The terminology of sprues and gates is important when discussing
sizes or procedures. Remember, the sprue is the straight piece that
the castings are attached to. Gates are located between the sprue and
the casting. It is the gate size that is critical in preventing
porosity and misruns. Remember, the sprue is the post attached to the
pouring button that you attach the castings to using gates.
If we all use different terminology it becomes very confusing.
I agree that correct terminology is necessary when discussing a
topic, and as a caster of 26 years I could not resist the temptation
to jump in on this one. Here is my 2 cents worth on the subject.
The Gate is actually your point of entry for the metal into the
cavity. This is usually dome shaped and is represented after the pour
by your button. You are correct in that the sprue is the vertical
feeding channel for the metal, but from the sprue to the casting is
actually called the runner. Therefore the correct feed is as follows.
Here are some other points:- From the casting you can attach what is
called a Riser which permits a reservoir of metal to be held and
re-drawn back into the casting as the metal cools to compensate for
shrinkage, thus reducing the risk of what is known as shrinkage
porosity. The riser is usually attached to the heavier part of the
piece and is usually a feeder with a ball on the end…sometimes
referred to as a feeder head.
Now from the riser, and only from the riser do you attach vents to
lead gasses out of the cavity, and always leading back towards the
button. (Personally I never use vents. )
In reference to another point made on the forum regarding turning the
flask gate up for the last 10 minutes of the casting cycle. There is
only one reason to turn a flask over during casting and it has to do
with maximizing the circulation of oxygen into the cavity to ensure
the ignition of the carbon residue or ash. This is even more important
when casting with stones in place due to the lower burnout
temperatures. If your flasks are raised by utilizing a grate, then you
surely will have sufficient airflow to obtain a proper burnout. By
turning the flask over during the last 10 mins is already too late,
this is due to the fact that the carbon elimination cycle has already
passed and therefore serves no purpose. This is usually done after the
first hour once you have melted the wax out of the cavity. After the
wax tray is taken out of the oven then is the time to turn it over.
Again, if your flasks are raised and air can enter the mold cavity
from underneath, then all is well and the turn is not necessary. In a
nutshell, you turn the flask over if the opening is blocked by the
flask being placed on a brick with a smooth surface. If it is a ribbed
ceramic with grooves that allow air to enter…have no fear.
I agree. I only burn out for 1 hr. so I am inverting my rings at
about the 50 min. level of the 1 hr. burnout. Dental high heat
investments work this way. As to the terminology used, so far
venting is the same, the gate is the same, the sprue is the same, the
ball on the sprue which needs to be at least double the sprue
diameter is called the reservoir. and there is also a runner bar.
One of the absolutely marvelous centrifugal casting ‘rings’ is put
out by Belle de St. Clare. The ‘rings’ are oval and there are 2 sizes.
The formers have insertable pre-made wax runner bars to which you
attach your wax-up. You can attach the waxes like little duckies all
in a row. This system reduces metal turbulence greatly.
Orchid Jewelry Listserve Member
"No man’s life, liberty or fortune is safe…while our legislature is in session."
the ball on the sprue which needs to be at least double the
sprue diameter is called the reservoir. Just to clear this point
which is 100% percentage correct, so that there is no confusion with
similar techniques or terminology that I mentioned before.
The ball or reservoir that Skip is referring to is attached as part
of the sprue which is indeed called the reservoir. The Ball that I am
referring to as the riser is attached to the casting and not to the
sprue. The reservoir offers a metal reserve for the whole sprue or
tree if you like, and is important that this as Skip mentioned is at
least double the size of the sprue in diameter. The riser acts in a
similar way to a reservoir, however being attached to the casting it
will feed that particular piece only with reserve material. We cast
approx. 2 Kilos a week in season and there are times when you need
both the riser and the reservoir due to the size of the trees. The
riser we will use on heavy pieces that are conducive to shrinkage
porosity or just just shrinkage in general, the reservoir we will use
on all flasks regardless. Anyway hope this helps. Best Regards. Neil