1.2.1 Shell Mold Casting

 

• The basic process for these molds is,

1. Create two mating patterns of desired shape.

2. Coat the molds with a shell (sand and binders, such as a resin) until desired thickness and other properties are obtained.

3. Cure the molds and remove the patterns.

4. The mold halves are mated and held firm while metal is poured.

5. The final part(s) is removed.

 

 

 

• This technique can be very economical.

 

• Special care must be taken to assure venting for gasses, as the mold media is less porous.

 

• This method can easily use cores and chills to make complex molds.

 

• Graphite molds can be used for materials that would normally react with other materials used for the molds.

 

 

1.2.2 Lost Foam Casting (Expandable Pattern)

 

• This process has a number of basic steps,

1. Make a mold for producing styrofoam patterns.

2. Make styrofoam patterns using inject molding of expanded polystyrene foam beads (or another low density monomer foam) This process can be automated.

3. Glue the parts foam patterns together, and glue to sprue/runner/gate systems as required.

4. For high quality surface finish the parts may be coated with a ceramic slurry and hardened in a drying oven.

5. Place the pattern in sand, taking care to compact the sand about the pattern.

6. Cast metal into the pattern. The foam will evaporate, and escape through the normal routes gas evacuates through.

7. Wait until the part is hard, and remove from the sand.

8. If a ceramic coating was used this can be removed using impact, vibration, or abrasive techniques as appropriate.

 

 

• This process can be automated, and can be very inexpensive in quantities.

 

• Complex parts can be made with relative ease by gluing together foam pieces.

 

 

1.2.3 Plaster Mold Casting

 

• This technique is basically,

1. Create a two part pattern.

2. A mold material is used that is a plaster of paris type mixture (fast setting) to make two cavities. This may have some additives to improve properties. Foamed plaster may be used to increase permeability.

3. After setting these cavities will be dried in an oven to remove moisture.

4. The Antioch process is optional and increases mold permeability by dehydrating in an autoclave, and rehydrating for a number of hours.

4. The mold halves are then mated and heated.

5. After reaching adequate heat levels the molten metal is poured. Mold porosity is low so pressure or vacuum must be used to encourage complete filling of the mold.

6. The final part is removed and cleaned

 

• This technique is known for its high level of dimensional accuracy.

 

 

1.2.4 Ceramic Mold Casting

 

• Also known as ‘cope and drag investment casting’.

 

• The basic process is,

1. A wood or metal pattern is placed in a flask and coated with a slurry of zircon and fused silica combined with bonding agents.

2. The mold is removed, cleaned and baked. The shells may be used as given, or they may have other materials, such as clay put on as backing materials.

3. The molds are then used as normal.

 

• This can make high temperature material parts.

 

 

1.2.5 Investment Casting

 

• The basic steps are,

1. An expendable mold of a part is made in wax, plastic, etc.

2. The part has a gate and runner attached to it, and all are dipped in a ceramic slurry.

3. The slurry is hardened, and the core is melted and/or burned out.

4. The core is burned out and the mold is preheated to the temperature of the molten metal

- 644°C for aluminum

- 1040°C for ferrous alloys

- etc.

5. Molds are filled by pressure, vacuum or centrifugal force.

6. After cooling, the mold is broken off, the sprues are cut off, and stubs are ground off.

 

 

• Many parts can be made at the same time by attaching them to a common gating system.

 

• Parts can be glued together to make shapes that would normally be too complex to mold.

 

• Typical methods used are,

- cast iron

- steel

- aluminum alloys

- brass

- bronze

- magnesium

- zinc

 

• The die used to make the mold cores can be used for thousands of parts.

 

• Typical large applications are,

- large propellers

- large frames

- nozzles

- cams

- valve parts

 

• Typical small applications are,

- dental

- jewelry

- orthopedic surgical implants

- camera components

 

• Advantages,

- fine details can be made

- thin sections are possible

- high accuracy

- weights from <1 ounce to > 100 lb.

- any castable metal can be used

- no parting lines

- good surface finish (60-220 μ in.)

- can be automated

- many parts can be made at once providing lower per piece cost

- high melting point metals can be used

 

• Disadvantages,

- less strength than die cast parts

- process is slow

- changes to the die are costly

- more steps are involved in production