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Shell moldingFrom Wikipedia, the free encyclopedia

Shell molding, also known as shell-mold casting,[1]

is an expendable moldcastingprocess that uses

aresincoveredsandto form themold. As compared tosand casting, this process has better dimensional

accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that

require high precision.[2]

Examples of shell molded items include gear housings, cylinder heads and connecting rods. It is also used

to make high-precision molding cores.

Contents

[hide]

1 Process

2 Details

3 Advantages and disadvantages

4 References

o 4.1 Notes

o 4.2 Bibliography

[edit]Process

The process of creating a shell mold consists of six steps :[2][3]

1. Finesilicasand that is covered in a thin (36%)thermosettingphenolic resinand liquid catalyst is

dumped, blown, or shot onto a hotpattern. The pattern is usually made from cast iron and is

heated to 230 to 315 C (450 to 600 F). The sand is allowed to sit on the pattern for a few

minutes to allow the sand to partially cure.

2. The pattern and sand are then inverted so the excess sand drops free of the pattern, leaving just

the "shell". Depending on the time and temperature of the pattern the thickness of the shell is 10

to 20 mm (0.4 to 0.8 in).

3. The pattern and shell together are placed in an oven to finish curing the sand. The shell now has

atensile strengthof 350 to 450 psi (2.4 to 3.1 MPa).

4. The hardened shell is then stripped from the pattern.

5. Two or more shells are then combined, via clamping or gluing using a thermoset adhesive, to form

a mold. This finished mold can then be used immediately or stored almost indefinitely.

6. For casting the shell mold is placed inside aflaskand surrounded withshot, sand, orgravelto

reinforce the shell.[4]

The machine that is used for this process is called a shell molding machine. It heats the pattern, applies the

sand mixture, and bakes the shell.

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[edit]Details

Setup and production of shell mold patterns takes weeks, after which an output of 550 pieces/hr-mold is

attainable.[citation needed] Common materials includecast iron,aluminumandcopperalloys.

[1]Aluminum

andmagnesiumproducts average about 13.5 kg (30 lb) as a normal limit, but it is possible to cast items in

the 4590 kg (100200 lb) range.[citation needed]

The small end of the limit is 30 g (1 oz). Depending on the

material, the thinnest cross-section castable is 1.5 to 6 mm (0.06 to 0.24 in). The minimumdraftis 0.25 to

0.5 degrees.[1]

Typicaltolerancesare 0.005 mm/mm or in/in because the sand compound is designed to barely shrink and

a metal pattern is used. The castsurface finishis 0.34.0micrometers(50150 in) because a finer sand

is used. The resin also assists in forming a very smooth surface. The process, in general, produces very

consistent castings from one casting to the next.[3]

The sand-resin mix can be recycled by burning off the resin at high temperatures .[4]

[edit]Advantages and disadvantages

One of the greatest advantages of this process is that it can be completely automated for mass

production.[2]

The high productivity, low labor costs, good surface finishes, and precision of the process can

more than pay for itself if it reduces machining costs. There are also few problems due to gases, because

of the absence of moisture in the shell, and the little gas that is still present easily escapes through the thin

shell. When the metal is poured some of the resin binder burns out on the surface of the shell, which

makes shaking out easy.[1][3]

One disadvantage is that the gating system must be part of the pattern because the entire mold is formed

from the pattern, which can be expensive. Another is the resin for the sand is expensive, however not much

is required because only a shell is being formed.[3]

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[edit]References

[edit]Notes

1. ^

a

b

c

d

Degarmo 2003, p. 310.

2. ^abcDegarmo 2003, p. 308.

3. ^abcdDegarmo 2003, p. 309.

4. ^abTodd, Allen & Alting 1994, p. 267.

2.

3. INTRODUCTION TO THE SHELL MOULDING

PROCESS

4.

5. Amit M Joshi

6.

7. Dept. of Metallurgical Engineering & Materials Science,

8. Indian Institute of Technology Bombay, India.

9. Email- amitjoshi@iitb.ac.in

10.mechamit2002@yahoo.co.uk

11.

12.Shell moulding is a process for producing simple or complex

near net shape castings, maintaining tight tolerances and a high

degree of dimensional stability. Shell moulding is a method for

making high quality castings. These qualities of precision can be

obtained in a wider range of alloys and with greater flexibility in

design than die-casting and at a lower cost than investment

casting. The process was developed and patented by Croning

in Germany during World War II and is sometimes referred to

as the Croning shell process.

13.

14.EXPERIMENTAL DETAILS

15.

16.Raw Materials, Equipments

17.

18. Resin coated sand.

19. Chromel Alumel Thermocouple, digital panel meter.

20. Muffle Furnace.

21. 2 Kg Aluminium Silicon Alloy (LM6) scrap.

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22. Clay Graphite crucible.

23. Fireclay.

24. Asbestos Gloves, Tongs, safety goggles.

25.

26.

27.THE PROCESS DETAILS

28.

29. The process was optimized to get a better shell by varying

the temperature of the metal pattern, holding time of sand

resin mixture and final curing time of shell and pattern.

30.

31.A metal pattern having the desired shape of the casting is heated

to 180

250o C. The pattern is sprayed with a solution of a

lubricating agent or a release agent containing silicone to

prevent the shell from sticking to the pattern. The sand is put in

the dumpbox in sufficient quantity. The pattern is then turned

down with its heated face inside the dumpbox. The dumpbox is

now inverted so that the sand resin mixture falls on the heated

metal pattern face. The sand mixture gets heated up and it

softens the resin forming a shell on the pattern. Initially the resin

becomes sticky. Further additional heat cures it. The dump boxis again turned to its original position. The excess sand falls in

the dump box leaving a shell on the pattern.

32.

33.The pattern along with the shell is again kept in the heating

oven. This cures the resin in the sand and shell acquires rigidity.

On the pattern ejector pins are provided to strip off the shell.

After the shells get cooled the two parts are joined properly to

form a mould. Fireclay is applied along the edges of the mouldto avoid leakage of the molten metal. Shell mould D was used as

it had better thickness and had no breakage.

34.

35.A 2 kg scrap of Aluminium Silicon alloy (LM6) was charged in

a muffle furnace in a clay graphite crucible. The dross floating

on the molten alloy was removed by a thin graphite plate. The

molten aluminium silicon alloy was poured in the cavity of the

shell mould.

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36.Later on the shell mould was broken to remove the casting. The

use of safety goggles, gloves, shoes is a must to avoid any

accident.

37.

38.

39.

40.

Shell

No.

Temperature of the

heated pattern

measured by

Chromel Alumel

thermocouple

Temp. of

pattern

(oC)

Holding

time

Final

Curing

time

Thickness

of the shell.

(cm)

A 12.5 mv 325 40 sec 60 sec 1.1

B 10.5 mv 260 60 sec 60 sec 1.65

C 10.7 mv 264 90 sec 90 sec 1.85

D 11 mv 270 120 sec 120 sec 2.0

41.

Copyright

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42.Metal Pattern

Heating Coil

Hole for

inserting

Chromel

Alumel

Thermocoupl

e.

45.

46.

47.Shell Moulding Pattern

48.

Copyright

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49.Two parts of the

Shell on the

heated pattern.

50.51.Shells formed on the heated metal pattern

52.

CopyrightAmit Joshi

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53.

54.

55.Shells removed from the heated pattern

56.

57.58.Shells matched to form a mould. (Mould preheated to avoid moisture)

59.60.

Copyright

Amit Joshi

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Aluminiu

m Silicon

alloy

(LM6)

beingpoured in

the mould

from the

graphite

crucible.

62.

63.

64.

65.

Copyright AmitM Joshi

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66.

67.

68.Aluminium Silicon alloy casting ring broken from the mould

after room temperature cooling.

69.

70.

71.

72.Advantages73.

74. Better surface finish

75. Better dimensional tolerances.

76. Reduced Machining.

77.

Less foundry space required.78. Semi skilled operators can handle the process.

79. The process can be mechanized.

80.

81.

82.

83.

Copyright

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(B) (A) (D) (C)

Less Holding time Over heated pattern, Shell with better Shell with

and curing time less holding and curing time thickness good thickness

Comparison of Shells

86.

87.

88.

89.Disadvantages90.

91. The raw materials are relatively expensive.92. The process generates noxious fumes which must be

removed.

93. The size and weight range of castings is limited.

94.

95.Applications

96.

97.-Crankshaft fabrication

98.-Steel casting parts, fittings99.-Moulded tubing fabrication

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100. -Hydraulic control housing fabrication

101. -Automotive castings (cylinder head and ribbed cylinder

fabrication).

102.

103.

104.

105. Amit M Joshi106. Dept. of Metallurgical Engineering & Materials Science,

107. Indian Institute of Technology Bombay,

108. (I.I.TBombay), India.

109. Email- amitjoshi@iitb.ac.in

110. www.iitb.ac.in

111.

112.

113. COPYRIGHT NOTICE:

114.

115. The information contained within the entire Metal Web

News, MWN (www.metalwebnews.com) pages are provided

for the personal use of the MWN readers. No commercial use is

authorized by the authors of the information provided. Each

individual article is copyright protected for the respectiveauthor. No information may be copied or sold for any

commercial purpose whatsoever, without written permission

from the author and an exact copy such written permission

delivered to Metal Web News prior to any authorized

commercial use.116.

Shell molding process offers better surface finish, better dimensional tolerances,and higher throughput due to reduced cycle times.

117. A heated (200 C / 392 F) metal pattern is covered with a mixture ofsand and thermoset plastic. This causes a skin of about 3.5 mm (0.125 in) of

sand/plastic mixture to adhere to the pattern. This skin is removed from thepattern to form the "shell mold". The two halves of the shell mold are securedtogether and the metal is poured in the shell to form the part. Once the metal

solidifies, the shell is broken.118. This process can produce complex parts with good surface finish 1.25 m

to 3.75 m (50 in to 150 in) rms, and dimensional tolerance of 0.5 %. Sizelimits of 30 g to 12 kg (1 oz to 25 lb). Minimum thicknesses can be as low as 1.5mm (0.062 in) to 6.25 mm (0.25 in), depending on the material. A good surfacefinish and good size tolerance reduce the need for machining.

119. A fairly high capital investment is required, but high production rates can

be achieved. The process overall is quite cost effective due to reduced machiningand cleanup costs.

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120. The materials that can be used with this process are cast irons, and

aluminum and copper alloys. Typical parts made with this process are connecting

rods, gear housings, lever arms etc.