still learning, what causes this texture

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Sep 2, 2011
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still learning casting and am getting usable parts but wanted to know why the rough texture? sand to wet maybe? im using a 30% bentonite from well drillers mud and 70% play sand. all sifted and hand mulled (that takes for ever and gives your hands a real good work out) this was just scrap aluminum from a junk mower engine




The surface finish could be a result of the sand grain, or from overheating the melt, or a little of both.

If you used a pyrometer and poured at 1,350 (+ -), then it was the sand.

If you don't have a pyrometer, then you can judge when you hit pour temperature by looking at the meniscus at the edge of the molten meltal and the crucible.
As soon as the meniscus goes flat, you should pour immediately.

High velocity and turbulence should be avoided in a metal pour.
When the metal hits the basin at the bottom of your spure, it will roll back on itself, and entrain air, sand, and slag.
Keep the lip of the crucible as close to the top of the sprue as possible, and avoid the waterfall effect.

I think you could use a shorter sprue, and use a smooth transition into the gate(s).
Most people use a spure that is way too tall.
You spure could be 1/2 the height you have it.

All things considered, you have a nice casting, with a few minor inclusions.

I am not a greensand expert, and so others will have to chime in about water content and makeup of your sand.
Greensand mixes can be a bit of an art.
Typically you only add as much water as required to get the sand and clay to adhere.

If you want a really good surface finish, use Petrobond (tm), or its equivalent home-blend.
There is also an art to keeping oil-based sand conditioned, so expect a bit of a learning curve with that too if you try it.
Don't add oil to oil-based sand after you mix it, since I ruined my sand that way.
Add alcohol to condition it.

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ok miniscus - is that where the metal sort of lips upward at the edge of the crucible? i dont have a pyrometer maybe Christmas this year but not yet. the center sprue is actually part of the piece so had to make the outer sprues the same length. yes i could cut down maybe half inch of the center piece but then i assume the outer sprues need to be that long as well in order to fill to the top?
on inclusions are you talking about what i circled in red? if so thats actually a piece of something black embedded into it. guessing something either in the metal or something in the sand that got past. it looks like a hole in it but its hard as a rock right there. which maybe another thing i need to learn about.
but thank you for the recomendations of shorter sprues and holding the crucible closer to the opening. i was holding up about 6 inches above the hole in my mold and then the long sprues on top of that

Problems occur when you get high velocity in the molten metal, and it starts washing and eroding the sand mold.

You want a slow even fill, with no splashing, no waterfalling, and as smooth a transition as you can make from the bottom of the sprue into the gate.
Sometimes people use a horizontal runner off the bottom of the sprue, but it is not absolutely necessary.

I have not used the miniscus method, but I have seen others use it with great accuracy.
I will look for a photo of a guy that uses the miniscus only (no pyrometer) and you can see the surface finish he gets with his greensand.

I am not sure why you have a sprue on both sides.
One sprue and gate should do it.

Here is an example of the surface finish you can get with green sand, aluminum, and miniscus only.
I am guessing he used the riser on the opposite side to ensure a complete mold fill, but it is not necessarily required, especially if you have that riser above the center of your casting.

And I don't think you need the vent holes in the top of your mold, since you have that riser that will do the same thing.

If you use a ceramic blanket furnace like the one in the photo below, be sure to spray on satanite to seal the fibers and prevent them from getting airborn.

We get smooth as finish...use dry and very fine sand, ours is like powder.
Thanks BlueJets, on the fine sand part, Growing up my parents owned a comercial pottery which sold plates bowls etc to Parks Belk and other stores. when they retired there were sacks of "silica sand" that was such a fine mesh you were to always wear a respirator when messing with it. back then it was mentioned that that was to fine. it was like flour. also had plenty of sodium silicate and fine mesh bentonite back then. wish that was still available to me but alas its not. guess what i am asking is - is there a limit on how fine the sand needs to be? like a mesh or anything like that? yeah i could just buy some oilbond but when i priced it, to fill 2 five gallon buckets it was a few hundred dollars.
I use OK85, which is a round grained sand from Oklahoma.
The paper below mentions that round-grain sand requires significantly less binder than non-round grain sand.

and another paper about sand and mesh size

I am not sure if the "85" in OK85 refers to the mesh size or something else, but this sand is about as fine as I would want to handle.
It is very dry, which is a requirement when using resin binder.

I very small hole in a container full of OK85 (like slightly larger than a pinhole) will allow the entire container of sand to drain out onto the floor, flowing like water.

I know Petrobond (tm) oil-based sand uses a very fine grain sand, but I am not sure the mesh size.
Petrobond normally comes pre-mixed with clay, and so the dust is contained by the oil in the sand/clay mix.

When loose sand gets too fine, you get dust everywhere, which can be a problem from many aspects.

A possible source of nice sand is a fracking company, the sand acts as a propent to prop the newly formed fractures open. The Saint Peter sand is often the source. A fracking company might even give you some, I think they get it in bulk.
but wanted to know why the rough texture?
Looking at your sand it is very coarse but what you can do is dust the mold with talcum powder and smooth it out using your finger or a spoon. pouring aluminium at a cooler temperature can give a smoother finish.

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