Types of Casting

Sand Casting

This type of metal casting involves making a mold in a sand mixture and then pouring liquid metal into the sand cavity. This is a simple six-part process: (1) Use a pre-existing pattern to create a sand mold, or craft one by hand. (2) Add your gating system (to control the liquid metal). (3) Remove the pattern (if used). (4) Pour in the metal. (5) Cool. (6) Remove the casting. This is the best form of casting for a small operation that will be making castings in small batches.

A variant of this is shell molding. This is similar to sand casting, except with the added touch of using resin to hold the sand together. This requires a much longer lead time and requires the heating of the mold between castings, but it has the advantage of allowing castings to be turned out more rapidly.

Plaster Casting

Using plaster to make a mold has the advantage that a plaster mold is easy to make. If the mold is damaged in ejecting a metal casting, turning out a new one is a simple affair (often, a plaster mold is, in itself, a plaster casting). This method is used for metal alloys based on aluminium, zinc and copper (i.e., not things like iron) for the manufacture of lock components, fittings, gears, ornaments and valves.

Die Casting

This involves forcing molten metal into cavities under high pressure. First, the mold is lubricated and closed. Then liquid metal is shot into the die/mold under high pressure. The pressure must be sustained during the casting process. Finally, the die is opened and the shots (not casting; shots differ from castings since there may be multiple cavities in a die, yielding multiple castings per shot) are removed. This is an industrial process for producing a large number of small- or medium-sized pieces that must have a high quality of detail and consistent features. It is commonly encountered when a toy or part is "die-cast."

Permanent Mold Casting

This is what most people think of when they imagine casting. Two halves of a mold are joined, and liquid metal is poured into the mold through a hole in the top. The metal is allowed to cool, and the casting is struck by separating the two halves of the mold. Old-fashioned bullet casting is done this way

Casting Process

The process cycle for sand casting consists of six main stages, which are explained below.

1)Mold-making - The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.

2)Clamping - Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.

3)Pouring - The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.

4)Cooling - The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can not be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.

5)Removal - After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely have some sand and oxide layers adhered to the surface. Shot blasting is sometimes used to remove any remaining sand, especially from internal surfaces, and reduce the surface roughness.

6)Trimming - During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.