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

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






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

      • 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 .
      • Core may be made of;
      • - metal
      • - plaster
      • - ceramic materials
      • - core sand
      • Cores must be collapsible
      • Metal cores ---- shape limitations
      • Sand cores ------ most frequently used
      • 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.
    • Cores
      • 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
      • CORE BAKING:
      • 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.
      • Core-baking Equipment:
      • A. Core ovens
      • 1. Batch-type
      • 2. Continuous
      • a. Horizontal
      • b. Vertical
      • B. Dielectric bakers
      • C. Radiant bakers
      • 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
      • 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
      • Finishing work may be classified as follows;
      • 1. Cleaning
      • 2. Sizing
      • 3. Core assembly
      • 4. Inspection
      • 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
      • 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.
      • 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.
      • 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.