Mold and Core Making Essentials

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1. Introduction to Mold and Core Making Mold making materials can be metallic or non-metallic. Non-metallic options include molding sands, plaster of Paris, graphite, silicon carbide, and ceramics. Molding sand is the most common due to its refractoriness, chemical/thermal stability, high permeability, workability, strength, availability, and low cost. Common Materials: Metallic: Cast iron, mild steel, alloy steels. Non-metallic: Molding sands, plaster of Paris, graphite, silicon carbide, ceramics. Advantages of Molding Sand: Refractoriness, chemical and thermal stability, high permeability, workability, good strength, cheap, easily available. 2. Molding Sand Sources: Sea beds, rivers, lakes, granular rock elements, deserts. Common Indian Sources: Batala sand (Punjab) Ganges sand (Uttar Pradesh) Oyaria sand (Bihar) Damodar and Barakar sands (Bengal-Bihar Border) Londha sand (Bombay) Gigatamannu sand (Andhra Pradesh) Avadi and Veeriyambakam sand (Madras) Types of Molding Sand: Natural: Contains sufficient binder. Synthetic: Artificially prepared (silica sand 88-92%, binder 6-12%, water 3-6%, additives). 3. Constituents of Molding Sand 3.1 Silica Sand Main constituent for refractoriness, strength, stability, and permeability. Impurities like iron oxide, alumina, limestone, magnesia, soda, and potash can lower the fusion point. Chemical Composition: Primarily granular quartz, with small amounts of impurities. Classification: Based on size (small, medium, large) and shape (angular, sub-angular, rounded). 3.1.1 Effect of Grain Shape and Size: Angular Grains: Large surface area, require more binder and moisture, lower permeability. Rounded Grains: Preferred for good molding, smooth surface, higher sinter point, better permeability and plasticity, higher bond strength, better compactability. Sub-angular Grains: Intermediate characteristics. Compound Grains: Cemented together, least desirable, require more binder/moisture, tend to disintegrate at high temperatures. Grain Size Distribution: Wide distribution: Improves compactability and green strength. Narrow distribution: Reduces green strength. Finer grains: Lower compactability, reduced green strength (thinner bentonite film). Coarser grains: Lower green strength (fewer contact points), higher void space, greater permeability. 3.2 Binder Can be inorganic or organic. Clay (Kaolinite, Ball Clay, Fire Clay, Limonite, Fuller's earth, Bentonite) is common inorganic binder. Organic binders (dextrin, molasses, cereal binders, linseed oil, resins like phenol formaldehyde, urea formaldehyde) mostly used for core making. Bentonite is most common clay binder, but requires moisture. 3.3 Moisture Typically 2-8% of molding sand. Fills pores between clay particles, held rigidly by clay, responsible for strength. Decreases permeability with increasing clay and moisture content. 3.4 Additives Materials added to enhance specific properties of molding/core sands: Coal dust: Produces reducing atmosphere, prevents metal oxidation, used for grey iron/malleable cast iron. Corn flour: Carbohydrate, increases collapsibility, volatilizes leaving space for sand grains, reduces mold expansion, improves strength. Dextrin: Carbohydrate, increases dry strength. Sea coal: Fine powdered bituminous coal, forms coke when heated, fills pores, restricts sand grain movement, reduces mold wall movement, reduces permeability, makes surface clean/smooth. Pitch: Distilled soft coal, enhances hot strength, surface finish, similar to sea coal. Wood flour: Fibrous material, prevents sand grains from contacting, volatilizes allowing expansion, increases mold wall movement, reduces expansion defects, increases collapsibility. Silica flour: Pulverized silica, increases hot strength, surface finish, reduces metal penetration. 4. Kinds of Molding Sand 4.1 Green sand Tempered/natural sand (silica sand, 18-30% clay, 6-8% moisture). Fine, soft, light, porous. Damp, retains shape, low cost. Used for ferrous/non-ferrous castings. 4.2 Dry sand Green sand dried/baked. More strength, rigidity, thermal stability. Suitable for larger castings. 4.3 Loam sand Mixture of sand, clay (30-50%), water (18%) to form thin paste. Patterns not used; shape given by sweeps. Used for large grey iron castings. 4.4 Facing sand Forms face of mold, direct contact with molten metal. Requires high strength and refractoriness. Made of silica sand and clay without used sand. Carbon forms prevent metal burning into sand. Layer 22-28mm, 10-15% of total molding sand. 4.5 Backing sand (Floor sand) Backs up facing sand, fills molding flask. Often used molding sand, sometimes called black sand due to coal dust. 4.6 System sand Used in mechanized foundries to fill entire flask, no facing sand. Cleaned, re-activated with water/additives. Requires higher strength, permeability, refractoriness. 4.7 Parting sand Without binder/moisture. Prevents green sand sticking to pattern, allows cope/drag separation. Clean, clay-free silica sand. 4.8 Core sand Used for making cores (oil sand). High silica sand mixed with oil binders (linseed oil, resin, mineral oil). Pitch or flours with water for large cores. 5. Properties of Molding Sand 5.1 Refractoriness Ability to withstand high temperatures without breaking down or fusing. Depends on $\text{SiO}_2$ content, grain shape, and size. Measured by sinter point. 5.2 Permeability (Porosity) Allows escape of air, gases, moisture during pouring. Function of grain size, shape, moisture, clay content, ramming. Increased by venting. 5.3 Cohesiveness Property for sand grains to interact and attract, enhancing binding capability, green, dry, and hot strength. 5.4 Green strength Strength to permit molding and handling. Requires adhesive (attaching to other body) and cohesive (sticking to each other) sand grains. Depends on grain shape, size, clay type/amount, moisture content. 5.5 Dry strength Strength after moisture evaporation to prevent erosion and enlargement of mold cavity by metallostatic pressure. 5.6 Flowability (Plasticity) Ability to get compacted and behave like a fluid, flow uniformly. Increases with decreasing green strength and grain size. Varies with moisture and clay content. 5.7 Adhesiveness Property to stick or adhere to foreign material (e.g., inner wall of molding box). 5.8 Collapsibility Mold collapses after solidification, allowing free contraction of metal, preventing tearing/cracking. Highly desired in cores. 5.9 Miscellaneous properties Should not stick to casting, not chemically react, cheap, available, reusable, low coefficient of expansion. 6. Sand Testing Tests to judge molding and casting characteristics: Moisture content Test Clay content Test Chemical composition of sand Grain shape and surface texture of sand Grain size distribution of sand Specific surface of sand grains Water absorption capacity of sand Refractoriness of sand Strength Test Permeability Test Flowability Test Shatter index Test Mould hardness Test 6.1 Moisture Content Test Determined by drying sample and weighing, or by speedy moisture teller (reacts water with calcium carbide, forms acetylene gas). Electrical conductivity can also indicate moisture. 6.2 Clay Content Test Clay in 50g molding sand defined as particles failing to settle at 1 inch/min in water (less than 20 micron). 6.3 Grain Fineness Test Sample of dry silica sand sieved through a series of sieves (U.S. Bureau of standard meshes 6 to 270) on a mechanical shaker. Grain Fineness Number (GFN) is calculated. 6.4 Refractoriness Test A.F.S. standard sand specimen heated to high temperatures (1000°C to 1300°C+, in steps). Cooled, examined under microscope or scratched with steel needle to check for sintering. 6.5 Strength Test Green and dry strength depend on bonding materials. Compression strength test (using compression sand testing machine) is common. Tensile, shear, transverse tests also performed. Sample preparation using rammer. Hydraulic press used for strength tester. The image below shows a Strength Testing Machine: Peep Hole Molding Sand Specimen Lugs Hand Wheel High Pressure Manometer Low Pressure Manometer Adjusting Cock Fig. 12.2 Strength testing machine 6.6 Permeability Test Measures porosity. Standard cylindrical sand specimen (50.8mm diameter, 50.8mm height) tested in a permeability meter. Permeability ($P$) calculated as: $P = \frac{vh}{pat}$, where $v$ is air volume, $h$ is specimen height, $p$ is pressure, $a$ is cross-sectional area, $t$ is time. A.F.S. standard permeability meter uses 2000 cc air, 5.08 cm height, 20.268 cm$^2$ area, 10 seconds, 10 gms/cm$^2$ pressure, yielding $P = 300.66$ App. The image below shows a Permeability Meter: Balanced tank Water tank Nozzle adjusting lever Nose piece for fixing sand specimen tube Dial meter Balanced tank Specimen tube Molding sand sample Pressure manometer Air passage Variable nozzle Mercury seal Fig. 12.3 Permeability meter 6.7 Flowability Test Determined by rammer plunger movement between fourth and fifth drops. Records actual movement of plunger. 6.8 Shatter Index Test A.F.S. standard sand specimen rammed by 10 blows, dropped on a half-inch mesh sieve from 6 ft. Weight of retained sand expressed as percentage of total weight. 6.9 Mould Hardness Test Based on Brinell hardness. Half-inch diameter steel hemi-spherical ball loaded with 980 gm spring force. Penetration into mold surface indicates hardness (0-100 scale). The image below shows a Mould Hardness Tester: Plastic Sleeve Metallic Sleeve Needle Dial Ring Tip Fig. 12.4 Mould hardness tester 7. Sand Conditioning Necessary for natural sands due to thinning of clay coating and strength decrease with continuous use. Achieves uniform binder distribution, moisture control, foreign particle elimination, and aeration. Foreign Material Removal: Nails, gaggers, hard sand lumps, metals removed by magnetic separators and riddles. Mixing: Sand constituents mixed thoroughly in a muller to distribute binders, additives, and moisture uniformly. 8. Steps in Making a Sand Mold Select molding box size. Place drag portion of pattern on ram-up board. Sprinkle facing sand around pattern. Fill drag with loose molding sand, ram uniformly, repeat. Remove excess sand with strike off bar. Roll drag over, sprinkle parting sand. Place cope pattern on drag pattern, align with dowel pins. Place cope flask over rammed drag, sprinkle parting sand. Place sprue and riser pins at suitable locations. Set gaggers in cope if needed. Strike off excess sand from cope. Remove sprue and riser pins, create vent holes. Sprinkle parting sand over cope, roll cope over on bottom board. Rap and remove cope and drag patterns, repair mold. Cut gate connecting sprue basin to runner and mold cavity. Apply mold coating, bake if dry sand mold. Set cores, close mold by inverting cope over drag. Clamp cope and drag; mold ready for pouring. 9. Venting of Molds Very small pin-type holes made in cope using a vent wire. Allows escape of gases (evaporation of water, steam, decomposition of organic materials, air expansion). Prevents casting defects such as trapped gases, back pressure, mold bursts. Many small vents are better than few large ones. The image below shows Venting of Holes in Mold: Riser Sprue Vent holes Cope Parting line Mold cavity Drag Gate Vents made in the mold Fig. 12.8 Venting of holes in mold 10. Gating System in Mold Elements that feed molten metal into the mold cavity. The image below shows a Gating System: Pouring basin Sprue Runner Gate Casting Riser Sprue base Runner extension Fig. 12.9 Gating System 1. Pouring basin: Conical hollow element, feeds molten metal, maintains flow rate, reduces turbulence, separates dross. 2. Sprue: Vertical passage, tapered, connects pouring basin to runner, feeds molten metal without turbulence. May have skim bob to collect impurities. 3. Gate: Small passage connecting runner to mold cavity, feeds liquid metal. 4. Choke: Smallest cross-section area in gating system. Gate or sprue can act as choke. 5. Runner: Channel connecting sprue to gate, avoids turbulence and gas entrapment. 6. Riser: Passage in cope, molten metal rises to compensate shrinkage, permits gas escape, promotes directional solidification. 7. Chaplets: Metal distance pieces (made of parent metal) prevent shifting of mold/core, support core. Melt and fuse with molten metal. 8. Chills: Metallic materials that rapidly extract heat from specific areas, creating hard surfaces (e.g., in Fig. 12.12). 11. Factors Controlling Gating Design Avoid sharp corners/abrupt changes to suppress turbulence. Proper relationship between cross-sectional areas. Shape, location, dimensions of runners, type of flow, position of metal entry. Gating ratio: total cross-section of sprue, runner, gate decreases towards mold cavity (choke effect). Keep runner away from mold cavity. Streamline gating system, use generous radii, taper sprue, provide basin instead of pouring cup. 12. Role of Riser in Sand Casting Compensates for metal shrinkage during solidification, feeds molten metal, permits air/gas escape, indicates mold fill, promotes directional solidification. 12.12.1 Considerations for Designing Riser: A) Freezing time: Molten metal in riser must freeze slower than casting for sound casting. B) Feeding range: For large/complicated castings, multiple risers. Divide casting into zones, each with a separate riser. Attach risers to heavy sections that solidify last. Maintain proper temperature gradients. C) Feed Volume Capacity: Sufficient volume to feed mold cavity. Metal in risers kept molten using exothermic compounds or electric arc feeding. Volume capacity based on freezing time and demand. 12.12.2 Effect of Riser: Riser size affects heat loss. Cylindrical risers are generally better due to longer solidification time. Insulation and shielding improve sound casting. 13. Green Sand Molding Most widely used. Mixture of silica, water, additives (10-15% clay, 4-6% water, rest silica sand). Mold prepared and poured while still green (not dried/baked). Advantages: Adaptable to machine molding. No mold baking/drying required. Less mold distortion. 14. Core Compact mass of core sand, prepared separately. Placed in mold cavity to create hollowness (internal cavities). Must withstand hot metal. Functions/Objectives: Produce hollowness. Sufficiently permeable for gas escape. Can form part of green sand mold. Improve mold surface. Provide external undercut features. Achieve deep recesses. Strengthen mold. Form gating system of large mold. 15. Core Sand Special molding sand for cores. Requires high refractoriness, high permeability (more than molding sands), minimal gas production, collapsibility. Main Constituents: Pure silica sand (high refractoriness) and binder. Coarse grain size for higher permeability. Binders: Hold grains, impart strength, collapsibility. Organic binders preferred (combustible, give collapsibility). Common Binders: Cereal binder: Green strength, baked strength, collapsibility (0.2-2.2% by weight). Protein binder: Increases collapsibility. Thermo setting resin: High strength, collapsibility, minimum gas (phenol formaldehyde, urea formaldehyde). Sulphite binder: Used with clay. Dextrin: Increases collapsibility, baked strength. Pitch: Increases hot strength. Molasses: Secondary binder, increases hardness on baking (sprayed liquid). Core oil: Liquid, forms coherent film when baked, holds sand grains. 16. Core Making Four stages: core sand preparation, core making, core baking, core finishing. 16.1 Core Sand Preparation Mechanical mixing (roller mills, vertical revolving arm, horizontal paddle mechanisms) for uniform, homogenous core sand with proper properties. 16.2 Core Making Process Using Core Making Machines 16.2.1 Core blowing machines: Fill core sand into core box using compressed air. High velocity deposits sand in remote corners. Classifications: small bench blowers, large floor blowers. 16.2.2 Core ramming machines: Cores prepared by ramming sand in core boxes using squeezing, jolting, and slinging principles. 16.2.3 Core drawing machines: Used for deep draw core boxes. Rapping action by vibrating vertical plate helps draw core from core box. 16.3 Core Baking Baking in ovens/furnaces drives away moisture, hardens binder, gives strength. Types: continuous (mass production, small cores), batch (variety of cores). Dielectric bakers (faster, good temperature control, no supporting plates). 16.4 Core Finishing After baking, remove fins/bumps, dimensional inspection. Coat with refractory/protective materials to prevent metal penetration. Use bars, wires, arbors to reinforce bulky cores; lifting rings for handling. 17. Green Sand Cores Made from green sand (moist, 5% water, 15-30% clay). Good permeability, avoids shrinkage/voids. Not dried. Preferred for simple, small, medium castings. Less strength, cannot be stored long. 18. Dry Sand Cores Produced by drying green sand cores to 110°C. High strength, rigidity, thermal stability. Can be stored long. Used for large castings. Good surface finish. Resist metal erosion. Requires more floor space, material, labor, equipment. 19. Classification of Molding Processes Classified by method or mold material. 19.1 Classification based on the method used: Bench molding Floor molding Pit molding Machine molding 19.2 Classification based on the mold material used: Sand molding: Green sand, dry sand, skin dried, core sand, loam, cement bonded, carbon-dioxide, shell. Plaster molding Metallic molding Loam molding 20. Molding Methods 20.1 Bench Molding For small jobs. Uses cope and drag flasks. Performed on a bench. 20.2 Floor Molding For medium/large jobs. Uses only drag portion of flask, floor as drag. Performed with dry sand. 20.3 Pit Molding For large castings, made in pits. Sand rammed by bedding-in process. Walls/bottom reinforced with concrete, coke layer for gas escape. One box for mold completion. 20.4 Machine Molding For mass production. Economical, accurate, uniform castings. Machines ram sand, roll over mold, form gate, rap pattern, withdraw pattern. 20.5 Loam Molding Uses loam sand (molding sand + clay paste). Patterns avoided; rough structure built with bricks/loam sand, shaped by strickles/sweeps. Baked for strength. Used for large castings, saves pattern cost, but requires skill and time. 20.6 Carbon-Dioxide Gas Molding Rapid hardening process. Mold material: pure dry silica sand, 3-5% sodium silicate binder, The image below shows Carbon Dioxide Molding: CO2 gas Mould Plyboard or hard board cover Rubber strip CO2 gas Mould Head Hardened face Hardened face (a) Using Muffle Board (b) Using Shower Curtain CO2 gas Pattern Probe CO2 gas Hollow pattern (c) Using a Simple tube to provide entry to gas (d) Using a Hollow Pattern Fig. 12.14 Carbon dioxide molding 20.7 Shell Molding Mass production of accurate, thin castings with smooth finish. Uses thermosetting plastic dry powder and fine sand (1:20 ratio). Pattern heated (205-230°C), covered with resin-bonded sand for 30s, forms hard layer. Baked (315°C for 60s), stripped from pattern. Shells joined to form mold cavity. Cores also made. Thin walled articles with close tolerance ($\pm 0.002$ mm). Shells 0.3-0.6 mm thick. Joined by clamps/adhesive. Supported by bulky permeable material. Advantages: High suitability for thin sections (e.g., petrol engine cylinder). Excellent surface finish. Good dimensional accuracy (0.002 to 0.003 mm). Negligible machining and cleaning cost. Occupies less floor space. Less skill required. Molds can be stored. Better casting quality. Mass production. Greater detail, less draft. Unskilled labor. Disadvantages: Higher pattern cost. Higher resin cost. Not economical for small runs. Dust-extraction problem. Complicated jobs/various sizes cannot be easily shell molded. Specialized equipment required. Expensive resin binder. Limited for small sizes. 20.8 Plaster Molding Mold material: gypsum/plaster of Paris with additives (talc, fibers, asbestos, silica flour) to control contraction/settling time. Slurry (130-180 pounds water/100 pounds plaster) poured over metallic pattern in flask. Vibrated to remove air. Plaster sets, pattern withdrawn. Mold dried in oven (200-700°C). Two halves joined. Sprue, runner cut. Advantages: Very good surface finish, reduced machining cost. Slow, uniform cooling, reduced stress concentration. Accurate control of metal shrinkage, avoids warping/distortion. Limitations: Steam evolution if not dried (dehydrated at high temperatures, but strength decreases). Low permeability (can be increased by removing bubbles). 20.8.1 Antioch Process Special plaster molding for high-grade aluminum castings. After plaster sets, autoclaved in saturated steam (20 psi). Dried in air (10-12 hours), then oven (250°C for 10-20 hours). Creates granular structure, increases permeability. 20.9 Metallic Molding (Permanent Mold/Gravity Die Casting) Long-life molds made of dense, fine-grained, heat-resistant cast iron, steel, bronze, anodized aluminum, graphite. Two halves for easy casting removal. Metal introduced under gravity. Pressure die casting when metal introduced under pressure.