Freeing the new casting from the mold
Although not as visually dramatic as melting and pouring, the shakeout of a new metal casting is still an impressive experience. When a casting is first pulled steaming from the mold, it is covered in sand debris and excess metal material. It must pass through shakeout, cleaning, gate & riser removal, and finishing machines, which make a bone-shaking racket as they do their work. When this process is done, the casting will be in its final shape, though heat treating, powder coating, and inspection may still be necessary before shipping.
Removing the casting from the mold
The process for removing a casting from a mold begins with determining the right time and temperature to do the shakeout. After the liquid metal has been poured, it must freeze before unmolding, but there are other timing considerations besides simple solidity. If pulled too soon, the surface of the metal may chemically react to the cool air with unwanted effects. Metal microstructures change based on its rates of heating and cooling. Pulling a too-hot casting into relatively cold air can cause the structure to become more brittle in a process known as “galvanizing embrittlement.” When casting ductile iron, the foundry is concerned with the ratio of pearlite, ferrite, or martensite structures, which can arise from the same type of metal, depending on the rate it cools.
There are formulae available for foundries to determine when a casting of a certain volume and grade of metal should be pulled from the mold, but many foundry workers use visual cues. For example, ductile iron is usually pulled from the mold after it has turned grey.
With sand casting, unmolding is known as “shakeout.” Initial unmolding may be done manually in small foundries or with small castings, where a foundry worker will dig into the mold with tongs to grab a sprue or runner. They lift the casting out while the mold crumbles behind it, usually tapping the casting with another tool to cause excess debris to fall away. More typically, however, this work is done on an agitation/vibration table that vigorously shakes as unflasked molds are dropped onto it. The shaking dislodges the sand which drops from the casting and sifts through holes in the table. The sand usually falls into a box or onto a conveyor, to be recaptured as molding sand.
Investment castings are made in a hard mold of ceramic sand. This mold needs to be cracked open manually or removed with powerful water jets.
Removing sand cores
Sand cores are made with resin or another substance that provides greater structure than basic foundry sand, and because of this they may need more than the vibrating table to remove them. Removal processes are dependent on the type of material used for the core. Some sand cores are constructed to be highly collapsible after sitting in the heat of the mold. Others maintain their structure throughout the pouring but can be baked out using lower temperatures for longer times. Some sand cores are built around a wire frame that will need to be mechanically extracted. Sand core removal will also be influenced by the placement and depth of the core.
The way to loosen some chemical binders is with the addition of other chemicals like sodium or potassium hydroxide, or with cold, water, or steam. Whatever process breaks down the sand core, the final object may go back to an agitation table to shake out the final sand.
Cleaning the casting
Castings do not come finished from the vibrating table. They must be cleaned in a process traditionally called fettling.
In all metal castings, there will be a sprue and gate left from where metal was poured into the mold. There will generally also be risers, gates, and runners—the infrastructure that was needed to make a sound casting. Before finishing a casting, the foundry must remove these gates and risers, as well as any small imperfections. The processes of cleaning the casting also removes the final remnants of clinging, burnt-on sand.
To fettle is to trim or clean the rough edges of something. The related phrase, “in fine fettle,” suggests that something is in good condition.
“Flashing” is a common small imperfection of sand and die casting. This excess thin skin of metal most commonly appears along the parting line of a sand mold or where ejector pins sit in a die cast machine. In each of these places, a small amount of molten metal leaks out of the mold. “Burrs” are raised edges or small pieces of material that come from slight shifting of the metal in the mold.
Very commonly, shot blasters are used in the first stage of cleaning. In these systems, steel balls or small iron stars are shot at the castings by an impellor. They strip off scale, flashing, burrs, and other impurities. Air-blast machines can also use a smaller abrasive.
These blast systems can work on conveyer belts, rotating tables, or in large rooms that allow the foundry worker to aim the abrasives. Another common machine at this stage of cleaning is the tumbler drum. In these tumblers, castings are placed in a drum to be turned over like clothing in a drier. Air or shot is blasted into the tumbling castings, ensuring all sides are cleaned.
Pneumatic hammers or drills are commonly used for sand core removal. They can be run by robot or by a foundry worker.
Water blast, in its most simple form, is pressure washing of the castings. Hydroblast systems might additionally add abrasive elements to the water to help with removal.
Removing Gates and Risers
Some gates and risers will not be removed by tumbling or air blasting, as the extra stock is too thick to be removed this way. These gates and risers might be cut off using an oxy-acetylene torch, hammered using chipping hammers or sledges, or be sawn off. If machining is part of the necessary finishing of a piece, some of the removal of gates and risers might wait until the fine grinding, lathing, or sawing is being done at the machining stage.
The removed stock metal of the gates and risers will be reused. It may be of better quality than incoming scrap metal, as it has had a chance to burn off any gas-producing hydrogen-rich material that piggybacked into the furnace, like paint or oil. Stock is therefore one of the ways a foundry can keep material consistent.
Sand casting does not generally provide a very fine, finished surface. If an item needs to be smooth, or needs precise dimensional tolerances, then it will be machined to match the item’s specifications. Since machining is an expensive process, the foundry will have optimized casting to produce an item as close as possible to final specification. Even still there may be surfaces that require extra precision.
Nearing the end of the line
By the end of shakeout, cleaning, and machining, the casting is in its final form. Further finishing touches might still be necessary—including heat treating, painting, powder coating, or assembly—but the work of making a new shape from metal is done. In a first production run of a new or innovated product, castings at this stage might be the first-run prototypes for a client or designer to check over for aesthetic value or proof of concept. Inspection and heat treating still need to provide quality assurance for working mechanical load, but at this point the client can see if the component meets their vision, or if there have been issues through the patternmaking process that need to be addressed. If any flaws in design are illuminated by these production prototypes, a designer may go back to casting metal design, wiser and more savvy for having come through the process!