Sodium Silicate for Core Making by Tom Cobett

Hi All! 

It has been a while since I have posted anything, but, I could not pass up a few 
comments on core making and the use of Sodium Silicate. You guys are in luck, 
since this happens to be my personal specialty. I actually have patents on using 
Sodium Silicate to make sand cores and molds! 

Let's talk about Sodium Silicate first. Sodium Silicate is made by melting 
Silica Sand and Soda Ash together in a furnace. The resulting glass can be 
dissolved in water, steam and much pressure, to make liquid Sodium Silicate. 
This is one of the oldest and cheapest industrial chemicals in the world. 

When you ask for "Sodium Silicate" (a.k.a. Water Glass) you may get just about 
anything. It's a bit like asking for "Motor Oil." The two things that describe 
any Sodium Silicate are the Weight Ratio and the Solids Content. The Weight 
Ratio is the ratio of the Silica to the Sodium Oxide. The most common Sodium 
Silicates are all made with a Weight Ratio of 3.22 parts of Silica to 1 part of 
Sodium Oxide. If you just ask for Liquid Sodium Silicate, this is most likely 
what you will get, a "3.22 Ratio Silicate." In bulk tankers, I buy it for about 
$.06/lb. It weighs in at about 11.6 lb. per gallon and has the viscosity of 
Olive Oil. 

This 3.22 Ratio Sodium Silicate usually has a solids content of about 36 to 38%. 
The balance is water. Treat this liquid like VERY strong industrial detergent. 
It has a pH of about 13.0 and stings like a bitch of you get it in your eyes or 
into a cut on your skin. Otherwise, it is completely water soluble. 

The largest manufacturer in the world is PQ Corporation. They sell this basic 
product as "N" Grade Liquid Sodium Silicate. You may be able to buy a 5 gallon 
pail of this stuff for about $50.00. Check the website www.pqcorp.com 

This is not the best choice for making sand cores and molds, since it does not 
provide very good strength. The preferred product would be a liquid sodium 
silicate with a weight ratio of about 2.4 to 2.6. It will cost slightly more, 
but, will make make much better cores. PQ sells a 2.4 Ratio liquid sodium 
silicate as "RU" Grade. It comes as a very thick liquid. You can add about 15% 
by weight water to get the solids content down to about 38 to 40%. 

Sodium Silicate can go from liquid to solid by simple de-hydration (baking), by 
passing Carbon Dioxide THROUGH it, or by reacting it with a liquid ester, like 
Propylene Carbonate or Glycerol Triacetate or Ethylene Glycol Diacetate. If you 
use the straight Sodium Silicate, the collapsability of the sand core won't be 
very good. Sodium Silicate does not burn out, so you will need to add something 
organic. The simplest thing to use is Sugar. Start by adding about 10-12% by 
weight of the liquid Sodium Silicate. It will mix in nicely, and stay as a 
stable liquid. 

You could also add something as simple as 0.25 to 0.5% by weight of the sand of 
wheat flour, starch, molasses, ground charcoal, or even kaolin clay to get 
better breakdown. The kaolin clay will provide some green strength which might 
make it possible to strip the core without gassing it, and then let it dry in an 
oven. You don't need to get it much hotter than 225°F. 

How much to use? If you use a clean, dry sand that has a grain fineness of 
between 60 and 90, I would start with a Sodium Silicate addition of 4.0% by 
weight of the sand. You can mix this with a simple kitchen mixer in a plastic 
mixing bowl. Mix for about a minute or two at high speed. Once the sand is 
mixed, put a lid on the bowl, or put the sand into a plastic bag to keep it from 
de-hydrating. The covered sand should stay good for MONTHS, so long as it does 
not dry out. 

If you want to use Carbon Dioxide Gas for hardening the core, you must allow the 
gas to pass through the entire sand mass. Simple exposure to the surface of the 
sand won't give you any appreciable hardening. You can get small bottles of CO2 
from the guys who service soda pop machines. YOU MUST USE A REGULATOR TO CONTOL 
THE PRESSURE OF THE CARBON DIOXIDE! The soda machine guys will sell you an used 
one for a few bucks. Generally, you don't need much more than 10 - 15 p.s.i. for 
the gas. 

You can use a nail or a wire to make a vent in the sand to pass the gas through 
it. If you were using Carbon Dioxide at 10 p.s.i. and you want to cure 1 lb. of 
sand, it will only take about 5-10 seconds of gassing. This is enough strength 
to get the core out of the core box. After you get the core removed from the 
form, just let it dehydrate overnight in a dry place. Or, you can bake it out in 
a warm oven or even a Microwave. (Microwave works REALLY well, just be carefull, 
since the Sodium Silicate attracts MW and the sand is likely to get extremely 
hot in just a minute or two.) 

If you opt to use the ester hardener, you need to add it to the sand mix at 
about 10% by weight of the liquid Sodium Silicate. Just keep in mind that once 
it is mixed together, you will have only a certain amount of time before the 
sand mix begins to harden. We call this "Work Time." You need to have all the 
sand in place before the end of Work Time. Then you need to wait until the sand 
is so hard that you cannot put a nail or knife into it. We call this point 
"Strip Time." 

If you use PQ "RU" Grade Silicate watered down to 40% solids, and then add 
Propylene Carbonate as the ester hardener, you will have about 10 minutes of 
Work Time and a 20 minute Strip Time at room temperature. If the sand is hotter, 
it goes faster. 

If you are casting Aluminum, you can get by without adding any organic additives 
to the sand mix, Just dunk the finished casting in water and the cores will 
flush out in a few minutes. If you are pouring Brass, Bronze or Iron, you will 
fuse the Sodium Silicate core into something that behaves like concrete, unless 
you have added some additional organic material. 

You can also make incredible cores with a two part epoxy binder. I would not use 
more than a total of 1.5 to 2.0% total epoxy binder and hardener on a 60 to 90 
Grain Fineness sand. Let it harden completely before you take it out of the box. 
Dust the box with baby powder before you ram the sand into it. This will help it 
to release from the core box. 

If you can get Corn, Potato, Rice or Tapioca Starch, you can make very nice 
cores by mixing the starch at about 2% and water at about 1% into the sand. You 
can harden the core by baking or microwave oven. These work really well for 
Aluminum castings, and they fall out of the hole! 

You can also use Linseed Oil or Tung Oil at about 1.0 to 2.0%, with a little 
Kaolin Clay and water as the binder. Strip the core green and then bake it at 
350-400°F for 10 minutes per inch of thickness. We call this "Baked Oilsand." 

You can also make great cores by boosting up the strength of greensand with more 
clay, corn starch and water. Let them dry out before you use them. 

I have been making sand molds and cores for over 35 years. there is no mystery 
to how this is done. There are several binders that can be used. Sodium Silicate 
is cheap and safe. Starch is a little more difficult to use, but shakes out 
better. Epoxy and other organic adhesives are expensive, but work extremely 
well. I can remember making water jacket cores for this guy who was building a 
12 cylinder Jaguar engine. That was a tough one. The epoxy worked really well. 

On many occasions, I started with a block of cured coresand, and then ground 
away the extra sand to get the shape that I needed. Recently, I have done this 
with a 3-axis milling machine. If you want to go this route, make a really 
strong core mix first. I use a grinding stone (like from a Dremel) instead of a 
milling cutter and the results are very nice. 

If you want to know more, just ask. 

Tom Cobett 
Cleveland, Ohio 

"In Pyro Veritas"