INTRODUCTION TO THE SHELL MOULDING PROCESS

 

Amit M Joshi

 

Dept. of Metallurgical Engineering & Materials Science,

Indian Institute of Technology Bombay, India.

Email - amitjoshi@iitb.ac.in

mechamit2002@yahoo.co.uk

 

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.

 

Experimental Details

 

Raw Materials, Equipments

 

    Resin coated sand.

    Chromel Alumel Thermocouple, digital panel meter.

    Muffle Furnace.

    2 Kg Aluminium Silicon Alloy (LM6) scrap.

    Clay Graphite crucible.

    Fireclay.

    Asbestos Gloves, Tongs, safety goggles.

 

 

The Process Details

 

       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.

 

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 box is again turned to its original position. The excess sand falls in the dump box leaving a shell on the pattern.

 

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 mould to avoid leakage of the molten metal. Shell mould D was used as it had better thickness and had no breakage.

 

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.

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.

 

 

 

 

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

 

Copyright

 

Metal Pattern

 

Heating Coil

 

Hole for inserting Chromel Alumel Thermocouple.

 

 

Shell Moulding Pattern

 

Copyright

 

Two parts of the Shell on the heated pattern.

 

Shells formed on the heated metal pattern

 

Copyright

Amit Joshi

 

 

Shells removed from the heated pattern

 

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

 

 

Copyright

Amit Joshi

 

Aluminium Silicon alloy (LM6) being poured in the mould from the graphite crucible.

 

 

 

 

 

Copyright Amit M Joshi

 

 

Aluminium Silicon alloy casting ring broken from the mould after room temperature cooling.

 

 

 

Advantages

 

       Better surface finish

       Better dimensional tolerances.

       Reduced Machining.

       Less foundry space required.

       Semi skilled operators can handle the process.

       The process can be mechanized.

 

 

 

 

Copyright

 

(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

 

 

 

Disadvantages

 

       The raw materials are relatively expensive.

       The process generates noxious fumes which must be removed.

       The size and weight range of castings is limited.

 

Applications

 

-Crankshaft fabrication

-Steel casting parts, fittings

-Moulded tubing fabrication

-Hydraulic control housing fabrication

-Automotive castings (cylinder head and ribbed cylinder fabrication).

 

 

 

Amit M Joshi

Dept. of Metallurgical Engineering & Materials Science,

Indian Institute of Technology Bombay,

(I.I.T Bombay), India.

Email - amitjoshi@iitb.ac.in

www.iitb.ac.in

 

 

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