Lets talk about definitions for metal casting, especially as related to hobby backyard castings for model engines, with a bit of preface.

I have seen some superb and basically flawless gray iron and aluminum model engine castings kit castings, and I have also seen casting kit castings in gray iron that were very expensive, but full of blow holes, and not really suitable for any engine use.

It seems to be accepted that castings with serious defects, and gray iron castings with chilled (very hard) spots in them are part and parcel to the hobby.

I am here to say that if I can make aluminum and gray iron castings without defects, and without hard spots, then everyone in the hobby should expect that the casting kit companies should also be able to do so when using "professional" foundries.

I have learned the metal casting hobby over the last 10 years, and I will share the secrets of making successful model engine castings.

But we need to discuss definitions first, and so the following:

Note that this is my slant on casting definitions, and perhaps not an exact thing.

1. Sprue: The sprue is the hole in which you pour your molten metal.

The sprue should be tapered, and should be small enough in diameter so as to fill quickly, and remain full at all times during the pour.

2. Pour basin: Many use a pour basin on the top of the mold, and they pour metal into the pour basin, which then overflows into the sprue.

Sometimes a basin is also used at the base of the sprue.

3. Runner(s): Runners are the horizontal passages that transmit the molten metal from the bottom of the sprue to the mold cavity. Runners are often V-shaped, probably since this makes it easy to withdraw the runner pattern from the sand mold. Runners are often located at the cope/drag interface, either in the cope, or in the drag. The cope is the top half of the sand mold, and the drag is the bottom half of the sand mold.

4. Gates: A gate is the passage or passages from the horizontal runner into the mold cavity. The purpose of a gate is to control the flow of metal into the mold cavity, introduce the molten metal into the mold cavity at the entry point(s) that you desire, and to skim the molten metal so as to catch any floating slag.

Gates can be many shapes, but are generally rectangular in shape, or sometimes a wide thin rectangle such as with a knife gate.

Knife gates are used when the casting is thin, such as with a plaque, and they allow the mold cavity to fill quickly across a wide area.

5. Riser: A riser is a chamber (void in the sand mold) located at strategic spots, with the intent of furnishing molten metal to a part of a casting while it is shrinking and soldifying. Often risers are used when the thickness of a casting is not uniform, so that you don't have the thin part of a casting solidify first, and draw metal from a thicker not-yet-solidified part of the casting; thus causing srhinkage defects and hot tears.

6. Hot Tear: A hot tear is what is sounds like. If part of a casting solidifies before the rest of the casting solidifies, it can create forces in the junction between these two areas, and the result is tears in the metal.

7. Inclusion: A n inclusion is something imbedded in the metal casting that is undesired, such as a piece of the sand mold, slag, etc.

8. Bubbles: Bubbles and similar round-edged defects in castings can be caused by absorption of hyrogen or other gasses in aluminum, too much water in the sand mold, resin molds that are not fully cured and flamed, or pockets of trapped air at the top of mold cavities that are not vented.

Bubbles can vary in size from very tiny pinhole size, to inches in diameter.

9. Spin trap: A spin trap can be used at the end of a runner, and it is a vertical shaft that often leads out the top of the mold. The purpose of a spin trap is to capture the initial debris that gets washed down the sprue and runner, capture any initial aspirated air that was mixed into the molten metal while the sprue was filling, and to prevent the molten metal from striking the end of the runner and splashing back into the mold cavity.

10. Cope: The top half of the sand mold.

11. Drag: The bottom half of the sand mold.

12. Flask: The wood or metal box into which the sand is rammed.

A flask often consists of two parts, which make the cope and drag molds.

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I have studied John Campbell's "10 Rules for Good Castings" for several years, and also studied the work of those who have successfully applied John's rules, in order to make flawless castings.

Some of the 10 rules are more critical on a hobby level than others.

Rule #1: Use quality scrap metal of a known composition.

I would add that you should use metal that will work well for the particular casting you are making, such as using bearing bronze for bearings, instead of some metal that has a very high stiction (don't ask me how I know this).

Velocity:

Probably the most critical thing in metal castings is controlling the velocity of the metal as it travels down the sprue, through the runner(s), through the gate(s), and into the mold cavity.

If you don't control your metal velocity, you basically get something similar to what happens with waves breaking on the shore. The metal folds over on itself like a wave, and then churns sand, air, slag, etc. into the molten metal. Churring/folding the metal as it flows should be avoided at all cost.

Metal velocity can be controlled several ways.

Some use the sprue as a choke to limit the metal flow rate.

Some use the gates to control the flow rate.

Some use ceramic sponge filters located either at the base of the sprue, or inline with the runner, to control flow rate.

I use the gates to control metal flow.

Mold Fill Rate:

The mold cavity should be filled as fast as possible, but not so fast that you cause splashing or turbulence of the molten metal.

You are basically looking for a laminar flow of the metal, where the wavefront of the molten metal as it travels down the runner and through the gates is never broken.

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Try to make the thickness of your casting consistent and constant.

If you look at sections of the old steam engine cylinders, you will see that almost every part of the cylinder has the same wall thickness.

You get even solidification when you use a constant casting thickness, without hot tears.

If you can't avoid using uneven casting thicknesses, you should consider using one or more risers, which allow you to avoid the small part of the casting solidifying first and drawing metal from the parts of the casting that are still molten.

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Vents:

There is some debate in backyard casting circles about venting a sand mold.

Some use sand that has enough permeability to allow the vents to be omitted.

I have had resin-bound molds trap large air bubbles at the top of the mold cavity, and so I always use vents at the high points of the mold cavity, in the cope or top half of the mold.

I make vents using a 1/16" diameter steel wire, which I poke from the interior of the mold out through the top of the mold.

I also vent my cores.

If my core is round, I normally make the core with a 1/4" wood dowel rod inside, and then withdraw the dowel, to leave a 1/4" passage through the core.

I use vertical vents from both ends of a core, 1/4" diameter, and these vents extend out the top of the mold.

I flame my cores lightly with a gentle propane flame.

Alternately, cores can be baked to drive off any residual moisture.

Many castings get ruined because the core was not vented or dried correctly.

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Pour Basin:

Pour basins are one of those things that have been used forever, and so it seems almost impossible to talk anyone out of using them.

I don't use a pour basin, but rather pour the metal directly down the sprue, with the crucible as close to the top of the sprue as practical.

You should fill the sprue as fast as possible to the top, and then keep the sprue completely full during the entire pour.  If you interrupt the pour before the mold cavity is completely filled, chances are you have aspirated air into the melt, and perhaps caused a cold joint inside the casting.

I use 1" section of 3" diameter heavy wall pipe above the sprue, and sometimes above riser openings.

The section of pipe acts like a pour basin to some extent, but also acts a a dam, to catch the metal when the mold gets completely filled, and to keep molten metal from running across the top of the mold.

The traditional pour basin is the base way I can think of to churn slag, air, and sand into the flow of metal, and ruin a casting.

People sometimes cling to their pour basins because "that is the way it has always been done".

And a lot of castings have defects too, but that is not a reason to keep making castings with defects.

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Edited By PatJ on 22/08/2022 19:20:43

Nigel-

I made a lot of bad castings before I got it figured out.

There is no reason for anyone to repeat all of my blunders, and so I am sharing what I know.

As for UK foundry terminology, I can't help with that.

I think John Campbell's book uses the terms that I use, and he is from the UK.

I have seen people pay good money for defective castings, and have done so myself.

All I can say is that castings defects are totally avoidable, both on the commercial and hobby levels, if one pays attention to details.

My castings don't have defects.

No gassing in aluminum castings.

No inclusions, hard spots, void, bubbles, chills, etc. in my gray iron castings, and very machinable gray iron castings.

I can help anyone trying to learn hobby casting, and save you a lot of headaches.

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Edited By PatJ on 22/08/2022 19:26:57

It should be noted that you can often cast a number of smaller castings in the same flask, and so the $6.00 per casting adder refers to castings that are so large that they take up an entire flask.

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Here are a few random responses to Jason's questions:

A commercial chemical respirator is perhaps $24.00, so no big deal.

A box of nitrile gloves is another $12.00.

Most commercial foundries use resin-bound sand of one variety or another, especially with ductile iron and steel castings. Resin bound sand with a ceramic mold coat makes an absolutely superb iron or steel casting.

My first order of resin binder was 5 years ago, and I am still using that product without problems.

The resin was difficult to find.

You really need to use a commercial foundry sand such as OK85 if you use a resin binder.

Bound sand is not easily recyclable, but you can get very significant casting improvements using bound sand.

Bound sand molds harden, and so you can use a far thinner and lighter mold than with green sand, so my resin-bound molds can be perhaps 30% the size of a greensand mold for the same part.

One less expensive alternative to resin-bound sand is sodium silicate bound sand, which is suppose to work well even with iron. I have used sodium silicate with some molds, but more with cores.

The best way to use sodium silicate as a binder is to use a catalyst, so that it hardens without CO2.

If you can't find the catalyst, you can harden sodium silicate bound sand with CO2, but don't over gas it (5 second gas only).

Sodium silicate does not have the problems as far as chemical exposure that resin-bound sand has.

I looked back in my albums, and you are correct about there being a lot of casting defects with my resin-bound sand molds.

The photos I took in 2012 show all sorts of casting defects, when I used resin-bound sand.

The difference between then and now is that in 2012, I had absolutely no idea what I was doing as far as foundry work, other than pressing a pattern in sand, and then attempting to get iron hot enough to pour into it.

In 2012, I was like a blind man stumbling through the forest, trying to find his way, and hoping for good luck, and ditto when I was trying to learn 3D modeling.

I did not know how to tune a burner so that I could reach a good iron pour temperature, and I did not have a clue about the correct sprue/runner/gate/riser configuration.

Fast foward 10 years, and have studied John Campbell's methods, read many many casting books, watched endless casting videos, etc.

I have now use a much lighter mass furnace now, which fires much faster and reaches iron pour temperatures quicker.

I am now very familiar with how to optimize the temperature of an oil burner.

I know now how to do sprues/runner/gates/spin traps, vents, risers, etc. in a repeatable and consistent way.

In 2012, I was lucky to get any type of casting at all.

Today, I can lay out a flask, pour a part in gray iron, and pretty much guarantee anyone that the part will be easily machinable, with no inclusions, hard spots, voids, bubbles, or any other defects.

People have said "I don't believe you can consistently make very high quality gray iron castings".

My response has been "Come over to my house, bring some patterns, and we will make some iron castings. You will very quickly become a believer". So far nobody wants to see it for themselves.

Talk is cheap, but throwing some high quality iron castings on the table pretty much speaks for itself.

I can match or exceed the quality of commerical grade iron castings easily, and repeatedly, but it took 10 years to learn the tricks.

Yes, that is the real cost.

Prices have been skyrocketing this year, and so you can be generous and say that using resin bound sand adds perhaps $12.00 per flask.

I see people "saving money" by using greensand, and often the have to re-cast a casting more than once to get it right.

Compared with the time spent trying to correct a bad casting, $12.00 per flask is very cheap in my opinion.

You have to factor in the cost of mis-casts that you get when using less reliable methods and sand/binders.

I can't remember the last time I had to recast anything; it just does not happen these days with me.

I have basically made every conceivable mistake that can be made in casting metal, and now I know exactly what to do, and more importantly what not to do.

The slurry that is used for lost-was and lost-PLA castings does appear to have a pretty short life, perhaps 1 year, and you cannot let that material freeze.

I am not trying to toot my own horn, but I do have the gray iron casting thing down very well, and while setting up a home foundry may a bit pricey, stepping up from greensand to resin-bound sand is about $12.00 per flask, best I can tell, and that is on a hobby level.

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Edited By PatJ on 23/08/2022 08:39:27

Here is a great example of why many castings have so many defects from excessive velocity and poorly placed gates.

Crankshaft fill simulation.

I have always said "If it works well for you, use it".

Everyone has to find the methods and materials that they are confortable with using, and the methods and materials that make sense to their specific application and uses.

There is no "one-size-fits-all" in hobby metal casting.

The hobby can range from simple open-faced molds, to the most complex lost wax castings you can imagine, or anywhere in between.

I must say that metal casting has been one of the most enjoyable and rewarding things I have ever tried, perhaps because I failed for so long, and then finally got it down pretty good.

Some of it is that you get to play with fire !

My wife tolerates my metal casting hobby, as long as I don't set the house or car on fire.

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Not yet.

I have had a "work-a-lanche" ever since covid started.

I really want to cast a green twin full size, which would have a 10" diameter flywheel.

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