Metal Casting Zone Info Blog

Posts Tagged ‘mold material’

A Brief Introduction of Molding Sand

When people speak of sand casting, the first thing that comes to their mind is the sand on the beach. They do not think that sand casting is one of the most popular methods which are used to produce metal parts. In order to help people get a general idea about it, the article will introduce some information about the sand casting.

Generally speaking, the sand used in foundry industry includes two kinds, namely molding sand and core sand. The molding sand is composed of 85% natural sand, 9% clay and 6% water. Sometimes, in order to improve its property, coal powder, vegetable oil and wood flour will be added into the mold sand.

Although the sand is the main material for sand casting, it is quite different from the common sand on the beach. Anyone who has built sand castles on the beach knows how fragile sand castles are. So it is impossible to use the beach sand to hold molten metal. One question that comes to the mind is what properties can help the molding sand hold molten metal. Generally speaking, the molding sand has the following features. What comes first is permeability. The ability of allowing the steam to pass through the walls of mold material is called permeability. After the high-temperature molten metal is poured into the mold, there are a lot of gases in the mold. If the gas can not be removed successfully, the casting will produce some defeats. In addition, the permeability is also influenced by the clay content, water content and graininess. The next feature is refractoriness. This is the ability of withstanding high temperatures of sand. From this point, people also can deduce that sand used for casting steel must be more refractory than one for brass or aluminum because of the greater pouring temperature. Also, a sand used to cast large heavy castings must be more refractory than one used for light thin castings of the same metal. The third feature is strength. The ability of resisting the external force is called strength. If the strength of mold sand is not high enough, the castings easily collapse. However, the strength can not be too high. Or else, the sand castings will produce some defeats due to the decrease of permeability. The fourth feature is plasticity. It refers to the ability of maintaining the shape under the effect of external force. Because the molding sand has the good plasticity, the molding operation is very convenient and the shape of mold is quite accurate.

Find More Sand Casting Foundry Articles

Jewelers who specialize in custom designing often use the lost wax casting method to create one-of-a-kind rings, charms, pendants, or other specialized items out of precious metals.

First the jeweler may sketch an idea on paper, especially if he or she is working with a client.  When the diagrammed sketch is approved, the jeweler then fashions an exact replica of the finished item in a soft, pliable wax.  The wax pattern, or maquette, is fitted with a stem called a sprue, which will create an exit for the wax when it is burned out.

This wax form is weighed to determine the amount of metal that will be needed.  It is then attached to a base and fitted with a tumbler that holds the mold material, which is plaster mixed with water to a cake batter consistency, called investment.  It must be free of bubbles before pouring into the tumbler, so it is placed in a vacuum to remove all the air.  The investment is poured into the tumbler and then taken to a kiln to have the wax burned out.

A jeweler’s centrifuge machine is the tool that injects the liquid metal into the mold.  The mold is taken from the kiln and placed in a holder that has a hollow arm attached to a cup that lines up with the hole in the mold. The pre-measured metal is placed in this small crucible, and then heated with a torch until it is liquid. 

The pin is then released on the centrifuge, and it spins rapidly around while the liquid metal is forced into the tumbler and mold. Once the spinning stops, the tumbler is removed from the centrifuge with tongs (it is extremely hot), dipped into cold water, and the plaster cracks and falls off of the metal inside.

At this point, the beautifully designed object created on paper does not even slightly resemble the blob of metal in the jeweler’s tongs.  This is because the base (or button) and the sprue are now metal like the rest of the design, and must be sawed and ground off with a jewelers’ Dremel tool, which is an instrument much like a dentist’s drill.

Much fine sanding and shaping is done with the Dremel and its attachments, and gradually the original design emerges.  Jewelers’ rouge puts a nice patina on the finish, and after much polishing, the piece is now ready for use.

Having a custom-made, jeweler-designed ring, pendant, charm, or other special item is something to be proud of, since it can define who you are, what your interests are, or what is important your life.