SIMPLE GATING RULES
Simple gating rules, sometimes not that simple. I could give you a lot of rules and
calculations to gate a casting,
but that probably wouldn't help much. So I'll talk about what I know and have used. I am by no means a gating expert but I have gated a lot of patterns
following the direction of the foundrymen that make it work. So lets get started.
I use what is called a "1-4-4 ratio". This simply means when you take a cross section of each area of the gating system a ratio is applied.
1 = the area of the smallest cross sectional of the sprue or choke.
4 =
four times the area of the sprue = the total cross sectional area of all the
runners.
4 = four times the area of the sprue = the total cross sectional area of all the ingates.
I have referred to this as a starting point to calculate gating sizes, because anything from a 1-2-2 to a 1-4-4 ratio will work, down sprues are normally stock sizes so
calculating to the closest stock size
is what normally happens. The difference in the ratio has more to do with the alloy and shape of the casting, and an expert could probably give you a much better answer than I could, as to what ratio
you should use with which casting. There are a lot of formulas used to calculate pouring time for the type of alloy your using which would give you the size of the ingate, I don't know what they are, but I'll add some links
that could get you there if you need them.
The gating system on this pattern uses a 1-2-2 ratio the ingates are in the cope, runners in the drag.
Putting the ingates in the cope helps keep the casting free of washed sand or any other kind of crud.
I make wood gating systems for anything that I think may be more that one casting or for any casting
that may have a more involved gate than I want to cut by hand.
It helps to make a series of stock gating
systems that can be easily modified for use on unmounted patterns.
As you look at these pictures notice I fillet everything, this helps the metal flow with less
turbulence, reduces
the chance of sand washing into the mold.
This is a work sheet I use to decide how I'm going to gate and riser a pattern. When using a ratio you'll need
to decide on the size and number of ingates required to fill the mold. The contact at the
pattern should be about
the same as the casting section at that area. This will give you the size of the runner(s) and sprue. This is where
experience will take over, by looking at what you just created you will at least know if your close and what to
do if your not.
Now I think would be a good time to describe all the different areas of a gating system. I'll start where the metal enters the mold. Some of the terms are different depending on the area your from, if I can I'll give all the terms to
describe
an area of the system. The same rules apply it you hand cut your gating systems, I try not to do this unless it's a simple gate to cut, it take just about as long to hand cut a gate as it does to make one out of wood. I make stock gating
pieces to make things easer when I do need a gate an unmounted pattern. With a wood gating system you can put all the fillets required to make the metal flow without
turbulence.
Basin/pouring cup: this is an area of the down sprue, it can be cut into the cope mold in the down sprue
and should be about 3 times the diameter of the sprue at the largest. Or is can be a
separate piece added
to the top of the cope and sealed in place with cope seal or sand around the base.
Sprue: this is simply a channel for the metal to get into the mold. It is actually 3 pieces to the gating system
the basin, sprue and choke.
Choke: the choke is the smallest area of the sprue or it can also be built into the runner as a necked down
area . It's used to control the flow of metal into the mold.
Basin/well: this is an reservoir at the base of the sprue, in the drag. It should be deeper and wider than the
runner(s). There should be good sized fillets
at the intersection of the runner and well.
Runner: runner(s) are used to distribute the metal to the ingate(s) of the casting. Depending on the gating
system the runner will be straight tapered or stepped. The cross section of the runner should equal
the cross
section of all the ingates that come after it, reducing in size by the size of each ingate, until you get to the
end of the runner.
Ingate: this is where the whole ratio thing starts, you need to figure out the size and shape ingate(s) will be, to
fill the mold. from this you can
calculate the runner and sprue size. The ingate can taper in height or be
uniform in thickness. The ingate should be wider than it is tall, this is to prevent a hot spot in a thin wall casting.
Riser: simply used to feed metal into an isolated area of the
casting, as it shrinks, that has a thicker section of
metal. There are many different types of risers, the can be straight,
neck down, knockoff and sleeved. The
riser shown is a neck down riser.
Chill: Maybe not considered part of the gating system in the strict since, but it does help control how the casting
cools. It's
usually a piece of steel that is rammed in the mold and is used to cool a section of casting that may not
be easily risered.
Putting everything together you might get something that looks like this. It's important to keep smooth transitions
every place the gating system changes direction.
This is why I try to mount every pattern on a snapboard. By
doing this
you'll be able to ram the mold harder, have a cleaner parting and have
fillets at all intersections, keeping
turbulence to a minimum.
As I get time I'll start adding pictures and sketches of various gating systems. This would also be a good time to send you to casting defects.
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