Sand mold Equipment

In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores.

Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mold before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off.

In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.

Sand

The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mold, which characterize the following four unique types of sand molds.

Greensand mold - Greensand molds use a mixture of sand, water, and a clay or binder. Typical composition of the mixture is 90% sand, 3% water, and 7% clay or binder. Greensand molds are the least expensive and most widely used.

Skin-dried mold - A skin-dried mold begins like a greensand mold, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mold strength. Doing so also improves the dimensional accuracy and surface finish, but will lower the collapsibility.  Dry skin molds are more expensive and require more time, thus lowering the production rate.

Dry sand mold - In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has high dimensional accuracy, but is expensive and results in a lower production rate.

No-bake mold - The sand in a no-bake mold is mixed with a liquid resin and hardens at room temperature.

The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures:

Strength - Ability of the sand to maintain its shape.

Permeability - Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains.

Thermal stability - Ability to resist damage, such as cracking, from the heat of the molten metal.

Collapsibility - Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand can not compress, then the casting will not be able to shrink freely in the mold and can result in cracking.

Reusability - Ability of the sand to be reused for future sand molds.

Packing equipment

There exists many ways to pack the sand into the mold. As mentioned above, the sand can be hand packed into the mold. However, there are several types of equipment that provide more effective and efficient packing of the sand. One such machine is called a sandslinger and fills the flask with sand by propelling it under high pressure. A jolt-squeeze machine is a common piece of equipment which rapidly jolts the flask to distribute the sand and then uses hydraulic pressure to compact it in the flask. Another method, called impact molding, uses a controlled explosion to drive and compact the sand into the flask. In what can be considered an opposite approach, vacuum molding packs the sand by removing the air between the flask and a thin sheet of plastic that covers the pattern.

The packing of the sand is also automated in a process known as flask-less molding. Despite the name of the process, a flask is still used. In conventional sand casting, a new flask is used for each mold. However, flask-less molding uses a single master flask in an automated process of creating sand molds. The flask moves along a conveyor and has sand blown against the pattern inside. This automated process greatly increases the production rate and also has many benefits to the castings. Flask-less molding can produce uniform, high density molds that result in excellent casting quality. Also, the automated process causes little variation between castings.