Casting Ch 6 (Cores) @ Www.07 Met.Tk

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  • 1.  
  • 2.
    • CORES
    • Cores are sand shapes which form the casting contours that cannot be molded with a pattern.
    • Exterior contours ----- pattern
    • Internal shapes/cavities ---- cores
    • Cores provide the casting process;
    • * its ability to make the most intricate of shapes,
    • * eliminate much machining,
    • * produce shapes which would be impossible to machine .
  • 3.
    • Core may be made of;
    • - metal
    • - plaster
    • - ceramic materials
    • - core sand
    • Cores must be collapsible
    • Metal cores ---- shape limitations
    • Sand cores ------ most frequently used
  • 4.
    • Other purposes:
    • i) Complete molds may be assembled of core-sand forms. Intricate shapes
    • ii) May be used to form a part of a green-sand mold.
    • Pattern contours with backdraft or projections which cannot be molded can be formed by placing a core in the mold after the pattern is drawn.
    • iii) To strengthen or improve a mold surface.
    • iv) May be used as a part of the gating system
    • v) Ram-up cores are used for several purposes.
    • * Located on the pattern and rammed up along with the molding sand, the core then forming a part of the mold face.
  • 5. Cores
  • 6.
    • Core sand mixtures ------ sand grains and organic binders
    • The binders provide green strength, baked strength and collapsibility.
    • Because the strength comes from core oil, an organic binder, the strength is lost and the core becomes collapsible when hot metal is poured around the core.
    • Core-making ------ manually or with machines
    • Cores with no flat surfaces must be supported on a core drier, until they are baked.
    • Core venting
    • Reinforcing wires
  • 7.
    • Temperature up to about 500 F.
    • Moisture is driven off first.
    • Then, the core oil or other binder changes chemically and molecularly from a liquid to a solid by oxygen absorption as the temperature rises 400 to 500 F.
    • Backing cycle --- 2 to 6 hrs.
    • Unbaked cores ---- gives of much gas.
    • Overbaked cores may collapse too soon.
    • When baked, a core-oil bonded core assumes a nut-brown color, darkness indicating overbaking and lightness underbaking.
  • 8.
    • Core-baking Equipment:
    • A. Core ovens
    • 1. Batch-type
    • 2. Continuous
    • a. Horizontal
    • b. Vertical
    • B. Dielectric bakers
    • C. Radiant bakers
  • 9.
    • Core Ovens:
    • Batch Type:
    • Drawer type
    • Gas or oil fired or coal
    • Fresh air -- oxygen is needed for many core oils to harden.
    • Continuous Type:
    • Conveyor belt
    • Cores are loaded on racks
  • 10.
    • Dielectric Baking:
    • Principle: heating of non-conducting materials on a molecular scale can be caused in a rapidly fluctuating electrostatic field.
    • A pair of flat plate electrodes to which is applied a rapidly oscillating alternating-current voltage.
    • Plate Voltage = 1000 to 5000 volts
    • Frequencies up to 20 million cycles
    • Metal driers or core plates are not used to support cores.
    • Driers can be made of plastics
    • A rapid process
  • 11.
    • Finishing work may be classified as follows;
    • 1. Cleaning
    • 2. Sizing
    • 3. Core assembly
    • 4. Inspection
  • 12.
    • Cleaning Operations:
    • All work done except sizing or assembly of cores.
    • Trimming, brushing, venting, coating and mudding.
    • Refractory coating to improve resistance to molten metal.
    • Coatings may be applied by spraying, dipping or swabbing.
    • Mudding
    • Graphite and red talc moistened with water to putty consistency may be used to make the cores completely smooth.
    • Another mud consists of 94 % silica flour, 3 % western bentonite, and 3 % dextrin moistened to a putty with water.
    • Drying to eliminate water
    • Vent hole --------- wax vent holes
  • 13.
    • Sizing Operation:
    • To make cores dimensionally accurate.
    • Gauges may used to check critical dimensions.
    • Sagging or slumping during baking.
    • For exact size ----- Cores may be made slightly oversize and ground to correct height.
  • 14.
    • Core Assembly:
    • Some cores may be of one piece
    • Other cores may be assembled of two or more pieces
    • Cores assemblies may be held together by pasting, bolting or leading.
    • Avoid sealing off vent holes and groves when cores are pasted
    • Pasted or mudded cores are best if dried to avoid the danger of blow defects in the castings.
    • Cores may be bolted together --- strong assembly.
    • Joining ------ by leading (molten lead)
    • Inspection:
    • The cores should be smooth, free of loose sand or projections, mudded if necessary, and dry, to be ready for core-setting operation.
  • 15.
    • Core Setting:
    • Core setting is the operation of placing cores in molds.
    • Correct size ----- positioned properly.
    • Cores are positioned in the mold by core prints.
    • May rise up with molten metal --- securely anchored.
    • Cores may require positive location in three directions, one vertically and two horizontally.
    • Chaplets:
    • These are metal forms placed between mold and core surfaces.
    • These are often used to overcome vertical movement of the core due to buoyancy .
    • Closing the Mold: Some sand falls during core setting – must be removed.