Question

Sand casting is one of the older techniques for manufacturing process. There are various steps involve in casting process including mold making and pattern making. Please draw a sand casting production sequence and explain for each processes.

Answer 1

Open riser Vent Pouring basin (cup) Flask Blind riser Sprue Cope Core (sand) Sand Parting line Drag Sand Mold cavity Choke Runner Gate

fig shows mold making process illustrating a sand mold with various features described below

Moulding flank half in case of be made and reaches the mould cavity. : It holds the sand mould in pasi on and top half is called "Cope' & bottom called dig. Any intermediate flask wed three piece moulding is called cheek? ( Pattern : It is the replica of the final object to is used for the making the mould cavity & Parting line. It is the dividing line 4w the two mould half Bottom board: This is wed at the start of moulding pow less where the pattern is placed on it, sand is oprin- kled and rammed in drag. (@ Moulding sand : It is a refractory material used for making mould carity ,A is a mixture of sand elay moisture . (2) Core This is wed for making hallow cavities in a (8) Pouring basin. It is a small funnel slope carity, at the top of mould into which the molten metal is poured. O SPRUE: The procese through which metal reaches the of mould cavity & controls the flow of metal into the mould (10) Runner: It is the passage way in the parting plane through which the metal flow is regulated before f the casting 2) Ingate the actial entryport through which the meted a enters the moved cavity.

Molding - Pattern making - Core making Gating system Sand Mold Melting of metal Pouring into mold Casting Heat treatment Cleaning and finishing Inspection - Furnaces Solidification Shakeout Removal of risers Additional heat treatment Defects Pressure tightness Dimensions and gates
fig shows outline of production steps in a sand casting operation

Pattern making is one of the first and most important steps in the casting process. The “pattern” is essentially a replica of the object about to be cast. Usually made out of wood, metal or model board, patterns are used to create cavities in moulds. It is through pouring molten metal into these moulds that aluminium castings are created.

In metal casting, patterns are the objects used to create expendable molds. A single pattern can make up to thousands of molds.here are different types of patterns, but they fall into two major subtypes: reusable ones for sand or plaster casting, which make impressions into many molds, or expendable patterns for investment casting, that are destroyed before casting during the process of making the mold. For either type of pattern, the patternmaker’s concern is not just replicating the desired object, but also helping direct metal into the mold at the right rate and into the right place. Metal cooling is a process of crystallization, and how the mold fills can influence the direction of freezing, which can be important for strength of the final object.

Allowances given during pattern making:  A good patternmaker knows how to adjust their pattern to compensate for how metal cools and what sort of finishing steps might be needed on the part.

Shrinkage Allowance is a correction to compensate for the solidification shrinkage of the metal and its contraction during cooling. These allowances vary with the type of metal and size of casting. Typical allowances for cast iron are 1/10 to 5/32 in/ft; for steel, 1/8 to 1.4 in/ft; and for aluminum, 1/16 to 5/32 in/ft. These allowances also include a size tolerance for the process so that the casting is dimensionally correct.

Machine finish allowance (“machining allowance”) is necessary if the cast object is going to be refined after it’s shaken out. The allowance must provide enough material that machining won’t make the object too small. Sometimes, knowing there will be machine finishing helps a wood pattern designer decide where to place the risers: the stock metal can be clamped while the casting is being machined.

Distortion allowance is an artful adjustment that is made to a pattern when the metal used will distort as it cools. If the displacement angles are known, the pattern can be made to compensate. This modification is called a “camber” and can make the original pattern look warped.

Draft allowance is a modification needed only for sand casting patterns, not investment patterns. Pattern makers need to make sure that their patterns can be removed from the sand without destroying the shape they’ve left behind. This is done by tapering the walls of the mold, just as the buckets for sandcastles are usually tapered. Sharp corners and straight edges are more likely to crumble when the pattern is removed from the molding substance.

Gating system: The system of channels to feed metal to all parts of the casting is support infrastructure that, when well designed, will decrease casting defects. These elements include:

• The pouring cup is a tapered entry into the mold. It helps control the flow of metal, as well as capturing slag during pouring.
• The sprue, sometimes called the “down-sprue,” is the main channel into the mold from the pouring cup. It is the river of metal that fills the casting.
• Runners are channels that allow liquid metal to flow from one area of the mold to another.
• Gates or “in-gates” are the entry spaces to the casting itself. Large gates are necessary for quick cooling metals, and smaller gates are used for slowly cooling ones.
• Risers are large-volume reservoirs of liquid metal that are part of the mold but will not be part of the casting. They can be “blind risers,” which are completely buried in the mold to feed tricky areas of the casting, or open risers that fill with metal—and can help indicate to the foundry worker that the mold is full. Risers have a greater volume of metal than any part of the casting so that they will stay hot the longest. Using gravity, risers continuously feed liquid metal into the cooling casting to help prevent holes or shrinkage.