Curing Green Sand

What does this green sand system do?

In a sand casting foundry, a green sand system would be either a water-clay (bentonite) -silica mix or an oil-oil activated clay (bentonite specially treated)-silica system used to make molds(moulds). Many foundries utilize Olivine ( a special crushed rock ) instead of silica due to the problem of silicosis.

This would be as opposed to using a self setting no-bake system or a shell system or a sodium silicate process ( CO2 process) which are also common in sand foundries.

Is there really silica?

A process for producing a self-curable and water-soluble mold, which comprises admixing a suitable amount of water with a mixture consisting of (A) 100 parts by weight of refractory sand particles composed of alumina, (B) 1 to 5 parts by weight of at least one alkali metal phosphate selected from the group consisting of potassium phosphate and sodium phosphate, and (C) 0.2 to 3 parts by weight of an aluminum powder; shaping the resulting mixture into a mold of the desired form; and then allowing the mold to cure spontaneously.

hello Remnan,

The question stays unanswered but I understand your words. You said; "If I where to add new silica sand on green sand system". The "if" is important. What's the new silica? What's the green silica system? When we know them, we could answer to your question, which could be interesting or not, depending of your explanation. All the best and wait, Gil.

Actually, the green sand i was referring to is clay-bonded silica sand, and the metal to be poured on the moulds is cast iron, aluminum, and bronze. The problem that i have encountered this past few months is scabbing and other sand erosion defects, looking at the history of my sand its been 2-3 yrs since "someone" added new silica sand in the sand system, and so i researched on many books and sites and they have one thing in common "replenish base silica sand" but i dont know what factors to consider in adding new sand. btw, happy hearts day.

I would be of a mind to say that you probably have a lot of dead (inactive) clay in your system and that your moisture levels are probably too high because of it. Dead clay wants water just like your active clay but it has little to no binding power. So now your people add more water to make the sand stick together. I'm surprised that you didn't mention pinholes from the water vapor since that is also a result.

Using only one sand system for that mix of metals is not the best. Aluminum poured at 1375F does not need the same sand requirement that cast iron at 2750F needs.

The iron will burn out the clay much faster than the aluminum or even the bronze which I would assume that you're pouring at 2050 to 2350F depending on the bronze alloy.

After that long a period without any new sand your fines (270 & Pan) must be pretty high from being worked so long without replenishment, probably well over 7% combined and therefore your permeability is probably not too good.

The most important thing that you should do is find a sand testing lab if your supplier doesn't have one and send out a sample. It is best to know where you are starting from.

Keep in mind that too slow a filling of a mold with a lot of surface area can also cause expansion defects like scabbing. It is not necessarily the casting weight that's important but the rate at which the sand surface feels the heat of the metal until that surface is wetted. At too slow a rate the molten metal starts drying out the surface of the sand and by the time the metal reaches the surface the sand has lost its strength and buckles or expands. Western bentonite generally helps to prevent that since it contributes much more to the hot strength than the southern does. Cast iron mixes are typically 5% western to 2% southern while your aluminum would be 5% southern to 1 to 2% western. I found that a 4% southern with 2% western was a good all around mix for aluminum and not too heavy bronze.

As I said in the earlier response you should be adding new sand at a rate of 100# per ton of aluminum poured (including gates and risers not just shipments) and 150# per ton of CI poured. Depending on the majority of your metal poured you can't hurt your situation by immediately starting to add new silica mixes to your existing system since it hasn't been done in such a long period and estimate from those numbers how much you should have added. I'm surprised that you have any left. Do not salvage the sand really stuck to the castings since that often is in the worse condition. If you try to save the really burnt out sand you will be adding more fines and dead clay to your system.

If you pour largely iron, then use 70-75 GFN silica with 4% western and 2% southern, 4% water. If more aluminum and bronze, 4% southern and 2% western and a 80-85 GFN silica and 3 1/2% water. On the initial mix use a wheel and plow muller and mull for no less then 7 minutes after adding your clay. But most importantly find a lab to test what you do have. Mix up a batch of your system sand like you've been doing all along and send them a 5-8# sample and have them run the standard green sand tests and post it here when you get it and we can go from there.

Iron foundries ofter add iron oxide to their mix and wood flour is used by a lot of non-ferrous people to help prevent expansion problems but a proper mix generally doesn't need those.