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Casting shrinkage?


#1

I’m having issues with a client of mine. He says from the first
generation metal ring to second metal ring, there is 10% reduction in
results. Is this % correct?

I know from other sources it is about 5-6% maximum! Can anyone please
elaborate! I need this so I can educate this individual. Why? He
wants to use a 6 mm stone down to a 5 mm in one rubber mold forming.
I say he’s ‘out to lunch’ but I need definite printed facts on this.

…Please help a.s.a.p.!!!..Gerry!
https://ganoksin.com/blog/gerrylewy/


#2

Hi Gerald,

It was suggested the other day at TAFE, that if you are to make a
ring out of wax that you make it 1/2 size bigger, I see this as a
ballpark figure.

Anyway what metals are you using, this has a bearing on the shrink
rate.

I’m looking for shrink tables for precious metals, but so far I’m
coming up with bupkiss :frowning:

Regards Charles A.


#3

Dear Gerald,

Good Morning Gerald.

What is the type of rubber he is using for the molding?

What is the process of vulcanizing? If it is the thermosetting type
by heat & pressure you can get that kind od shirkange.

If it is thermoforming type with the silicons you will not have
shrinkage.

Please ask him to give a detailed step by step operation process
stating all the materials being used.

I will mail you the details during the day.
Regards,
Umesh


#4
I'm having issues with a client of mine. He says from the first
generation metal ring to second metal ring, there is 10% reduction
in results. Is this % correct? 

When you make a mold, depending on what type of mold material, there
can be shrinkage from the mold, and shrinkage from casting. When I
make rings, I usually make the ring 10% larger than I want so that
after molding and casting again, it is the size I want

elaborate! I need this so I can educate this individual. Why? He
wants to use a 6 mm stone down to a 5 mm in one rubber mold
forming. I say he's 'out to lunch' but I need definite printed
facts on this. 

If I understand, if you mold a 6mm setting, it will be 5.4 mm. You
need to take into consideration having to burr a seat, do you need
thicker wall and slightly smaller opening?

Richard Hart G.G.
Denver, Co.


#5

Dear Gerry,

The rate of casting shrinkage is not automatic but is dependent on a
wide variety of factors, all of them introduced by the user and his
or her procedures, level of skill, attention to detail, knowledge,
experience, etc. Casting shrinkage rates can vary, very widely
because of this. There is no “maximum.”

Shrinkage is created in the spruing process, in the mold-making
process, in the wax injection process and in the casting process.
There are 100’s of factors.

The simplest thing to say that that caster A typically gets one
shrinkage rate while caster B typically gets another rate.

Hope this helps.
Michael Knight


#6
there is 10% reduction in results. Is this % correct? I know from
other sources it is about 5-6% maximum! 

Gerald, what I learned in the trenches is about 8% for a vulcanized
rubber mold and finishing. When you are talking about 20-some mm for
a ring diameter, you can do that. Making a setting that’s 5-6mm means
you may as well just use 10%, as the difference between.5 and.6 (10%)
on a stone that size is negligable. Even at those numbers, a 5mm
setting would be 5.5mm in a model, not 6mm. Even no-shrink rubbers of
course shrink, they just shrink less. And of course all of us real
jewelers know that you can buy 10 different 6mm findings and they
will all fit a 6mm stone differently, sothere’s that.


#7

What kind of rubber? Lots of variables here but somewhere between 5%
and 10% is not unrealistic from metal master to finished second
generation. But 6mm to 5mm is more like 17% so I think your client
is OTL on that count.

James Binnion
James Binnion Metal Arts


#8
He says from the first generation metal ring to second metal ring,
there is 10% reduction in results. Is this % correct? 

It’s actually pretty easy to get even more shrink than that if you
really cook a traditional rubber mold while vulcanizing it, even with
low shrink rubber. 15 minutes too long and ten degrees too hot will
do the trick. The effect can be increased by cranking up the
temperature of the wax pot, squeezing the tar out of the mold while
it’s being shot and using a heavy hand while finishing it. Using the
newer generation zero-shrink RTV (room temp. vulcanizing) rubber and
careful experimentation with wax temperature, air and mold pressure,
shrinkage of 5% or less is pretty easy to achieve, so it doesn’t have
to be that high, even for someone with very little experience.
Cutting the mold and dealing with the results of that will usually
cause far more grief for the rookie than shrinkage.

Another consideration concerning shrinkage from master to 2nd
generation is that it is generally more of a volumetric kind of thing
as opposed to a dimensional thing. A 2nd G casting might be a half
ring size smaller and 5% lighter, but a bezel in a heavy part may
actually end up slightly larger in inside diameter as the wax / metal
shrink away from the hole, towards the center of the heavy part. In
other words, all of the solid parts get volumetrically smaller. When
using RTV for a single stone setting, I don’t worry too much about
shrinkage. I just try to make sure the stones can be made to fit if
it doesn’t shrink much and leave enough metal on the outside in case
it does.

Why? He wants to use a 6 mm stone down to a 5 mm in one rubber mold
forming. 

You didn’t say what kind of setting he might be thinking of, a prong
style or a bezel for instance, or how many stones are involved. A
setting for a single 5mm to 6mm shouldn’t be that big of a deal for
either style, I don’t think. If it’s prongs, I’d make the master
using fairly heavy prongs, say 16 or even 14 gauge, taper the shape a
bit top to bottom, and construct it so that it will fit a 5mm stone
set low, with a shallow cut on the seat and a 6mm a little higher
with a heavier cut to the seat. For a bezel, just think of a tapered
tubular shape with an ID of something like 4.7mm and an OD of 6.8 or
so at the top on the master. If he doesn’t want to cut and fit that
much, or if he wants something much more delicate and one-off
looking, then he might just be out of luck.

But we’re talking about mass-production tradework, right? I mean,
times being what they are Gerry, if that’s what the man wants, you
really ought to think about making it for him. If you don’t want the
work or to be associated with “that kind” of work, I’ll bet you can
find some takers here to pass it on to. Heck. PM me. I’m not that
proud, and I’ve got a payroll to meet. ;

In my humble opinion and certainly without the benefit of seeing the
project, a 5mm to 6mm tolerance shouldn’t be completely out of the
realm of possibility unless there are extenuating circumstances like
multiple stones set girdle to girdle at the same height. Something
like that would make it a different story altogether.

Hope this helps, Gerry!
Dave Phelps