Properties of moulding sandporosity or permeability

It is the property of sand which permits the steam and other gases to pass through the sand mould. depends upon grain size, grain shape, moisture and clay components of the moulding sand.

If the sand is too fine, the porosity will be low.Plasticity

It is that property of sand due to which it flows to all portions of the moulding box or flask. The sand must have sufficient plasticity to produce a good mould.

AdhesivenessIt is that properties of sand due to it adheres or cling to the sides of the moulding box.

CohesivenessIt is the property of sand due to which the sand grains stick together during ramming. It is defined as the strength of the moulding sand.

Refractoriness The property which enables it to resist high temperature of the molten metal without breaking down or fusing.

Collapsibilitydetermines the readiness with which the moulding material will break down in knockout and cleaning operations.

Classification of Moulding sand according to their useGreen sand

The sand in its natural or moist state is called green sand.It is also called tempered sand. It is a mixture of sand with 20 to 30 percent clay, having total amount of water from 6 to 10 percent. The mould prepared with this sand is called green sand mould, which is used for small size casting of ferrous and non-ferrous metals.

Dry SandThe green sand moulds when baked or dried before pouring the molten metal are called dry sand moulds.The sand of this condition is called dry sandThe dry sand moulds have greater strength, rigidity and thermal stability. These moulds used for large and heavy casting.

Loam SandA mixture of 50 percent sand grains and 50 percent clay is called loam sand. It is used for loam moulds of large grey iron casting.

Facing SandA sand which is used before pouring the molten metal, on the surface is called facing sand.It is specially prepared sand from silica sand and clay.

Backing or Floor SandA sand used to back up the facing sand and not used next to the pattern is called backing sand.The sand which have been repeatedly used may be employed for this purpose. It is also known as black sand due to its colour.

System SandA sand employed in mechanical sand preparation and handling system is called system sand. This sand has high strength, permeability and refractoriness.

Parting SandA sand employed on the faces of the pattern before the moulding is called parting sand The parting sand consists of dried silica sand, sea sand or burnt sand

Core SandThe cores are defined as sand bodies used to form the hollow portions or cavities of desired shape and size in the castingThus the sand used for making these cores is called core sandIt is sometimes called oil sand. It is the silica sand mixed with linseed oil or any other oil as binder.

Functional requirements of moulding materials

A foundry moulding mixture passes through four main production stages preparation and distribution

mould and core productioncastingcleaning and reclamation

The principal properties required at the moulding stage flowabilitygreen strength

flowability: measure of the ability of the material to be compacted to a uniform density balance of these properties depends largely upon the intended method of compaction (vary from hand ramming with tools to jolt)

green strength: The need for green strength arises when the pattern is withdrawn and the mould must retain shape independently without distortion or collapse.

Moving to the pouring stagemany moulds are cast in the green state,but others and for heavy castings - hardened to generate greater rigidity under the pressure and erosive forces of the liquid metal. This state was formerly achieved by the high temperature drying of clay bonded sands or the baking of traditional core sands, but this has been largely outdated by the chemical hardening of sands containing reactive binders of the modern organic and silicate types. At this stage, dry strength i.e. strength in the hardened or dried condition is significant even in greensand practice dry strength is required to avoid friability mould partially dry out during standing before casting.

Moulding practice and the special requirements of coresandsMany castings, including most of those made by machine moulding, are cast in greensand moulds, Introduction of high pressure moulding machines enabled even castings in the tonnage weight range to be produced to acceptable quality standards. There are strong economic incentives to use this low cost system, but hardened moulds are preferred in many cases, particularly for heavier castings.

Greensand practice1. Clay binders involve low material costs and avoid the additional costs of mould hardening, whether by chemical or thermal means.2. The rapid turn round of moulding boxes and the smooth moulding andcasting cycle are advantageous in mechanized systems.3. The sand is readily reconditioned, since there is little dehydration of theclay bond.4. Greensands, having lower compression strength, offer less resistance to contraction than hardened moulds, so that the risk of hot tearing isreduced.5. Moulds joint closely, leaving little flash for removal by fettling.6. The process is environmentally friendly.

Hardened mould or dry sand practice1. Hardened moulds offer maximum resistance to distortion- metallostatic pressure- mould erosion during prolonged pouring. They are therefore suitable for castings of the largest dimensions and provide high standards of accuracy. 2. In the production of cast irons in particular, mould rigidity contributes to

internal soundness as well as to dimensional accuracy.3. The venting problem is reduced in the absence of steam generated from

moisture.4. impermeable mould surfaces are readily attainable, since sands of lower permeability can be used, with coatings where required.5. Surface chilling is greatly reduced, facilitating metal flow in thin sections.6. Problems of drying-out during delays in casting, leading to surface friability,

are avoided.

Mould hardeningChemical hardening is accomplished either by liquid or gaseous reagents.Liquid reagents are commonly employed in the cold-set mode and are blended into the sand at the mixing stage, in some cases at the point of entry to the mould, so that no separate hardening operation is required. Gas or vapour hardening is performed after compaction and is mainly applied to cores and smaller mould parts. Full mould drying has been largely superseded by the above practices, but stove are still used in some circumstances. These are usually operated in the temperature range 200–400°C. Surface drying can also be carried out, using gas torches or hot air drying hoods. Surface hardening can be enhanced by prior treatment with sprays of water or dilute binder solutions, and inflammable mould coatings can also assist in the process.

Sand testing techniquesSpecimens for bulk testing• Mechanical properties and certain other characteristics are determined on

specimens compacted to a bulk density similar to that encountered in a well rammed mould.

• Many tests utilize the 2 in X 2 in cylindrical AFS specimen or its 50mm X 50mm DIN equivalent, prepared by subjecting a weighed quantity of sand – to a selected number of blows from a compatible standard rammer,– transmit to a close fitting piston in a tubular mould

• The weight of sand is adjusted to produce a close tolerance specimen, which can be expelled from the tube on a stripping post.

• This specimen may be used for– permeability testing and for a number of green and dry strength

measurement behaviour during ramming can be used as a criterion of compactability or flowability.

Green and dry strength tests• The principal strength tests measure stress to failure under a constant

rate of loading. • In the low green strength range, compression testing can be carried out

on a simple, manually operated, spring loaded machine, • But most strength testing is carried out on universal machines• the load is applied by means of a pivotted weight, progressively brought

to bear on the specimen as the motorized pusher arm climbs the calibrated rack.

• For very high strengths machines such as that illustrated in Figure are available.

• In this example motorized loading up to 20 kN can be applied to the selected test specimen

• the force being measured through a strain gauge load cell. • The unit symbolize computer display and recording of test results.• Green strength tests are carried out on newly made specimens• dry or cured strength tests on specimens hardened in a standard manner

Testing machine for higher strength materials

Universal sand strength testing machine

Compression tests• The cylindrical specimen is axially loaded through

flat faced holders • Standardization of the rate of loading is particularly

important in the green compression test, since creep of greensand occurs readily under constant load.

Shear tests• The compressive loading system is modified to

provide offset loading of the specimen • Under most conditions the results of shear tests

have been shown to be closely related to those of compression tests, although the latter property increases proportionately more at high ramming densities.

The tensile test• A special waisted specimen is loaded in tension through a pair of grips.The transverse test• A plain rectangular specimen is supported on knife edges at the ends and

centrally loaded to fracture• Tensile and transverse tests are commonly applied to high strength sands• The conditions being especially relevant to the stresses incurred in cores during

handling and casting. • Both tests, unlike the compression test, provide values well within the working

ranges of normal types of universal sand testing machine.• Versions of these two tests are also used for the evaluation of shell moulding

mixtures, being carried out on cured specimens approximating to the thickness of a production shell.

• Studies of correlations between various strength properties are particularly valuable and can greatly reduce the volume of testing

Transverse test

The shatter index• shatter test is used as an indicator of sand toughness• The ability to deform rather than fracture under shock loading. • The standard compression test cylinder is ejected from its mould and allowed

to fall from a fixed height of 1.83m on to a flat steel anvil.• Fragments retained on a concentric 13.2mm aperture BS410 sieve are weighed,

together with the residual core from the anvil, • the shatter index is this weight expressed as a percentage of the total weight of

the specimen• A low value is an indication of poor lift, or friability in pattern withdrawal and

subsequent handling, whilst too high a value is associated with unsatisfactory moulding qualities resulting from excessive clay or water content.

• Values of 50–85 represent the mouldable range.

Surface hardness• The hardness of a compacted sand surface can be determined using portable

spring loaded indentation testers• For the measurement of green hardness, a spherical or conical indenter is used

depending on the expected hardness level• the depth of penetration from the flat reference surface of the instrument

corresponds to an empirical scale of hardness in the overall range 0–100.• The hardness can be measured on actual mould surfaces to check the degrees

of uniformity and the ramming efficiency. Deformation• The deformation behaviour of moulding sands has received less consideration

than the properties reflected in the above tests.• Deformation can be determined during the green compression test by

measurement of the decrease in length of the cylindrical specimen to the point of failure, whilst more comprehensive data can be derived from full stress–strain curves,

Permeability• Permeability is determined by measuring the rate of flow of air through a

compacted specimen under standard conditions. • The test for green permeability is carried out on the A.F.S. standard cylindrical

specimen, retained in its ramming tube• The permeability meter, incorporates a graduated bell of 2 litres capacity

containing a volume of air over water. • A tube from the air enclosure communicates directly with the specimen tube,

placed over an ‘O’-ring seal, so that the air can escape through the specimen as the bell descend.

• The time for exhaustion of 2 litres of air is determined.• The permeability number P is defined as the volume of air in cm3/min passing

through a specimen of length 1 cm and cross-sectional area 1 cm2, under a pressure difference of 1 cm water gauge:P = Vh/atpwhere V = volume of air, cm3

h = height of specimen, cma = c.s.a. of specimen, cm2

t = time, minp = pressure difference, cm water.

Using the standard apparatus and technique, the permeability may be derived directly from the formulaP = 3007.2/twhere the time t is expressed in seconds.

• A standard orifice, of small cross section in comparison with the porosity of the specimen, is placed between the pressure chamber and the specimen.

• The pressure between orifice and specimen now lies at a value intermediate between the chamber pressure and atmospheric pressure, depending on the permeability of the specimen.

• The value is derived directly from this pressure, measured by calibrated water manometer.

• A similar principle is embodied in the portable quick reading instrument • The required air pressure is in this case generated by a high speed electric fan and the

pressure drop is indicated on a sensitive gauge calibrated directly to read the permeability number.

• Either type of permeability meter can be used in conjunction with a flexible tube and contact pad to provide a direct indication of the level of permeability at any flat mould surface.

• Permeability determinations on dried or hardened sands require a modified specimen holder enabling the cylindrical walls of the test piece to be sealed against the bore surface of an enlarged tube, using molten wax or an inflatable rubber sleeve

Bench life• Compression tests are made on specimens rammed at successive intervals of five

minutes from mixing.• The bench life is defined as the time to reach the value of 10 kN/m2, beyond which the

strength of subsequently compacted material will be prejudiced by interruption of the bonding process already under way in the loose sand.

• Other relevant time intervals can be applied, depending on the sensitivity of the mixture under test.

Strip time• Numbers of compression test specimens are moulded in multi-gang boxes immediately

after mixing. • Tested at intervals to detect the time at which the strength reaches350 kN/m2.• A full strength–time plot of the type shown, can be developed from this form of test to

characterize the setting behaviour in full, including the maximum potential strength of the material.

Scratch hardness• A four point spring loaded penetrator is contained within a concentric reference

surface which is pressed against the hardened sand surface.• The penetrator is manually rotated though a fixed number of revolutions,

normally two, and the depth of penetration is shown on a dial gauge as an indication of relative surface density and scratch resistance.

Impact penetration• A sharp spring actuated probe graduated in 1 cm divisions, is repeatedly triggered by

manual pressure against the sand surface, • And the number of blows required to reach a given depth represents the relative

resistance of the sand body to penetration. • The test can be carried out on production moulds or on sample 10 cm cubes, molded

and tested whilst in the corebox. • A valuable feature of this test is its ability to explore the rate and depth of hardening

and to detect any lack of through cure• plots of numbers of impacts against penetration depth enable the degree of uniformity

to be investigated.Gas evolution• The rate and volume of gas evolved from a dried sample of hardened sand can be

determined using a sealed, temperature controlled, silica tube furnace with provision for a nitrogen atmosphere.

• The weighed sample is propelled into the hot zone of the inert gas-filled tube, normally at a temperature of 850°C, and the pressure rise is continuously recorded until a maximum is attained.

• The pressure readings can be converted to volumes, using a calibration chart derived from the known total evolution from a standard substance.

• Both rate and total volume are significant in relation to potential gas defects in castings.

Impact penetration tester with internal spring loaded hammer

Hot distortion• The hot distortion test is carried out on a flat strip sand specimen which is fixed

at one end and loaded in cantilever mode at the other• The underside is heated and the deflection at the loaded end continuously

measured. • Upward movement due to initial expansion of the lower layers of the strip is

followed by sagging with the progressive thermal softening and eventual breakdown and collapse of the material.

• Strain–time curves provide a guide to the behaviour of different binders and curing conditions.

• Typical curves representative of hot box, cold set and silicate bonded sands, including the influence of iron oxide additives, were produced in a comprehensive study by Morgan and Fasham

Schematic arrangement of hot distortion test

High temperature properties• High temperature tests are particularly relevant to the study of mould and core

materials, since they simulate in some degree conditions met in the casting process.

• more suited to longer term laboratory assessments than to routine testing• Gas evolution and hot distortion tests - given their particular value in relation to

the chemically hardened sands. • hot strength test include the determination of compressive strength to failure at

specified temperatures, -prior soaking periods for the specimen to reach the test temperature

• measurements of total deformation can also be obtained from the same test• To assess the retained strength, a property relevant to knockout behaviour, the

specimen is put though the thermal cycle, but cooled to room temperature before testing to failure

• Collapsibility can be assessed through determination of the time to failure under constant load

• more suitable than isothermal hot strength tests for organically bonded sands, in which the bond can be completely decomposed during the soaking period.

• Free expansion in the absence of loading can also be measured using a dilato meter• this type of measurement can be relevant to certain types of sand defect incastings

High temperature and retained strengths of sodium silicate bonded sand

Moisture testing• Moisture content is readily determined by loss in weight on drying at 110°C.• Various rapid aids are employed, including balances directly calibrated to

read the moisture content of a standard sample after drying; • samples can be dried by the passage of hot air through a filter cloth tray.• For shop control the Speedy moisture tester can be employed. • This makes use of the reaction

CaC2 + 2H2O = Ca(OH)2 + C2H2

• The sample is placed in the cap of a metal flask fitted with a pressure gauge and an excess of the carbide reagent placed in the flask body.

• The flask is closed, clamped and shaken and the acetylene pressure provides a direct reading of moisture content on the gauge.

• Electrode probe devices have been used to determine the moisture content of loosely heap sand, but are not employed for the accurate assessment of samples.

• an electrical method employing measurements of microwave absorption in compacted samples is employed with success to control moisture in mechanized system sands

Active clay• The live bentonite clay present in a sample of reconditioned sand can be

determined by the methylene blue test, • discriminate between active clay and other particles of similar size

which are included in the normal clay grade determination.• Methylene blue dye, when added to an acidified slurry of clay and water,

is adsorbed by the clay to a point at which the appearance of excess dye can be observed by spotting drops of the liquid on to a filter paper.

• weighed sand sample is agitated and heated in the selected initial solution, using stirring or ultrasonic means.

• The standard methylene blue solution is then progressively added from a burette until the spot tests reveal the end point, which is represented by a halo around the spot.

• The volume of the standard solution required can be directly related to the active clay content using an appropriate calibration curve.

pH and acid demand• The pH value of a solution, a reciprocal function of the hydrogen ion concentration, is

the standard representation of the degree of acidity or alkalinity on a scale from 1 to 14.

• Values from 1–7 represent acid• 8–14 alkaline condition• The pH value can influence the behaviour of clay binders • readily measured using a meter depending on electrochemical potentials.• In chemically bonded sands employing acid catalysts, account needs to be taken of the

presence of alkalis already present in the base sand before the addition of the binder. • Since pH measurements are only influenced by substances in solution, the purpose of

the acid demand test is to assess the full effect of insoluble alkalis as well.• The test involves the introduction of a standard volume of hydrochloric acid of known

concentration to a sample of the sand suspended in water.• The acid reacts with the whole of the alkali content, leaving an excess of HCl.• This can be quantified by titration with a standard solution of sodium hydroxide, so

enabling the true acid demand value of the original sample to be determined by difference.

Loss on ignition• Loss on ignition is employed to determine the presence of organic

and other gas forming materials present in the sand mixture or its individual constituents, including new and reclaimed sands.

• A weighed sample of pre dried material is fired in a silica crucible held in a muffle furnace at 925°C for 2 hours.

• The percentage loss in weight is determined and arises from the volatilization, oxidation and decomposition of substances forming gaseous products.

• These include additions or residues of carbonaceous additives such as coal dust, chemical binders, cereals, and carbonates in sea sands.

• Although the test does not discriminate between such sources, it does in practice provide a check on the consistency of binder contents and the condition of new and reclaimed sands.

• Actual volatiles can if necessary be separately determined by similar tests conducted in inert atmospheres.