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 coresand forms. Intricate shapes ii) May be used to form a part of a greensand 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) Ramup 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.
CORE MAKING • 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. • Coremaking manually or with machines • Cores with no flat surfaces must be supported on a core drier, until they are baked. • Core venting • Reinforcing wires
• 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 coreoil bonded core assumes a nutbrown color, darkness indicating overbaking and lightness underbaking.
Corebaking Equipment: A. Core ovens 1. Batchtype 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 nonconducting 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 alternatingcurrent 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 OF CORES: 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 coresetting 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.