Mold mixtures - Nguyen Ngoc Ha

through transportation and storage • Uniformly fills flask or container • Can be compacted or set by simple methods • Sufficient elasticity to remain undamaged during pattern removal • Can withstand high temperatures and maintain dimensional stability until solidification • Sufficiently permeable to allow gases to escape 1.1. Desirable Properties of Sand-Based Molding Materials • Sufficiently dense to prevent metal penetration • Sufficiently cohesive to prevent wash-out of mold material into the pour stream • Chemically inert to the metal being cast • Can yield to solidification and thermal shrinkage, preventing hot tears and cracks • Recyclable 1.2. Desirable Mold Properties • Strength - to maintain shape and resist erosion • Permeability - to allow hot air and gases to pass through voids in sand • Thermal stability - to resist cracking on contact with molten metal • Collapsibility - ability to give way and allow casting to shrink without cracking the casting • Reusability - can sand from broken mold be reused to make other molds? 2. MOLD SAND 2.1. Introdution • 80-98% mass of mold mixture • Grain size: 0,016-2mm • The most important properties: - Chemical composition - Size and shape of grains - Melting temperature - Thermal expansion - Bulk density 2.2. Selection of sand • Most sand casting operations use silica sand (SiO2), because it is inexpensive and is suitable as mold material because of its resistance to high temperatures. There are two general types of sand: naturally bonded and synthetic sand. Because its composition can be controlled more accurately most foundries prefer synthetic sand. 2.2. Selection of sand • Several factors are important in the selection of sand for sand molds. Sand having fine, rounded grains can be closely packed and forms a smooth mold surface. Good permeability of molds and cores allows gases and steam evolved during casting to escape easily. • The selection of sand involves certain tradeoffs with respect to properties. For example, fine-grained sand enhances mold strength, but the fine grains also lower mold permeability. 2.2. Selection of sand • Sand is typically conditioned before use. Mulling machines are used to uniformly mull (mix thoroughly) sand with additives. For example clay 9bentonite) is used as a cohesive agent to bond sand particles, giving the sand strength. • Zircon (ZrSiO4), olivine (Mg2SiO4), and iron silicate (Fe2SiO4) sands are often used in steel foundries for their low thermal expansion. Chromate (FeCr2O4) is used for its high heat transfer property. 2.3. Sand Parameters • Grain Size – measured by sifting sand through sieves • Moisture Content – measured with moisture meter • Clay Content – measured by weighing a sample of sand before / after washing • Permeability – AFS permeability number measured using “standard rammed sample” • (Green) Compressive Strength – measure of mold strength before pouring • Hardness – resistance of packed sand to penetration Grain Shapes of Sands Effect of grain size and shape on mould quality Bigger grain size results in a worse surface finish Irregular grain shapes produce stronger mold Larger grain size ensures better permeability Effect of moisture, grain size and shape on mould quality 2.4. Sand Properties and related Defects • The characteristics of the sand granules can be very influential in determining the properties of the molding material. Round grains give good permeability and minimize the amount of clay required because of their low surface area. • Angular sands give better green strength because of the mechanical interlocking of the grains. 2.4. Sand Properties and related Defects • Large grains provide good permeability and better resistance to high temperature melting and expansion, while fine-grained sands produce a better surface finish on the final casting. • Uniform-size sands give good permeability, while a wide distribution of sizes enhances surface finish. 2.4. Sand Properties and related Defects • When hot metal is poured into a silica sand mold, the sand becomes hot, undergoes one or more phase transformations, and has a substantial expansion in volume. • Because sand is a poor' thermal conductor, only the sand that is adjacent to the mold cavity becomes hot and expands. 2.4. Sand Properties and related Defects • The remaining material stays fairly cool, does not expand, and provides a high degree of mechanical restraint. Because of this uneven heating, the sand at the surface of the mold cavity may buckle or fold. • Castings having large, flat surfaces are more prone to sand expansion defects since a considerable amount of expansion must occur in a single, fixed direction. 2.4. Sand Properties and related Defects • Sand expansion defects can be minimized in a number of ways. Certain sand geometries permit the grains to slide over one another, thereby relieving the expansion stresses. • Excess clay can be added to absorb the sand expansion, or volatile additives, such as cellulose, can be added to the mix. • As the sand becomes hot, the cellulose bums, creating voids that can accommodate the sand expansion. 2.4. Sand Properties and related Defects • Castings can also contain voids that form where the molten metal is held back by trapped or evolved gas. • These are usually attributed to low sand permeability and/or large amounts of gas evolution caused by high moisture or excessive amounts of volatiles. • If adjustments to the mold composition are not sufficient to eliminate the voids, vent passages may have to be cut, a procedure that adds significantly to the mold-making cost. 2.4.Sand Properties and related Defects • The molten metal can also penetrate between the sand grains, causing the mold material to become embedded in the surface of the casting. • Penetration can be the result of high pouring temperatures (excess fluidity), high metal pressure, or the use of high-permeability sands with coarse, uniform particles. • Fine-grained materials, such as silica flour, can be used to fill the voids, but this reduces permeability and increases the likelihood of gas and expansion defects. 2.5. Sand Quality Tests • Periodic tests are necessary to determine the essential qualities of foundry sand. Various tests are designed to determine the following properties of molding sand. a) Hardness Test (Mold Hardness): A spring loaded (2.3 N) steel ball 5.08 mm in diameter is pressed into the surface of the mold and depth of penetration is recorded as hardness. Medium hardness is about 75. 2.5. Sand Quality Tests b) Fineness Test: It is used to obtain percentage distribution of grain sizes in the sand. Sand is cleaned and dried to remove clay. It is placed on graded sieves, which are located on a shaker. Standard sieve sizes (mesh) are 6,12,20,30,40,50,70,100, 200 and 270. Shaking time is 15 minutes vibrator 6 12 270 2.5. Sand Quality Tests c) Moisture Content: Measure the weight of the given sand sample. Dry it around 1000C and then weigh it again. Calculate the percentage. d) Clay Content: A sample of sand is dried and then weighed. Then clay is removed by washing the sand with caustic soda which has absorbed the clay. Sand is dried and weighed again. The percentage gives the clay content. e) Strength Test: Most common compressive test. A universal strength tester loads a 50 mm long 50 mm diameter specimen by means of dead weight pendulum with a uniform loading rate. 2.5. Sand Quality Tests f) Permeability: It is measured by the quantity of air that passes through a given sample of sand in a prescribed time under standard pressures. g) Refractoriness Test: High temperature withstanding ability of sand is measured. piston 2.6. Types of mold sand • Silica (SiO2) • Zircon (ZrSiO4) • Olivine (Mg2SiO4) • Iron silicate (Fe2SiO4) • Chromate (FeCr2O4) a. Silica Sand • Silica: - SiO2 - = 2,5-2,8 kg/dm 3 - Tmelting  1680 – 1713 0C - Color: grey, yellow, black • Felspat: - MeO.Al2O3.6SiO2 (Me: K,Na) -Tmelting=1170-1550 0C • Mica: - K2O.3Al2O3.6SiO2.H2O - Tmelting= 1150-1400 0C a. Silica Sand • Ferrous Oxides: hematite (Fe2O3), magnetite (FeO.Fe2O3), ilmenite (FeO.TiO2) • Ferrous hydroxides • Cacbonates: - MgCO3, CaCO3 - Tm= 500-900 0C • NaCl, KCl • Clay 2.6. Types of mold sand b. Zircon Sand • ZrSiO4 • High Tmelting: 2400C • High Density: 4.7 • Low thermal expansion • High cost c. Olivin Sand • Mg2SiO4 • Tmelting= 1750-1830 0C • Low thermal expansion 3. BINDERS USED WITH FOUNDRY SAND • Sand is held together by a mixture of water and bonding clay – Typical mix: 90% sand, 3% water, and 7% clay • Other bonding agents also used in sand molds: – Organic resins (e g , phenolic resins) – Inorganic binders (e g , sodium silicate and phosphate) 3.1. Mold Clay a. Bentonite Clay • The main mineral: Montmorillonite (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O • Chemical composition: (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6.nH2O • High swelling in water • Sodium bentonite; Calcium bentonit • Bentonite has been widely used as a foundry- sand bond in iron and steel foundries. Sodium bentonite is most commonly used for large castings that use dry molds, while calcium bentonite is more commonly used for smaller castings that use "green" or wet molds b. Kaolinite Clay • The main mineral: Kaolinite • Chemical composition: Al2O3.2SiO2.2H2O • Kaolinite has a low shrink–swell capacity and a low cation-exchange capacity (1–15 meq/100 g) 3.2. Water Glass • Sodium silicate is the common name for compounds with the formula Na2(SiO2)nO. A well known member of this series is sodium metasilicate, Na2SiO3. • Also known as waterglass or liquid glass, these materials are available in aqueous solution and in solid form. The pure compositions are colourless or white, but commercial samples are often greenish or blue owing to the presence of iron- containing impurities.