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Manufacturing & Production Engineering DISCUSSION
I am sand casting a simple aluminium alloy plate, 200 × 100 × 20 mm, from a wooden pattern made to the finished size. The castings come out slightly undersized, about 2.5 mm short on the 200 mm length, and the top face has a sunken area in the middle with some porosity under it.
I understand both problems are called shrinkage, but are they the same thing? How much should the pattern be oversized, and how do I work out the riser size?
They are different stages of the same cooling. Metal contracts three times: as a liquid while it cools to the freezing point, during solidification, and then as a solid down to room temperature. The first two are volumetric and show up as sinks, cavities and porosity in the last region to freeze. They are fed by risers. The third is the dimensional contraction of the solid casting, and the pattern allowance compensates for it.
So the short length is a pattern problem and the sunken top is a feeding problem, and each needs its own fix.
The pattern maker's allowance for aluminium alloys is commonly taken as about 1.3 percent. For your 200 mm length the pattern should be 200 × 1.013 = 202.6 mm, which matches the 2.5 mm you are losing. Apply the same factor to the 100 mm and 20 mm dimensions.
Typical figures for other metals are about 1 percent for grey iron and about 2 percent for steel, but the exact value depends on the alloy and on how much the mould restrains the casting, so foundries adjust it after measuring the first castings. Add machining stock on faces that will be machined and a draft of 1 to 2 degrees on the vertical faces so the pattern draws cleanly.
For the riser, use Chvorinov's rule: solidification time is proportional to (V/A)², where V/A is the volume divided by the cooling surface area, called the modulus. The riser must freeze after the casting, and a usual target is a riser modulus about 1.2 times the casting modulus.
Plate: V = 200 × 100 × 20 = 400,000 mm³ and A = 2 × (20,000 + 4,000 + 2,000) = 52,000 mm², so M = 7.7 mm. The riser needs M of about 9.2 mm. A cylinder with height equal to diameter, cooling on all faces, has M = D/6, so D = 55 mm and H = 55 mm.
A riser of the right modulus can still fail if it is too small in volume or in the wrong place. Taking about 6 percent solidification shrinkage as a working figure, the plate needs roughly 24,000 mm³ of feed metal. A 55 mm riser holds about 130,000 mm³, and a plain open sand riser delivers only a modest fraction of its volume, often quoted at around 15 percent, so the margin is thin. Going up to about 65 mm, or using an insulating sleeve, is the safer choice.
Place the riser on the thickest or last-freezing region and keep its neck short and thick so the neck does not freeze first. Porosity spread through the whole section rather than in one sink can also be hydrogen gas, which needs degassing of the melt, not a bigger riser.
