Sand casting is one of the most used metal production processes. They are very important and they constitute a very big market around the world. As it implies with its name, sand is used in mold making. These sands are very special sands and they need to have some properties to obtain good casting operations. In here, we will explain;
- General ingridients of mold sands that are used in sand casting market.
- General properties of mold sands to achieve succesful sand casting operations.
- Classification of mold sands that they are avaliable in sand casting market.
What Are The Constituents In Mold Sands?
As you know that, sand is obtained from silica, and mold sands are generally mixed with other minerals. So, in general what makes mold sand is special is, the ingridients.
In general, a mold sand includes around 90% of typical sand, 7% of clay which binds the sand to obtain mold shape and 3% of water for shaping purposes. Clay is very important ingridient to obtain sand mold. After the shaping of mold sand, solidification of clay binds the sand grains to obtain solid mold structure.
Sometimes, other phenolic resins can be used instead of clay. Thermosets solidifies with the application of light, from which we can obtain solid mold structures with this nature.
How The Mold Sands Are Shaped?
After mixing up the constituents of mold sands, the shaping of this mixtur before solidification is very important. Before shaping of mold sand, general system to obtain mold must be built. In this system, there are parts of system;
- Flasks constitute the external boundry to fill inside it with mold sand. After the sand mold is solidified, mold is obtained in the shape of flask.
- To obtain mold cavity with mold sand, patterns are used. Patterns and gating systems are placed inside the flask, and sand mold is spilled inside it. After solidification, mold cavity and gating systems are obtained.
‘Packing’ of sand inside the flask is an also very important process. Sand must be packed inside the flask to obtain a solid mold structure. There are some mechanisms to pack the sand inside flask.
- Sand is packed inside flask by slinging the sand in high speed.
- Pneumatic machines can be used to pack the sand inside flask by squeezing it.
- Jolting of sand from an another flask into new flask to obtain mold shape.
What Are The Required Features Of Mold Sands?
Mold sands and molds that are created from sands must have some properties to meet with the situations originated from metal casting.
- Thermal Stability: This is the most important characteristics of mold sands. The constituents of mold sands must be prepared to withstand the high temperatures that are originated from metal casting processes. The surface of internal cavity of molds must withstand the thermal effects from liquid metal inside them.
- Permeability Of Sand Mold: Gases are produced in the pouring process of liquid metal. Also, there is an atmospheric gases inside the mold cavity. When the liquid metal is poured inside the mold, these gases must be evacuated from the mold cavity. Otherwise, porosities and unintended shaped can be obtained from sand casting operation. So, required permeability of sand mold is very important, and grainy structure of sand provides this property. Permeability of sand mold also changes with the changing grain shape of sand. When the sand grains are round in shape, their permeabilites will be much more higher. But, when the sand grains have very different shapes, very good packing and good surface characteristics are obtained.
- Collapsibility: Molds that are produced from sand and other constituents must be collapsed after the casting operation is done. This sand will be used in another casting operations.
- Strength Of Mold: Good packing must be established to obtain required strength to withstand different conditions that are originated from casting operations. As we stated above, the strength of mold depends on the sand grain structure, quality of the used binders etc.
- Reusability: Once the mold is collapsed, the extracted mold sand must be used in an another sand casting operations. Reusability is very important in terms of costs.
Classification Of Mold Sands
In general, there are three types of conventional sand molds that are used in sand casting operations.
- Dry-Sand Molds: Organic binders are used instead of clay products. In baking owens, prepared sand molds are baked at 200-300C degrees of temperatures. With this kind of sand molds, better surface characteristics and dimensional properties can be obtained. But, this method is much more expensive rather than other methods. Baking time is an also obstacle on the serial production manner. Thus, this method is used in the production of medium to big sized metal products, in small quantities.
- Skin Dried Molds: With the addition of special binders into the mixture of sand mold, only the surfaces of mold cavities are dried with the application of light, heat etc. Around 10-20mm depth of cavity surface is generally dried. It is much more economical compared with dry-sand molds.
- Green Sand Molds: Green sand mold method is the most used and most conventional method to obtain sand molds. They have a percent of moisture in pouring time. Required permeability, strength, collapsibility is obtained with the smart design of mold structures. But the moisture inside the sand mold structure can cause some defects in casting. And surface characteristics are lower than other methods.
All the explanations about conventional sand molds can be explained like above. Do not forget to leave your comments and questions about sand molds below.
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