The Challenge - Using a "patio stone" rock as a pattern, make a "negative" of the rock. Then use that "negative" pattern to make a mold to make an aluminum casting. The aluminum casting will then be used by the landscape customer to make concrete "patio stone" rocks.
The Craftsmen - Marco and Paul, two of the principals of Alumaloy and their employees.
The Circumstances - A landscape customer wants to make concrete "patio stone" rocks out of a slurry mix of concrete. They sculped the "patio stone" pattern out of the slurry mix. They now need to make a "negative" mold out of this "patio stone" in order to to produce dozens of stones that are similar in shape, size, and look. Using the "patio stone" master for their pattern, they tried making the negative out of plaster of Paris without success. The landscape customer has now come to Alumaloy for a solution. And I've been fortunate to have just arrived on the scene with a challenge of my own (a much smaller one) to offer Alumaloy - but that's another story for another day.
Making The "Negative" Pattern
In a previous blog, I've discussed using sodium silicate and silica sand to make sand cores. Alumaloy uses the stuff to make their one-of-a-kind sand cores all the time. There's always a barrel or two of the stuff mixed up and ready to use at a moment's notice.
Paul put the patio stone face down on a mold board inside the cope-half of a flask that had lots of draft on the sides. He then stuffed some mixed sand along the edges to get rid of the undercuts (there were a few on the back of the stone) and oversprayed the mixed sand with some brown lacquer. This would help him to identify where the edges of the stone were. He added some 1/8" steel rod as reinforcing steel and some hooks so that they could lift the negative pattern out of the green sand mold. He also drilled some holes into the sides of the flask to insert some steel rods to tap the patio stone when it came time to remove the stone from the sand mix.
2) into the sand mix. The CO2 would create a chemical reaction with the sand mix to harden the sand into a solid block of sand.
I wouldn't have believed it if I hadn't seen it with my own eyes! Using an air hose attached to the cylinder of CO2, he gave each hole a two-second (it could have been a 4 or 5 second) shot of CO2. He repeated this again. In less than 5 minutes, he had a solid chunk of sand which had the exact pattern (ridges, indentations, grain, etc) of the face of the patio stone.
Problem was he now had a 150 lb block of sand that he had to turn over to remove the original patio stone pattern from the block of sand. Calling on Marco (who was always on hand offering advice and going for things to keep things moving along), they both managed to turn the flask and block over so that the patio stone was exposed. A bit of digging and jabbing here and there and the patio stone came out. They next sprayed the sand casting with lacquer to seal the block of sand. As seen in the photo below, they now had the "negative" pattern that they needed to make the green sand mold and an aluminum casting. You can clearly see the fine detail of the rock that has been impressed into the "negative" sand pattern.
He also added a sprue cutter to form a sprue that would be used to pour molten aluminum into the mold.
They next placed the cope back on top of the drag. They clamped the four corners of the flask so that the cope wouldn't "float" off the drag when the molten aluminum was poured into the mold. The mold was now complete and ready for the aluminum pour.
The flask was upended and the casting pulled out of the green sand. The green sand was steaming hot. Don't touch! It will badly burn your fingers to a crisp before you can say "Ouch!!".
After letting the casting cool overnight, it was moved over to the clean-up part of the shop where Paul wire-brushed the remaining green sand off the casting.Once the green sand had been brushed off, you could clearly see the detail of the rock in the aluminum casting. The sodium silicate and silica sand had clearly captured the detail of the "patio stone" rock.
110 pounds of solid aluminum, ready to receive the concrete slurry to make "patio stone" rocks.