Patterns and Molds

A pattern is a replica of the part that will be cast, with the same (or very similar) shape and dimensions. It is used to create a cavity in a mold material, where molten metal will be poured to form the part. Depending on the casting process, a pattern can be reusable or disposable. Reusable patterns are more durable and can make multiple molds, while disposable patterns are destroyed after making one mold. In sand casting and shell mold casting, the pattern is used to make many molds. While a new pattern is needed for each casting using investment and lost foam casting.

Green Sand Casting

Green-sand casting is a process in which a mold is made of moist sand mixed with clay and organic additives based on a multi-use pattern. The mold is then filled with molten metal and allowed to cool. After solidification, the mold is broken and the casting is removed.

Advantages:

• It is relatively inexpensive and easy to set up
• It can produce large and complex casting
• It can use both ferrous and non-ferrous metals

Drawbacks:

•  It can produce poor surface finish and dimensional accuracy
• It can cause defects such as sand inclusion, gas porosity, and shrinkage

An example of a part commonly manufactured using green-sand casting is an engine block.

Shell Molding

Shell molding is a process in which a thin shell of sand and resin is formed around a metal pattern. The pattern is then heated to cure the resin and harden the shell. The pattern is removed and the shell halves are joined together to form a mold. The mold is then filled with molten metal and allowed to cool. After solidification, the shell is broken and the casting is removed.

Advantages:

• It can produce high dimensional accuracy and surface finish
• It can produce thin and intricate castings
• It can reduce machining and cleaning costs
• It can use both ferrous and non-ferrous metals

Drawbacks:

• It is more expensive and complex than sand casting
•  It can produce limited size and weight of castings
• It can cause defects such as gas porosity, hot tearing, and core shift

An example of a part commonly manufactured using shell molding is a valve body.

Investment Casting

Investment casting, also known as lost wax casting, is a process in which a wax pattern is coated with refractory material to create a mold. The wax pattern is then melted out and the mold is filled with molten metal and allowed to cool. After solidification, the mold is broken and the casting is removed.

Advantages:

• It can produce high dimensional accuracy and surface finish
• It can produce complex and intricate castings
• It can produce near-net-shape castings with minimal machining
• It can use a wide range of metals, including alloys and superalloys

Drawbacks:

• It is more expensive and time-consuming than other casting methods
• It can produce limited size and weight of castings
• It can cause defects such as gas porosity, shrinkage, and cracking

An example of a part commonly manufactured using investment casting is a turbine blade.

Lost Foam Casting

Evaporative casting, also known as lost foam casting, is a process in which a foam pattern is coated with refractory material and placed in a sand mold. The mold is then filled with molten metal and the foam pattern evaporates. After solidification, the sand mold is broken and the casting is removed. This type of casting can also be useful in generate small batches or single parts in the prototyping stage as well as full production.

Advantages:

•  It can produce high dimensional accuracy and surface finish
• It can produce complex and intricate castings
• It can produce near-net-shape castings with minimal machining
• It can eliminate the need for cores or risers

Drawbacks:

• It can produce poor mechanical properties due to gas entrapment
• It can cause defects such as sand inclusion, gas porosity, and shrinkage

An example of a part commonly manufactured using evaporative casting is an engine cylinder head.