Sand casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.

Sand casting

Sand casting is one of the most popular and simplest types of casting that has been used for centuries. Sand casting allows for smaller batches to be made compared to permanent mold casting and at a very reasonable cost. Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small size operations. From castings that fit in the palm of your hand to train beds (one casting can create the entire bed for one rail car), it can all be done with sand casting. Sand casting also allows most metals to be cast depending on the type of sand used for the molds.[28] Sand casting requires a lead time of days for production at high output rates (1–20 pieces/hr-mold) and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2,300–2,700 kg (5,100–6,000 lb). Minimum part weight ranges from 0.075–0.1 kg (0.17–0.22 lb). The sand is bonded together using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.

Typical Cross-Section of a Two-Part Sand Casting Mold

The above diagram represents the most basic casting process used today. This process is used worldwide regardless of metal, alloy or molten liquid material being cast. Man used this concept at the beginning before Biblical times to
manufacture objects for decoration, weapons, utensils and hardware. This process is commonly known as sand casting
since sand, which is held together with a binder, is the medium to form the mold cavity.
Definitions
Core: A sand shaped insert placed in the mold cavity to produce internal features on the part.
Cope: The upper half of the sand mold.
Drag: The lower half of the sand mold.
Flask: A box made of wood or metal to contain the sand.
Gates: Multiple openings in the mold to allow the molten brass to flow into the mold cavity.
Gating System:
A passage where the molten brass flows into the mold. The gating system is made up of the pouring
cup, sprue, runner and gates.
Mold: A cavity or matrix by which molten brass is shaped into a desired product.
Parting Line: The line where the top and bottom halves of the sand mold meet.
Pattern: A representation of the final product used to imprint the shape into the sand.
Risers:
Reservoirs called risers inside the mold, which is filled with the molten brass to compensate for shrinkage or
"feed" the mold cavity during the solidification process.
Runner: The horizontal part of the gating system, which supplies molten brass to the gates.
Sprue:
The vertical part of the gating system, which is connected to the pouring cup at the top and feeds the runner with molten brass at the bottom.
Vent: An opening in the mold to allow the escape of hot gases.
Simple Casting Process Description
The flask is comprised of a bottom and a top half and together, they form the flask assembly. The upper section of the flask assembly is known as the cope and the lower section is termed the drag. The first step in the casting process is the mold must first be made. The drag or bottom half of the flask is filled with sand, which is pre-mixed with a binding agent to keep it intact. Inside the drag, there is a pattern, which will create the impression that will form the mold cavity (the patterns for the
top and bottom may be different depending on the object that is to be cast). The sand is compacted around the pattern by manual and mechanical means to ensure all the air is removed and that a smooth mold cavity is formed around the pattern. This procedure is repeated with the cope or upper half of the flask. Depending on the complexity and size of the part and the number of castings being made at one time, the pouring cup, sprue, runners, risers and gates will also be imprinted onto the sand or an expendable core will be used that will burn away. Once the sand has set, the cope and drag are flipped to remove the pattern thus leaving a void in the sand. Cores may be added, if necessary to create hollow and internal features on the final product. The next step is to close the mold and lock it in place using weights or clamps to keep the mold from splitting open at the parting line during the pouring process.
Now that the mold is complete, the molten brass is poured into the mold cavity via the pouring cup or hole. Once filled, the entire assembly is allowed to cool and solidify. Depending on the size of the casting, cooling time can take several hours
(in industry, some castings take over a week to solidify!). Once the molten brass has solidified, the flask assembly is taken apart. The sand mold is then broken apart to expose the product by hand tools, vibration, rotary drums or a combination of these techniques. At this point, the gating system is also removed, and the cores are disposed of.
Once the product is cool to the touch, they go through a rigorous cleaning and finishing process. The parting line on the casting is blended and any spikes or splinters are removed by mechanical means. Once the rough finishing is complete,
the parts are polished to a smooth finish in various grinding and buffing stages. Depending on the colour (natural brass, verdigris or black) the products go through a final finishing process to give it the desired colour. Once complete, the final products are ready for shipment around the globe.

Why Titanium ??

Pure Titanium

Titanium is a light metal having a density of about 4540 kg/m³. This compares to steel at 7900 kg/m³ and Aluminium at 2710kg/m³.   Titanium has a melting point of about 1668^oC which is higher than iron at1560^oC. Titanium has a Modulus of Elasticity of 110 x 10⁹ Pa. compared to steel at 210 x 10⁹ Pa. Therefore Titanium has a significantly high deflection under the same load than steel.  Pure Titanium can be cold rolled to 90% reduction in thickness at room temperatures without cracking.

Titanium does not occur free in Nature. However, when combined with other elements, it is quite abundant, occurring in small amounts in most of the volcanic, sedimentary and metamorphic rocks.
Its more important minerals are ilmenite, rutile, arizonite (iron titanate), brookite, anatase, leucochene (titanium dioxide), perovskite (calcium titanate), and others. The first two have commercial importance, and can be found in deposits spread all over the world. There are important rutile and ilmenite deposits in Australia, Argentina, USA, Central Africa, Brazil, Canada, Egypt, India and Norway. The largest well-known deposits of rutile are located in Australia.
Titanium and its alloys are relatively new engineering metals since they have been in use only since about 1952. They are extremely attractive materials for engineers because they have a high strength to weight ratio, high elevated temperature properties to about 550^oC, and excellent corrosion resistance particularly in oxidising acids and chloride media.  This metal is being increasingly used for marine applications. Its resistance to seawater attack combined with its mechanical properties make it a prime choice for equipment operating within the sea or transferring seawater.
Titanium is not an 'exotic' metal, it is the fourth most abundant structural metal in the earth's crust, and the ninth industrial metal. This metal has become the prime selection for a wide range of critical and demanding applications.

Titanium Alloys

There are two crystallographic forms of titanium:

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      α-titanium, in which atoms are arranged in Hexagonal Closest Packing (HCP) crystal lattice;

    *
      β-titanium, in which atoms are arranged in Cubic Body Centered (BCC)crystal lattice;

Pure titanium exists in form of α-phase at temperatures above 1621°F (883°C) and in form of β-phase at temperatures below 1621°F (883°C).

The temperature of allotropic transformation of α-titanium to β-titanium is called Beta Transus Temperature.

Alloying elements in titanium alloys may stabilize either α-phase or β-phase of the alloy.

Aluminum (Al), gallium (Ga), Nitrogen (N), Oxygen (O) stabilize α-phase.

Molybdenum (Mo), vanadium (V), tungsten (W), tantalum (Ta), silicon (Si) stabilize β-phase.

Titanium alloys are classified into four groups according to their phase composition:

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      Commercially pure and low alloyed titanium alloys

Commercially pure titanium consists of grains of α-phase and dispersed spheroid particles of β-phase. Small amounts of iron, present in the alloys, stabilize β-phase.

Commercially pure titanium has relatively low mechanical strength and good corrosion resistance.

    *
      Titanium alpha and near-alpha alloys

α-alloys consist entirely of α-phase. They contain aluminum as the major alloying element, stabilizing α-phase.

α-alloys have good Fracture Toughness and Creep resistance combined with moderate mechanical strength, which is retained at increased temperatures.

They are easily welded, but their workability in hot state is poor.

Near α-alloys contain small amount of ductile β-phase. Besides α-phase stabilizer (aluminum), near α-alloys are alloyed by 1-2% of β-phase stabilizers (molybdenum, silicon).

Mechanical properties of near α-alloys are similar to those of α-alloys, however due to the presence of β-phase these alloys may be heat-treatable and are forged in hot state.

        

*

Titanium alpha-beta alloys

α-β alloys contain 4-6% of β-phase stabilizers; therefore they consist of a mixture of α and β phases.

α-β alloys are heat-treatable. They have high mechanical strength and good hot formability.

Creep resistance of the alloys is lower, than that of α- and near α-alloys.

   * Titanium beta alloys

β-alloys are rich of β-phase. They contain substantial amount of β-phase stabilizers, preventing β-α transformation at high cooling rates of quenching.

β–alloys are heat-treatable to very high strength and have good hot formability.

Ductility and fatigue strength of the alloys in heat-treated conditions are low.

Titanium alloys are designated according to their compositions:

Ti-5Al-2.5Sn identifies titanium alloy, containing 5% of aluminum and 2.5% of tin.

Ti-6Al-4V identifies titanium alloy, containing 6% of aluminum and 4% of vanadium.

In parallel to this designation system other systems for designation titanium alloys exist (ASTM, IMI, military system).

Notice

The Alpha group contain most importantly aluminum and tin. They can also contain molybdenum, zirconium, nitrogen, vanadium, columbium, tantalum, and silicon. Alpha alloys are not suitable for heat treatment.  Alpha alloys are used for aircraft parts and cryogenic equipment.
The Alpha-Beta group can be strengthened by heat treatment. The alloys are used in aircraft and aircraft turbine parts, chemical processing equipment, marine hardware.
The Beta Alloys have good hardenability. Beta alloys are slightly more dense than other titanium alloys, having densities ranging from 4800 to 5050 kg/m³.  They are the least creep resistant alloys, they are weldable, and can have yield strengths up to 1345 x 10⁶ Pa.(Solution treated and age hardened)   Beta alloys are the smallest group. They are used for heavier duty purposes on aircraft.