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1 Sand Casting

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  • 06-11-2014

    1

    Metal Casting Sand casting Sand casting uses ordinary sand as the primary

    mould material. The sand grains are mixed with small amounts of

    other materials, such as clay and water, to improvemouldability and cohesive strength, and are thenpacked around a pattern that has the shape of thedesired casting.

    The pattern must be removed before pouring, themold is usually made in two or more pieces.

    An opening called a sprue hole is cut from the top ofthe mold through the sand and connected to asystem of channels called runners. Contd.

    The molten metal is poured into the sprue hole, f lowsthrough the runners, and enters the mold cavitythrough an opening called a gate.

    Gravity f low is the most common means ofintroducing the metal into the mold.

    After solidification, the mold is broken and thefinished casting is removed.

    The casting is then fettled by cutting off the ingateand the feeder head.

    Because the mold is destroyed, a new mold must bemade for each casting.

    Contd

    Sequential steps in making a sand casting A pattern board is placed between the bottom (drag)

    and top (cope) halves of a f lask, with the bottom side up.

    Sand is then packed into the drag half of the mold.

    A bottom board is positioned on top of the packed sand, and the mold is turned over, showing the top (cope) half of pattern with sprue and riser pins in place.

    The cope half of the mold is then packed with sand.

    Contd

    The mold is opened, the pattern board is drawn(removed), and the runner and gate are cut into thesurface of the sand.

    The mold is reassembled with the pattern boardremoved, and molten metal is poured through thesprue.

    The contents are shaken from the flask and the metalsegment is separated from the sand, ready for furtherprocessing.

  • 06-11-2014

    2

    Casting Terms Flask: A moulding flask is one which holds the sand

    mould intact. It is made up of wood for temporary

    applications or metal for long-term use.

    Drag: Lower moulding flask.

    Cope: Upper moulding flask.

    Cheek: Intermediate moulding flask used in three-

    piece moulding.Contd

    Pattern: Pattern is a replica of the final object to be

    made with some modifications.

    Parting line: This is the dividing line between the two

    moulding flasks that makes up the sand mould.

    Bottom board: This is a board normally made of wood,

    which is used at the start of the mould making.

    Contd

    Moulding sand: The freshly prepared refractory

    material used for making the mould cavity. It is a

    mixture of silica, clay and moisture in appropriate

    proportions.

    Backing sand: This is made up of used and burnt

    sand.

    Core: Used for making hollow cavities in castings.

    Pouring basin: A small funnel-shaped cavity at the top

    of the mould into which the molten metal is poured.

    Sprue: The passage through which the molten metal

    from the pouring basin reaches the mould cavity.

    Runner: The passage ways in the parting plane through

    which molten metal f low is regulated before they reach

    the mould cavity.

    Gate: The actual entry point through which molten

    metal enters the mould cavity in a controlled rate. Contd

  • 06-11-2014

    3

    Chaplet: Chaplets are used to support cores inside the

    mould cavity.

    Chill: Chills are metallic objects, which are placed in

    the mould to increase the cooling rate of castings.

    Riser: It is a reservoir of molten metal provided in the

    casting so that hot metal can flow back into the mould

    cavity when there is a reduction in volume of metal due

    to solidificationContd

    Padding Tapering of thinner section towards thicker section

    is known as 'padding'. This will require extra material. If padding is not provided, centre line shrinkage or

    porosity will result in the thinner section.

    IES-2001

    The main purpose of chaplets is

    (a) To ensure directional solidification

    (b) To provide efficient venting

    (c) For aligning the mold boxes

    (d) To support the cores

    IES-1996Which of the following methods are used for

    obtaining directional solidification for riser design

    1. Suitable placement of chills

    2. Suitable placement of chaplets

    3. Employing padding

    Select the correct answer.

    (a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1, 2 and 3

    IES 2007

    Which one of the following is the correctstatement?Gate is provided in moulds to(a) Feed the casting at a constant rate(b) Give passage to gases(c) Compensate for shrinkage(d) Avoid cavities

    PatternA pattern is a replica of the object to be made by thecasting process, with some modifications.

    The main modifications are The addition of pattern allowances, The provision of core prints, and Elimination of fine details, which cannot be obtained

    by casting and hence are to be obtained by furtherprocessing

  • 06-11-2014

    4

    Pattern Allowances1. Shrinkage or contraction allowance

    2. Draft or taper allowance

    3. Machining or finish allowance

    4. Distortion or camber allowance

    5. Rapping allowance

    Shrinkage allowance All metals shrink when cooling except perhaps

    bismuth.

    This is because of the inter-atomic vibrations which

    are amplified by an increase in temperature.

    The shrinkage allowance is always to be added to the

    linear dimensions. Even in case of internal dimensions.

    Contd

    Liquid shrinkage and solid shrinkage Liquid shrinkage refers to the reduction in

    volume when the metal changes from liquid tosolid state at the solidus temperature. To accountfor this, risers are provided in the moulds.

    Solid shrinkage is the reduction in volumecaused, when a metal loses temperature in thesolid state. The shrinkage allowance is provided totake care of this reduction.

    Pattern AllowancesCast Iron 10 mm/mBrass, Copper, Aluminium 15 mm/mSteel 20 mm/mZinc, Lead 25 mm/m

    In grey cast iron and spheroidal graphite iron, theamount of graphitization controls the actualshrinkage. When graphitization is more, theshrinkage would be less and vice versa.

    IES-1995Which one of the following materials will require

    the largest size of riser for the same size of casting?

    (a) Aluminium

    (b) Cast iron

    (c) Steel

    (d) Copper.

    IES-1999In solidification of metal during casting,

    compensation for solid contraction is

    (a) Provided by the oversize pattern

    (b) Achieved by properly placed risers

    (c) Obtained by promoting directional

    solidification

    (d) Made by providing chills

  • 06-11-2014

    5

    Draft To reduce the chances of the damage of the mould

    cavity at the time of pattern removal, the vertical faces

    of the pattern are always tapered from the parting line.

    This provision is called draft allowance.

    Inner surfaces of the pattern require higher draft than

    outer surfaces.

    Draft is always provided as an extra metal.

    DRAFT ALLOWANCE

    Shake Allowance

    At the time of pattern removal, the pattern is rapped

    all around the vertical faces to enlarge the mould

    cavity slightly to facilitates its removal.

    It is a negative allowance and is to be applied only to

    those dimensions, which are parallel to the parting

    plane.

    Distortion Allowance A metal when it has just solidified is very weak and

    therefore is likely to be distortion prone.

    This is particularly so for weaker sections such as longflat portions, V, U sections or in a complicated castingwhich may have thin and long sections which areconnected to thick sections.

    The foundry practice should be to make extramaterial provision for reducing the distortion.

    Pattern Materials Wood patterns are relatively easy to make. Wood is not

    very dimensionally stable. Commonly used teak, whitepine and mahogany wood.

    Metal patterns are more expensive but are moredimensionally stable and more durable. Commonly usedCI, Brass, aluminium and white metal.

    Hard plastics, such as urethanes, and are often preferredwith processes that use strong, organically bonded sandsthat tend to stick to other pattern materials.

    In the full-mold process, expanded polystyrene (EPS) isused.

    Investment casting uses wax patterns.

    The pattern material should be Easily worked, shaped and joined

    Light in weight

    Strong, hard and durable

    Resistant to wear and abrasion

    Resistant to corrosion, and to chemical reactions

    Dimensionally stable and unaffected by variations in

    temperature and humidity.

    Available at low cost.

  • 06-11-2014

    6

    IES-1994Which of the following materials can be used for

    making patterns?

    1. Aluminium 2. Wax 3. Mercury 4. Lead

    Select the correct answer using the codes given below:

    Codes:

    (a) 1,3 and 4 (b) 2,3 and 4 (c) 1, 2 and 4 (d) 1, 2 and 3

    Types of PatternSingle Piece Pattern

    These are inexpensive and the simplest type ofpatterns. As the name indicates, they are made of asingle piece.

    Gated PatternGating and runner system are integral with thepattern. This would eliminate the hand cutting ofthe runners and gates and help in improving theproductivity of a moulding.

    Types of PatternSplit Pattern or Two Piece Pattern

    This is the most widely used type of pattern for intricatecastings. When the contour of the casting makes itswithdrawal from the mould difficult, or when the depthof the casting is too high, then the pattern is split into twoparts so that one part is in