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Edge gating technique

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CI:
Here is a video of someone casting some parts for a 1/4 scale V8 engine.
This individual has done a lot of work making lost foam castings using a CNC router and pink hardware store foam.

The lost foam method is well established in the industrial world, and used for engine block castings, intricate pump castings, etc.
Typically the industrial method uses beads that are expanded inside of a heated metal mold, to make the foam pattern.
The molten metal vaporizes the foam as it flows through the mold cavity.
The key is to get an exact pour rate, pour temperature, and an investment coating that has enough porosity to allow the vaporized gasses to pass out of the mold cavity in front of the molten metal wavefront.

While the work I have seen with hobby lost foam castings is impressive, I don't plan on using this method because the surface finish is not as good as I want, there are potential cold joints where molten metal meets from two sides (the mold filling process is very slow), and there are sporatic defects visible in some of the castings.
For many, these castings would be perfect for their build, and the quality is most impressive.
The fact that these thin castings were even made, by any method, is a significant achievement.

The fins and lettering on top of the valve covers is really challenging, and it is difficult to pull patterns like that from the sand without damaging the mold.
CNC'ed foam patterns can be made with little or no draft angle on things like the lettering and fins on the valve cover, and that is not a problem with the lost foam method since you coat whatever shape your pattern is with investment coating, and things like draft angle don't matter.
From a structural standpoint, draft angle on a pattern makes a stronger casting I think, and the radius at corners eliminates stress concentrations at adjoining surfaces.

One drawback to the lost foam method is that a new pattern has to be CNC'ed out every time you make a casting, since the pattern is melted in the process.
Not really a big deal if you have a good CNC machine.

The fact that there is so much visible gassing out the sprue during the pour is concerning, since the gasses are flowing past the clean metal that is flowing into the mold cavity, and that is going to entrain slag, sand, air bubbles, etc. into the pour.

What is impressive about this video though is how thin the castings are.

The other takeway from this video is the edge-gating system that is used.
I have used a lot of knife gates, and they work very well, and keep metal velocity and turbulence to a minimum.
The gate used in this video is almost the entire edge of the casting, and there is a large runner behind the gate.
I tend to use oversized runners and let the gates regulate flow, and this is pretty much what I think I am seeing here.

Quite an interesting video from many aspects; hobby/foundry/model engine/3D modeling (SketchUp was used).

http://www.youtube.com/watch?v=Mmze9FxQ1XQ

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CI:
I saw a thread yesterday that I guess is several years old, but had forgotten about, which was a Merlin engine built from a set of those fantastic investment castings that someone in California made.

I think potential problems can multiply when one gets smaller than perhaps a 1/3 scale with a complex engine like a V8 or V12.
The Merlin thread mentions some tedious looking casting straightening, which is really risky since the method used was heating the casting and bending it.
My experience with heating aluminum casting and trying to bend them is that they break.

A 1/3 scale V8 would be much more forgiving in my opinion.

Everyone has their favorite scale.
I admire those who can build working engines on a tiny scale.

If the previous post's foam castings are not exactly square, then they will have to be either machined flat if possible, or straightened using some method.

I also wonder whether machining allowances were included on the above patterns, or whether the patterns are a CNC of a finished engine.
I suspect it is CNC'ed patterns of the finished engine, so that is not going to work out too well when you start machining.

If close attention is not paid to the shrinkage factor, then the castings will be too far out of spec, and will not be machinable to a size that matches the drawings.

Many factors to consider for sure.

The evolution of the hobby into 3D modeling, and then the offshoots into various casting methods like lost foam and lost PLA is quite interesting though, and I think these techniques will see further refinement over time.

For a while, I thought that they would perfect the wax filament for 3D printers, for use with investment castings, but I don't recall any viable uses of that in the hobby.

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vtsteam:
I've done lost foam casting in zinc, aluminum, and one failure attempt in iron. It's not my favorite method of casting, but there are times when, for me at least, it's useful. Mainly for intricate thin walled castings. It's not an enjoyable process, and for what I build I generally I don't need a thin walled intricate casting. I do enjoy conventional greensand casting in all of its more traditional aspects, but if I need something in whatever method is required, I'll use it.

CI:
That is some impressive lost foam casting work, especially that extrusion bracket thing !
I can see that it seems to work pretty well.
You must have the coating and pour process figured out.

I wonder if the iron failure was due to the higher iron temperature creating gas faster than the coating could dissipate.
I know they use the expanded bead lost foam method with cast iron engine blocks successfully, but hadware store foam is not expanded beads.

Edit:
I have seen some discussions concerning hardware store foam density, and apparently it can vary.
Hardware store pink and blue floam seem pretty lightweight, but the white expanded bead foam seems to be lighter.
I would guess the lower the foam density, the faster the mold fill, and probably a higher success rate.

There are also discussions about machinability of hardware store foam, and apparently the more dense foam is more easily machined (CNC'ed).


I wonder if 3D printed sand mold machines will ever be cheap enough to hit the hobby market.
That would allow some very complex molds to be made, with internal passages, etc.

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Jasonb:
Question

If you are coating the foam with layers of investment and forming a shell why does the foam have to remain and be burnt out by the metal.

Could the foam not be melted/burnt out once the investment shell had been dried avoiding the need for the slow pour and gasses that Pat mentions? In effect you are using the foam like you would a wax.

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