Casting Advice (no laughing please)

Morning

Is there anyone out there who has advice for a total beginner at sand casting?

I made a waste oil burner and furnace, after a lot of tinkering it’s working pretty well and can get up to 1100 deg C or so without too much trouble.

My first project is to make some brass sanding blocks for making woodwork moulding planes. I made a pattern on a 3d printer. I used play sand (fine and consistent) and about 10% fireclay as DIY greensand.

I used a cast iron flask, packed down the pattern in the sand vertically and removed it carefully. I melted down the brass and poured it in. The result was, well, roughly the right shape, but it had a big nodule on the bottom (presumably caused by the pouring brass) and a terrible pitted finish.

of course I can sand out some of the pitting, but that’s not the point.

Does anyone with experience have any tips? See pics. Yes it’s terrible, I know.

Steve

Edited By Steve355 on 02/07/2022 10:01:42

There's Luker, Pat J, and Me for a start, we will all help ! Are you in the UK. ? I would try to get some better sand, that in part may account for the rough finish. I would put a small taper on your pattern. For brass the temperature is quite critical, for this job about 1000/1010*c. You have to contend with shrinkage at about 3/16" per foot. The big hollows are shrinkage. Did you use an open mold ? Gating and feeding are another problem. I will send a PM. Noel.

If you are getting up to 1100 degrees then the zinc in the brass will boil away (boiling point Zn 907C) so the first suggestion would be to keep the temperature down. Zinc fumes can be dodgy too.

Aluminium which melts at 660C as against the 930C for brass might be an option.

Oh great, thanks both

Yes, it was an open mould.

I could buy some commercial sand, but it isn’t cheap, and really this is a learning activity for casting a slideway and carriage I want to make for a bench tail vice. So I’d need a lot of it. Is there a cost effective way to get good sand?

I’d say the bottom of the casting is about 1/32 wider than the top. I assume that’s because of the weight of the liquid brass above it. I’m thinking that I might be better to try casting it properly using a cope and drag type flask.

the critical bit on this piece is the curve - it must be accurate to form an accurate (+/0.005) sole on the moulding plane. I could use aluminium, but I wanted some nice brass ones. TBH the pattern actually works fine as a sanding block for the purpose intended!

And yes, at about 1000 deg there was lots of white flame and smoke, which I assume was the zinc burning off. Quite spectacular.

thanks

Steve

Reading Pat’s excellent posts, it doesn’t sound like a particularly cheap hobby. It may not be feasible on a tight budget.

I don’t think it’s something you could easily teach yourself, there are too many things to consider and ignorance can be very dangerous if not deadly. The only casting I did was many years ago during apprenticeship at college, but went to many foundries in my working life both small and large. The risk of explosion is very real from damp sand. Unless you have a ready supply of identifiable scrap then it would be cheaper to machine most parts from solid.

Green sand will give spectacular results, see my album foto of the showmans loco and 2" gear wheel the wheel is as cast ! One well known model supplier argued for years that a casting supplied by me had been done lost wax " because you cannot get that sort of detail with sand". Only when he attended a demo of mine did he accept having seen it, did he believe it ! I use mansfield sand and it is over 25 years old- indefinite ? The only thing you need to replace is the water ! Try using a fine white sand, ie as brushed into slabs and a small amount of Bentonite clay - Terry Aspin mentions "wind blown sand from Skegness beach " I bought some Petrobond but have never used it ! In commercial foundries the used sand is a hazardous waste and as such is expensive to dispose of to the extent that many have built reprocessing plants to re use ! Yes to be successful will require some expenditure, but will largely depend on what can be begged borrowed or otherwise aquired ! I had an unfair advantage over many - I ran a waste disposal business for 20 years ! Noel.

I taught myself; if I can do it anyone can.

Don’t worry about the furnace temperature; the melt temperature is more important. 50Deg superheat is what you looking for; you just need to pour quicker. My furnace runs much hotter than the melting point of iron, and pouring brass etc. is no problem. For fuming; somewhere on the forum I posted a DIY crucible topping mix to help with this, but if the melt is fuming it’s probably already too hot.

If you spend a lot on backyard foundry work you doing something wrong. The cruicible is really the only consumable, and when I started I even made those. My waste oil cost nothing. My lining has lasted 10 years, and my green sand is still what I mixed all those years back. I’m not sure how it will ever go off?

What you trying to do is actually difficult. It’s much easier to cast something with a large surface area to volume ratio (casting modulus), like cylinders etc. than a block like you want to cast. You on the right track with an open mould like you cast but I suggest looking at the following:

Ram on the split line, and pour down the split, with the open end like you did it. Straight after pouring sprinkle some coal over. This is like hot capping and forces the draw from the top surface to prevent shrinkage defects in the mould. The pattern needs to be longer and the top is skimmed. Alternatively core the part.

The reason for splitting is to improve ramming. If you dust the surface with cake flour and go over it with a feather duster you’ll get a very good surface finish.

Check the play sand, the way it’s washed rounds the particles making it less suitable for casting. You want flat ‘sides’ of different shapes when casting to improve the binder and sand lock. My sand is from the local river with bentonite and other additives from the grocer/ garden shop. Practically most sand will work but the binder needs to be adjusted for the surface area etc.

Edited By Luker on 02/07/2022 12:50:25

That's not entirely true. Green sand has 5-10% water, I would call that damp?

Every time you throw away a little motor you throwing away copper which is the base metal for all the bronzes and brasses. Die cast toys are a good source of zink, in fact if you cant find the base metals lying around you not looking hard enough...

Lets encourage!

Fascinating! Thank you for an introducing such an interesting thread.

The only two-pennorth I can offer is that I don't think we've said DON'T BREATHE THE FUME loudly enough. I have had zinc fever as a result of welding galvanised sheet and the experience was nasty. It's described as being a lot like 'flu, I can vouch for that. I'm not too sure of the long term effects (I've lived to tell the tale) but the short term consequences were awful. Don't go there.

Rgds Simon

Getting the right molding sand is really the most difficult part of casting things in my opinion.

Building a furnace and the associated equipment is a mechanical thing.

Making good green sand is somewhat of a magical black art.

Green sand (sand mixed with clay and a small amount of water) is dynamic, and since the water evaporates, you really need to mull green sand before you use it, generally adding a bit of water. A muller is like a large mixer, with one or more wheels, and plows to turn the sand over and then smear it with the wheel(s).

I was told by someone with a ton of iron experience in Australia that I should expect it to take up to 20 years to master green sand.

I first tried Petrobond (tm), which is an oil-based foundry molding sand.

Petrobond is a mix of clay, sand, non-detergent motor oil, and a slight amount of alcohol (I am told it should really be a special alcohol, and I will look up exactly what that type is).

One of the things the backyard casting community did not tell me was that if you pour iron (or perhaps bronze too) into Petrobond, and then open the mold while the casting is hot, a cloud of smoke will be created, and if you lean forward to look at the casting, the smoke can light off into a fireball. My eyebrows have since grown back, and luckily I saw the fireball coming and closed my eyes.

Every time I used Petrobond, the alcohol in it had dried out too much, and so it did not have any molding strength. Not knowing any better I added non-detergent 30 weight motor oil, and by adding too much oil, I ruined 5 gallons of Petrobond. I should have added alcohol only.

I am aware of one person in the UK who puts his Petrobond on a tarp on the floor, and then folds the tarp, and walks on the Petrobond, in order to mull it. He reports that this method works.

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I had the luck of having a local art-iron group in the City, and so I approached them for advice on foundry sand.

They use a lot of Petrobond for their art classes, and produce excellent castings with it.

I asked them about reusing burnt Petrobond, and they said yes, you can reuse the burnt sand; it does not have to be discarded. They have a larger commercial muller at the art-iron foundry.

What they do with their Petrobond is to use a thin layer of new Petrobond as facing sand, ie: the sand right around the pattern, and then they backfill around that with the used Petrobond. This method works well.

When they get too much burned sand in their Petrobond, they mix in some new Petrobond, and all is well again.

I still had problems getting my Petrobond to have good green strength, and so I want back to the art-iron foundry, and asked them what other foundry sand solutions the had.

Their molding sand of choice for iron sculptures is resin-bound sand, which is not reusable, but extremely flexible material, that sets into a hard mass, and can then be cut, cemented, drilled, almost like wood.

The sand that they use with the resin is called OK85, and it is a fine grain, and round grain sand, with a very low moisture content. I am pretty sure the OK85 is oven-baked, and its moisture content is extremely low.

Resin binder requires sand that has a very low moisture content.

I purchased some OK85 and resin binder, and overnight, all of my sand problems vanished.

Resin bound sand is not reusable (not easily anyway), and so my solution to that is to build custom wood flasks that wrap very closely around the pattern, thus minimizing the sand usage.

Resin bound molds can be very thin, and as thin as 1/2" thick in places, and so a I probably use 25% or less of the sand that would be used in a normal green sand flask.

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In summary, I tried oil-based Petrobond sand, with some success, but variable success due to spotty results trying to get the sand conditioned correctly.

I tried resin-bound sand using very dry commercial OK85 sand, and that worked well, and continues to work well.

A commercial chemical-rated respirator must be worn at all times when mixing resin-bound sand.

I then discovered a third type of binder with is sodium silicate.

The art-iron folks use this material as a binder with the same very dry OK85 sand.

Sodium silicate is not considered toxic like resin, although I would still wear nitrile gloves when handling it.

I am told that sodium silicate molds can be used with iron castings, and they have a surface finish almost as good as resin-bound sand.

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lots to think about there…

I wish I could get the furnace up to iron melting temperature. It’s nearly there, but I can’t seem to get it over 1200. Perhaps where the lid meets the body needs insulating - flame shoots out of it. Or perhaps, it’s simply not big enough - so much heat gets exhausted out of the hole in the top.

what do you mean by “ram on the split line”? (I don’t know enough casting lingo here!) what’s ramming?

I like the idea of coring it - if that means making a sand core. It would use less metal that way.

I will have a play at doing that this afternoon. Not sure I can run the furnace again today though because it scares everyone.

I looked for a source for sodium silicate, and found that it is used somehow in the pottery business, along with some fine grained sand that seems to be similar to OK85.

One source on this side of the lake for sodium silicate is Clay Planet.

https://shop.clay-planet.com/sodium-silicate-solution---pint-1.aspx

Clay planet also offers a catalyst that can be used with sodium silicate, so that the SS bound sand sets just as it does with resin. The catalyst eliminates the need for setting the SS sand with CO2.

JasonB mentioned a pottery material source on the other side of the lake, but I forget the name.

So purchasing some sodium silicate and sand from a pottery supply house is one option.

If you make your own CO2, which I have seen some do, be cautious with the pressures that can be generated.

One fellow put dry ice in a fire extinguisher container, and the pressure skyrocketed to something like 1,500 psi in a few minutes. Don't blow yourself up with CO2.

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And the last option that I am aware of is to do what you have done, which is make your own greensand.

You could try naturally sourced sand and clay, or purchase sand and clay from a pottery supply house.

I tried some ready-made store-bought green sand with iron about 3 years ago, just to see what the results would be, and the surface finish was terrible, but otherwise the part (a flywheel) was solid.

Too much water in greensand can cause a steam explosion, so be cautious of that.

If you want to use greensand, and can machine off the surface, then you could cast the piece oversized, and trim off the rough surface.

It should be noted that all metal should be poured at the lowest temperature possible, while the metal is still hot enough to completely fill the mold.  Surface finish is directly related to metal pour temperature.

Overheated metal degrades the sand mold, and you can easily see this in action if you measure and record your pour temperatures.

I know of one individual who pours jumbo coins, and he has the most magnificent surface finish.

I noticed that he did not use a pyrometer, and so I asked him "How do you always have perfect pour temperature on your aluminum".

He said he observes the meniscus at the edge of the melt, and when the aluminum goes completely flat across its surface, then you should pour immediately.

I was shocked that this worked, but it did for him.  I have not tried this method.

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Edited By PatJ on 02/07/2022 14:31:10

If I have an existing part, and want to make a casting of it, it should be noted that the casting will be smaller than the original part.

The way I work around the shrinkage issue is to glue tongue depressor sticks or thin plywood onto the surfaces that will be machined, to build them up a bit.

You can do that with your pattern to give a bit more machining allowance.

I generally use the same shrinkage factor for all the metals I use (aluminum 356, bronze, and iron), which is about (LOL, I forget the number; I will look that up).

The shrinkage varies a bit between these metals, but not enough to make a difference for any part that I cast.

.

Seems like I have been using a factor of perhaps 1.015 for shrinkage.

My memory is not what it use to be.

So if you want to cast a part that is 6" long, make the pattern 6 x 1.015 = 6.09" long.

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I'm sure you will, just remember flames outside the furnace are doing nothing for temperature inside the furnace. With waste oil its all about getting the balance right. It also helps to mix a little diesel in. My furnace isn't that big, and I manage just fine.

Ramming is compressing the sand around the pattern, either by using your fingers or a wood mallet. Here's a video I did for someone, which should give you a few pointers...