5.moulding sand

moulding sand

1. Moulding Sand

2. TYPES OF BASE SAND The primary and basic material used for preparing moulds is sand , due to its high refractoriness . Sand usually referred to as 'base sand‘ Nearly 90 - 95 % of the total moulding sand is occupied by sand and the remaining is binder and additives . Basic types of base sand are given below Silica Sand Chromite sand Zircon Olivine sand 7/4/2014 2

3. I. Silica Sand Silica sand is essentially silicon dioxide (Si0 2 ) found in nature on the bottoms and banks of rivers, lakes and seashore . Silica deposits tend to have varying degree of organic and contaminants like limestone, magnesia, soda and potash that must be removed prior to its use, otherwise which affects castings in numerous ways . Silica sand is available in plenty , less expensive and possess favorable properties. Thermal expansion leads to certain casting defects; the reason for which not being used in steel foundries . However, silica sand when mixed with certain additives like wood flour, (corn flour), saw dust etc., defects can be eliminated . These additives burn by the heat of the molten metal thereby creating voids that can accommodate the sand expansion. 7/4/2014 3

4. II. Olivine sand Olivine sand is typically used in non-ferrous foundries . With its thermal expansion about half of that of silica sand , makes it suitable for production steel castings . But the high cost restricts its wide use III. Chromite sand This is African sand with cost being much higher compared to other sands. Due to its superior thermal characteristics , it is generally used in steel foundries for both mould and core making. IV. Zircon or Zirconium silicate This sand possesses most stable thermal properties of all the above sands. The choice for this type of sand arises when very high temperatures are encountered and refractoriness becomes a consideration. But the major disadvantage is that, zircon has trace elements of Uranium and Thorium which is hazardous in nature thereby restricting its use in foundries. 7/4/2014 4

5. PROPERTIES OF MOLDING SANDS The very important characteristic of a molding sand is that it should produce sound castings . For doing so, the molding sand should possess certain desirable properties and they are: Flowability Green Strength Dry Strength Hot Strength Permeability or Porousness Refractoriness Adhesiveness Collapsibility Fineness Bench Life Coefficient of expansion Durability 7/4/2014 5

6. Flowability Flowability is the ability of the molding sand to get compacted to a uniform density. Flowability assists molding sand to flow and pack all-around the pattern and take up the required shape. Flowability increases as clay and water contents increase. 2. Green Strength It is the strength of the sand in the green or moist state A mold having adequate green strength will retain its shape, Will not distort, Will not collapse, even after the pattern has been removed from the molding box. Green strength helps in making and handling the molds. 3. Dry Strength It is the strength of the molding sand in the dry condition. A mold may either intentionally be dried or a green sand mold may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured. The sand (of molding cavity) thus dried must have (dry) strength to withstand erosive forces due to molten metal, withstand pressure of molten metal, and retain its shape. There should be an optimum balance between dry strength, and collapsibility of the molding sand. 7/4/2014 6

7. 4. Hot Strength It is the strength of the sand (of mold cavity) above 212°F. In the absence of adequate hot strength, the mold may Enlarge, break, erode or, get cracked. 5. Permeability or Porousness The moisture, binders (organic compounds) and additives present in mould sand core produce steam and other gases. Though much of these gases escape through vents and open feeder heads, yet a good amount of the same tends to pass off through the pore spaces of the molding sand. Thus to provide a path for free escape of the gases, the molding sand should be permeable or porous. Sands which are coarse (Bigger in size) or have rounded grains exhibit more permeability . Soft ramming and clay addition in lesser amounts also improves permeability . In the absence of adequate permeability , defects like surface blows , gas holes , mold blast etc. may be experienced. 6. Refractoriness It is the ability of molding sand to withstand high temperatures (experienced during pouring) without fusion, Cracking Buckling experiencing any major physical change. As compared to castings of low melting point alloys, refractoriness is much more essential in the production of high melting point alloy castings (e.g. steel etc.). 7/4/2014 7

8. 7. Collapsibility Collapsibility is that property of the molding sand which determines the readiness with which the molding sand or mold, automatically gets collapsed after the casting solidifies , and breaks down in knock out and cleaning operations . If the mold or core does not collapse , it may restrict free contraction of the solidifying metal and cause the same to tear or crack . 8. Fineness Finer sand molds resist metal penetration and produce smooth casting surfaces . Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results. Fineness and permeability, both the properties of the molding sand can be maintained by using mold coating on highly permeable mold cavity walls . 9. Bench Life It is the ability of the molding sand to retain its properties during storage or while standing (i.e., in case of any delay). 7/4/2014 8

9. 10. Coefficient of expansion Molding sands should possess low coefficient of expansion . 11. Durability The molding sand should possess the capacity to withstand repeated cycles of heating and cooling during casting operations. Molding sand should be chemically immune to molten metals. Molding sand should be reusable . 7/4/2014 9

10. TYPES OF SAND MOULDS Moulds prepared with sand are called 'sand moulds' or 'temporary moulds', as they are broken for removing the casting. The different types of sand moulds are: Green sand mould Dry sand mould and No-bake sand mould 1. Green sand mould The word 'green‘ signifies that the moulding sand is in the moist state at the time of metal pouring. The main ingredients of green sand are silica sand, clay and moisture (water). Additives may be added in small amounts to obtain desired properties of mould/casting. Nearly 60 % of the total castings are prepared from green sand moulds. 7/4/2014 10

11. Advantages of green sand moulds Preferred for simple, small and medium size castings. Suitable for mass production Least expensive Sand can be reused many times after reconditioning with clay and moisture Disadvantages Moulds/cores prepared by this process lack in permeability, strength and stability. They give rise to many defects like porosity, blow holes etc., because of low permeability and lot of steam formation due to their moisture content. Moulds/cores cannot be stored for appreciable length of time. Not suitable for very large size castings. Surface finish and dimensional accuracy of castings produced are not satisfactory. Difficult to cast thin and intricate shapes. Mould erosion which is common in green sand moulds is another disadvantage. 7/4/2014 11

12. 2. Dry sand mould The word 'dry' signifies that the mould is dry or free from moisture at the time of metal pouring . The absence of moisture makes dry sand moulds to overcome most of the disadvantages of green sand moulds. A dry sand mould is prepared in the same manner as that of green sand mould, i.e., by mixing silica sand, clay and water. The entire mould/core is dried (baked ) in ovens to remove the moisture present in them. Baking hardens the binder thereby increasing the strength of moulds/cores. The temperature and duration of baking ranges from 200 - 450°F and from a few minutes to hours respectively depending on the type of metal being poured and size of the casting. 7/4/2014 12

13. Advantages Strength and stability of moulds is high when compared to green sand moulds. Baking removes moisture and hence, defects related to moisture are eliminated . Better surface finish and dimensional tolerance of castings. Disadvantages Consumes more time, labor and cost due to baking process. Hence, not suitable for mass production . Not suitable for large and heavy size castings , as they are difficult to bake. Capital cost of bake ovens . Under baked or over baked moulds/cores is another disadvantage. 2. Dry sand mould 7/4/2014 13

14. 3. No-Bake sand moulds A no-bake or self-setting sand mould is one that does not require baking. The main ingredients of no-bake sand are silica sand, binder (resin type), hardener and a catalyst or accelerator (if necessary). The bonding strength developed in moulds/cores is by means of a self-setting chemical reaction between the binder and the hardener. In some cases, a catalyst or an accelerator is added to speed up the chemical reaction. 7/4/2014 14

15. Advantages Higher strength - about 50 to 100 times that of green sand moulds. Patterns can be stripped within a few minutes after ramming which is not possible in both green and dry sand moulds. Moulds/cores can be stored for longer periods . Highly simplified moulding. Hence, reduced need for skilled labour. Better dimensional accuracy and stability . Improved casting quality with increased freedom from defects . Surface finish is excellent . In many cases, castings can be used in as-cast condition without machining. Disadvantages Use of resins and catalysts causes lot of environmental problems both within (i.e., during mixing and pouring) and outside (dumped sand) the foundries. Resins and catalysts are expensive . Unsafe to human operators . Due to high strength and hardness of moulds/cores, sand reuse is a slightly difficult process . 3. No-Bake sand moulds 7/4/2014 15

16. Skin-dried molds Sands used for making skin dried molds contain certain binders like linseed oil which harden when heated. The mold is made with the molding sand in the green condition and then the skin of the mold cavity is dried with the help of gas torches or radiant heating lamps. Unlike dry mold, a skin dried mold is dried only up to a depth varying from 6 mm to 25 mm. A skin-dried mold possesses strength and other characteristics in between green and dry sand molds. If a skin-dried mold is not poured immediately after drying, moisture from green backing sand may penetrate the dried skin and make the same ineffective. 7/4/2014 16

17. MOULDING SAND MIXTURE- INGREDIENS FOR DIFFERENT SAND MIXTURES A moulding sand is a mixture of base sand, binder and additives. ingredients of green sand no-bake sand mixture dry sand mixture 7/4/2014 17

18. Ingredients for Green Sand Mixture Green sand mixture is composed of base sand, binder, moisture and additives. Base sand Silica sand is used as the base sand . It possesses favorable properties , inexpensive and can be reused many number of times. The amount of silica sand added may vary from 85 - 92 % depending on the requirements. Binder Bentonite (clay binder) is the widely used binder for bonding sand particles. It is activated in the presence of water . A best bond between the sand particles can be obtained with Bentonite varying from 6 - 12 % and water 3 - 5 %. Additives Additives are added in small quantities to develop certain new properties, or to enhance the existing properties of moulding sand. Sea coal, silica flour, wood flour and iron oxide are a few commonly used additives . 7/4/2014 18

19. Ingredients for No-bake sand mixture Ingredients of 'alkyd binder system ' which is one of the most widely used binder system in Indian foundries is discussed below. Base sand Silica sand is used as the base sand. Binder The alkyd binder system consists of three parts : Part A (binder), Part B (hardener) and Part C (catalyst). Part A (Binder): The binder is an alkyd resin which is obtained by reacting linseed oil with a polybasic acid like isopthalic and solvents like turpentine, kerosene or mineral spirit to improve flowability . Its addition ranges from 2 - 5 % based on weight of sand . Part B (Hardener): The hardener is a reacted product between cobalt/lead salts and napthanic acid . Its addition ranges from 5 - 10 % based on weight of binder . Part C (Catalyst): Methylene-diphenyl-Di-isocyanate commonly known as MDI is used as catalyst to speed up the chemical reaction. Its addition ranges from 20- 25 % based on weight of binder . 7/4/2014 19

20. Ingredients for Dry Sand Mixture Ingredients for dry sand mixture is similar to that of green sand. 7/4/2014 20

21. Loam sand ingredients Loam sand contains much more clay as compared to ordinary molding sand. The clay content is of the order of 50% or so. The ingredients of loam sand may be fine sands , finely ground refractories, clays , graphite and fibrous reinforcement. A typical loam sand mixture contains silica sand 20 volumes, clay 5 vols, and moisture 20%. Molds for casting large bells etc., are made up of brick framework and lined with loam sand and dried. Sweep or skeleton patterns may be used for loam molding . 7/4/2014 21

22. MOLDING SAND BINDERS Binders produce cohesion between the molding sand grains in the green or dry state (or condition). Binders give strength to the molding sand so that it can retain its shape as mold cavity. Binders (to the molding sands) should be added as optimal minimum. Increasing binder content reduces permeability of molding sand. Increasing binder content , increases green compression strength up to a limit ; after which green compression strength remains practically unchanged with further increase in binder content. Clay binders are most commonly used for bonding molding sands. The best clay is one which imparts the optimum combination of bonding properties, moisture, life and cost of producing the required casting. 7/4/2014 22

23. MOLDING SAND BINDERS Clay binders are most commonly used for bonding molding sands. Clay binders can be classified as: Fire clay Bentonite Sodium montmorillonite Calcium montmorillonite Illite Kaolinite Bentonite: The most commonly used clay binders are Bentonites as they produce strongest bonds in foundry molding sands. Bentonite deposits are available in India in Rajasthan and Bihar Bentonites are the weathered product of volcanic ash and are soft creamy white powders. 7/4/2014 23

24. Fire Clay: Fire clay is a refractory clay usually found in the coal measures. Fire clay particles are about 400 times as large as compared to those of Bentonite ; hence the same percentages of fire clay produce lower strengths . Illite Illite is the decomposition product of micaceous materials due to weathering. Illite is found in natural molding sands. Illite has softening point of about 2500°F. Illite does not swell in the same way as bentonite but gives reasonable strength. Illite particles have thickness and width of 20 and 100-250 milli-microns respectively. Kaolinite Kaolinite is the residue of weathered granite and basalt . Kaolinite binder has its composition 60% kaolinite, 30% illite and 10% quartz, Kaolinite gets Very low swelling due to water and is non-gel forming. Kaolinite particles possess thickness and width of 20 and 100-250 milli-microns respectively. MOLDING SAND BINDERS 7/4/2014 24

25. WATER The amount of water may vary from 1.5 to 8%. water is responsible for the bonding action of clays . Water activates the clay in the sand and the clay-sand mixture develops strength and plasticity. Water added to the sand mixture, partly gets adsorbed by clay and partly remains free and is known as Free Water. The absorbed water is responsible for developing proper bond and the green strength . The free water acts as a lubricant , It increases plasticity It improves moldability , but It reduces strength of the sand mixture. For a given type of clay and its amount, there is an optimum required water content. Too little water will not develop proper strength and plasticity . Too much water will result in excessive plasticity and dry strength. The amount of water required to develop the optimum properties can be found out experimentally . 7/4/2014 25

26. ADDITIVES The basic constituents of molding sand mixture are; Sand binder and water Materials other than the basic ingredients are also added to molding sand mixtures, of course in small quantities , in order to enhance the existing properties . To develop certain other properties. to give special qualities like resistance to sand expansion, defects etc. Some of the additive materials along are given below. 7/4/2014 26

27. Facing materials: Facing materials tend to obtain smoother and cleaner surfaces of castings and help easy peeling of sand from the casting surface during shake out . A few facing materials are Sea coal Graphite Coke Silica floor Cushion materials: Cushion materials burn when the molten metal is poured and thus give rise to space for accommodating the expansion of silica sand at the surfaces of mold cavity. In the absence of cushion materials , large flat surfaces of castings may buckle due to thermal expansion of silica sand grains. A few cushion materials are: Wood floor Cellulose Other special Additives: Finely ground corn floor, Iron oxide, Boric acid, diethylene glycol 7/4/2014 27

28. MOULDING SAND TESTING METHODS: (1) Grain Fineness Test: Grain size of a sand is designated by a number called “Grain Fineness Number” that indicates the average size as well as portions of smaller & larger grains This number corresponds to a no. of meshes in a standard sieve of 280 mm diameter Grain Fineness No. = Total Product / Total % of sand retained on screen (2) Moisture Content Test: The moisture content is calculated by difference between the moist and dry weights of the sample in grams divided by 50 gram gives per the % of moisture content in the given sand

29. (3) Clay Content Test: the method for determining the clay content of moulding sand consists of agitating the sand in water. So as to separate the clay from the sand particles and then removing the clay which remains suspended in distilled water at room temperature (4) Permeability Test: Permeability is measured by the quantity of air that will pass through a standard specimen of the sand under a given pressure in a prescribed time Permeability Number = v × h / p × a × t Where, v = Volume of air in cc, h = Height of the sand specimen in cm, p = Air pressure in gm / cm², a = Cross-sectional area of the specimen in cm², t = Time in mins