Casting is the process of producing objects by pouring molten material into a cavity called a mold, which is the negative of the object, and allowing it to cool and solidify. Shrinkage allowance: The pattern must take sufficient account of shrinkage; these are called contraction allowances and their exact values depend on the alloy being cast and the exact sand casting method used. Some alloys have a total linear shrinkage of up to 2.5%, while other alloys experience essentially no shrinkage or increase in size during the casting process.
Draft or Taper Allowance: The model should include adequate draft allowances, meaning that its sides are tapered so that when it is pulled through the sand, it will tend not to drag the sand out of the site with it. Allowance for distortion: it has been found that large castings tend to warp or distort during the cooling period due to the size, shape and type of metal. Such an allowance depends on the judgment and experience of the model maker, who knows the shrinkage characteristics of the metal.
Finishing or Machining Allowance: The rough surfaces of the casting must be finished or machined. Therefore, the rough casting must be made larger than the actual component in size, and therefore the pattern should also be larger in size than the actual component. Shaking or impact allowance: When the pattern is patted or shaken to easily remove it from the cavity, it appears that the cavity in the mold is slightly increased in size.
In small and medium castings this allowance may be ignored, but in large castings, or in those which must fit together without machining, or where high precision is required, shake allowance is made by making the pattern slightly smaller.
TYPES OF PATTERN
One Piece Pattern: This type of pattern is made without seams, splits or any loose parts. Split pattern: A split pattern is made in two or more pieces joined together with dowel pins. Loose-piece model: The loose-piece model is made of loose component parts assembled together with dowel pins.
In mass production if the gate is made by hand for each small mold, it will take a long time. Match Plate Pattern: When split patterns are mounted with half on one side of a plate and the other half directly on the other side of the plate, the pattern is called a match plate pattern. Coping and pulling model: If the casting is large, the complete mold is very heavy and difficult to handle by a single operator.
After removing the two halves from the molding sand, the bracket and pull are then assembled to make the complete mold. Follow the board pattern: A pattern with thin parts tends to distort or collapse during tamping.
MOLDING
PROPERTIES OF MOLDING SAND
It is this property that allows the molding sand to be successfully retained in the molding flask and not fall out of the flask when removed. Plasticity or fluidity: It is the property of the molding sand by which it assumes a predetermined shape under pressure and retains it when the pressure is released. Once the mold has been made by fixing and ramming the pattern into the sand, the shape of the mold should not distort after the pattern is removed from the sand.
TYPES OF MOLDINGS
Dry sand casting: Dry sand casting is similar to green sand casting, but the mold is dried or baked before pouring until the moisture is driven away. The dry sand molds are stronger and can be handled more easily with less damage and can be stored for a long time. Dry sand molds are generally stronger than green sand molds and can therefore withstand a lot of extra handling.
The improved quality of the sand mix due to the removal of moisture can result in a much smoother finish on the castings than when made in green sand molds. Where molds are properly washed and sprayed with refractory coatings, the casting finish is further improved. This type of casting is much more expensive than green sand casting and is not a.
Dry Skin Molding: Dry skin molding is sometimes preferred over green sand molding, where it is required to ensure that surface moisture and other gas-forming materials are reduced. By drying the skin of the mold face after special binding materials have been added to the sand molding mixture, a hard mold face similar to that obtained in the dry sand practice is produced. The mold shake is almost as good as that obtained with greensand molding.
Injection casting, centrifugal casting and permanent casting are the processes that use metal molds.
CASTING
TERMINOLOGY
PROCESSES
SAND
STEPS IN SAND CASTING
ELEMENTS OF A GATING
SYSTEM
Guides: Guides are passages that distribute the liquid metal to different areas within the mold. Master Cavity: The impression of the actual part to be cast is often called the master cavity. Vents: Vents help escape gases that are released from the molten metal during the solidification phase of the metal casting process.
They feed this material to parts of the mold to compensate for shrinkage as the casting solidifies.
CHI LLS
ANIMATION OF SAND CASTING STEP
Gravity Casting: The following figure is similar to sand casting except that the mold is machined from solid metal, usually cast iron. Being metal, the mold can be accurately machined and, with good thermal conductivity, allows the casting to cool quickly. The surface finish is better than what can be produced by sand casting, but since metal molds are required, product sizes are generally smaller than what is possible with sand casting.
Obviously, the metal being cast must have a lower melting point than the cast metal. Pressure Casting: It is a development of gravity casting, in which the molten metal is injected into a steel mold under pressure; it is the metal equivalent of die casting. Pressure injection molding is faster than sand and gravity injection molding, and because the fluid is pressurized, finer surface details can be replicated.
It is commonly used for door handles, electrical iron bases, and hollow sections that require fine detail, such as carburetor housings. Suitable for metals with a relatively low melting point (8710C) such as lead, zinc, aluminium, magnesium and some copper alloys. Die casting is limited to metals with high fluidity (zinc, aluminium, magnesium, copper, lead and tin).
INVESTMENT OR LOST- WAX CASTING
CONVENTIONAL CASTING: In the conventional process, the shell is filled with molten metal by gravity casting. As the metal cools, the parts and ports, sprue and casting cup become one solid casting. KNOCKOUT: When the metal has cooled and solidified, the ceramic shell is broken down by vibration or water jets.
CENTRIFUGAL CASTING
SQUEEZE CASTING
COMPARISON OF CASTING PROCESS
CASTING DEFECTS
SURFACE DEFECT
INTERNAL DEFECTS
VISIBLE DEFECTS
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