Shrinkage in rings
MattGroves
casting a ring in 14k gold?
Does the investment allow for shrinkage without breaking? or does the metal
simply try to shrink, is thwarted, and places a lot of stress within itself.
oh, so many questions
GeneLeis
>casting a ring in 14k gold?
>does the metal
>simply try to shrink, is thwarted, and places a lot of stress within itself.
If you are carving the same wax you plan to cast and wear then you will have no
problems with shrinkage.
The investment, if you use a good brand, is designed to almost precisely
compensate for the shrinkage of the metal. When properly mixed the investment
will expand with heating to an amount almost equal to the amount that the metal
will shrink. Nice feature.
Gene
jred...@gmi.net
mattg...@aol.com (MattGroves) wrote:
> Generally speaking, how much shrinkage must you allow for (in ring size) when
> casting a ring in 14k gold?
>
> simply try to shrink, is thwarted, and places a lot of stress within itself.
>
Matt We do lost wax casting with vacumn and do not experience any noticable
shrinkage. If the ring size is to small, it can be stretched on a ring
mandrel. A stone setting should be undersized and routed out with a flex
shaft. Regards Joe
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LeeCo11
>casting a ring in 14k gold?
>
I usually give an eigth of a size for a ring that will be sold to a customer.
If you are going to make a mold of the ring afterwards go a little bigger and a
little heavier in the shank because that will shrink a bit too.
Lee
Esther Heller
>
> >Generally speaking, how much shrinkage must you allow for (in ring size) when
> >casting a ring in 14k gold?
>
> >simply try to shrink, is thwarted, and places a lot of stress within itself.
>
> problems with shrinkage.
>
> The investment, if you use a good brand, is designed to almost precisely
> compensate for the shrinkage of the metal. When properly mixed the investment
> will expand with heating to an amount almost equal to the amount that the metal
> will shrink. Nice feature.
>
> Gene
Waittaminite, lets make sure I have this straight (especially after
Peter's explanation to the folks with the mercury spilled on a gold
ring)....
Are you saying that I could carve a wax to exactly properly fit my
finger and if cast with a good brand of investment it will come out
the same size when done? Being an amateur observer here and a more
experienced woodworker who knows something about making woodworking
tools, the mind boggles. You can usually tell a craftmen copy of
a number of old commercial tools by the fact that they used the
original as the model and the brass or whatever they cast in shrank
so the size differential from standard is the tip-off. Sounds like
if I can come up with a brass/bronze alloy with about the same shrink
rate as 14K I can make an exact copy.... There are a lot of neat old
tools that simply aren't made any more and I am told you can melt
bronze with a blacksmith's forge.... Also lots of hardware you would
like to match that simply isn't available.
Have I missed something? Recommended "good brands"? Sounds like
it would be functionally equivalent to making an "important"
piece of jewelry in a less expensive metal.
Thank you all!
--
Esther Heller eoh at kodak dot com
Peter W. Rowe
<munge...@kodak.com> wrote:
>>Waittaminite, lets make sure I have this straight (especially after
>>Peter's explanation to the folks with the mercury spilled on a gold
>>ring)....
>>
>>Are you saying that I could carve a wax to exactly properly fit my
>>finger and if cast with a good brand of investment it will come out
>>the same size when done?
Well, in practice, sorta yes. but here's how that works in detail. The
investment is at a fairly high temp, like 800 to 1200 F when you pour the metal
in. Thermal expansion will have expanded the whole flask and investment, so the
metal is being poured into a mold cavity that generally is a bit bigger than the
actual size of the wax. Then, the metal will solidify in this larger cavity and
proceed to shrink a bit as it solidifies and cools. The degree to which this is
matched is not so much a factor of the quality of the investment, as it is the
purpose the investement is designed for, and it's type. High crystobalite
invesements have high thermal expansion rates, and these can sometimes even
produce a casting larger than the wax, for some metals. In platinum casting,
too, this can happen, though it's variable a good deal with the exact temps used
for a given cast. The Crystobalite investments are generally dental types, and
are specifically designed so that the casting, in a dental alloy, will be the
exact same size as the wax it was cast from. That's so the inlay for your teeth
that's just been cast, will fit without gaps...
Jewelry investments are generally engineered a bit differently. Among other
things, they need greater strength, and need to be used in larger flasks, which
isn't so compatible with high expansion investments.
But you don't really WANT a cast ring to end up the exact size, in the casting,
as the wax, if you think about it. You're going to take that casting and file
and sand and polish the inside of the shank as well as outer details. That
makes the size a tad larger. And in many cases, you may need to do a little bit
of trueing up and rounding out and the like, on a mandrel with a mallet... All
in all, by the time you've taken that cast ring from it's raw cast state to a
finished ring, you'll end up the right size, or pretty close. So the end result
is that the castings shrink a little, yes. but the end size of the finished
jewelry is close to the ring size of the wax.
However, this doesn't change the fact that there is some shrinkage, both in the
raw casting and in the end result. In terms of thickness of prongs and metal
cross section, the finished piece of jewelry will have a bit of shrinkage.
Polishing alone can often make a piece a lot thinner than the original model
If you wish to cast things where the finished casting really is the same size as
the original, you can use those dental investments for small things. For larger
ones, consider sand casting. Here, you can simply wiggle the model a little in
the sand impression as you remove it from the sand, slightly increasing the size
of the mold cavity. Since sand casting is often the method by which those old
brass and bronze items were made in the first place, the end result should be
good. But sand casting in and of itself isn't such an easy thing to learn
either....
Another option is to take the original and electroplate it heavily, building up
the thickness a bit before molding and casting the duplicates.
hope this helps.
Peter Rowe
LeeCo11
casting, as the wax, if you think about it. You're going to take that casting
and file and sand and polish the inside of the shank as well as outer details.
Personally, YES I do want it to be the exact same size. Aside from the piece
shrinking in the casting procedure (won't get into molds, though as a side note
the pink mold rubber is really good at eliminating shrinkage) Filing and
sanding reduce it more. Yes it increases the size but reduces the thicknesses
all around.
It is really annoying though when you are casting a piece that has multiple
parts all cast in different colors. Somehow they can fit perfectly together in
the wax but after casting the exactness dissappears and modifications need to
be made.
>And in many cases, you may need to do a little bit
>of trueing up and rounding out and the like, on a mandrel with a mallet...
You shouldn't have to. The wax should be exactly how you want the finished
piece to be. If it is out of round you did a bad job. In theory (with an
accent on theory<g> ) the cast piece should just need a light sanding and
polishing (after the sprues are removed of course) and thats it.
>So the end result
>is that the castings shrink a little, yes. but the end size of the finished
>jewelry is close to the ring size of the wax.
Close is fine in ring size but again if it is a multi piece casting you want
the pieces to all fit together exactly not just 'close'.
>he fact that there is some shrinkage, both in the
>raw casting and in the end result. In terms of thickness of prongs and metal
>cross section, the finished piece of jewelry will have a bit of shrinkage.
I always try to make the prongs a bit heavier in the wax to compensate for the
shrinkage (they also cast better and pull out of the mold better when they are
heavier.
>If you wish to cast things where the finished casting really is the same size
>as
>the original, you can use those dental investments for small things.
Unfortunately this is only good if you are doing your own casting. If not you
are at the mercy of the caster and have NO idea what they are doing, even if
they tell you what they are going to do. <g> You just have to grin and bare
it.
Wouldn't it be nice if someone made a set of measuring devices that are
slightly larger than they should be, to compensate for shrinkage.
Of course the most frustrating part, IMHO, is that when there is a complaint by
a jeweler about shrinkage (or anything else that goes wrong) the caster blames
the model maker saying it had to do with the wax. :-(
Thats it for my complaining,
Lee
Donna
wrote:
<snip>
>ones, consider sand casting. Here, you can simply wiggle the model a little in
>the sand impression as you remove it from the sand, slightly increasing the size
>of the mold cavity. Since sand casting is often the method by which those old
>brass and bronze items were made in the first place, the end result should be
>good. But sand casting in and of itself isn't such an easy thing to learn
>either....
Peter, you always explain things well. Can you instruct in sand
casting, please?????
Peter W. Rowe
wrote:
>>>> consider sand casting.
>>Peter, you always explain things well. Can you instruct in sand
>>casting, please?????
>>
Instruct? Um. that will take a while. but I can explain the gist of it. Sand
casting is a very old and traditional method of mold production, still common in
foundry work, but more and more replaced by ceramic shell sorts of molds in may
uses. it used to be the predominant form of casting operation however.
The basic process is that a model is made, usually from wood, but other
materials work just fine for jewelry use. The model must have no undercuts for
a simple mold. Complex models may be several pieces, and two piece models,
which seperate around a center parting line are common.
the model is placed on a flat surface, and a mold frame (simply a box of one
sort or another. In jewelry scale sand casting, it's often a small cast iron
frame) is place around it. Specially prepared sand (fine grain sand, mixed with
either water, sometimes glycerine, or an oil based binder of some sort) is
sifted around and over the model, and then packed tightly down around it,
filling in the frame to a level surface. Often mallets or other means of really
tightly packing the sand are used.
Then the frame is turned over, exposing the model's flat bottom side imbedded
tightly in the sand. This whole surface, sand and model, is lightly dusted with
a parting powder (talcum powder works), and the other half of the mold frame is
placed on top of the first one. If the model has a second half, this is now
placed over the first half, and more sand is rammed into the second half of the
modl frame same as the first. When the two halves of the mold are then
seperated, the model must be freed from the sand and removed. Gently rapping
it/wiggling it can give it the clearance needed to do that. Care is taken to
avoid fracturing the packed sand, and a sprue hole is then cut into the sand
leading from the cavity left by the model to the outside of the mold frame.
Vent holes are also put in, any loose particles of sand are gently brushed away,
and the mold is closed again. Now you've got a packed sand mold, with a sprue
to pour into. Depending on the type of sand and binder, and the metal being
cast, some molds are now gently baked to drive off moisture or increase the
strength of the mold by drying the binder a bit. Others are not. Molten metal
can be poured into that sand mold, and after a bit of time for solidification,
the sand can simply be broken away. Most of it can be reused, again depending
on the type of binder.
Molds of this type are more limited than lost wax casting, because of the need
to remove the model from the mold. But models can be quite complex, and one can
make seperate "cores" of sand that get inserted into parts of the cavity left by
a master model to create things like the hole for the finger in a ring, or for
the stone in a gypsy setting design, or the like. In industry, cores can become
very complex affairs.
Advantages of sand casting, however, are that with the right type of sand, you
can go from a model to a finished casting in about ten minutes sometimes.
Vigor produces a little basic jewelers sand casting kit which is reasonably
servicable.
More expensive, but a little easier to use is a kit called the "delft clay"
process. It's not a clay, but simply a trademarked name for what I'm told is a
standard type of very fine grained casting sand, and the kit comes with a usable
little casting mold well suited to jewelry scale work. You can carve a quick
model in wax or wood, or use an existing object, so long as it can withstand
having the sand packed tight (hammered) around it. As soon as you've finished
preparing the sand ("delft clay") mold, you can immediately melt and pour your
metal. Gold comes out of the mold with a surprisingly good surface and a almost
bright, perhaps because of organic binders in the clay providing a reducing
surface in the mold. I use this process sometimes when I need certain shapes of
ingot, or types of heavy gold shanks for a design, and it's quicker to cast one
up this way than to fabricate it. Actually, what I often cast up is a heavier
blank form, which I forge down into the finished shank, thus giving me the
advantage of forged metal, while still saving me much time over starting with a
plain bar of metal, and I end up having to file less off this way as well, thus
saving metal. For a few designs, though, I cast the actual desired finished
form, which often requires only a little more clean up than a lost wax casting
would do.
Like I said, it's a limited process, and not applicable to every use. But where
it's appropriate, it's easy, cheap, and fast.
Peter Rowe
Ronald Arney
>
Snip
> Molds of this type are more limited than lost wax casting, because of the need
> to remove the model from the mold.
I was doing a show some years ago, and saw a fellow with medieval looking
pewter "Ash Trays" that appeared to have been cast in volcanic molds. At least
that is what the surface looked like. Listening to his spiel, I heard him say
that they were sand cast. During a lull in the show I accused him of doing a
bit of leg-pulling, asking how in blazes he could remove a model required to
to get that texture out of sand. He said he didn't, he left it in... The best
I could come up with was a blank look.
Using a hot-wire, he carved thick sheets of styro-foam. The hot metal did the
burn-out as he poured. Using a dowel of foam on the bottom as a sprew saved
another step.
Another time I watched a fellow pack a frame without a pattern of any kind,
and then carve directly into the sand. After the pour, he removed the, in this
case, belt buckle, from the sand; did a bit of artistic adjustment to the sand
and poured again, obtaining another buckle. He told me that he sometimes got
as many as 6 or 7 buckles this way, each different, without need to pack
another flask. For brass buckles at $75 to $125 each, he had a pretty good
thing going.
Putting the two techniques together, using the soft, flexible sheets of 1/8th
foam used for protecting breakables for shipping, I've cut shapes and done a
bit of sand scratching myself. I, however, have no talant for carving negative
images in anything. Those who do, may find these techniques both interesting
and profitable.
Ron