Basin and Sprue Design

[GreenTwin]

Here is a video by "Olfoundryman", who is from Australia.

I make the habit of comparing notes between every source of foundry information I can find, and he is an important source for me as far as casting aluminum.

I had not paid much attention until recently to olfoundryman's videos because I assumed he was just another ytube amature like the rest of us.
About a year ago I watched one of his videos, and he mentioned a lot of do's and don't with aluminum casting, and I pretty much agreed with all of them.

I was able to track down his email address, and it turns out he was the metalurgist for a commercial foundry, and he really knows how to make superb castings. He can also mass produce superb castings, such as the aluminum auto carburetor bodies he makes, and each is flawless.

One of the things that surpised me was his use of a pouring basin, but after I watched his video, I realized why it worked so well.
His pouring basin is very small, and he basically fills it almost instantly, along with the spure.
He keeps the lip of the ladle very close to the top of the mold, to avoid adding velocity to the metal, and to keep from breaking up the pouring stream (you want to maintain a laminar or smooth unbroken flow).

I don't use a pouring basin, but I often use a metal ring at the top of the sprue, and so basically I am doing exactly the same thing as olfoundryman, ie: filling the sprue almost instantly.
The basin he uses, or the ring of metal I use, basically gives a little more area to pour into, and prevents spilling if you don't pour exactly down the sprue.

I think olfoundryman is by far the best aluminum ytube caster, but he comes from a commercial foundry background, and is not an amatuer.

 

[GreenTwin]

Bob Puhakka is a guy who originally started a consulting company, with the intent of teaching commercial foundries how to use John Campbell's 10 rules for good castings, along with Bob's ideas, to reduce their casting defects.

Bob visited a lot of commercial foundries, and realized that the defect rate was much higher than it should be.
He also realized that most commercial foundries do things the way they have always done things, and they were not open to any new ideas from anyone.

Out of frustration, Bob purchased an aluminum foundry in Canada, and set up the entire process.
His foundry became so efficient that he prompltly put the foundries in the surrounding area out of business.

Most of the hobby casting folks I have talked to dismiss Bob as some "crazy foundry guy".
But the reality is what Bob Puhakka told me, which is that one example of what he does routinely is make all the aluminum castings for an entire $100 million dollar ship, with every casting perfect, and xray-verified to be defect-free (I have photos of his xrays).
Some of his castings are very large; 6 foot in diameter or larger.

Bob has made several videos, some specifically aimed at the hobby crowd.
The video about bifilms below was particularly useful to me, with regards to aluminum castings.

 

[GreenTwin]

One thing to understand about Bob Puhakka is that some of the methods he discusses in his videos are now obsolete.
Bob is constantly changing and improving his process, and as he told me, the methods he used in the past are not the methods he is now using, and probably won't be the methods he uses in the future.
There are still many lessons to be learned from his work though.

Bob Puhakka is a religious follower of John Campbell's "10 Rules for Good Castings", and I also fall into that category.
I don't worship John Campbell, but I admire his work, and his rules have worked very well for me.

Waterfall effect:
33:42 in the video above is why you want to avoid the waterfall effect.

Sprue Well:
34:25 for why you should not use a sprue well.
The spure well is a basin that many foundries use at the base of the sprue.

Meniscus Oxide Film:
49:25 on how the bifilm is being formed on the walls of the runner, and why you want smooth flow to avoid mixing bifilms into the metal.

Pouring Basin:
57:18 shows the the pour basin, and 58:30 shows why I don't use a pour basin, more accurately, why I use a very small pour ring similar in size to what Olfoundryman uses.
The metal hits the bottom of the pour basin, splashes back upwards like an upsidedown mushroom, and entrains air, slag, bifilms and sand into the molten metal.

Stoppered Pour Basin:
59:50 shows a stoppered basin, where the basin is filled before the stopper is raised to allow metal to flow.
You can still churn a lot of bifilms, air, slag and sand into the melt before the basin gets full, but an improved basin method.
Bob was using internal traps in the runner system to remove any residual air bubbles that made it down the sprue.

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[GreenTwin]

After reviewing a multitude of information over about 6 years, I finally decided to use a gate-regulated runner system; ie: the gates control the flow of metal into the mold cavity.

It appears that olfoundryman uses a narrow sprue base to regulate flow/velocity.

The reason I use a very generous sprue and runner size, with a spin trap at the end, is to wash the initial entrained slag/air/loose sand down into the spin trap.
And a secondary reason is that the melt is cooling as it traverses the runner, so if you have a large stream of metal wash down the runner, it heats the runner, and washes the cooled metal into the spin trap.

When the runner is finally fulled to the top, and the sprue is full and no longer aspirating air, then the metal reaches the gates at the top of the runner, and flows into the mold cavity.
I often use rectangular shaped gates that are not very tall, and often use a gate on either side of the mold cavity, especially if the casting will be thin.

Aluminum seems to be much more fluid than gray iron, and I don't have any way to superheat my iron, so I have to use whatever temperature an oil burner will reach as a maximum. And iron seems to cool more rapidly than aluminum as it flows through the runner/gate system, and the chances of a short are higher I think than aluminum.
And if the iron is too cool, it will chill when it enters the thin parts of the mold, so every trick possible needs to be used to get the entire mold filed with iron that is as hot as possible, and fill as fast as possible without creating turbulence.

Many methods work for hobby purposes, but this is the method I use, and I have had very good results with it with gray iron, so this is the method I use.

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