Basics of Sand Moulding

Sand Casting:

Casting is a solidification process. Therefore, the microstructure can be finely tuned, such as grain structure, phase transformations and precipitation. However, defects such as shrinkage porosity, cracks and segregation are also intimately linked to solidification. These defects can lead to lower mechanical properties. A subsequent heat treatment is often required to reduce residual stresses and optimize mechanical properties.

Sand casting uses natural or synthetic sand (lake sand)  which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cutoff and re-used also.

The process is fairly straightforward: you make a pattern of what you want to cast, then use the pattern to make a sand mold, and then pour molten metal into the mold. After the metal freezes you end up with the piece that you want.

The sand used for green sand molding is critical and determines the favorable or unfavorable outcome of the casting.  It controls the tolerances, surface finish and the repeatability while in production.  Remembering that the tolerances on sand castings are usually wider than the other casting methods.

Ex: Gears, Pulleys, Crankshafts, Connecting Rods, Propellers, heavy Machine base etc.

The most common metals are Iron, Steel, Bronze, Brass and Aluminium. The process is to make medium to large parts like Valve bodies, Locomotive components and Construction Machinery. Likewise small parts of Buckles, Handles, knobs, and Hinges.

The sand casting process involves the use of a furnace, metal, pattern and Sand mould. The metal is melted in a furnace and then ladled and poured into the cavity of the sand mould, which is formed by the pattern. The sand mould separates along a parting line and the solidified casting can be removed.

Advantages:

1. Low cost of mould materials and equipment

2. Large casting dimensions may be obtained

3. Wide variety of metals and alloy (Ferrous and Non-Ferrous) may be cast

Disadvantages:

1. Rough surface

2. Poor dimensional accuracy

3. High machining tolerances

4. Coarse grain structure

5. Limited wall thickness (2.5 – 5 mm)

Mould making

is the crucial step in the production of castings. Making a mould properly means a casting half done. A sand mould is formed by packing sand into each half of the mould. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mould.

Sand is the most commonly used mould material for casting. Other materials used include metals, plaster and ceramics. Sand moulds are made in moulding boxes or flasks which are metal frames with sufficient adhesive area to support sand. In large flasks additional adhesive area is made available by providing cross bars in the flask.

Most moulds are made in two part flasks, the top part called “Cope” and the bottom part called “Drag”. Copes and Drags are held in definite relation to each other by aligning pins and lugs. For some moulds additional intermediate boxes called “Cheeks” may be required. If the production rates are high, moulds may be arranged in the form of stack moulds with a number of such intermediated boxes.

Also lubrication is often applied to the surfaces of the mould cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.

Characteristics:

1. The mould should be strong enough to resist erosion by the flow of the metal and to take the weight of the metal.

2. The mould material should not produce too much of gases as the gases may enter the mould cavity and get entrapped with metal or violently boil out the metal.

3. The mould should be made in such a way that gases generated are vented out of the mould.

4. The mould should be refractory enough to withstand the high temperature of liquid metal.

5. The cores should be collapsible enough to permit contraction of the metal after solidification.

6. The passages for the flow of liquid metal into the mould should be designed to provide smooth non turbulent flow of metal, effective trapping of slag and proper directional solidification.

7. There should be a proper riser system in the mould to provide sufficient extra metal to account for the liquid and solidification shrinkage.

8. The mould material and moulding process should be such that cleaning of castings is facilitated.

Moulding Tools:

1. Shovel:It is just like rectangular pan fitted with a handle. It is used for mixing the moulding sand and for moving it from one place to the other.

2. Riddle:

It is used for removing foreign materials like nails, shot metal splinters of wood etc from the moulding sand.

3. Rammer:

It is a wooden tool used for ramming or packing the sand in the mould. Rammers are made in different shapes.

4. Strike-off bar:

It is a cast iron or wrought iron bar with a true straight edge. It is used to remove the surplus sand from the mould after the ramming has been completed.

5. Vent wire:

It is a mild steel wire used for making vents or openings in the mould.

6. Lifter:

It is a metal piece used for patching deep section of the mould and removing loose sand from pockets of the mould.

7. Slick:Different types of slicks are used for repairing and finishing moulds.

8. Trowel:

It contains of a flat and thick metal sheet with upwards projected handle at one end. It is used for making joints and finishing flat surface of a mould.

9. Swab:

It is made of flax or hemp. It is used for applying water to the mould around the edge of the pattern.

10. Draw spike:

It is a metal rod with a pointed or screwed end. It is used for removing the pattern from the mould.

11. Gate cutter:

It is a metal piece to the gate the opening that connects tee sprue with the mould cavity.

12. Rapping plate (or) Lifting plate:

It is used to facilitate shaking and lifting large pattern from the mold.

13. Spirit level:It is used to check that the sand bed, moulding box or table of moulding machine is horizontal.

14. Clamps:

Clamps are used to hold the cope and drag of the complete mould together so that the cope may not float or rise when the molten metal is poured into the mould.

15. Gaggers (or) Lifters:

These are iron rods bent at one or both ends. These are used to reinforce the moulding sand in the top portion of the moulding box and for supporting hanging sand.

16. Moulding box:

Sand moulds are prepared specially constructed boxes called the moulding boxes or flasks. The function and construction of a molding box have already been described.

17. Moulding flasks:

1. Snap flask

2. Tapered slip flask

3. Box flask (or) Tight flask

4. Wooden flask

Moulding Sands:

Most sand casting operations use Silica sand (SiO₂). Usually sand used to manufacture a mould for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be 89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when manufacturing parts by sand casting.

Characteristics of Moulding Sand:

Important characteristics are:

1. These sands are refractory in nature and can withstand temperature of the metal being poured, without fusing.

2. The moulding sands do not chemically react or combine with molten metal and can therefore be used repeatedly.

3. The sands have a high degree of permeability and thus allow the gases formed during pouring to escape.

4. The strength, permeability and hardness of the sand mix can be varied by changing the structure or ingredients of sand.

Properties of moulding sands:

The important properties are:

1. Strength

2. Permeability

3. Grain Size and Shape

4. Thermal stability

5. Refractoriness

6. Flow ability

7. Sand Texture

8. Collapsibility

9. Adhesiveness

10. Reusability

11. Easy of preparation and control

12. Conductivity

Classification of moulding sands:

According to their use in the foundry, moulding sands are classified into following categories:

1. Green sand:

• It is sand used in the wet condition for making the mould. It is mixture of silica sand with 15-25 per cent clay and 6-8 per cent water
• As explained earlier green sand moulds are not dried and metal is poured in them in the wet condition
• Being damp the sand can be easily worked with hand to give it any desired shape
• This sand is used for producing small to medium sized moulds which are not very complex

2. Dry sand:

• Dry sand is the green sand that has been dried or baked after preparing the mould.
• Drying sand gives strength to the mould so that it can be used for larger castings

3. Loam sand:

• Loam sand is sand containing up to 50 % clay which has been worked to the consistency of builder mortar.
• This sand is used for loam sand moulds for making very heavy castings usually with the help of sweeps and skeleton patterns.

4. Parting sand:

• This sand is used during making of the mould to ensure that green sand does not stick to the pattern and the cope and drug parts can be easily separated for removing the pattern without causing any damage to the mould.
• Parting sand consists of fine grained clay free dried silica sand, sea sand or burnt sand with some parting compounds.
• The parting compounds used include charcoal, ground bone and limestone, groundnut shells, talc and calcium phosphate.

5. Facing sand:

• Facing sand is the sand which covers the pattern all around it. The remaining box is filled with ordinary floor sand.
• Facing sand forms the face of the mould and comes in direct contact with the molten metal when it is poured.
• High strength and refractoriness are required for this sand.
• It is made of silica sand and clay without the addition of any used sand.
• Graphite, mollases, plumbago etc. may be added to the facing sand. Thickness of the sand layer varies from 20 to 30 mm.

6. Backing sand:

• Backing sand is the bulk of the sand used to back up the facing sand and to fill up the volume of the flask.
• It consists mainly of old, repeatedly used moulding sand which is generally black in colour due to addition of coal dust and burning on contact with hot metal.
• Because of the colour backing sand is also sometimes called black sand.
• The main purpose for the use of backing sand is to reduce the cost of moulding.

7. System sand:

• This is the sand used in mechanized foundries for filling the entire flask.
• No separate facing sand in used in a mechanized foundry.
• Sand, cleaned and reactivated by the addition of water and binders is used to fill the flask. Because of the absence of any fresh sand, system sand must have more strength, permeability and refractoriness compared to backing sand.

8. Core sand:

• Core sand is the sand used for making cores. This is silica sand mixed with core oil. That is why it is also called oil sand.
• The core oil consists of linseed oil, resin, light mineral oil with some binders.
• For larger cores, sometimes pitch or flour and water may also be used to save on cost.

Properties of moulding sands:

The important properties are:

1. Strength:

· The sand should have adequate strength in its green, dry and hot states

· Green strength is the strength of sand in the wet state and is required for making possible to prepare and handle the mould.

· If the metal is poured into a green mould the sand adjacent to the metal dries and in the dry state it should have strength to resist erosion and the pressure of metal.

· The strength of the sand that has been dried or basked is called dry strength

· At the time of pouring the molten metal the mould must be able to withstand flow and pressure of the metal at high temperature otherwise the mould may enlarge, crack, get washed or break

· Strength of the moulding sand depends on:

1. Grain size and shape

2. Moisture content

3. Density of sand after ramming

· The strength of the mould increases with a decrease of grain size and an increase of clay content and density after ramming. The strength also goes down if moisture content is higher than an optimum value.

2. Permeability:

· The moulding sand must be sufficiently porous to allow the dissolved gases, which are evolved when the metal freezes or moisture present or generated within the moulds to be removed freely when the moulds are poured. This property of sand is called porosity or permeability.

3. Grain size and shape:

· The size and shape of the grains in the sand determine the application in various types of foundry. These are three different sizes of sand grains.

1. Fine

2. Medium

3. Coarse

· Fine sand is used for small and intricate castings. Medium sand is used for benchmark and light floor works. If the size of casting is larger coarse sand is used

· Sand having fine, rounded grains can be closely packed and forms a smooth surface. Although fine-grained sand enhances mould strength.

4. Thermal stability:

· The sand adjacent to the metal is suddenly heated and undergoes expansion. If the mould wall is not dimensionally stable under rapid heating, cracks, buckling and flacking off sand may occur.

5. Refractoriness:

· Refractoriness is the property of withstanding the high temperature condition moulding sand with low refractoriness may burn on to the casting

· It is the ability of the moulding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the Silica sand is highest.

6. Flowability:

· Flowability or plasticity is the property of the sand to respond to the moulding process so that when rammed it will flow all around the pattern and take the desired mould shape. High flowability of sand is desirable for the sand to get compacted to a uniform density and to get good impression of the pattern in the mould.

· Flowability is also very important in machine moulding

· Flowability of sand increases as clay and water content are increased.

7. Sand texture:

· As mentioned earlier the texture of sand is defined by its grain size and grain size distribution.

· The texture chosen for an application should allow the required porosity, provide enough strength and produce the desired surface finish on the casting.

8. Collapsibility:

· The moulding sand should collapse during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the moulding material should be cheap, reusable and should have good thermal conductivity

9. Adhesiveness:

· It is the important property of the moulding sand and it is defined as the sand particles must be capable of adhering to another body, then only the sand should be easily attach itself with the sides of the moulding box and give easy of lifting and turning the box when filled with the stand.

10. Reusability:

· Since large quantities of sand are used in a foundry it is very important that the sand be reusable otherwise apart from cost it will create disposal problems

11. Easy of preparation and control:

· Sand should lend itself to easy preparation and control by mechanical equipment

12. Conductivity:

· Sand should have enough conductivity to permit removal of heat from the castings.

Methods of Sand testing:

The moulding sand after it is prepared should be properly tested to see that require properties are achieved. Tests are conducted on a sample of the standard sand. The moulding sand should be prepared exactly as it is done in the shop on the standard equipment and then carefully enclosed in a container to safeguard its moisture content.

Sand tests indicate the moulding sand performance and help the foundry men in controlling the properties of moulding sands. Sand testing controls the moulding sand properties through the control of its composition.

The following are the various types of sand control tests:

1. Moisture content test

2. Clay content test

3. Grain fitness test

4. Permeability test

5. Strength test

6. Refractoriness test

7. Mould hardness test

Moisture content test:

Moisture is the property of the moulding sand it is defined as the amount of water present in the moulding sand. Low moisture content in the moulding sand does not develop strength properties. High moisture content decreases permeability.

Procedures are:

1. 20 to 50 gms of prepared sand is placed in the pan and is heated by an infrared heater bulb for 2 to 3 minutes.

2. The moisture in the moulding sand is thus evaporated.

3. Moulding sand is taken out of the pan and reweighed.

4. The percentage of moisture can be calculated from the difference in the weights, of the original moist and the consequently dried sand samples.

Percentage of moisture content = (W1-W2)/(W1) %

Where, W1-Weight of the sand before drying,

W2-Weight of the sand after drying.

Clay content test:

Clay influences strength, permeability and other moulding properties. It is responsible for bonding sand particles together.

Procedures are:

1. Small quantity of prepared moulding sand was dried

2. Separate 50 gms of dry moulding sand and transfer wash bottle.

3. Add 475cc of distilled water + 25cc of a 3% NaOH.

4. Agitate this mixture about 10 minutes with the help of sand stirrer.

5. Fill the wash bottle with water up to the marker.

6. After the sand etc., has settled for about 10 minutes, Siphon out the water from the wash bottle.

7. Dry the settled down sand.

8. The clay content can be determined from the difference in weights of the initial and final sand samples.

Percentage of clay content = (W1-W2)/(W1) * 100

Where, W1-Weight of the sand before drying,

W2-Weight of the sand after drying.

Grain fitness test:

The grain size, distribution, grain fitness are determined with the help of the fitness testing of moulding sands. The apparatus consists of a number of standard sieves mounted one above the other, on a power driven shaker.

The shaker vibrates the sieves and the sand placed on the top sieve gets screened and collects on different sieves depending upon the various sizes of grains present in the moulding sand.

The top sieve is coarsest and the bottom-most sieve is the finest of all the sieves. In between sieve are placed in order of fineness from top to bottom.

Procedures are:

1. Sample of dry sand (clay removed sand) placed in the upper sieve

2. Sand is vibrated for definite period

3. The amount of same retained on each sieve is weighted.

4. Percentage distribution of grain is computed.

Permeability test:

The quantity of air that will pass through a standard specimen of the sand at a particular pressure condition is called the permeability of the sand.

Following are the major parts of the permeability test equipment:

1. An inverted bell jar, which floats in a water.

2. Specimen tube, for the purpose of hold the equipment

3. A manometer (measure the air pressure)

Steps involved are:

1. The air (2000cc volume) held in the bell jar is forced to pass through the sand specimen.

2. At this time air entering the specimen equal to the air escaped through the specimen

3. Take the pressure reading in the manometer.

4. Note the time required for 2000cc of air to pass the sand

5. Calculate the permeability number

6. Permeability number (N) = ((V x H) / (A x P x T))

Where,

V-Volume of air (cc)

H-Height of the specimen (mm)

A-Area of the specimen (mm²)

P-Air pressure (gm / cm²)

T-Time taken by the air to pass through the sand (seconds)

Strength test:

Measurements of strength of moulding sands can be carried out on the universal sand strength testing machine. The strength can be measured in compression, shear and tension.

The sands that could be tested are green sand, dry sand or core sand. The compression and shear test involve the standard cylindrical specimen that was used for the permeability test.

a. Green compression strength:

Green compression strength or simply green strength generally refers to the stress required to rupture the sand specimen under compressive loading. The sand specimen is taken out of the specimen tube and is immediately (any delay causes the drying of the sample which increases the strength) put on the strength testing machine and the force required to cause the compression failure is determined. The green strength of sands is generally in the range of 30 to 160 KPa.

b. Green shear strength:

With a sand sample similar to the above test, a different adapter is fitted in the universal machine so that the loading now be made for the shearing of the sand sample. The stress required to shear the specimen along the axis is then represented as the green shear strength. It may vary from 10 to 50 KPa.

c. Dry strength:

This test uses the standard specimens dried between 105 and 1100 C for 2 hours. Since the strength increases with drying, it may be necessary to apply larger stresses than the previous tests. The range of dry compression strengths found in moulding sands is from 140 to 1800 KPa, depending on the sand sample.

Steps involved are:

1. Specimen is held between the grips

2. Apply the hydraulic pressure by rotating the hand wheel

3. Taking the deformation use of the indicators.

Refractoriness test:

The refractoriness is used to measure the ability of the sand to withstand the higher temperature.

Steps involved are:

1. Prepare a cylindrical specimen of sand

2. Heating the specimen at 1500 C for 2 hours

3. Observe the changes in dimension and appearance

4. If the sand is good, it retains specimen share and shows very little expansion. If the sand is poor, specimen will shrink and distort.

Mould hardness test:

Hardness of the mould surface can be tested with the help of an “indentation hardness tester”. It consists of indicator, spring loaded spherical indenter.

The spherical indenter is penetrates into the mould surface at the time of testing. The depth of penetration w.r.t. the flat reference surface of the tester.

Mould hardness number = ((P) / (D – (D²-d²))

Where,

P- Applied Force (N)

D- Diameter of the indenter (mm)

d- Diameter of the indentation (mm).