binnion
James Binnion Metal Arts


After watching this thread for a while, and valiantly resisting
chipping in my two cents worth - I gotta give in and let it out.

Number one - Unless you have an agreement with the cell phone
manufacturer - in advance - you are setting yourself up for a
lawsuit. Copying the shell or cover IS copyright infringement.

The design belongs to the cell phone company. You could, perhaps,
legally make your OWN design to fit their phone, as an “aftermarket
accessory,” but you cannot copy what is already on the phone. (And it
better not look like theirs - that idea that constantly floats around
about how you can change 20 or 30% of someone’s design and call it
yours is absolute BS)

Number two - While there are lots of things that lend themselves
well to small studio casting, this is NOT one of them. The myriad
problems you will encounter in trying to make a functional mold,
cast, hand fit, and finish this article would make the cost so
prohibitive as to wipe out any real profit. Profit is usually the
reason that we take on jobs. (grin)

Number three - The advice you’ve gotten thus far about casting an
article with this large and thin an area, seems to have come from
people who have not tried something like this. I have. Not cell
phones, but a similar enough item…

The first thing that has to be done is to allow for shrinkage.
Everyone seems to agree on this. Sure there are investments that will
cut that down, but you still have the shrinkage of the injection wax,
the liquid metal when it cools, and the fitting and finishing will
remove more metal.

IF I were to attempt something like this (and I wouldn’t) I would
first ADD to the outside thickness of the cover. Then I would grind
away enough of the inside of the cover to have a loose fit to the
innards of the phone. The exact amount would have to be done by trial
and error, and it would have to be done perfectly evenly throughout
the entire inside. Probably lots of errors… Several molds, several
failures, to get it even close to right - so that after casting and
finishing it would fit as well as the original.

One other factor is that plastic flexes differently than metal. Is
the original made of plastic or metal? Cast metal will be more rigid
than thin sheet metal and may damage the inner workings of the phone
when pressed into place. I don’t know enough about the model of phone
you’re trying fit to say much about this. Is it a snap fit? Castings
are not “springy,” but could be coaxed a bit by some judicious
hammering. But all of this depends on your getting an almost flawless
casting to work with!

Number four - No one seems to have mentioned that forming thin sheet
metal would be a far better option with more chance of success.

I’m sure that if he wanted to, Valentin Yotkov could do this by
hand, with nothing more than some hammers and a stake shaped like the
working part of the phone.

Another method, better suited to multiple copies - would be to make
a steel die set, and form it in a 50 ton or larger press. You could
make it of very lightweight sheet metal, and this would have a bit of
"spring" to it if you need a “snap fit” to attach the cover.

I am not in any way trying to discourage the idea of making cell
phone covers. Only trying to point out some pitfalls in the
"duplicate" casting method and offering ways to get around them -
legally, and technically - and without suffering as much as you might
if you pursue the direction(s) discussed so far.

All of us have taken on ill-advised or miserable jobs at one time or
another. This one sounds like it could have some of the makings to
become one of those…

Brian P. Marshall
Stockton Jewelry Arts School
Stockton, CA USA

Hi Welly,

Did you try lubricating the plastic cases, or spraying them with mold
relaeasing agents? What about casting them in other materials, like
one of those RTV silicone rubbers? Given the innate oiliness of those
products, I’d expect your releases to be immediate and effortless, or
nearly so. Also, many of those are practically nil on the shrinkage
scale, so if the 1-3% they claim turned out not to be true, perhaps
you could make a mold, blow a wax, cast in silver, add a coating of
easy solder to that, then adjust for the shrinkage by working that to
the spec’s you want, +5%, and then recast. Tedious and
time-consuming, to be sure, but certainly a reasonable investment for
the return you’re seeking.

By the way, have you built in any fail-safes against torquing and

Douglas Turet,
Turet Design, LLC
P.O. Box 242
Avon, MA
Tel: (508) 586-5690
Fax: (508) 586-5677

I have tried this type of thing before and by and large I have been
unsuccessful. Not so much in the casting, even though that’s
sometimes a difficult thing… Some plastics burn out well, some
kill your cast, some make amazing patterns. That’s not the biggest
mission… The biggest hassle is the click thing. I have cast two
fitting click pieces before in silver and then a different one in 9kt
gold, ( for the springiness) and when I clicked them together that
was all she wrote. The metal didn’t have the give that plastic had
and the more I pried and twisted the worse is became. Once clicked,
never to let go. I think that there would have to be quite a lot of
re-engineering to make metal to metal click. Say for instance, of the
battery cover. Cast metal, clicking on plastic, worked better. I am
not at all suggesting it is impossible, but it is something that has
to be worked around. I imagine that cell phones are made with
CAD/CAM. If one could get those files. From the manufacturer, I
hasten to add. They might be more easy to modify in a metal friendly
manner. I have never cast cell phone things, but it’s a great idea. I
would very much like to hear of your progress, one way or the

Cheers, Hans Meevis.

Hi, You should have no particular trouble getting good castings
burning out the plastic. Bring the temp up slowly to 1375 and hold
it for at least 5 hours. bring the flask down to your favorite
casting temp ( everyone has a different favorite ). Make sure the
flask looks clean and white. If there is still grey around the
opening, you must burn it out longer. I’ve cast a good many pieces
this size using a vacume well, but again, every one has a favorite.
E-mail me if I can help in any way.

Have fun. Tom Arnold

You might try electroforming the plastic piece to build up
dimension. May have to trim the inside, depending on how it attaches
to the phone If you can measure precisely how much shrinkage your
process is yielding you can ‘oversize’ your model accordingly. Use a
non-vulcanizing mold compound.