The sprues shown are HUGE. The sprue for the pendant (the second
casting shown), must be at least 1/2" wide! I guess that explains
how it is done.
I remember your original post, but could not contribute at the time.
There are only so many hours in a day.
I am not doing casting for a living, but in college we had a gravity
casting lab to understand how in the past craftsmen could achieve
such a good results and I still remember few things.
You are correct in your assumption that proper sprue placement is
the key, but determination of the size is not straight forward. We
employing the following process:
- Determine sprue attachment points.
You are trying to create and uninterrupted flow of metal. Look for
abrupt changed in direction of flow like sharp corners and the like
and put a sprue there. I realize that it will be the hardest place to
remove the sprue from, but let’s worry about casting first. Do not
position sprues on the same vertical level.
Visualize a cylinder with the sides tangent to the extremities of
the model and position sprues on helical path with counter-clockwise
rotation ( north hemisphere only, reverse for southern hemisphere due
to coriolis effect ).
- Determine sprue diameter.
Every sprue is responsible for filling adjacent area of a model
lying bellow point of attachment.
Metal remains liquid very short time ( exact determination depends
on temperature of the form and temperature of the metal ) The sprue
should be large enough to allow required quantity of metal to flow
For example: you estimate that a sprue should fill the area of the
mold which weighs 10 grams and metal would remain liquid for 1
second. That means that sprue should be able to supply 10 grams of
metal per second.
Make weight of the sprue at least double the weight of the area the
sprue should fill. It should give you enough margin for error.
The distance of sprue attachments to the gate should vary as well.
The father the metal has to flow, the colder it gets. Try to vary the
distance in accordance with Golden Ratio. You do not have to be
exact. Approximately is good enough.
What we found out was very important, is the cross-section of the
sprued model should be the same( approximately will work )
throughout the height of the model.
Once you know the total weight of all your sprues, the gate
should double that.
None of these rules are cast in stone. Think about it as something
which is nice if it can be done; but if for some reason it cannot,
just try to get as close as you can.
If the first setup would not work, notice which area of the model
gives you the problem and which sprue is responsible for filling that
area and adjust accordingly.