8/2/2019 Shell Mold

1/11

8/2/2019 Shell Mold

2/11

Shell moulding is a process for producing simple orcomplex near net shape castings, maintaining tighttolerances and a high degree of dimensionalstability. Shell moulding is a method for makinghigh quality castings. These qualities of precisioncan be obtained in a wider range of alloys and withgreater flexibility in design than die-casting and ata lower cost than investment casting. The process

was developed and patented by Croningin Germany during World War II and is sometimesreferred to as the Croning shell process.

8/2/2019 Shell Mold

3/11

Hole for inserting Chromel AluThermocouple.

Heating Coil

Two parts of the Shell on theheated pattern.

8/2/2019 Shell Mold

4/11

The process of creating a shell mold consists of six(6) steps:

1. Fine silica sand that is covered in a thin thermosetting phenolicresin (3-6%) and liquid catalyst is dumped, blown, or shot onto ahot pattern. The pattern is usually made from cast iron and isheated to 230 to 315 C (450 to 600 F). The sand is allowed to siton 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 dropsfree of the pattern, leaving just the "shell". Depending on the timeand 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 a tensile strength of 350 to 450psi (2.4 to 3.1 MPa).

8/2/2019 Shell Mold

5/11

4. The hardened shell is then stripped from thepattern.

5. Two or more shells are then combined, viaclamping or gluing using a thermoset adhesive,to form a mold. This finished mold can then beused immediately or stored almost indefinitely.

6. For casting the shell mold is placed insidea flask and surrounded with shot, sand,or gravel to reinforce the shell.

The machine that is used for this process iscalled a shell molding machine. It heats thepattern, applies the sand mixture, and bakes theshell.

8/2/2019 Shell Mold

6/11

8/2/2019 Shell Mold

7/11

Setup and production of shell mold patterns takes weeks, after whichan output of 550 pieces/hr-mold is attainable. Common materialsinclude cast iron, aluminum and copper alloys.Aluminumand magnesium products average about 13.5 kg as a normal limit,but it is possible to cast items in the 4590 kg (100200 lb)range. 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 to0.24 in). The minimum draft is 0.25 to 0.5 degrees.

Typical tolerances are 0.005 mm/mm or in/in because the sandcompound is designed to barely shrink and a metal pattern is used.The cast surface finish is 0.34.0 micrometers (50150 in) becausea finer sand is used. The resin also assists in forming a very smooth

surface. The process, in general, produces very consistent castingsfrom one casting to the next. The sand-resin mix can be recycled by burning off the resin at high

temperatures.

8/2/2019 Shell Mold

8/11

Better surface finish Better dimensional tolerances. Reduced Machining. Less foundry space required. Semi skilled operators can handle the

process.

The process can be mechanized.

8/2/2019 Shell Mold

9/11

The raw materials are relativelyexpensive.

The process generates noxious fumeswhich must be removed.

The size and weight range of castings islimited.

8/2/2019 Shell Mold

10/11

8/2/2019 Shell Mold

11/11

Crankshaft fabricationSteel casting parts, fittingsMoulded tubing fabricationHydraulic control housing fabricationAutomotive castings (cylinder head and

ribbed cylinder fabrication).