Understanding Casting Shrinkage

Neil,

I just returned from vacation and saw your detailed post, which took
some work, thanks.

I think I understand what you are saying, and will try to reiterate
some of the main points, hopefully correctly, to see if I get it.

On initial flow, the cooling action between the mold and metal will
create an air gap, and thus a volumetric reduction for both inner and
outer diameter surfaces. This form of shrinkage explains why it is
sometimes necessary to create a bulge in a wax when a flat plane
surface is desired in the final casting.

But there is also a second form of shrinkage occurring, that which
happens when the metal crystallizes into a complete solid. It is for
this type of shrinkage that inner and outer diameter surfaces can
vary closer to or farther away from the mold walls. A combination of
surface tension, gravity and orientation produce a “desire” of the
molten metal to reform in a particular shape. Thus parts of the mold
will be fighting the metal’s force from cooling, while other parts of
the mold will have an increased air gap and be unstressed by the
casting.

If only the first type of shrinkage occurred, draft angles would not
be required for release for multi-use die cast molds. But they are
needed, which means that the crystallization shrinkage force is
dominant over the lava-tube like volumetric shrinkage.

Predicting and compensating for this shrinkage with risers, ribs,
draft angles, radii and all the other tricks you mentioned are what
make casting an industrial science, a profession I now more fully
appreciate (from a respectful distance).

I hope that isn’t a mischaracterization of your words.

Ben Steiger

I just returned from vacation and saw your detailed post, which
took some work, thanks. 

Your Welcome

I think I understand what you are saying, and will try to
reiterate some of the main points, hopefully correctly, to see if I
get it. 

ok

On initial flow, the cooling action between the mold and metal
will create an air gap, and thus a volumetric reduction for both
inner and outer diameter surfaces. This form of shrinkage explains
why it is sometimes necessary to create a bulge in a wax when a
flat plane surface is desired in the final casting. 

The volumetric shrinkage occurs first thus creating an air gap.
Convexing a surface to allow for volumetric shrinkage was quite
common, and in all honesty I’ve never personally approached a flat
surface in this manner. For me, it was all about material delivery.

But there is also a second form of shrinkage occurring, that which
happens when the metal crystallizes into a complete solid. It is
for this type of shrinkage that inner and outer diameter surfaces
can vary closer to or farther away from the mold walls. 

Whatever happened during volumetric shrinkage, will be reflected in
the final casting. Once it becomes a solid, it will be dimensionally
stable for as long as it stays at that temperature. However, as soon
as you quench, what everyone else told you comes into effect and that
is a predictable shrinkage on mass towards the center point of the
casting and not to the center of mass as was the case with the
initial pour.

A combination of surface tension, gravity and orientation produce
a "desire" of the molten metal to reform in a particular shape. 

Well you added some points that I hadn’t touched on :-), but close
enough.

Thus parts of the mold will be fighting the metal's force from
cooling, while other parts of the mold will have an increased air
gap and be unstressed by the casting. 

On a poorly designed mold as in die casting, this could be the case,
and is something you try to avoid. In lost wax casting not
necessarily
so. If that flask is held at casting temp indefinitely, the air gap
remains. Allow the flask to cool to room temp, then there will be
pressure on the mold and normally it will shrink onto the core.

If only the first type of shrinkage occurred, draft angles would
not be required for release for multi-use die cast molds. But they
are needed, which means that the crystallization shrinkage force is
dominant over the lava-tube like volumetric shrinkage. 

Neither is dominant, because at the pre solidification and at
elevated temps, the volumetric shrinkage is occurring. Once the
casting is solid, it will remain stable until it cools below that
solidification temp, then and only then will the global shrinkage
come into effect. Therefore, they are non-competing reactions to keep
it simple, and merely an evolution through process. In die cast
molds,
1 degree of draft will usually do it. However, there are many
instances where you can get away with no draft at all, but in
reality, the mold will last much longer because of less friction
during ejection. Now in sand casting, that draft is critical to allow
the pattern to release efficiently and leave a nice clean impression
in the drag base.

Predicting and compensating for this shrinkage with risers, ribs,
draft angles, radii and all the other tricks you mentioned are
what make casting an industrial science, a profession I now more
fully appreciate (from a respectful distance). 

You and me both :slight_smile:

Best Regards.
Neil George
954-572-5829

Cheap jewelers working with quenching a flask might for instance
use a whole lot of wine corks, so many that they are about 2-3
thick on the surface. You quench through this and the emitted
particles are reduced. So ask your friends to collect corks for you
or have a really big party.

Or cooking oil, which floats. I use acetone and methylene
chloride soaks in my work and use water as a "cover" liquid." 

Come on folks none of these methods are safe, period end of
discussion. The only safe methods are to not quench and remove
castings cold or quench with proper ventilation and respiratory
protection.

James Binnion
@James_Binnion
James Binnion Metal Arts

360-756-6550

To all casters past and present.

I have been casting and quenching my flasks since 1971. At that time
I even used asbestos flask liners. Currently I practice casting and
quenching like I have described in my comments for years with an
update of a OSHA fit mask. I have cast between once and five times a
week ever since then. Each cast still consists of several flasks of
various metals and volume; centrifugal as well as vacuum casting. I
have made several modifications on equipment as well as build my own
machines. The casting process of I have described are what I know are
industry standards.

Maybe Dan from Racecar could add more detail from his perspective.

Bob the caster who taught me many of his tricks is now 70 years old
and in perfect health. He even smoked for a few years in his teens.
He cast and quenched for at least thirty years with no venting what
so ever.

What gives? Is casting to be viewed like sucking unfiltered Camel
cigarettes? I will always advocate the safest and practical way to
do things. Are those who can only research about technique the only
real authority on jewelry making? I don’t make this stuff up folks.
If you are lucky enough to ever get into some of the industry casting
facilities as I have you would see how it is done.

Ongoing criticism like this is what will bring forums like this to a
decline.

Quiet now for a while to come…

Todd Hawkinson

Bob the caster who taught me many of his tricks is now 70 years
old and in perfect health. He even smoked for a few years in his
teens. He cast and quenched for at least thirty years with no
venting what so ever. What gives? Is casting to be viewed like
sucking unfiltered Camel cigarettes? I will always advocate the
safest and practical way to do things. Are those who can only
research about technique the only real authority on jewelry making?
I don't make this stuff up folks. If you are lucky enough to ever
get into some of the industry casting facilities as I have you
would see how it is done. 

I have cast 18-27 3x7 inch flasks every week for 12 years, quenching
in a 5 gallon plastic bucket. I have been casting for over 20 years,
not great investment hygiene. I live at 5200 ft above sea level, and
I have normal lung function. I was not going to start in as I usually
do, but since you mention it… I believe in being careful. I was
self
taught as a silversmith and for 20 years observed no safety
precautions for torch use or for soldering fumes. I did wonder how
many on this forum observe safety precautions, but smoke cigarettes?

I do tend to be more careful, and more paranoid from the posts on
this forum. However, at 60, however careful I am with diet, care with
chemicals, ect. from what I observe from those around me at the same
age or younger, good health is a crap shoot. There is a fine line
between being cynical or realistic, and I find being realistic a
challenge as whatever I can conceive of to protect myself does not
include what I do not take into consideration that sneaks up behind
me and bites meon the tush.

Richard Hart