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Casting: Feed Sprues and Gates


#1

A very high percentage of investment casting defects can be directly
or indirectly attributed to the design of the sprue system feeding
metal to the pattern. Defects such as incomplete pattern filling and
shrinkage porosity are easily identified as directly related to poor
sprue design. Gas porosity might be caused by a poor burnout but it
could also be caused by casting at a higher than necessary
temperature. And the elevated temperature could be necessary for
complete pattern filling through an inadequate sprue system; this is
an example of the sprue system indirectly causing a defective
casting…

Read more…


#2

well, first of all, I do cast at a higher than usual temp, out of
necessity, as I cast filigree…and barring underfills, or
cracks/blow outs I often get 90% plus fill rate with usually 50+
items in a tree …I don’t seem to have too much trouble with
incomplete patterning except at the bottom of the tree, where I
usually put extra waxes, but not usually my necessary pieces. I cast
using vacuum. I was looking at the page referenced in the last email
(see below)

and was noticing in the diagrams that the sprued models are
generally being sprued straight out…at almost a right angle from the
central sprue…I was taught to sprue at a steeper angle…see

http://www.jeannius.com/castprocess/

for examples of my trees, both wax and cast. To me, it makes sense
to sprue more in an angle compatible with the draw, either vacuum or
centrifugal, rather than straight out…this seems to me like it
could cause incomplete fills on it’s own. BTW…I use rather thin
sprues myself since my castings are thin. The sterling filigree wire
I create my original models with is only 1 mm X 0.5 mm…so I am
casting very fine stuff.

When I do get defects, they are usually because something has gone
wrong (crack in plaster, or a model was touching the wax web, or I
undercalculated my wax:silver ratio…or because I didn’t
sufficiently debubblize my investment. It’s very easy to get trapped
bubbles with filigree.

FYI…I cast at 1150 deg F out of the kiln and my electromelt is set
to slightly above 1900. This is in order to get a proper fill. I also
do a steam dewax prior to burnout.

Jeanne
jeannius.com


#3
http://www.ganoksin.com/borisat/nenam/wgc-sprues.htm 

Eddie Bell’s description of solving casting problems is damned good.
Anyone with problems should read and memorise it, twice. Lots of
variables when casting but he covers the really main one.

Cow dung with straw or fancy investment, vac or spin or twirling
over your head on a vine, fancy computer controlled kiln or charcoal
don’t really make a difference if the sprue/gate is all wrong.

Think and feel how the metal flows and freezes and the battle is
mainly over.

The main magic explained.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#4

Hi Jeanne,

... rather than straight out.....this seems to me like it could
cause incomplete fills on it's own. 

The angle up angle down seems to depend on the bent of the caster.

Having a tree with the models straight out, may seem strange, but
then you have to consider what physics are applied to the models.

If the hub of the tree is spinning around itself (as opposed to the
spinning action of a Kerr spin casting machine) then horizontal
placing of the models takes full advantage of centrifugal action.

Regards Charles A.


#5

Goodmorning to all from the wet side of Europe,

By all means, it’s not that simple by thinking how metals flows!
Water will flow from the highest point to the lowest and THAT is
simple.

Every metal/alloy has it’s specifications you have to deal with and
you have to know.

I made uncountless mistakes by castings because I thought that I
knew how metals flow. Let me tell you that I was completly on the
wrong way of how to perform a nice casting.

I saw how it’s had to be done and it looks so easy but never thought
of having the knowledge in order to make this happend. Jepp, its quiet
simple if you see how one does it. Heat it up in the kiln, let it
burn out, melt the metal, cast it and there you go. What about the
exact temperature, burnout cycle, fluidity of the metal, alloy you
use, thickness of the items you like to cast, type of investment,
consistency of the investment, numbers of items you like to cast,
room temperature, moisture of the environment etc Not everything is
made out of wax and they need a different approach, other burnout
cycles and temps.

I know how to cast about 15 up to 20 items in one casting but I paid
for all the mistakes, paid for all the stupidity I’ve done by
believing that casting is a simple process.

In order of having the perfect finish with no pits, no porosity,
complete castings and shining results without any other color except
the color of the metal, I went to Germany and took very specific
education by a company named Horbach. It was expensive BUT they
showed me all the little know-hows and details you have to look for
and exactly these little details are making the big differents…
but you need to know. I’m still greatful because they showed me many
wise conciderations to take, the complete and generous information
the gave, the opportunity to perform a casting from the beginning
till the end.

To end this subject I need to tell that after all the mistakes and
money I invested in this part of jewelry, it’s fun and I love it
because it gives me some kind of satisfactory but I still don’t know
everything. Sometime mistakes just take place because of…
something I don’t know?

If you like to go for casting, by all means dig in and get smart,
read books but don’t take it the easy way. Satisfactory, craftmenship
and proudness are the big differents what makes customers buy your
product… or not.

Enjoy and have fun.
Pedro


#6

I am using vacuum, so to me, it makes more sense to line up the
models in the direction of the vacuum pull. Earlier, I did standard
centrifugal…


#7

Straight out 90 degrees from the tree works fine with vacuum no need
to angle down.

James Binnion
James Binnion Metal Arts