MME 345, Lecture 22 Moulding and Casting Methods 2. Greensand system Ref: [1] Heine, Loper & Rosenthal, Principles of Metal Casting, McGraw-Hill, 1976 [2] Beeley, Foundry Technology, Butterworth-Heinemann, 2001 Topics to discuss today 1. Introduction to greensand moulding aggregates 2. Characteristics of greensand moulding aggregates 3. Ingredients of greensand moulding aggregates 4. Clay-water bonding 5. Testing and control of moulding sand aggregate
Introduction Green moulding sand process has a long history. A large production of small and medium castings is cast in greensand moulds. Uses natural sand as the main ingredient for moulding. Benefits of greensand systems over dry sand or other sand systems: 1. The sand is readily reconditioned 2. Reduced risk of hot tearing in castings 3. Moulds join closely producing little flash 4. Suitable for mechanised systems. 3/27 Recent developments in greensand system: 1. Modernised melting technique to use inexpensive molten metal. 2. Development of sand treatment, its equipment and sand control system. 3. Mechanisation of moulding, shake-out and sand transport. Moulds for making a ton of castings may require 4 to 5 tons of moulding sand aggregate The sand-metal ratio may vary from 10:1 to 0.25:1 depending on the type and size of castings and moulding methods employed In any case, the tonnage of sand which must be handled in a sand casting foundry is large, and its quality must be controlled to make good castings. 4/27
Characteristics of greensand moulding aggregates A foundry moulding mixture passes through four main production stages: (1) preparation and distribution, (2) mould and core production, (3) casting, and (4) cleaning and reclamation. The property requirements of the materials are generally determined by moulding and casting conditions for integrated sand systems, the preparation and reclamation stages are also considered From a general viewpoint, the moulding sand aggregate must be readily mouldable and the mould cavity should retain its shape till the molten metal solidifies to produce a defect-free casting. 5/27 1. At the moulding stage: Flowability ability of the material to be compacted to a uniform density Green strength ability to retain the shape of mould during handling 2. During casting: Thermal stability ability to retain shape at high temperature Refractoriness ability to withstand high temperatures without fusion Dry strength to withstand erosive forces and pressure of liquid metal Hot strength to withstand distortion and deformation during heating at high temperature Collapsibility ease to break down in knockout Permeability a path for the escape of gases Fineness to prevent metal penetration and produce smooth casting surfaces 3. At storage: Bench life ability to retain moulding properties on standing or storage Durability capacity to withstand repeated cycles of heating and cooling (reusability) 6/27
Ingredients of greensand moulding aggregates 1. Sand granular materials of size 0.05 2.0 mm produced from disintegrated or crushed rocks sand denotes a class of several minerals: silica, zircon, olivine, chromite, and ground ceramic minerals 2. Clay added 2 to 50 % of sand as a binder clay are residual or weathered products of various silicate rocks; groups of clay minerals used in foundry include: fireclay (kaolinite), bentonites (montmorillonite), and some special clays (halloysite and illite) plate or flake structures form of particle size range 20 to 0.1 micron. 7/27 3. Moisture as essential as the clay substance itself; bonding strength is developed by the sand-clay-water system about 1.5 to 8 % moisture is added to activate the clay in the sand, causing the aggregate to develop plasticity and strength 4. Other additives different organic or inorganic materials are added to impart certain properties Purposes As binder Increase collapsibility Reduce expansion problems Increase green strength Increase dry strength Increase hot strength Reduce metal penetration Improve surface finish Increase flowability Increase bench life Additives Cereal, molasses, resin, linseed oil, water glass Cereals, molasses, fibrous materials Cereals, coal dust, fibrous materials Coal dust, molasses Cereals, resin, molasses, coal dust Iron oxide, silica flour Coal dust, iron oxide, silica flour Coal dust, resin Resin, fibrous materials Cereals, resin, molasses 8/27
effect of clay the basic function of clay is bonding. forms a thin coating around each grain and produce cohesion between the grains. if the layers become continuous and then progressively thicker, further increase in bonding cannot be achieved. bentonite-bonded silica sand Dry compressive strength developed: Western bentonites : > 80 psi Southern bentonites : 40 80 psi Fire clays (kaolinite) : moderate Fire clay plus : > 200 psi western bentonite 9/27 Clay-saturated sands The most versatile greensand Reduces casting defects due to sand expansion, erosion etc. High strength (GCS = 14 20 psi) Require adequate ramming (> 85 mould hardness) bentonite clay-sand-water mixtures Clay requirement for saturation Depends on purity and type of clay, base sand, and additives 8 12 % bentonites; 20 25 % fire clay (Sand fineness AFS 60 100) Clay-unsaturated sand systems 4 9 % bentonites; 10 15 % fire clays Used for lighter castings where expansion defects, erosion etc. are lesser problems. 10/27
effect of water Temper water and free water. The green strength increases up to a maximum and then decreases. Dry strength continues to increase due to improved distribution of the binder and the higher bulk densities attainable. Too much water causes excessive plasticity and dry strength. Too little water fails to develop adequate strength and plasticity. bentonite-bonded silica sand control of moisture in the moulding sand to develop the best properties is a necessary basis of sand control 11/27 effect of mould hardness effect of clay content on relationship between mould hardness and green compressive strength 12/27
southern bentonite sand mixture kaolinite sand mixture western bentonite sand mixture 13/27 southern bentonite sand mixture western bentonite sand mixture kaolinite sand mixture 14/27
Clay Water Bonding bonding between dry clay does not occur bond is developed only when clay particles are hydrated Several theories proposed for clay-water bonding: 1. electrostatic bonding 2. bonding due to surface tension forces, and 3. bonding due to inter-particle friction 15/27 electrostatic bonding bonding between a network of dipolar forces operating at the sand-clay and clayclay interfaces, initiated by the preferential adsorption of positive ions and negative ions on combined water and clay (hydrated) surfaces when water is added to dry clay, the negative hydroxyl (OH ) ions are adsorbed on the nuclei of the clay atoms, owing to unsatisfied valence bonds at the surface of the clay crystal, and form an integral part of the crystal. so the clay-water particle becomes negatively charged. the positive (H + ) ions of water are attracted by the negative clay ions, but repelled by the nuclei of the clay atoms, with the result that the positive ions take up equilibrium positions. the hydrogen ions and the adsorbed hydroxyl ions about the clay particle comprise a so-called double diffuse layer. This formation is called a micelle. 16/27
clay particle a clay micelle surrounding the clay particle are negatively charged hydroxyl ions positioned at varying distances from the particle. outside this layer, positively charged hydrogen ions also are located at various distances from the clay centre; hence the term double diffuse layer applies. this layer is rigidly attached to the surface of the clay particle and is considered to behave as a solid. 17/27 particles of sand (quartz) also form micelles by the adsorption of hydroxyl and hydrogen ions. when quartz and clay micelles are formed in each other s presence, the hydroxyl ions of the clay micelle exhibit an attraction for the hydrogen ions contained in the quartz micelles. thus a clay dipole is formed and the result is an electrostatic bond between sand and clay particles and between clay particles x hydrated quartz grain excess water (a) Micellular dipoles, showing the localized concentration of adsorbed negatively charged hydroxyl ions and positively charged ions (b) Schematic sketch showing disposition of clay and quartz dipoles. In green sand the intermicellular voids are filled with water. clay micelles 18/27
The maximum attractive force obtained at an optimum distance of intermolecular spacing x. There are many such dipoles in a clay-water medium. Depending on the type of clay, a maximum degree of hydration is necessary to develop a dipole completely. This is why the strengths of clay-bonded sands increase with increasing amounts of water, up to a maximum value. As the amount of water is increased further, water enters the spaces between the dipoles to a distance greater than x, resulting in a decrease in the net intermicellular force. x+z x+z z = excess water 19/27 bonding by surface tension forces bonding resulted due to the surface tension of the water surrounding the clay and clay-sand particles surface layers of water acting on a stretched membrane of hydrated clay, forcing the particles together when the water layer becomes thinner by drying, the forces holding the particles together increase. 20/27
bonding due to inter-particle friction bonding resulted following the principles of block and wedge theory. the inter-particular friction developed between sand particles under pressure particles are jammed against their neighbours during ramming. opposes further deformation, and causes a bridging action between long rows of favourably oriented particles and the sides of the flask. 21/27 Testing and control of moulding sand aggregate Periodic tests are necessary as properties changes during use. Test may be either chemical or mechanical in nature. Various tests are designed and standardised To determine the properties of moulding sand To have uniform properties, control is necessary. Even though control is done, sand properties will change from time to time. To have good control of the sand properties, control values are to be determined. 22/27
sand testing techniques Majority are designed to measure the bulk properties of an aggregate. Sensitive to small variations in mixing conditions and specimen preparation Rigid standardisation is needed at all stages. Some of the most important tests routinely performed in foundry: 1. Loss on ignition 2. AFS sieve analysis 3. AFS clay content 4. Moisture content 5. Mould hardness 6. Compression strength 7. Permeability 8. Flowability 9. Collapsibility 10. Shatter index 11. Surface stability index 23/27 greensand control properties 1. Moisture Content 2. Permeability 3. Green Compressive Strength (GCS) 4. Green Surface Stability Index 5. Flowability 6. Clay Content 7. Loss on Ignition 24/27
control properties and standard control values Properties Steel Cast Iron Copper Aluminium Alloy Moisture content 3.2 3.2 4.0 4.0 Permeability 200 150 20 20 GCS, kg/cm 2 0.8 1.0 0.6 0.5 Green surface stability index 92 88 86 85 Flowability 72 75 75 75 Clay content 8 10 12 14 Loss on ignition 3 4 5 6 25/27 control property and resulting problems Property Too Low Too High Moisture Permeability Strength Surface Stability Index Flowability Low green strength, Bad surface stability, Edge friability Blow holes (pin holes), Misruns, Hard castings, Difficult pattern draw Bad surface stability, Friable mould, Sand inclusion, Mould erosion, Scabbing, Poor pattern strip Edge friability, Sand inclusions, Mould erosion, Scabbing Soft mould, Sand inclusions, Swollen castings, Bad surface stability mould Reduced permeability, Low green strength, Reduced sand flowability, Low mould hardness, Blow holes (pin holes), Swollen castings Rough surface, Metal penetration Bad flowability, Under compaction, Rough casting surface Clay Content (Active Clay) Low strength, Bad surface stability, Friable mould, Sand inclusions, Erosion defect, Scabbings Loss on ignition Poor surface finish, Scabbing Blow or pin-holes Reduced permeability, Knock-out problems, Increase of moisture content, Cracked castings 26/27
Next Class MME 345, Lecture 23 General Methods of Moulding Casting 2. Permanent mould casting and die casting