Ben,
First of all, outside of jewellery, this is a huge subject that
cannot be answered in one e-mail. However I will try to point out
some things out that may interest you.
Your post and others have convinced me that my model for
understanding shrinkage is wrong".
I don’t agree.
You are all correct, just correct at different points and under
different circumstances. All of the responses have provided you with
what I consider to be the basics, and the basics being what we all
did in science class with expansion and contraction experiments with
the ring and ball apparatus and a bunsen burner. Very easy and very
predictable for a wedding band. More complex shapes become a little
more interesting.
When I say at different points, everyone else is looking at the
shrinkage factor from a solid perspective and as a finished casting,
and whether you relies it or not, you are looking actually and
thinking more like a foundry man than a jeweler, which is a good
thing. As jewelers or jewellery casters, what do we pay attention
to?, other than a few criteria’s relating to proper investment and
spruing techniques, flask and metal temperatures and burn out cycle
times, clean metal etc, pretty much nothing else of significance
seems to matter. A foundry man is a totally different science and
truly an art in itself. The pattern will come with shrink allowances
built in, much the same way a caster will receive a wax lets say. The
jewellery caster will cast without any further consideration to
shrinkage control what so ever. The foundry man on the other hand
will very carefully design a full flow system to efficiently deliver
that material, especially in precision casting. He will have gates,
sprues with multiple runners all connecting to risers. Not so easy
to now maintain everything to spec. The foundry man/engineer is
responsible to maintain the shrinkage factor within the toleranced
specifications and fully specified in the engineering drawings. The
jewellery caster has no such responsibility, and the attitude is, it
is what it is and shrinkage is to be expected. Yes shrinkage is to be
expected, however there are ways to influence shrinkage, and other
industries use it to their advantage.
The flow of material will initially hit all surfaces and for
arguments sake, create a layer/crust of material against all the
surfaces and then molten metal will flow through the casting much
like a lava tube. Here is where you will get your thoughts played out.
The ID and OD surfaces will move away from the mold surface creating
what is known as an air gap between the casting and the mold and it
happens due to volumetric shrinkage. The easiest way to explain it,
is think of a sink in a particular surface of a casting. It created a
sink hole in the surface, because of the lack of material, hence
volumetric shrinkage. At this point and in relation to the mold, the
OD is smaller than the mold and the ID is larger than the mold core,
and therefore is also thinner in relation to the mold pattern. When
the whole casting as a solid cools, the whole solid will of course
become smaller. It is at this point of pre-solidification, that it is
critical to maintain a good flow of material through the casting to
not only avoid further volumetric shrinkage, but to just as
importantly maintain consistent heat distribution throughout the
material to avoid hot or chill spots. The foundry man, will rely
heavily on risers, especially on large and complex castings.These
risers are designed to solidify after the main casting therefore
allowing additional material to feed the casting for a denser and
fully filled final product. To collaborate the volumetric shrinkage
point, and that the cast surface separates from the mold, observe the
direction that the sinking/shrink happens, the shrinkage moved or
shrank towards the core of the material and not towards the center
point of the object. An approximate calculation for volumetric
shrinkage is around 1% for every 210 degrees (F) depending on the
metal. It can be more or less.
On another tangent, Draft angles in die cast molds and even plastic
injection molds do more than just aid in release. They also play a
role in many instances of influencing shrinkage. Because the thinnest
area of a drafted surface will cool faster than the heavy base it
creates what is called directional shrinkage. Many features in a mold
create directional shrinkage in a controlled manner to distort the
piece in a way to come as close to the desired end product as
possible. When everything is ideal, and the part is uniform and
simple, well it’s not a big deal, but when dealing with thick and
thin sections, how that part solidifies and cools is critical. A
casting engineer with good flow analyses can add or delete features
to push pull and distort sections and somewhat control how that end
product ends up, and therefore using shrinkage to his advantage.
Depending on the size of the ID and its relation to the OD and flow
length which is rated at twice the section thickness from points of
entry, the size and shape of that hole within reason can be
manipulated by calculating the area of cooling surface at that feature
etc.
As I said before, this is a huge subject with a lot of complexities,
at least at the industrial level. Items many times will have ribs in
place and are integral features that impart strength into machined
parts with thin and hollow sections, but are also designed at the
same time, to be conducive for casting. Ribs can have straight walls,
they can have a draft to enhance directional shrinkage, the point
being is at the machining level they have a purpose, and at the
casting level, additional purposes come into play. Same thing with
Radii that are machined at the intersection of adjoining faces, they
help to reduce the possibility of shearing in parts. Add casting into
the mix and they help reduce turbulence and sharp changes in direction
in casting. The direction of features many times and even their
position can alter a casting significantly. A straight flow direction
of material travel will have totally different effects on the part
than a cross flow metal delivery system, and it can become quite a
task to calculate.
Anyway, enough for now.
Best Regards.
Neil George
954-572-5829