Sand, when mixed with the correct ratio of sodium silicate, rammed into a core box, and then exposed to carbon dioxide (CO2), will result in a very hard and durable sand core. Never having used sodium silicate (and never having made sand cores before!), this was an excellent lesson in learning what works and what doesn't.
PQ Corporation is the largest manufacturer of sodium silicates, one of the most widely used chemicals in the world. Their brochure on sodium silicate has this table which describes the various strengths of sodium silicate available.
The key measure of sodium silicate is the weight ratio of its two major components - Silica DiOxide (SiO2)to Sodium Oxide (Na2O) (the column titled "Wt. Ratio SIO2 / NA2O"). The most commonly available sodium silicate has a weight ratio of 3.22 parts of Silica DiOxide to 1 part of Sodium Oxide with a solids content (active ingredients) of 37.6% (8.90+28.7=37.6 from Table 2 above). The rest (62.4%) is water. This is sold as "N" grade sodium silicate and is NOT the best choice for making sand cores as it doesn't provide very good strength to the sand core. The sand core will slowly disintegrate when handled. This type of sodium silicate has the viscosity of a cheap liquid dishwashing soap. Table 2 above describes it as a syrupy liquid.
I made the mistake of using the 3.22 N-grade sodium silicate (it may even have been a weaker solution) with very poor results. Even after 24 hours and constant exposure to CO2, the sand cores wouldn't hold together. For those that did stick together, I would end up with loose sand grains in my hand whenever I handled them.
RU-Grade Sodium Silicate
The best type of sodium silicate for making sand cores has a weight ratio of 2.40 parts of Silica DiOxide to 1 part of Sodium Oxide with a solids content (active ingredients) of 47.05% (13.85+33.2=47.05 from Table 2 above) - a 25% increase in active ingredients over the N-Grade stuff!! This is typically sold as "RU" grade sodium silicate and has the viscosity of concentrated liquid laundry detergent - it pours very slowly. Table 2 describes it as a heavy syrup.
I got some 2.4 RU-grade sodium silicate from CM and today mixed up a batch of sand to make some sand cores.
Mixing The Sand And Sodium Silicate
I first got all of my supplies, cups, bags, and stir sticks together and laid them all out on a sheet of plastic to make the cleanup easier. (The McDonald's cup is my supply of sand - easier to pour from a small cup than from a 25 kg bag, eh!?) I then put a smaller sheet of plastic down on top of the larger sheet so that I could easily recover any spilled sand.
To make the sand cores, I first filled the core box with dry 90m silica sand and poured it into a Ziploc bag. I added about 10% more dry sand as it will compact more when the sodium silicate is added to the mix.
I poured the liquid into the bag of sand and rolled the sand, sodium silicate, and bag between my hands until the sand and sodium silicate were well and uniformly mixed. The mixture felt only slightly damp but would clump together when squeezed.
Stuffing The Core Box
To make sure the CO2 would penetrate the sand core, I placed 1/4" steel rods into the middle of the core box so that I would have holes through the middle of the sand core. I spooned a small amount of sand mix into the core box and rammed the sand mix around the sides of the box and the steel rod.
1/4" holes through the middle of the sand core.
Using The CO2 Gas
I wouldn't have believed it if I hadn't seen it in person but.... it only takes a few seconds of CO2 gas to turn the loose sand into a hard sand block!! The secret is in how the CO2 is applied to the sodium-silicate-sand mix.
On the right-side of the photo above, you can see a block of wood with a couple of holes in it. And, in the photo below, you can see a plastic container with a hole in the top. Using my blow gun attached to the CO2 cylinder, and pressing down on the top of the plastic container, I slowly gave a 2-second shot of CO2 into the container. This immediately hardened the surface of the sand core.
I then undid the screws of the core box. Voila, the sand core easily separated from the sides of the wooden core box. In less than 20 seconds from the time of applying the CO2 to starting to undo the screws, I had a solid sand core! Whoodathunkit, eh!!??
2 process for my second sand core. The solidified sand core easily slid from the core box.
Ratio of Sodium Silicate To Sand (By Weight!!) Is Very Important!!
The whole secret in using sodium silicate is in the sand mix and the application of the CO2.
In my first try at using sodium silicate, I was short about 3 tablespoons of sand mix. I hastily mixed up a small batch that had about 30% sodium silicate. Bad news!! It wouldn't hold its strength even when repeatedly exposed to the CO2. The sand core was still as soft as when I had rammed it into the core box. I presume the extra liquid prevented the CO2 from penetrating the sand core. So, whether you use a 6% ratio or a 10% ratio, the relative ratio (by weight) of sodium silicate to sand is very important. It doesn't take a lot of sodium silicate.
How You Apply The CO2 Is Very Important!!
While CO2 is heavier than air, my first attempts at using sodium silicate weren't that good. I placed the wet sand cores into a plastic bag and applied the CO2. On my trip to Alumaloy Castings, I saw how they applied the CO2 to the sand - a small cup-like device attached to their CO2 hose, and a piece of wood with a hole in it held on top of the sand core and aligned with the holes in the sand core. A couple of 2-3 second shots of CO2 and the sand core was as solid as a rock.
It was obvious I needed to apply the CO2 in a more "aggressive" fashion. So I modified my process using a 1-litre plastic container with a hole drilled in the top. I was then able to drive the CO2 right into the exposed surfaces of the sand cores.
2 into the holes created by the 1/4" steel rods, I simply drilled a couple of holes into a piece of 1/4" plywood so that I could drive the CO2 right through the middle of the sand cores.
In any event, I'm very pleased with the results. Now we go into full-scale production with the sand cores using sodium silicate and CO2.