Casting: Fundamentals of Casting and Steps in Casting

Casting

Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part.

Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.

Casting may be used to form hot, liquid metals or melt-able plastics (called thermoplastics), or various materials that cold set after mixing of components such as certain plastic resins such as epoxy, water setting materials

Substitution is always a factor in deciding whether other techniques should be used instead of casting.

Casting
Casting




Fundamentals of Casting

Six basic factors involved in the casting process:

Mold cavity

Melting process

Pouring technique

Solidification process

Part removal process

Post processing

Casting Terms

Pattern

Flask

Cope

Drag

Core

Core Box

Core Print

Mold Cavity

Riser

Gating System

Pouring Cup

Sprue

Runner

Gate

Parting Line

Draft

View of the Mould

Cope, Drag and the pattern

Placement of Runner and Riser

Cope and drag

Placement of runner and riser

Initial view of the cope and drag



POURING OF MOLTEN METALS FROM LADLE

Pouring of molten metal

Pouring of White hot molten metal

Steps in Casting

1. Drag portion of pattern is placed in flask

The diagrams to the left shows the pattern on a flat board and a casting box called a โ€˜dragโ€™ being placed over it

2. Drag is packed with sand

Special casting sand will soon be packed around the pattern.

To ensure the easy removal of pattern from the sand, parting powder is sprinkled over and around it. (parting powder is similar to talcum powder).

It stops the casting sand sticking to the pattern and pulling away with it when the pattern is finally removed from the sandย 

Casting sand is then shaken through a sieve.

only fine particles fall around the pattern. This is called facing sand and it must be fine so that detail on the pattern shows up on the final casting

3. Packing of sand with ram

The drag is then packed with more casting sand. It is a good idea to sieve all the sand being placed above the pattern and then ram it down firmly using a ramming tool.

The tool has two ends,

Cylindrical (used for general packing down of the sand)

Pointed (Used for packing sand close up to the pattern).

4. Leveling

When the drag is packed fully it is leveled off (called โ€˜strickled offโ€™) using a straight steel bar

5. Turn Over Drag

The entire drag and its contents are then turned over so that the base of the pattern can be seen.

6. Fixing and positioning Locating Pin

A top box called a โ€˜copeโ€™ is then placed on top of the drag and locating pins are put in position so that the casting boxes cannot move sideways

7. Insert Sprue and Risers

Sprue pins are positioned. One usually on the back of the pattern and the other to the side. These will eventually provide an entrance and exit for the molten aluminum when it is poured into the sand.

The sand is packed/rammed into the cope in the same way as the drag. Parting powder is first applied, followed by facing sand.

The sprue pins should be taller than the box and stand out from the sand when it is leveled with a strickling bar

8. Pack the cope with Sand

Small depressions are dug into the sand at the top of the two sprue pins. These are useful when the aluminum is poured.

The depressions are called the pouring basin and feeder

9. Gates and runners cut into mold

The top box (the cope) is then removed and if all is well the cope with the sand inside should lift off the drag (bottom box) without the sand falling out. A small โ€˜gateโ€™ is cut below the position of one of the sprue pins.

This will help the molten aluminium flow into the cavity left by the mould. Small tools are available or can easily be made to dig a variety of shapes in the casting sand. They are similar to small trowels

10. Removal of Pattern

The pattern is removed using a โ€˜spikeโ€™. The end of the spike can be threaded and so it can be screwed into the softwood pattern.

Before removing the pattern it is a good idea to gently tap the spike so that it loosens the pattern from the sand. It can then be lifted away from the casting box (drag).

11. Closing cope and drag

The cope (top casting box) is placed back on top of the drag and the locating pins put in position.

Before this is done vents can be created using a thin piece of welding rod, pushing it through the sand . This allows gases to escape once the aluminium is poured

12. Pouring of Molten Metal

The aluminium is poured with great care.

The aluminium is poured down the hole left by the first sprue pin (now called the โ€˜runnerโ€™).

As it runs down the runner it flows through the โ€˜gateโ€™ cut by the trowel, into the cavity left by the pattern and up the riser (the hole left by the second sprue pin).

The casting should be left for sufficient time before removal from the sand.

13. Final Casting

When removed from the sand, the runner and riser are cut away and the casting is ready for machining



Process Factors

Molten metal problems

Reaction of the metal and its environment can lead to poor quality castings. Oxygen and molten metal react to form โ€œslagโ€ or โ€œdross.โ€ These impurities can become trapped in castings to impair surface finish, machinability, or reduce the mechanical properties of the castings.

Fluidity

Molten metal must flow then freeze into the desired shape. Incorrect flow characteristics can result in โ€œshortโ€ shots, incorrect part tolerances, cracks in castings, voids, etc.

Gating System

Correct design of the gating system is a must. Gating system controls the speed, rate, and delivery of molten material into the mold cavity.

Patterns

Shrinkage allowance

Cast Iron = 1/10 โ€“ 1/8 in/ft

Aluminum = 1/8 โ€“ 5/32 in/ft

Brass = 3/16 in/ft

Amount of draft

Finish material allowance

Final dimensional accuracy of the casting

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