castings - concordia nrskumar/index_files/mech311/lectures/...directional solidification ... •no....

Download Castings - Concordia nrskumar/Index_files/Mech311/Lectures/...Directional Solidification ... •No. of castings is small ... the mold (made of steel) •Cores –made usually of dry

Post on 12-Mar-2018

213 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • Castings

    65tons, 7m high, 43cm dia,

    1600years old

  • At 500/550 pound in weight .

    The biggest Gravity Die Casting in The World at the Timehttp://www.rainwater.demon.co.uk/bertha.htm

  • CASTING AND RELATED PROCESSES

    Fundamentals: 6 steps are involved in the casting process:

    1. A mold/ cavity

    A mold for each cast part (Single-Use Molds)

    Permanent mold (Multiple-Use Molds, Metal or graphite)

    Shrinkage allowance

    2. Melting (proper temperature, desired quantity& quality, reasonable cost)

    3. Pouring (gas escaping, free of defects)

    4. Solidification (the nucleation, the growth, free of cracks)

    5. Mold removal (draft angle, extractor)

    6. Cleaning, finishing & inspection

    Casting : Forming the metal in liquid state by solidification in molds of desired

    shapes.

    Inexpensive, economic (no chips formed)

  • SAND CASTING for any metal

    Pattern, Flasks, Molds, Core, Cope, Drag, Risers, Gating, Runners, Sprue

    Sand : a refractory material with additives will solidify (for molds)

    -The pattern simulates the part when making the mold and packing the sand

    -The pattern has to be next removed form the mold to leave place for metal

    -The mold has to be made such that

    it could be divided

    -An inlet is required to bring the metal

    to the mold sprue hole

  • - System of channelsrunners, are required to bring metal to every place of

    the mold :Gating system (controls the metal flow)

    - Sometimes, a Core is required (for hollow parts) cavity inside the mold

    - PINS are required to position the two holding boxes: Flasks: DRAG & COPE

  • Solidification: 1. Nucleation

    Undercooling

    Impurities, Grain refinement

    2. Growth

    Directional Solidification

    Cooling rate:

    Faster Finer grains

    Slower Larger grains

  • Solid-liquid phase cooling curve: shows the type of alloy

    (Eutectic alloy)-Alloy with freezing range

    High super heat Larger time to fill delicate shape

    Thermal arrest plateau of latent heat of fusion

  • Undercooling recalescence

  • Prediction of total solidification time: Chvorinovs rule

    ts =B (V/ A)n

    V- volume of casting

    A- surface area

    B- mold constant

    n- [1.5, 2]

    Ts of Riser >> Ts of casting (In general 1.25 times)

    Different Cooling rates, Ts Different grain and material properties

    Faster Cooling Finer grains

    Slower Cooling Larger grains

    Cast Part Zones: Rapid Zone, Columnar Zone, Equi-axed zone

  • Pattern is not identical with the part to be cast

    size modification ,

    shape modification and

    shape modifications according to the requirements of casting process

    - made of wood, plastic on certain easy-to-machine metals

    Pattern Design

    - Dimensions of the pattern have to incorporate shrinkage allowances differ,

    depending on the kind of metal, typically 1-2%

    - Special shrink rules are used by the pattern makers

    - Large holes and Surface are tapered to allow its withdrawal Draft angles (1%-4%)

  • - Draft angle requirement affect the thickness of the wall weight of the casting

    - If machining is followed some finishing allowance have to be provided,

    depending on the casting experience Shrink Allowances must be considered.

    - Shrinking can cause a distortion of the casting allowances may be required as per

    experience of designer to avoid such distortions.

  • Length:196mm, dia: 96mm

    Different ways of doing it:

    Machining allowance: 2mm

    Melting point: 1520C, Room temp:20C

    LCT: 20ppm 20um/m/C

    Shrinkage Allowance on length = (196+2*2)*20e-6*1500=6mm

    3mm on both sides (3%)

    Shrinkage allowance on dia = (96+2*2)*20e-6*1500=3mm on dia.

  • Different Ways of Designing

  • Gating Flow System:

    Reduce Gas Absorption (Less Turbulence, No vortex)

    Regulate Flow ( larger area, sprue well, runner well)

    Gas Release, Reaction with mold gas release porosity, voids

  • TYPES OF PATTERNS: For removable patterns

    A. Solid Pattern simple shapes made of one piece only

    B. Split Pattern More complicated , made of two parts.

    - half of the pattern will rest in the lower part of the mold (drag)

    - the second half in the upper part (cope)

    - the split is at the parting line

    C. Loose piece pattern pattern must be divided to allow its removal

    D. Gated pattern for serial production made of metal and there are multiple

    pattern on the same main branch

    E. Match plate pattern for production they include runners and gate systems

    F. Follow Board for patterns difficult to split this avoids making a cavity for a

    solid pattern for symmetrical patterns

    G. Sweep Patterns- for rotary parts & straight sweep pattern for grooves

  • Simple Shapes

    No. of castings is small

    Needs follow board to position the pattern

  • Split Pattern

    Moderate quantities

    Pattern is split along the parting line

    Pattern halves are aligned with tapered pins

    Cope and drag molds are made with split patterns

  • Match Plate Pattern

    Large quantities of duplicate castings

    Split patterns are fastened to wood/metal match-plate

    Pattern plate is aligned to flasks with alignment pins

    Cope and drag molds are made separately and assembled

    Independent molding of cope and drag

    Gating, runner, riser can be part of match plate.

  • Loose Piece Pattern

    Sand castings of complex shapes

    When pattern removal is difficult with protruding sections

    Loose patterns are held by beveled pins or grooves

    Once the mold is formed, main segment is removed followed by loose

    pieces

    Expensive

    Substitute for full-mold or investment casting

  • SAND MOLDS:

    Made of SiO2 Traditional molding material

    Requirement for Sand: in molds

    1. Refractoriness withstanding high temperature - basic to sand

    2. Bond- retaining of shape, strength

    3. Permeability allowing the gases to escape

    4. Collapsibility permitting the metal to shrink mold would not collapse

    For molds, Green Sand is used and reused, (recycled)

    Additives: mixed with the sand to obtain the required properties (mainly 2,3,4)

    (clay (9%), Water (3%))

    For Uniform Mixture, sand put into a muller

    Other additives: Silicate of Sodium

    (Na2 SiO3) 4%

    - hardened when exposed to CO2 gas

  • SAND TESTING

    STANDARDIZED METHODS to control the sand quality (size of grains,

    moisture content, clay content, mold, hardness, permeability and bond ).

    Size of grains controlled by 11 standard series in decreasing mesh size

    shaken for 15 min and weighed.

    Moisture by electrical conductivity of small samples or by weight loss of 50g

    sample after heated @ 1100 C

    Clay- by washing the clay out of a standard amount of foundry sand (50g)

    Permeability- with special equipment

    Strength test Special equipment (moisture gives the strength)

    Hardness Penetration of a ball loaded by a spring the some Brinell hardness test

  • Sand mold preparation

    Machines are used + help of an operator

    JOLT SQUEEZE, Roll over molding machine

    Two halves of a flask they can be slightly opened

    Highest density of sand should be obtained close to the pattern

    Special flexible diaphragm squeezing process can be applied for very

    accurate casting.

    Before pouring metal, heavy weights are placed on top of the mold to prevent

    separation of mold sections

    Great importance is attached to the hardness of the skin of the molds. There are skin

    drying process involving heating of the surfaces and adding of some binders.

    Large floor molds made on the floor sand slings have been developed for

    tamping of the sand (up to 12 m long molds. Pneumatic sand rams in helping in

    extra tamping Larger molds can be in Pits on the floor Sections of dried sand

    can be used to make modular mold

    Sand conditioning before the use (properties of conditioned sand)

    Sand is uniformly mixed with additives and has uniform moisture content

    Sand is aerated at uniform room temperature such that sand can be

    processed through machines to make a mold

  • CORES: Allow producing holes and cavities inside the casts.

    Different methods in using of cores

    Cores may be classified as

    - green sand cores made of the same sand as the molds

    - dry sand cores with binders of increasing strength and are

    dried more expensive sand

    Green sand cheaper good enough Made in special core boxes, sometimes

    using special machines

    Cores strength increased by adding silicate of soda Na2SiO3 followed by CO2 process

    Na2SiO3 + CO2 Na2CO3 + SiO2 (colloidal)

    Core making process the same as for molds

  • Cores need to be installed in molds

    molds require recesses to accommodate cores, called CORE PRINTS.

    Core require sometimes support, which is done by chaplets

    which become a part of the mold (not removed)

    Core manufacturing mechanised machines for very complicated cores,

    for engine blocks

    Cores can be avoided by making the patterns

    and molds in a special way shell molds

  • SHELL MOLDING

    For smaller, more precise or 2 sizes much large than the third one parts

    MOLD:mixture of sand and resin (thermosetting plast

Recommended

View more >