Shell molding also known as shell-mold casting. • As compared to sand casting. • It is used for small to medium parts that require high precision.[1] is an expendable mold casting process that uses a resin covered sand to form the mold. a higher productivity rate.Definition • Shell molding. and lower labor requirements. this process has better dimensional accuracy.[2] . 39 to 0.[4] .4 to 3. The pattern is usually made from cast iron and is heated to 230 to 315 C° (450 to 600 °F). – Two or more shells are then combined. or gravel to reinforce the shell.79 in). This finished mold can then be used immediately or stored almost indefinitely. The shell now has a tensile strength of 350 to 450 psi (2. The sand is allowed to sit on the pattern for a few minutes to allow the sand to partially cure.Process • The process of creating a shell mold consists of six steps:[2][3] – Fine silica sand that is covered in a thin (3–6%) thermosetting phenolic resin and liquid catalyst is dumped. to form a mold. or shot onto a hot pattern. – The pattern and sand are then inverted so the excess sand drops free of the pattern. blown. – For casting the shell mold is placed inside a flask and surrounded with shot. – The pattern and shell together are placed in an oven to finish curing the sand.1 MPa). sand. leaving just the "shell". – The hardened shell is then stripped from the pattern. Depending on the time and temperature of the pattern the thickness of the shell is 10 to 20 mm (0. via clamping or gluing using a thermoset adhesive. . [1] Aluminium and magnesium products average about 13. aluminium and copper alloys.059 in) to 6 mm (0. the thinnest cross-section castable is 1. The process. The resin also assists in forming a very smooth surface. The minimum draft is 0.3–4. Depending on the material. produces very consistent castings from one casting to the next.0 micrometres (50–150 μin) because a finer sand is used.5 kg (30 lb) as a normal limit.Details • Setup and production of shell mold patterns takes weeks.5 degrees.[4] .25 to 0.[1] • Typical tolerances are 0. in general.24 in).5 mm (0. after which an output of 5–50 pieces/hr-mold is attainable. but it is possible to cast items in the 45–90 kg (99–200 lb) range.[3] • The sand-resin mix can be recycled by burning off the resin at high temperatures.005 mm/mm or in/in because the sand compound is designed to barely shrink and a metal pattern is used.[citation needed] The small end of the limit is 30 g (1 oz). The cast surface finish is 0.[citation needed] Common materials include cast iron. low labor costs.[3][1] . • When the metal is poured some of the resin binder burns out on the surface of the shell. • 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. which makes shaking out easy.[2] • The high productivity. and precision of the process can more than pay for itself if it reduces machining costs. good surface finishes.Advantages & disadvantages • One of the greatest advantages of this process is that it can be completely automated for mass production. however not much is required because only a shell is being formed.Advantages & disadvantages • One disadvantage is that the gating system must be part of the pattern because the entire mold is formed from the pattern.[3] . which can be expensive. • Another is the resin for the sand is expensive.