Sand preparation and Mould making

The main Raw materials used in mould making is the moulding sand.

Moulding sand may be defined as Granular particles resulting from the breakdown of rocks,due to the action of natural forces such as frost, wind, rain , heat and water currents. Rocks have a complex composition and sand posses almost all of them.

In nature it is found on the bottom and banks of rivers and lakes.

The properties of sand differs from region to region depending upon the natural environment.

Natural sand : Also called as green Sand and is collected from natural resources.It contains water as only binder.It has the adv of retaining its moisture content for a long time. This permits easy patching and finishing of moulds

Synthetic sand : This is obtained by mixing relatively clay free sand, binder ( Water and Bentonite) and other materials as reqd.It is a better moulding sand its properties can be controlled by varying the mixture composition.

The principle constituents of the molding sand are Silica : 86 – 90 % Alumina 4 – 8 %Iron oxide 2 – 5 % and smaller amounts of oxides of Ti, Mn, and Ca and some alkaline compounds.

Silica in the form of granular quartz is characterized by high softening temp. and thermal stability. It imparts refractoriness, chemical resistivity and permeability to sand. It is specified according to the shape and size. Sand grains could be round, medium, fine coarse, semi angular, angular and compounded.

Fine sand is desirable for small and intricate castings.As fine grains lie close permeability is poor.Med grnd sand is usd in bench work and light floor work,Coarse grains are used for large castings which helps to eascape the gasses. Roubnded grains have least contact with one aother and are less strong, permeability is high.Sub angular are comapritively less permeable than round grains. Angular grains having defined edges gives more strength and less permeability.Compound grains are hard lums and not preferred.

Silicon oxide is obtained form quartz rocks or by decomposition of granite.( composed of quartz and felspar) feldspar when decomposed becomes clay which imparts plasticity to the moulding sand,in the moist state i.e imparts necessary bonding action and strength in the presence of moisture and increases its strength after drying. Normally the amount of clay in silica sand varies from 6 – 10%.Clay actually consists of fine silt. and fine clay imparts the necessary bonding strength to the Mould so that the mould will not decompose after ramming. However it decreases permeability. However, too much of clay causes cracking of the mould after drying.

A good moulding sand contains impurities below 2 % . Green sand moulding is carried out with sand of low moisture content. (3 – 5%)In dry sand moulding, more water is present when making mold, as it is beneficial in promoting dry bond strength after storing. Many times coal dust is also added which makes the sand more open and helps to cool the mould after metal is poured. It absorbs fairly high a/m of heat preventing the sand grains from overheating and fusing.It releases co2, whose protective film helps to keep the metal and sand separated from each other.

Sand conditioning : Properly conditioned sand is an importenet factor in obtaining good castings. New sand and the used sand must be properly prepared before it can be used.

Conditioning of sand by hand is a difficult process. Hence appropriate equipments are provided for this purpose in foundry shop.

One such equipment is a mixer, which consists of a circular pan in which mounted a combination of plows and mullers driven by vertical shaft.This arrangement gives a shoveling action to the sand turning it over on itself, and lining it in front of the mullers which provides an intensive kneading and rubbing action.The result is through distribution of sand grains with the bonding materials. After sand is mixed it is discharged through a door, in the bottom of the pan. Both green sand and core sand are prepared in this manner.

The sand conditioning is done to get the follwing results:1. It distributes binder unifirmmly around the sand grains.2. It controls the moisture contents. It is important that all sand particle

surfaces to be moistened.3. It eliminates foreign particles from sand.4. It aerates the sand so that it is not packed and is in proper condition

for moulding.5. It delivers the sand at proper temperature.

Properties of moulding sand :The properties for moulding sand are determined by chemical composition, a/m of ckay, moisture content , by the shape and size of silica grains in the sand.

1. Permeability : ( Also called as porosity) : The sand must be porous enough to allow gasses, water and steam vapor to escape freely when the molten metal is poured into the mould. Insufficient porosity of the moulding sand leads to casting defects such as gas holes and pores. Hard ramming lowers the permeability but this is relieved by liberal venting.

2. Cohesiveness: The ability of sand partices to stick together is known as cohesiveness or strength of the moulding sand .The strength is determined by alumina ( Clay ) content. Clay should be present as thin, tenacious film on each grain of the sand. Sharp sand having smooth oval grains are not easily bonded clay helps bonding. Moulding sand must have a good strength other vice it may lead to collapse of the mould or its

partial destruction during conveying, turning over or closing. Cohesion must be retained when molten metal enters the mould.

3. Adhesiveness :The sand particles must be capable of sticking to other bodies particularly to the moulding box and is due to only this property,that the sand mass is held in the mould box properly and does not fall when the mould is moved. At the same time the sand must not stick to the casting and strip off easily leaving a clean surface.

4. Plasticity : In order to have a good impression of the pattern in the mould, moulding sand must have good plasticity.generally fine grained sand has better plasticity. It depends on the content of the clay which absorbs moisture when sand is dampened

5. Refractoriness: The ability of sand to withstand high heat with out breaking down and fusing. The fusion point of moulding sand can be increased by removing the impurities particularly metallic oxides. Sand also should not form glassy mass which hampers stripping.

6. Chemical resistivity : the moulding should not chemically react or combine with so that it can be used again and again.

7. Binding property : Binder allows the sand to flow and take the shape of the pattern. It must not be so strong that break out becomes difficult. Nor it should it be so weak that it allows the surface skin of the casting to break.

8. Flowability : Sand must be able to transmit the blows through out during ramming.

Sand moulds A sand moulkd may be defined as a preformed sand container into which molten material is poured and allowed to solidify. The mould will have a opening at the top into which molten metal is poured. The mould is designed such that the molten metal spreads to all pats of the mould by gravity.Samll or medium sized castings are generally made from a small rectangular box shaped containers without top and bottom. ( It is called as flask) A flask may be made

up of 2 or 3 parts, fitted together by dowel pins. The clamping of the flask before pouring molten metal is important which othervice can cause buoyant effect of the molten metal from lifting the top part of the flask.

Depending on the sand material used, the mould could be classified as : Green sand mouldSkin dry mould,Dry sand mouldCement bonded mould.

In green sand moulds moisture is present in the sand at the time of pouring the molten metal. The grins are held together by moist clay.Moisture level has to be controlled carefully; these are used for casting all ferrous alloys. Green sand is available in many forms .These moulds are less expensive and gives larger out put. Cycle time is less. They do not require any backing operations or equipments.The disadvantages of these moulds are that they cannot be stored for longer periods. Not strong, liable to breakage during handing or by metal erosion. Because of the moisture in the sand , the blow holes and gas holes may occur. The finishing may not be very smooth.If coal dust or orgnic matls are added, it is called as loam moulding.

Skin dry mouldsThese are made of green sand with dry sand baking. In some cases moisture is dried from the surface layer of the ramed sand to he depth of 25mm by heaters or gas torches. These are more common in large moulds and can be used for casting practically all ferrous and non ferrous alloys. These are less expensive to construct than dry sand moulds but more expensive than green sand moulds of the same size, It has the adv of less equipment, cheaper materials, less

time for preparation, and less floor space when compared to dry sand mldg. However these are not as strong as dry sand moulds and can’t be stored for long times as moisture may migrate through the dry skin.

Dry sand moulds These are made with that sand which does not require moisture to develop the strength. These are backed to increase strength , resist erosion and improve surface condition . The dry sand moulds may be used for many alloys but are more commonly used for steel castings.Mainly used in small and medium sized operations. For large sized operations, the dry sand moulds are made in sections and assembeled after baking.The dry sand moulds are more stronger and can be handled easily with less damage and also can be stored for longer times. These resist metal erosion and tendency for moisture related defects are eliminated. The disadvantage of these moulds are they require more expensive moulding material, labour costs are high an d extra operation, equipment and space are needed.

Cement bonded mouldsIn these moulds silica sand bonded with Portland cement is used as the moulding material which dries up in air. These moulds are more commonly used for very large ferrous work in pit moulding and in other cases where baking is impossible. It has high strength and possess all advantages of dry sand. For these moulds extra space for air drying operation has to be provided. The matl used in these moulds cannot be used again like other moulds, thus the process becomes expensive.

Various specialized sands used for making moulds are :

Co2 sand ---- In this type sand silica grains instead of being coated with natural clay are coated with sodium silicate. The mixture is then packed around the pattern and then hardened by passing Co2, through the interstices for about a minute. The sand thus sets hard and produces a strong mould.

Shell Sands : These are synthetic sand coated with phenol or urea –formaldehyde resins and cured against a heated pattern to produce very strong thin sheel. No back up sand is required to provide support for the weight of the casting. Since alloys solidify at high temperatures, the resins are not disassociated. But the moulds disintegrate when casting has solidified due to breaking up of chemical bond by heat from solidifying casting.

Facing sands :Usually facing sand is first applied on the pattern, so that only it comes in contact with the molten metal. The sand is refractory enough so as not to get fused and burnt on to the metal.

Backing sands These are applied as back up mechanical support to facing sand. These are permeable to allow the gasses to escape.

Mould washes : These are slurries of fine ceramic grains, These are applied over the mould surface to minimize the fusing of facing sand grains.These also produce smother surface on casting due to filling up of interstices.

Sand moulding methods

1.Bench Moulding : Small castings are made in this method. They are done on a bench. It is a slow and laborious process.It requires hand ramming with loose patterns . Green sand, dry sand and skin dry sand moulds can be made.2. Floor moulding : Employed for large and medium moulds. Green sand, dry sand and skin dry sand moulds can be made directly on the floor . It is also a slow and laborious process.and requires hand ramming with loose patterns.

The floor moulding is carried out two part boxes ( Top and bottom ) called cope and drag. These boxes are fitted on to a stout frame with pins and holes to ensure accurate locations. The ground is leveled, half part of the pattern is placed on it, box frame is placed around the pattern, Box is filled with sand and sand is rammed and leveled off. The packed box is then turned oyer and the second box is placed on the first, ensuring that they are tightened properly. The other half of the pattern is placed on the box, The sand is filled rammed and leveled off. Two boxes are then opened and pattern is removed.If any repair is reqd same is carried out and gates for pouring metal made.If cores have to be located to form holes these are placed in position. Boxes are again put back onto the position and clamped to prevent the upper box floating on the liquid metal.

3. Plate Moulding : In floor moulding the patterns are made up of wood, and are split at a appropriate position on a horizontal line. These two halves are to be located with the help of the dowels. In plate moldings instead of wood metal plates are used to increase the life. The portions of the pattern permanently assembled in alignment on each side.For large quantity productions and heavy castings occasionally two plates can be used. One is to assist in making of copes and the other forn drags. The plate incorporates some locating arrangement for the moulding boxes,which could be pegs in the plates, but more occasionally consists of holes for locating pegs. The use of plate normally calls for moulding boxes which incorporate lugs having holes for location. Typical double sided plates used for hand or machine mouldings are accomplished with provision for 6 castings from one pour. The plate includes runners, gates, part of pourer, and part of riser thus reducing considerably the time to finish the mould after extraction of the pattern equipment. This method is usually employed for large quantity production. The use of particular plate is restricted to a certain range of moulding boxes.

3. Pit moulding : The moulding is carried out in pits in this method. Green sand and some times the cement bonded sands are use. This is the only method is used for moulding large castings5. Machine moulding : Verities of machines are used for carrying out the moulding of small, medium and large castings. This method is faster and gives uniform moldings but requires mounted patterns .By this method also green sand, dry sand and skin dry sand moulds can be prepared.