Properties of a Sand Casting Mixture

 Properties of a Sand Casting Mixture:

        Type and Content of Binder and Other Additives:

                 As mentioned, controlling the type and content of the sand binder and other additives is the key to controlling the properties of the casting's mould sand mixture.

Moisture Content:

               Moisture content affects the other properties of the mixture such as strength and permeability. Too much moisture can cause steam bubbles to be entrapped in the metal casting.

Grain Size:

                   Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.

Shape of Grains:

                   This property evaluates the shape of the individual grains of sand based on how round they are. Less round grains are said to be more irregular.

Strength:

               The explanation of strength is, the ability of the sand casting mixture to hold its geometric shape under the conditions of mechanical stress imposed during the sand casting process.

Permeability:        

                 The ability of the sand mould to permit the escape of air, gases, and steam during the sand casting process.

Collapsibility:

                    The ability of the sand mixture to collapse under force. Collapsibility is a very important property in this type of casting manufacture. Collapsibility of the mould will allow the metal casting to shrink freely during the solidification phase of the process. If the moulding sand cannot collapse adequately for the casting's shrinkage, hot tearing or cracking will develop in the casting.

Flow ability:

                    The ability of the sand mixture to flow over and fill the sand casting pattern during the impression making phase of the manufacturing process, more flow ability is useful for a more detailed casting.

Refractory Strength:

                 During the pouring of the molten metal in sand casting manufacture, the sand mixture in the mould must not melt, burn, crack, or sinter. The refractory strength is the ability of the mould sand mixture to withstand levels of extreme temperature.

Re-usability:

                         The ability of the mould sand mixture to be reused to produce other sand castings in subsequent manufacturing operations. When planning the manufacture of a particular casting, remember some properties of a sand casting mould mixture are contradictory to each other. Tradeoffs in different properties are often needed to achieve a compromise that provides a sand casting mould mixture with adequate properties for the specific part and casting application. There are some things to consider when selecting a sand mixture for a manufacturing process. Small grain size enhances mould strength, but large grain size is more permeable. Sand casting moulds made from grains of irregular shape tend to be stronger because of grain interlocking, but rounder grains provide a better surface finish. A sand casting mould mixture with more collapsibility has less strength, and a sand casting mixture with more strength has less collapsibility.

Sand Conditioning For a Metal Casting Operation:

                If the sand is being reused from a previous sand casting manufacturing process, lumps should be crushed and then all particles and metal granules removed, (a magnetic field may be used to assist in this). All sand and constituents should be screened. In industrial practice shakers, rotary screens, or vibrating screens, are used in this process. Then continuous screw-mixers or mulling machines are used to mix the sand uniformly.

 Basic Steps of Casting:

Three steps are involved in a casting process:

            1) Heating metal till it becomes molten

            2) Pouring the molten metal into a mould

            3) Allowing the metal to cool and solidify in the shape of the mould.

     Casting is used in the automobile industry to produce engine blocks or cylinder heads. Metal casting is vital to our economy and security. Different metals are cast by many different processes for different applications. Cast metal products and processes offer advantages unavailable from products made by other metal forming and fabricating techniques. Castings are used in areas like transportation, aerospace, defense, mining, construction, maritime, fluid power, & domestic household. Some cast components include: engine blocks, suspension parts for automobiles & fluid flow components like valves, pumps, pipes, and fittings. To cut the emissions there is a need to improve the fuel efficiency and make the vehicle lighter in weight. Non ferrous metal like aluminum is lighter than steel and has density one third of that of steel. Aluminum has a lower density of 2.7 gm/cc compared to 7.8 gm/cc of steel. Aluminum and aluminum alloys are lightweight with good corrosion resistance, ductility and strength.

           Aluminum castings are more expensive than ferrous based castings. The greater use of aluminum can decrease vehicle weight, improve its performance and reduce fuel costs. Pure aluminum possesses relatively poor casting features and for this reason castings are repaired from aluminum alloys. The main alloying elements are silicon, zinc. Aluminum silicon alloys have good casting and corrosion resistance properties. The fluidity increases with silicon addition. The addition of copper to aluminum increases its strength and hardness. The aluminum copper alloys are heat treatable and possess good Machinability.

Advantages of Casting Process:

                    Certain advantages are inherent in the metal casting process. These often form the basis for choosing casting over other shaping processes such as machining, forging, welding, stamping, rolling, extruding, etc. Some of the reasons for the success of the casting process are:

1. the most intricate of shapes, both external and internal, may be cast. As a result, many                                             other operations, such as machining, forging, and welding, can be minimized or eliminated.

2. Because of their physical properties, some metals can only be cast to shape since they    cannot be hot-worked into bars, rods, plates, or other shapes from ingot form as a preliminary   to other processing.

3. Construction may be simplified. Objects may be cast in a single piece which would   otherwise require assembly of several pieces if made by other methods.

4. Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly. For example, in the automotive industry hundreds of thousands of cast engine blocks and transmission cases are produced each year.

5. Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housing, valves, and hydro-electric plant parts weighing up to 200 tons illustrate this advantage of the casting process.

6. Some engineering properties are obtained more favourably in cast metals.

Examples are:

         a) More uniform properties from a directional standpoint; i.e., cast metals exhibit the                                              same properties regardless of which direction is selected for the test piece relative to the           original casting. This is not generally true for wrought metals.

         b) Strength and lightness in certain light metal alloys, which can be produced only             as castings.

         c) Good bearing qualities are obtained in casting metals.

7. A decided economic advantage may exist as a result of any one or a combination of points mentioned above. The price and sale factor is a dominant one which continually weighs the advantages and limitations of process used in a competitive of enterprise.                

                   There are many more advantages to the metal-casting process; of course it is also true that conditions may exist where the casting process must give way to other methods of manufacture, when other processes may be more efficient. For example, machining procedures smooth surfaces and dimensional accuracy not obtainable in any other way; forging aids in developing the ultimate of fiber strength and toughness in steel; welding provides a convenient method of joining or fabricating wrought or cast products into more complex structures; and stamping produces lightweight sheet metal parts. Thus the engineer may select from a number of metal processing methods that one or combination, which is most suited to the needs of his work.