Thursday, August 26, 2010

A Morning At Alumaloy Castings (1990) Limited - How To Make A "Negative" Casting

The Company - Alumaloy Castings (1990) Limited, 424 Birchmound Rd, Toronto, ON, Canada, M1K 7M6
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.
He filled the flask with the sodium-silicate-and-sand mix, tamping the sand tightly around the patio stone.  When the flask was full, he struck off the excess sand.  Next, he made about a dozen holes in the sand using a 1/8" steel rod.  These holes would be used to get carbon dioxide (CO2) 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. 
Placing the negative pattern on a pallet truck, they wheeled the 150 lb pattern into the mold-making part of the shop, lifted the heavy pattern face-up onto a mold board and then placed the drag-half of a large 30"x 36" flask over the pattern.  This provided just enough clearance around all sides of the pattern. 
 Marco then gave the pattern a good dusting with talc parting powder to make sure the pattern would easily release from the green sand when the pattern was lifted out of the drag-half of the mold.  He next riddled a fine layer of green sand over all of the pattern, ramming the sand with his fists as he covered the pattern.  If he had used an air-ram at this point, he would have cracked the pattern.
He continued layering in green sand, making sure it was rammed tightly into the drag.  At this stage, he started to use the air-ram to make sure the drag was well packed.  
When the drag was full, he struck off the excess green sand and called on Paul and another worker to turn the drag over.  The whole affair - pattern, sand, drag and mold board - weighed about 400 pounds.
With the drag and pattern now face-up, he gave the pattern and the top of the drag-surface of the mold a good dusting of talc parting powder to make sure the cope-half of the flask would readily lift off of the drag-half.
He next placed the cope-half of the flask on top of the drag and riddled a layer of green sand on top of the pattern.  Again, using his fists so as not to crack the pattern, he rammed the green sand all around the pattern.
 He next set some wedge-shaped chunks of steel on top of the pattern to create risers.  As explained in a previous blog, the risers allow the molten aluminum to come up into the voids created by the wedges and allows the gases to escape as the aluminum cools.  If the voids created by the risers are large enough, the voids also serve as a reservoirs of molten aluminum that keep the mold filled as the aluminum cools and shrinks.

He also added a sprue cutter to form a sprue that would be used to pour molten aluminum into the mold. 
In order to create a sufficiently large reservoir of molten aluminum, he removed the chunks of steel used to create the risers and replaced them with large ceramic-like cylinders (the material is similar to that found in some professional back-yard foundries and kilns).  He continued to add and ram more sand until the top of the cope was full.  He next struck off any excess green sand and used a tamper to smooth the top of the surface. 
 Marco and another worker then lifted the cope off the drag while Paul rolled the drag out of the way.  The cope alone weighed more than 200 lbs so it was strictly a "lift the cope", "roll the drag out of the way" (thank god for those rollers, eh!), and "set the cope on its side".  A minimum amount of moves in a minimum amount of time.  Any miscalculations and the cope would have ended up on the floor as a big pile of green sand. 
 They next lifted that 150 lb pattern out of the drag.  It was definitely not an easy job as it took three of them to do it.  Here's what the drag-half of the mold looks like.  You can see the pattern in the mold that carries all of the fine detail of the original "patio stone" rock.   
But first Paul and Marco worked on the cope-half of the pattern cleaning and blowing off the loose sand and fixing any imperfections that might have been on the cope-half of the pattern.  After cleaning out the holes for the sprue and risers, they cut gates into the mold so that the molten aluminum would flow evenly and continuously into the mold. 
Marco and Paul next turned their attention to the drag-half of the pattern, blowing off the loose sand and cleaning up any imperfections.  They had to cut into the sides of the green-sand mold to remove the pattern but these were easily fixed up with handfuls of green sand and smoothing with a trowel.

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 key thing in an aluminum (or any) pour is to keep a steady "flow" of molten aluminum pouring into the mold - particularly with a mold this large.  It took two "pots" and 6 "ladles" of molten aluminum to fill the mold and to leave a good supply of molten aluminum in the risers. 
Marco then "babysat" the mold for the next 45 minutes periodically pouring a ladle or two of molten aluminum into the top of the riser tubes as the aluminum in the mold cooled, drawing down the reservoir of molten aluminum in the riser tubes.
After a couple of hours, the casting had cooled sufficiently to knock it out of the green sand mold.

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.
Next, the sprues and risers (now solid) were cut off and any sharp edges on the sides of the casting ground down.
The casting was next transported over to the weigh scales and weighed. 
110 pounds of solid aluminum, ready to receive the concrete slurry to make "patio stone" rocks.
A job well done!  And I had a first-hand look at how aluminum castings are made.