Sand casting

Sand casting is defined as pouring of molten metal into a sand mold (molds are generally

provided with a cavity of the shape to be made) and allowing it to solidify inside the mould.

Various patterns are used to create cavity in the molds wherein, pattern can be said as the

replica of the final object to be made with some modifications. Depending on production

quantities, different pattern materials namely wood, aluminum, ferrous metals are used in

practice. These materials are used for low, moderate and high production quantities

respectively. Figure M2.1.1 shows a typical mould arrangement for a sand mold casting.

Figure M2.1.1: Typical mould arrangement for a sand mold casting

The composition of “sand” refractory is usually a mixture of high purity silica sand, bentonite

clay, organic additives, and water. The cavity is formed by packing the moulding sand around

a pattern by ramming and squeezing. Holes and internal cavities in the casting are produced

by placing an accurate strong component called cores. After the refractory has compacted or

chemically hardened, the mould is opened at the parting line and pattern is removed. The two

halves of the mould are placed together by using a pin called dowel pins. Metal is poured in

to the mould cavity through a previously prepared opening called pouring cup.

Pouring cup

Core Drag

Cope

Cast metal in

cavity

Down sprue

Riser

Table M2.1.1: Metal commonly used in sand casting (Source: Design for Manufacturability

Handbook by James G Bralla, 2nd Ed)

Common metals and alloys Tensile strength,

MPa

Remarks

Cast iron G1800 124 Used where high strength is not a

requirement; best machinability, damping

properties, and resistance to thermal stress

Cast iron G2500 172 Used for small cylinder blocks, pistons, gear boxes, clutch plates, and light-duty brake drums

Ductile iron (60-40-18) 410 Used for auto crankshafts, hubs, parts requiring shock resistance

Magnesium AZ63A 200 Good castability; general casting alloy

having good strength, ductility, and

toughness

Copper alloys (Leaded semi red)

235 For low-pressure valves and fittings,

hardware parts, brass plumbing fixtures;

Leaded red brass 255 Good general-purpose casting alloy; used

forfire-equipment fittings, small gears,

small pumpparts;

Aluminum (C355.0) 248 Crankcases, gear housings air compressors, fittings

Stainless steel (CF-8M) 550 Similar to wrought 316; used for aircraft

parts,chemical processing, electronics,

nuclear equipment, food processing,

mining, fertilizer equipment, missiles

Nickel CZ-100 alloy 345 Standard grade nickel casting alloy with

excellent castability; used for pressure tight

components, pumps, valves, equipment for

processing caustics at elevated temperatures

Typical characteristics of a sand cast part

Complex castings can be produced by the use of sand moulds. For example: Intricate

shapes (under cuts, complex contours), both internal and external can be made in the

above method which is generally difficult to machine for achieving such shape.

The metals those can be melted can be used for casting in this method. Table M2.1.1

shows the list of metal commonly casted in the sand molding process.

Further, casting of any size and weight even as high as 200 Tons can be made in the

above method.

Cast components are usually stable, rigid and strong as compared to products which

are produced in other manufacturing process.

Generally sand mold casted products are somewhat irregular and grainy surfaces and

hence machining is required to get a better surface finish product.

Sand casting processes are used in cylinder blocks, machine tool beds, pistons, water

supply pipes, bells etc.

Design considerations and recommendations

The following important recommendations are need to be considered while designing the

sand casted products.

Shrinkage: As the molten metal cools and solidifies in the mould, the natural shrinkage

occurs. The dimension of the casted product gets reduced as compared with the mold

cavity. The amount of shrinkage depends upon the type of metal. In order to compensate

the shrinkage allowance for outer dimension, the size of the pattern is made over size

and for inner dimension like hole; the pattern is made under size. It has been observed

that shrinkage happens towards the material side. Table M2.1.2 shows shrinkage of

various metals commonly cast in sand mould.

Table M2.1.2: Shrinkage Allowance for Metals used in Sand Moulds (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

Metal Allowance (%)

Gray cast iron 0.83-1.3

White cast iron 2.1

Ductile cast iron 0.83-1.0

Malleable cast iron 0.78-1.0

Aluminum alloys 1.3

Yellow brass 1.3-1.6

Gunmetal bronze 1.0-1.6

Phosphor bronze 1.0-1.6

Aluminum bronze 2.1

Manganese bronze 2.1

Parting line: The parting line is a continuous line around a part that separates two

halves of the mould. Straight parting lines are more economical than the stepped parting

lines as shown in the Figure M2.1.2.

Figure M2.1.2: Recommended straight parting line

Draft: For easy removal of pattern from the moulding sand, some degree of taper or

drafts are provided. With the provision of little or no draft, there are chances that the

pattern may damage the mould rather than slipping out smoothly. Various factors

responsible for selecting the proper drafts are: method of moulding and drawing of the

pattern, pattern material, surface smoothness and degree of precision. Table M2.1.3

summarizes the recommended draft angles for outside surface of the sand moulded

casting. Often risers are provided to compensate the shrinkage. Figure M2.1.3

Figure M2.1.3: (a) Pattern withdrawal problem for no draft (b) smooth withdrawal of pattern

from Mould

Not recommended

Straight parting line

Recommended

Pattern Pattern Taper surface

Table: M2.1.3 Draft angle for outside surface for sand molded casting (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

Pattern material

Wood Aluminum Ferrous

Pattern-quality level

Ramming method Normal High Normal High Normal High

Hand 5° 3° 4° 3° - -

Squeezer 3° 2° 3° 2° - -

Automatic - - 2° 1° 1½° ½°

Shell molding 1° ¼°

Cold cure 3° 2° 2° 1° - -

Placement of risers: Risers are generally attached to the heaviest section. Heavier

sections are closer to the riser and the thinnest sections are farthest from the risers due to

faster solidification in thinner section. This minimizes the chances of getting voids.

(Refer Figure M2.1.4.)

Figure M2.1.4: Incorrect and correct designs of castings and riser location

Ribs and webs: In case of heavier sections, rib intersection with the casting wall can

cause hot spot shrinks. The number of intersecting ribs should be minimized to avoid

hot spot shrinks. Whenever it is necessary to bring all the ribs to a single point, a cored

hole would help in faster solidification, thereby avoiding hot spot shrinks. (Figure

M2.1.5. to M2.1.7.)

This Not this

Risers Risers

Not this This

Figure M2.1.5: Incorrect and correct casting-rib design.

Poor Much Improved Much Improved

Figure M2.1.6: Reduce the number of reinforcing ribs that intersect at one point

Not this This

Poor Better

Figure M2.1.7: Design alternatives to prevent hot-spot voids at rib and casting wall

intersections.

Best

• Corners and angles: Hot spot are most common defect in corners and angles of

casting design. Use rounded corners having same radius for both internal and external

corner. Again too much rounding promote shrink defect in the corner. In particular, in

case of T sections, larger inside radius can be used to minimize stress concentration and

hot spots. Use of dished contours one on each side of the center legs are also affective.

Further, intersection of two walls of the casting should be at right angles to each other if

possible to minimize heat concentration. This feature is clearly shown in Figure M2.1.8

& Figure M2.1.9.

Figure M2.1.8: Sharp corners cause uneven cooling

Not this This

Figure M2.1.9: Avoid sharp-corner and acute angles that cause areas of uneven cooling

Wall thickness: If the metal is flowing for a longer distance in the mould, then the

section should be heavier. But heavier sections also cause problem with voids and

porosity. Keep the wall thickness as uniform as possible (Figure M2.1.10).

Cold spot

Severe hot spot

Sharp

Corner

Void

Figure M2.1.10: Keeping wall thicknesses uniform promotes sounder castings

Table M2.1.4: Recommended wall thickness. (Source: Design for Manufacturability

Handbook by James G Bralla, 2nd Ed)

Section length To 300 mm To 1.2 m To 3.6 m

Aluminum 3-5 mm 8 mm 16 mm

Ductile iron 5 mm 13 mm 19 mm

Gray iron, low strength 3 mm

Gray iron, 138-Mpa

4 mm 10 mm

Gray iron, 207-Mpa

5 mm 10 mm 19 mm

Gray iron, 276-Mpa tensile strength

6 mm 13 mm 25 mm

Gray iron, 345-Mpa tensile strength

10 mm 16 mm 25 mm

Magnesium alloys 4 mm 8 mm 16 mm

Malleable iron 3 mm 6 mm

Steel 8 mm 13 mm 25 mm

White iron 3 mm 13 mm 19 mm

Original Design Preferred Design

Internal porous area

Section changes: Abrupt changes in the section must be avoided. The relative thickness

of the adjoining section should be less than 2:1. If heavy section is unavoidable then a

taper of 4:1 is advisable.(Figure M2.1.11)

Figure M2.1.11: Design rules for areas where section thickness must change Interior wall and sections: These members should be 20% thinner than the outside

members, since they cool more slowly. ( Refer Figure M2.1.12)

Not this This

Figure M2.1.12: Design for interior walls (20 % thinner than exterior walls) Lightener holes: To reduce the weight in low stressed area, lightener holes can be

added.

Holes and pockets: The draft on the inside of a pocket must be twice as on the

surrounding outside surface. The depth of hole or pocket should not be more than 1.5

times its narrowest dimension if it is in the drag half of the mould and this depth should

If heavy section is unavoidable use 4:1 taper

Bad Good

Bad

t

>2t

t

<2t

Good

t

>2t

L= 4 (T-t) T

If heavy section is unavoidable use 4:1 taper

be no more than the narrowest dimension if the hole or pocket is in the cope half of the

mould.(Figure M2.1.13 to M2.1.14)

Figure M2.1.13: Recommended hole drilling after casting (diameter less than 19 mm)

Figure M2.1.14: Extra material around the hole as reinforcement in a highly stressed section.

Bosses and pads Bosses: pads and lugs should be minimized as it creates voids and hot spots.(Figure M2.1.15)

Figure M2.1.15: Design suggestions for minimizing material thickness at bosses

Cores: It is recommended to avoid the use of cores as it is expensive to make and

handle. Often use of cores are unavoidable and are used to make holes. In such case,

the core diameter should have at least equal to the surrounding wall thickness and

preferable twice the wall thickness or more. If possible, side bosses and undercuts

should be avoided. In case internal cores are used, addition of venting holes are

required for removing the gases that are generated while the core comes in contact

with the molten metal.(Figure M2.1.16 to Figure M2.1.18)

Figure M2.1.16: Minimize the need for cores as much as possible by eliminating

undercuts.

Figure M2.1.18: Avoid small cored hole

Figure M2.1.19: Internal pockets in castings to facilitate cleaning after casting.

Gears, pulleys, and wheels: To minimize the stress proper balance between the section sizes

of the rim, spokes and hub must be attempted. It is recommended to have odd number of

spokes with curved in shape. Excessive surface variation is to be avoided.(Figure M2.1.20

to M2.1.21)

Figure M2.1.20: Incorrect and correct proportions of elements of pulleys and gear blanks.

Figure M2.1.21: An odd number of curved wheel spokes to dissipate cast-in stresses.

Incorrect Correct

Lettering and other data: Any lettering should be parallel to the parting plane. These

data need to be placed in such a way that these will not interfere with the machining.

These can be either sunken or raised above the surface.

Weight reduction: Casting weight is minimized by removing the metal from low

stress region and adding to high stress area by the use of simple inexpensive pattern

change.(Figure M2.1.21)

Insert of different metals: It is sometime desirable in casting to incorporate a section

of different material either harder or softer than the base metal depending on the

purpose and is proves to be economical.(Figure M2.1.22)

Figure M2.1.22: A cast-iron wear-surface insert in an aluminium aircraft-brake casting.

Design to facilitate machining: Sharp corners and edges are avoided by making

sufficiently rounding edges and corners.

Machining allowance: After casting, machining is required to achieve better surface

finish. Table M2.1.5 provides the guidelines about the machining allowance.

Cast iron insert

Aluminum casting

Table M2.1.5: Guidelines for machining allowance (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

Allowance(mm)

Casting size (overall casting length), mm

Drag and sides Cope surface

Gray iron Up to 150 150-300 300-600 600-900

900-1500 1500-2100 2100-3000

2.3 3 5 6 8 10 11

3 4 6 8

10 13 16

Cast steel Up to 150 150-300 300-600 600-900

900-1500 1500-2100 2100-3000

3 5 6 8 10 11 13

6 6 8

10 13 14 19

Ductile iron Up to 150 150-300 300-600 600-900

900-1500 1500-2100 2100-3000

2.3 3 5 6 8 10 11

6 10 19 19 25 28 32

Nonferrous metals

Up to 150 150-300 300-600 600-900

1.6 2.3 3 4

2.3 3 4 5

Dimensional factors and tolerance recommendation: Different factors which

influence the variation of dimension of cast pieces are: use of different methods,

pattern inaccuracies and difference in mould hardness, internal stress and many more.

Table M2.1.6 provides the guidelines about various tolerances.

Table M2.1.6: Recommended tolerances are provided in under average condition. (Source:

Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

Location Dimension Tolerance

One side of parting line 0-25 mm 25-75 mm

75-150 mm 150-230 mm 230-300 mm 300-400 mm

400-500 mm 500-600 mm 600-760 mm 760-900 mm

± 0.6 mm

± 0.8 mm

± 1.2 mm

± 1.5 mm

± 2.3 mm

± 2.6 mm

± 2.9 mm

± 3.2 mm

± 3.5 mm

± 3.8 mm

Area at parting line

Additional tolerance for dimensions across parting line (tolerance to be added to that above)

6-65 cm²

65-320 cm²

320-650 cm²

650-1600 cm² 1600-4000 cm² 4000-6500 cm²

± 0.5 mm

± 0.9 mm

± 1.0 mm

± 1.3 mm

± 1.5 mm

± 2.0 mm

Dimension

Between two cores 0-75 mm 75-150 mm 150-230 mm 230-600 mm

600-1500 mm Over 1500 mm

± 0.8 mm

± 1.5 mm

± 2.3 mm

± 3.0 mm

± 4.5 mm

± 6.3 mm

Cores: shell, hot-box, cold-cure, etc. (one side of core box)

0-25 mm 25-50 mm 50-75 mm

75-150 mm

150-230 mm 230-300 mm

Over 300 mm (over 12 in)

± 0.15 mm

± 0.30 mm

± 0.45 mm

± 0.75 mm

± 1.0 mm

± 1.3 mm

± 1.3 mm plus 0.2%

Shift, mold or core; largest

casting dimension A

greater than smallest B

0-200 mm

200-450 mm

450-900 mm

900-1500 mm

± 2 mm

± 3 mm

± 5 mm

± 6 mm