SAND CASTINGS

Sand casting refers to those processes where the castings are manufactured in sand molds. Generally, sand foundries also use sand for their core processes. The molds are used one time and destroyed during the process of the molten metal entering and solidifying in the mold. The sand is usually silica although olivine, chromite or zircon may be used. There are two major headings of sand molding processes: green sand and no-bake (also called airset).

Green sand Molding is undeniably the most common process known. More tonnage is made in green sand processes than all others combined. Green sand is a process in which the molds are made of a clay bonded sand that is tempered by water. The sand grains are coated with clay. The clay becomes sticky when water is added. The whole mixture has enough strength to hold its shape when pressed against a pattern. The word "green" comes from the presence of water and has nothing to do with the color of the sand. Most green sand is actually black. There are many different methods of compacting green sand ranging from loose floor molds where the sand is compacted with a hand held pneumatic hammer to highly automated molding machines that can make as many as 450 molds per hour.

The green sand process originated when people found deposits of naturally bonded sand. Naturally bonded sand is found with clay already coating the sand grains and moisture present. From this, people learned to experiment with synthetic bonding systems. In the nineteenth century horse manure was a commonly used binder. In those days when someone said they had crappy castings, they meant it.

No-bake or airset methods are chemically bonded sand systems. Typically a resin and catalyst are mixed together. Through a chemical reaction the resin hardens into a very strong bond. Sometimes an accelerator may be added to speed the hardening process. There are many binder systems used, but most are variations on a few basic chemicals. Some of the chemical systems used are furan, phenolic urethane and sodium silicate.

Shell Process (Core and Molding) is a sand process. The sand, usually silica, is coated with a plastic resin. The sand is blown or "invested" into a heated metal box. The heat of the box cures the resin thus hardening the sand. Shell cores can be made quickly, accurately and economically provided the tooling can be amortized. Usually shell molds are very thin and they may be stacked. This process is economical for small, semi-intricate parts.

Coldbox refers to any of several core processes where a chemical resin coating sand grains is cured by a gas passing through the core while it is still in the corebox. It is a very fast process. The core cures instantly when gassed. SO2, CO2 and Phenolic Urethane are all binder systems used in coldbox.

Oilsand is a sand used for both cores and molds, but more commonly cores, which is bonded with oil. Usually oilsand mixtures include other ingredients including, a cereal such as wood flour, water, iron oxide, etc.. Oilsand, especially when used for molding, is sometimes referred to as Petrobond, a proprietary name.

Warm box is a core process developed by Quaker Oats. Chemically coated sand is blown into an iron corebox heated to an intermediate temperature. The heat starts a chemical reaction that instantly cures the core. The core is solid and cured completely through.

OTHER CASTING PROCESSES

Centrifugal Casting is a method in which the molten metal is spun in a mold which is revolving around an axis. The axis may be horizontal, vertical or moving. The centrifugal force presses the metal against the walls of the mold until it solidifies. It also forces gases out of the metal. This process offers good economies and quality. It is limited to cylindrical shapes.

Die Casting is a permanent mold process used for very high production. Metal is forced into the mold or die under pressure. The tooling for this process is very expensive.

In the "H" Process molds are booked together horizontally so that as each successive mold fills the metal flows over the top into the next mold. During pouring, a continuous, hot feeder head is passing over the casting cavities until the last mold is filled. High yields and very accurate castings are reported. The molds are typically made of resin bonded core sands.

Investment casting is a very precise, high tolerance casting process involving the use of wax patterns. It is labor intensive and so is more expensive than other types of casting. In certain situations it can be economical. There are many steps to investment casting. First a wax pattern is made by injecting wax into a mold or die. Several of these patterns may be assembled onto a single sprue. The resulting assembly is called a tree. The tree is dipped into a ceramic slurry and then stucco coated with sand. When the slurry has dried the tree is dipped and stucco coated again. This may be repeated many times depending on the amount of metal that isto be poured into the tree. Once a sufficient shell of ceramic and sand is built up around the wax pattern the tree is placed upside down in an oven or autoclave. The wax is melted out and the ceramic shell is fired to cure it. The shell mold is placed in a container and either sand or steel shot is packed around it. The shell mold is preheated and then molten metal is poured into it. When the metal has cooled the ceramic shell is broken, the parts are separated from the gates and feeders, cleaned and, if necessary, heat treated.

Lost Foam, known also as full mold and evaporative pattern casting (EPC), places a pattern made of expanded polystyrene (EPS) in a packing of dry sand. Molten metal is poured in on the styrene melting it away and displacing it. This process has proven itself in high production aluminum parts, but has been problematic in other alloys and where there is not sufficient volume to justify the development costs. One notable success is the use of styrene patterns at Hodge foundry in Pennsylvania to make iron castings ranging in excess of 120,000 pounds. Each pattern is made by hand, and backed by a no-bake mold that is heavily vented. The results are astounding.

Lost Wax is a general term that is loosely applied to any process in which wax is melted from a mold to leave a cavity into which metal can be poured. Lost wax refers to a type or broad class of casting. It includes some types of plaster mold and investment. Not previously mentioned is a lost wax method in which no-bake sand is packed around a wax mold. The wax is melted out through vent holes that have been left in the bottom of the mold. The holes are plugged with sand prior to pouring metal into the mold.

Permanent Mold Casting is a process where castings are made in reusable molds. The molds are usually made of metal, but some other materials are being used including, graphite, ceramic and composites.

Plaster Molds are made in a flask like greensand castings and with lost wax methods similar to investment casting.

In the "V" Process a thin plastic sheet is drawn over a pattern that is mounted on a special, perforated board and to which a vacuum is applied. The pattern is placed in special flasks. Dry, unbonded sand is poured into the flask. A strong vacuum is placed on the sand through the flask. The vacuum holds the sand together very tightly, retaining the shape of the pattern after the pattern is removed. Metal can then be poured into the cavity. The vacuum draws away all the gases from the metal and pulls the metal into even the thinnest sections of the cavity. Castings can be produced with very thin sections and almost no draft in this process. The castings are extremely sound because of the absence of gas porosity. They are usually fine in detail and surface finish.