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Casting Notes - #1

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CI:
I am gonig to start a new thread, since otherwise the original molding thread will be highjacked.
Here is a comment by "petertha" that I will respond to from the other thread.

I'm not into casting (unless you include YouTube grazing, in which case I'm practically Yoda level haha). I'd like to one day, but it seems to be a whole hobby & specialized equipment setup unto itself. But I have a naïve, novice question after watching videos like RotarySMP casting aluminum parts for his Redrup radial replica. He uses 3DP parts to make patterns, but many of the casting molds are 'plain' sand with water glass (sodium silicate) mixed, tamped around the pattern & cured by CO2 gas. Aside from all the common trials & tribulations specific to preparing the aluminum melt, temp, ingredients, mold design... the resultant parts look pretty darn nice. Many castings with more complex internal shapes requiring cores. Why isn't this method a lot more popular with model engineers vs Petrobond or shop made oil/sand/clay recipes & the muller & recycling the more valuable sand? It would seem to me like a no brainer to buy very inexpensive clean sand, the goop, CO2 gas from any weld shop & you are off to the races. Maybe even finer sand mesh from ceramic supply shops for better detail. Yes, the spent molds become waste products, no recycling of material. OTOH essentially low strength 'rocks' wherever one can safely toss them. What am I missing, surely something?

https://www.youtube.com/@RotarySMP

CI:
I watched several of rotarysmp's videos, and he collaborates with "olfoundryman" on ytube.
I was able to chat with olfoundryman (Martin) via email, and compare foundry notes with him.
Martin was a metalurgist at a foundry, and he is very good at making aluminum castings, especially permanent mold castings such as carburetor bodies, which are stunningly well made.

While I agree with most of what Martin says, I don't agree with everything.
Many folks make a big deal out of the use of a pouring basin, but I and several other backyard folks I know just pour straight down the sprue, with perhaps just a metal ring around the top of the sprue, which I guess is like a small basin directly over the spure (more like a funnel actually).

Aluminum should never be stirred for any reason.
Melt aluminum as fast as possible, skim, and pour at about 1,350 F or sometimes slightly cooler.

I ran across material by John Campbell, which Rotarysmp mentions, including the "10 Rules for Good Castings".
https://puhakka.ca/the-10-rules/
I follow those rules.

John Campbell wrote "The Complete Casting Handbook", and it is a pretty deep book, and not necessarily a backyard casting hobby book.
John dedicated his book to a guy named "Bob Puhakka", who runs an aluminum foundry in Canada.
Bob Puhakka used John Campbell's methods to build a successful aluminum foundry, and Bob came up with new and inovative ideas to improve upon John Cambell's idea.
Bob has a video that discusses how to make high quality aluminum castings without major defects.
http://www.youtube.com/watch?v=V7vxHl2pNY8
In a nutshell, with aluminum, you want a smoother laminar flow of molten metal, with very low turbulence, and you don't want the aluminum to fold over on itself, since that mixes the thin surface film (bifilm) into the moten metal, which becomes a defect in the casting.

I was able to exchange a few emails with Bob Puhakka, and while we did not get into casting details, I must say he is an impressive guy.
Many do not like his flamboyant style, but I focus on the qualit of his castings, not his personality, and I don't really have a problem with his personality either, since we agree on many/most worldly things.

CI:
Airmodel said in the other post:
"The foundry supplier said to me that any fool could use sodium silicate to make a mold and it was a replacement for green sand and skilled molders."

There is truth to this statement, but the flip side is that greensand, and oil-based sand (often Petrobond tm), have to be mulled in a large mixing machine (called a muller).
When I started in the hobby, I assumed I would use oil-based sand, or greensand, like everyone else.
I purchased greensand, and the result with gray iron was a terrible surface finish.

I purchased oil-based sand, and got a much better surface finish, but molten iron tended to wash out parts of the mold.

The problem with using greensand or oil-based sand is that there is an art to mulling it.
With greensand, the sand/clay/water mixture (plus any additives) has to be perfect.
Too much or too little water, and you will have either a weak mold, or a mold that excessively steams/vents if it is too wet (or explodes).

Oil-based sand has sand, clay, oil, and often alcohol in it.
There is also a bit of an art to mulling oil based sand correctly.
Too much oil and the sand has no strength.
Too much or too little alcohol, and again, no strength.

What typically happened when I tried to use oil-based sand is that I would ram up the mold, pull out the pattern, and either damage the sand when removing the pattern, or have the entire cope sand fall out when I attempted to lift the cope and set it on the drag.

As I mentioned, the surface finish I got with gray iron and oil-based sand was good, but the sand would wash and cause sand inclusion defects in the casting (assuming a mold could be successfully made).
And if you open an oil-based mold too soon with an iron casting, the cloud of smoke will blow up in your face (don't ask me how I know this, but my eyebrows did grow back eventually).

.

CI:
For me, it finally boiled down to finding a mold making material that was easier and more reliable to use than greensand or oil-based sand, or quit trying to make backyard castings.
I attended a local art-iron festival, and notices they were using sand molds that were rigid, and discovered resin-bound sand molds.
The binder they were using is called "LINOCURE", which is an Alkyd no bake Material with Isocyanate curing (three part system, with catalyst to control set time).
Linocure molds work well with any type of metal, but it works especially well with gray iron castings, especially when a sprayed-on ceramic mold coat is used to prevent burn-in.

The sand they were using is called "OK85', and it is a fine round-grained commercial foundry sand, baked to a very dry state.

I tried some OK85 with Linocure, and the results were very good.
Bound molds eliminate most (or all) of the problems that are associated with greensand or oil-based molds.
A good commercial chemical respirator and nitrile gloves much be worn when handling Linocure.

The art-iron folks were also using sodium silicate (often called "waterglass for some reason) as a binder, with the same OK85 sand.
They report that the quality of iron castings with sodium silicate molds is almost as good as with resin-bound molds, and I think the same ceramic mold coat can be sprayed onto sodium-silicate molds.
I have made a few sodium silicate molds, and they are quite sticky, and required religious waxing of the patterns (or other non-stick applied pattern coatings).
Sodium silicate molds can be hardened with CO2, or with a catalyst.

I discovered that over-gassing sodium silicate molds and cores ruins them (they have no strength, and just crumble on their own).
Most folks who try soduum silicate end up over-gassing it, so they increase the ratio of sodium silicate to sand, which makes cores and such pretty much impossible to remove.
5 seconds is the maxium gas time for sodium silicate molds.

If a catalyst is used, then CO2 is not required, and the entire mold or core will set by itself in about 45 minutes or less.

CI:
I made the common mistake that I think many would make, when I started using bound sand.
I used standard flasks, which are perhaps a 4" tall cope, and a 4" tall drag.
That is a lot of wasted sand, and the reality is that bound sand is very strong, so the mold can be basically shape-formed around the pattern(s), and can be as thin as 1/2" in places with iron.
Bound sand flasks can be very thin, such as 1" tall.

If some creativity is used, the amount of discarded bound sand can greatly be reduced.
I would guess a bound mold could be 30% (or less) the weight of a typical greensand mold.

Bound sand is regarded as a pottery hobby discard, and is approved for disposal with the regular trash, as long as it is not an excessive amount every month.
For hobby folks, you generally are not discarding much bound sand.

The beauty of using resin-bound sand is that it works very well with gray iron, and assuming you understand how to design the sprue, runner(s), gates, and sometimes risers, then you can pretty much get a quality casting without any major defects first-time, every-time.
For the small engine castings I make, the only other material I would consider using besides resin-bound sand is sodium-silicate bound sand.
I found greensand and oil-based sand to be a time-consuming hassle, with a lot of mold failures, and the resulting frustration of having to recast 1/2 of what I molded.

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