Sunday, February 13, 2011

Casting The Patterns - Another Day At Alumaloy Castings

The day had finally arrived when I had the patterns ready after a bit of fine tuning and Alumaloy Castings Inc in Toronto were going to pour some #317 aluminum alloy.  A few other stars and planets had also come into alignment for some personal business that warranted the trip from Ottawa to Toronto.  In testing the patterns in my own green sand, I had discovered a couple of "flaws" that I wanted to correct which also led to some delays in the final casting.  In hindsight, these are "lessons learned" for my next patterns.

The master mold-maker was Marco Stabilini, one of the principles of Alumaloy.  Marco's been in the aluminum business since he was knee-high to a grasshopper as his father had started the business many decades ago.

The first step was to select a flask (cope and drag) large enough to hold the two patterns.  The drag was set bottom-side-up on a mold board.  Alumaloy is a production facility so they have the specialized equipment for large-production green sand mold making as we shall see in the photos below.
That "block" of wood between the two patterns will form a hollow "gate".  Aluminum cools as it shrinks.  Without the gate, the shrinkage will occur within our hollow mold and distort the pattern.  Along with a large cylindrical riser, the gate will act as a reservoir that allows additional  molten aluminum to flow into the mold so that shrinkage will occur outside the mold.

As with any mold making, the patterns were next dusted with parting powder.  The fine white talc keeps the green sand in the drag from sticking to the green sand in the cope and to the pattern.  It allows us to readily separate the cope from the drag.
Once the dust had settled, Marco riddled (to the uninitiated, that's "screened") some green sand over the patterns.  Riddling the sand ensures there are no lumps or unwanted pieces lying against the patterns so that we have smooth castings.
With the patterns nicely covered with riddled sand, Marco then filled the drag with more green sand from the hopper above his head.  By simply pulling on the lever over his head, he controlled the flow of green sand into the drag.  Before ramming the sand, he pressed the sand around the patterns so that the patterns wouldn't move and to make sure that all cavities were tightly filled with sand. 
As he filled the drag with sand, Marco rammed the sand tightly with the air ram.  This ensured there were no lightly-compacted spaces in the drag.  The air ram hopped around the drag as he went around the edges. 

Once he was satisfied that the drag was full, he placed another mold board on the drag, moved the squeeze plate into position and squeezed the bejesus out of the green sand.  The sand sank at least another inch as the pressure was applied.
With the drag part of the mold now finished, Marco and another employee turned the drag over - right-side-up.
 Paying close attention to the outline of the patterns, Marco then scraped and brushed green sand away from the edges of the pattern.  There was one part to each pattern where the green sand had to be molded down.  The darker areas in the photo below show the areas that received extra attention.  You can clearly see how the block of wood will form a hollow gate between the two patterns.  This gate (filled with aluminum) will be sawed off on the bandsaw during the final steps. 
More parting powder was shaken onto the top so as to ensure that the cope didn't stick to the drag.  As Marco said, the role of parting powder is to stop sand from sticking where you don't want it.  Our patterns are "split patterns".  That is, there's a bottom part that is molded in the drag and a top part that is molded in the cope.  The top part fits into the bottom part with two holes and wooden dowels that match up to each other. You can see how the top part has been fitted to the bottom part in the photo below. 
On closer examination of the space between the top of the two patterns, Marco decided to "mold up" the space between the two as leaving this space to be filled during the regular filling and ramming of the cope would leave a weak spot.  This weak spot might disintegrate and loose sand would flow into the mold, thereby corrupting the casting with sand holes.

You can see the alignment pins in the drag in the the front of the photo below.  It is these alignment pins (one on each side) that ensures that the cope will fit exactly into the correct position on the drag after the pattern has been removed from the mold. 
With the top of the drag finished off and dusted with parting powder, the cope was next fitted over the alignment pins and onto the top of the drag.  Marco then set a hollow ceramic cylinder on top of the gate and inserted a sprue cutter (the hollow brass cylinder in his hand).  The cylinder acts as a riser which ensures that the mold is completely filled with aluminum.  It also allows any gas that dissipates from the molten aluminum to rise to the top.  As the molten aluminum is poured into the sprue, it flows through the gate and into the mold.  As the mold is filled, the aluminum rises in the mold and into the riser.  As the molten aluminum cools, it shrinks.  At this point, more molten aluminum is poured into the riser.
Green sand was next riddled over the top of the dusted patterns, tamped into place, more sand added, rammed and then the top tamped smooth.
Next the cope was lifted from the drag with the top half of the pattern staying in the cope and the bottom pieces still in the drag.  The dowels in the top half of the pattern were loose enough that the pattern split very evenly.  The cope was placed on a stand that allowed easier access for finishing off the top half of the mold. 
The next task was to remove the top half of the pattern from the cope.  Rapping the dowels gently, the top halves of the patterns came out cleanly from the mold.  (Notwithstanding that the flask (cope and drag) are made from aluminum, that's about 100 lbs of green sand in the cope!  Not exactly a light load, eh!?)
The next step was to clean up any loose sand, making sure that it was all blown off the cope, cylinder, and sprue hole, in addition to rounding out the top of the sprue hole.
With the cope cleaned up, it was now time to focus attention on the drag and removing the bottom half of the split patterns from the sand mold.  After blowing loose sand from the top of the drag, Marco then wicked water around the perimeter of the patterns.  This was to ensure that the sand didn't break off from the edge of the mold when the patterns were lifted out.  Green sand has a specific moisture content which allows the sand to clump and stick together.
And here's close-up of what the drag looks like before the patterns have been rapped and removed.  Each one of the two patterns plus the block to form the gate will have to be rapped (tapped) and slowly lifted out.  It is this part of the operation that will determine whether the pattern maker has done his job by including enough draft (slope) on the sides of the pattern, there are no hidden undercuts incorporated into the pattern that will lift out the sand and ruin the mold.  This is one of the reasons why a pattern should be "tested" separately in green sand before being taken to the foundry for pouring.   You can clearly see how the drag was "molded up" between the two patterns. 
First the block that forms the gate is removed and loose sand gently blown off the mold.  The block is rapped to release it from the green sand, a screw (inserted onto the end of a piece of wood) is inserted in the hole in the block of wood.  When making patterns, these holes should be included in the pattern so as to facilitate lifting the patterns from the mold.
With the block of wood now lifted out of the mold, loose grains of sand are blown out of the mold.  It may also be the case that the sand may have to be "vacuumed" from the mold.  This is done with a piece of 1/2" pipe which has a "T" fitting soldered to the top.  A steady blast of air is blown into one side of the "T" which creates a vacuum down the tube.  The loose sand is sucked up the pipe and out the other side of the "T" fitting.  Removing loose sand is very important at each step as loose sand in the casting creates "sand holes". 
A channel needs to be cut from the sprue to feed molten aluminum into the gate.  Using a molder's spoon, he simply scoops some sand from the gate back to the hole made by the sprue cutter.  The loose sand is blown off of the mold. 
Once the block of wood is removed from the mold, the sides of the pattern are rapped - both on top and on the sides - to release it from the mold.  This is very critical in ensuring that the patterns can be lifted from the mold without breaking any of the sand from the mold.  If this happens, the mold will be finished and we will have to start all over again.
Inserting two screws into the holes provided in the pattern, the first pattern is carefully lifted out of the mold. 
And the same process is repeated with the second pattern.  This is where the location of the screw holes in a pattern becomes very important.  If the holes aren't included, or they're put in the wrong place, the green sand can be broken when the pattern is lifted.  You can see in the photo below the outline of the mold for the first pattern.  
With the patterns successfully removed, the mold is cleaned up, inspected for loose sand, dusted with parting powder and then lightly sprayed with kerosene.  This will help to keep the green sand in place as the sand cores are inserted into the cavities in the mold.
I had previously made the sand cores from a mix of silica sand and sodium silicate which was hardened with CO2 (You can find out about that step on the end of this link.)  The sand cores were then carefully inserted into the mold.  Because of adjustments I had made when "fine-tuning" the pattern, the sand cores had to be sanded so that they had a better fit when inserted into their cavities.
With the sand cores adjusted and fitted into their cavities, it was time to fit the cope back onto the drag.  Here's what the drag part of the mold looked like before the cope was placed back on. It's at this stage of the molding process that you can visualize the purpose of the yellow blocks of wood attached to the sides of the patterns. 
Marco and one of the workers then carefully fitted the cope over the alignment pins in the drag (refer to the two pins sticking up on each side in the above photo) and down onto the drag.  When the cope was in place, it was then raised about 6" and the drag examined to ensure that the top parts of the sand cores hadn't knocked loose any sand.
And here's our green sand mold ready to be charged with molten aluminum.  The flask has been weighted down with some small aluminum ingots so that the cope doesn't lift off the drag and spoil the casting.
 While Marco was preparing the mold, one of the workers was charging the foundry with aluminum ingots and scrap that had been cut off from other castings.  Pure aluminum hardly ever exists on its own.  It is usually alloyed with other non-ferrous metals.  Because there are different alloys of aluminum, the scrap had been thrown into different barrels to be remelted with their specific alloy.  In addition to castings that haven't met muster, most of the scrap comes from the sprues and gates that have been cut from their castings.
With the molten aluminum now reaching 1,380 degrees Fahrenheit, the aluminum was ready for pouring.  Flux was added to the foundry to bring the dross up to the top and to de-gas the aluminum.  The ladle was scooped into the molten metal and carried over to the mold.  The liquid aluminum was slowly poured into the sprue hole until it rose completely in the riser.
As the molten metal began to cool, the aluminum in the riser began to shrink until it was almost 1" below the level when it had first been poured.  It was hard to believe that the metal had shrunk that much in the cooling process.  It was very easy to visualize the shrinkage that would have occurred within the pattern if risers hadn't been added to the mold. (I've added red lines to indicate the amount of shrinkage.) 
Extra molten aluminum was poured into the top of the riser.  The "weight" of the molten aluminum in the riser provides pressure to the molten aluminum inside the pattern.  Any shrinkage of aluminum inside the pattern is replaced by the pressure of molten aluminum in the riser.

The mold and casting were left for 45 minutes until the aluminum had solidified.  The sand mold was then dumped into the "recycle" hopper.  The hot steam from the damp sand rose up as the perfect casting (complete with sprues and risers) appeared on the scene for the first time.
The casting was placed outside in the cold for an hour to quickly cool off (I had to return back to Ottawa that day).  Once cooled, the sand cores were vibrated from the casting by an air-impact hammer vibrating the aluminum riser.  With the sand cores removed, you could clearly see the outline of the two castings.
It was next over to the metal band saw to cut off the sprue, gate and riser.  This saw sliced through the aluminum like a knife through butter.
The castings were then cleaned up on the belt sanders and angle grinders until all of the "flash" had been cut from the castings.
And here's what the castings finally look like.
The Top View:
End Views:
And here's a view of the original casting that I started out with when making the two patterns:
Not bad, eh!!??

All in all, it's been a very interesting year getting involved in this project.  Starting with learning how to make patterns, experimenting with sodium silicate and CO2 to make sand cores, visiting with Marco, Paul, and Susi at Alumaloy on several occasions to see how it's done professionally, to the final steps of actually making the castings - it's been a very good educational experience for me.

Now to continue from here with trying some back-yard metal casting.  But first, we gotta finish the Mikey Burner and get our foundry built.