Foundry sands: the key to producing high-quality foundry cores

The essential component for the production of foundry moulds is sand. For greater castability, some foundries also use water-soluble moulds. Sand moulds are difficult to remove during high-pressure casting, while salt moulds can be easily removed by immersion in water.

Normally refractory sands such as silica, zirconium, chromite, olivine, KerphaliteTM (natural sand based on andalusite), cerabeads® (synthetic alluminosilicate sand) are used; to obtain the necessary shapes and characteristics, a binder is added to the sand.

The crucial factor is the grain size, which determines the quality of the casting surface, gas permeability, and binder distribution.

The criticality of refractoriness is not an important factor for aluminium foundry, while permeability is essential. The mould must be composed of material that allows for the free passage of gases and vapours, and the optimal conditions are achieved with spherical sand grains and high dimensional homogeneity.

As the grain size decreases, permeability reduces, but at the same time, the distribution of the binder improves because the available surface area for bonding between the individual grains increases. Also, the casting quality improves as the possibility of metal penetration into the mould is lower.

Regarding grain size, the ideal condition is therefore achieved with fine sand and low gas or vapor production during casting. However, the binder also has an essential role. Organic binders emit combustion and thermal cracking elements during casting, which can vary significantly, even for the same polymer type, depending on the type of casting, the casting temperature, and if reused or new sand with the same granulometry is used.

The use of fine-grained sand is possible thanks to inorganic binding systems because they are composed of materials that do not undergo thermal degradation because they cannot “burn”.

The sands used for mould production undergo a series of controls before acceptance:

Sieve analysis: a sand sample is sifted through a series of standardized sieves, weighing the fractions remaining on the different sieves and reporting the result to 100;

Percentage of fines (<0.125 mm) and dusts (<20 mm);

Fineness index (IF): the average measurement of the sand grains checked. After sieving, the amount of sand remaining on each sieve is weighed, and the IF is obtained through the formula

IF is the fineness index; p are the percentages retained by each sieve; a are factors that depend on the mesh light and the number of meshes per cm2 of the sieves.

Furthermore, other checks are also performed such as:

• Microscopic analysis of grain shape;
• Permeability: indicates the volume of air in cm3 that passes through 1 cm2 of a 1 cm thick sample tube in one minute, under the pressure of a 1 cm water column;
• Loss on calcination (P.A.C.), which evaluates the weight change after being heated in a furnace;
• Acid demand (ADV), which evaluates the degree of acidity of the sand sample through a drop-by-drop titration with 0.1N NaOH until a pH value of 5 is reached.

Finally, through a thermal and/or mechanical process, the sand can be regenerated to free the grain surface from the binder. Organic binders can only be treated thermally, while inorganic binders also require a mechanical treatment to obtain perfectly unbound sand.

The cores and forms are influenced by the type of surface of the refractory sand grains, as follows:

• Binder dosage: rounded shape and smooth surface reduce binder consumption;
• Permeability: a rounded shape increases gas permeability during casting;
• Flowability: a rounded shape improves flowability during forming;
• Penetrations: penetration problems are reduced with an angular shape that allows the creation of more compact artifacts.

The most commonly used Fineness Index (IF) is considered optimal between 40 and 60 and typical between 40 and 90.

It should be noted that sands with a high IF require a larger amount of resin, worsen the mix flowability, increase gas development and reduce the permeability of the core.

• Spherical: for high resistance requirements.
• Sub-angular: for reducing the risk of mouse tail or crestine defects.

Acid Demand/Requirement

• Ideal: between 0 and 5
• Acceptable: between -5 (acidic) and +5 (basic)
• Acid demand is a relevant parameter as a high value (i.e. an alkaline sand) reduces the life of the Cold-Box mixture as parts A and B react spontaneously with each other.

pH:

• Ideal: as close to neutrality (7.0) as possible
• Highly alkaline sands (pH > 7.0) decrease the shelf life of the Cold-Box mixture. Therefore, pH is a relevant factor.

Contaminants:

• Ideal: limited presence of clay oxides
• There are practical limitations on the use of contaminants:
• Oxides: between 0 and 0.3%
• Clay: between 0 and 0.3%

Temperature:

• Ideal: between 20°C and 27°C
• Acceptable: between 10°C and 40°C
• The temperature of the sand must be between 10°C and 40°C because:
• Too high temperatures accelerate the spontaneous reaction between components A and B within the mixer, reducing the shelf life of the mixture;
• Temperatures lower than 10°C increase the viscosity of components A and B, preventing proper mixing with the sand.

Moisture content

• Ideal: between 0 and 0.1%
• Acceptable: up to 0.25%

Attention should be paid to the moisture content, as if it exceeds 0.25%, the quality of the cores deteriorates significantly; water:

Decomposes part B;

Reduces the fluidity of the mixture;

Decreases resistances and undermines the hardening and rigidity of the core.

The sum of the product present in the last two sieves plus the bottom is the fines content. The acceptability limit is usually ≤1% but depends on the origin of the product.

Silica sands

Silica sands are the most widely used due to their chemical purity and advantageous thermal properties, and are compatible with all types of foundry binding systems. The density of silica is about 2.2-2.4 g/cm3 and has a high melting point, over 1690°C.

SiO2 silica, at atmospheric pressure, exists in three stable crystalline forms in the following temperature ranges:

Quartz -> 870°C <- Tridymite -> 1470°C <- Cristobalite -> 1710°C (m.p.)

In the case of aluminium foundry, temperatures are not reached high enough to allow the transformation from quartz to Tridymite, so we will focus on quartz.

The stable low-temperature phase is referred to as “β-quartz”. At 573°C there is the transformation: β -> α quartz, which is reversible and occurs precisely at the indicated T. This causes a volume expansion greater than 4%, which can cause the breaking of the cores and casting defects such as veins and mouse tails.

The expansion of silica sands is compared to that of other common sands. However, this expansion can be compensated by the use of complex binding systems and additives.

The use of agents that improve the flowability of sand, such as calcium, sodium, potassium, and iron and their oxides, due to their alkaline nature, can significantly lower the melting temperature. For example, for a 99.8% silica sand, a decrease in temperature from 1700°C to less than 1200°C is noted.

Silica sands also have an acidic character, but the presence of impurities such as FeO-TiO2, magnetite (Fe3O4), and olivine raises their pH, making some binding systems unusable. Furans catalysed with acid do not harden, while phenolic urethanes react and harden instantly.

In addition, like all sands, they have low thermal conductivity and a variable thermal expansion coefficient depending on the size of the grains and impurities.

Sands can be natural or synthetic, but since natural sands contain residues and impurities, only artificially treated synthetic sands are used in the foundry.

Regarding the shape factor, although a smaller grain size corresponds to better binder distribution, it is also known that a smaller surface area leads to less binder use, with a consequent decrease in the economic impact on the profitability and competitiveness of a foundry. Tables and formulas are used for calculating sphericity. If you imagine having perfect spheres, with the right vibration, perfect spherical sand grains would compact as much as possible, giving a theoretical permeability of zero, so the gas developed in the casting would not pass.

This theory has allowed us to establish the shape and distribution of the sand.

Regarding the shape of the sand grains, specific terms can be used to describe the shape, such as “well rounded”, “rounded”, “sub-rounded”, “sub-angular”, “angular”, and “very angular” (from high to low sphericity).

To optimize compaction and permeability, it is advisable to distribute sand grains over four different sieve sizes to create a bell-shaped distribution, similar to a Gaussian distribution.

The shape of the sand grains is critically important for mechanical strength, therefore it is recommended to use a mixture containing about 60% spherical sand and 40% angular sand.

The sands currently used in foundries come from various sources, such as quarries, lakes, or recovery, and can be classified based on their chemical composition, such as silica sands, zircon, olivine and chromite sands.

Zircon and chromite sands are exclusively used as additives to eliminate specific defects in castings due to their properties and high cost.

Olivine sand is difficult to use with traditional binders due to its chemical properties and is therefore used less and less.

There are different qualities of silica sand available on the market, characterized by different origins, physical properties and prices. The choice will depend on the specific needs of the foundry and the type of production.

The sand is normally supplied in bags, metal hoppers, tanks with pneumatic discharge or tipping trucks.

Other types of sands

There are different types of foundry sand: chromite, zircon, olivine, aluminium silicate.

Chromite is composed of an iron spinel oxide and chrome (FeCr2O4) with a high Cr2O3 content, usually greater than 37%. It has a melting point of 2180°C and a density of 4.3-4.5 g/cm3. Its grain is generally angular, it is highly chemically inert and refractory. Good thermal stability and conductivity make it suitable for castings of cast iron or steel with hot spots or high weight. However, it may contain hydrated impurities that can cause defects in castings. It has an AFS between 50 and 80.

Zirconium silicate ZrSiO4 constitutes the majority, i.e. more than 90% of the composition of zircon sand. It has a melting point of 2550°C and a density between 4.4 and 4.7 g/cm3. Its thermal expansion is limited, while its thermal conductivity is high and its reactivity with metals is low. The shape of the grains is generally rounded, reducing the amount of binder required.

Olivine is a natural, anhydrous and silica-free magnesium silicate. Unlike silica sand, olivine sand is basic. The melting temperature is about 1400°C. Olivine sand has a lower coefficient of expansion than silica sand and a virtually linear and continuous thermal expansion curve, avoiding the formation of defects such as mice tails and pockets. The density is 3.0 g/cm3 and it has a high thermal conductivity, reducing the use of coolers. The grain shape is angular and the AFS is between 60-180.

Aluminium silicate

Aluminium silicate (Al2SiO5) exists in three forms: kyanite, sillimanite, and andalusite. At high temperatures, these materials transform into mullite and silica. The grain form is highly angular. These materials have high refractoriness, low thermal expansion, and high resistance to thermal shock. They are used in microfusion, often in combination with zircon sand.

There are also special sands used when silica sands do not meet the chemical and physical specifications required for metal production or the mould-to-core ratio.

The main ones are:

Chromite is characterized by its high heat transfer capacity, which speeds up solidification of the liquid metal. It is a widely used material in steel foundries.

Zirconium, once used as the ultimate anti-chill sand, has been drastically reduced in consumption due to the discovery of natural, slight radioactivity. One of its applications in fine AFA 90-100 granulometry is in REPLICAST (or Ceramic Moulding System), where it is used for the layer in contact with the cast, mainly for steel casts.

Andalusite or kerphalite, used as moulding material, allows foundries to obtain cores with the lowest thermal expansion and highest surface quality capable of resisting high casting temperatures, even in the most critical areas. This sand guarantees a dense surface of the mould and therefore effectively resists penetration of the liquid metal.

Mullite synthetic sand or cerabeads has a significantly lower thermal expansion behaviour in the temperature range of 20-600°C, higher thermal conductivity, fire resistance, and other physical parameters compared to quartz sand.

Fused bauxite or sand has high heat transfer capacity, which speeds up solidification of the liquid metal. It is widely used in steel foundries.

Special sands, unlike quartz sand, have better physical and chemical properties that favour their use.

Particle size analysis: it must be compliant with the desired one.

All supplies must have the same characteristics; any difference would result in the need to revise the percentages of binders and the consequent impact on the castings.

• Moisture: the sand should theoretically not contain any. The increase in the percentage of moisture causes a weaker form and greatly lowers the storage time.
• Loss on ignition or calcification: the presence of impurities in the sand is checked as they can cause anomalies that are difficult to identify.
• Acid demand: the higher the value, the lower the duration of the mixture, with a consequent decrease in the mechanical resistance of the cores or molds produced. Foundries that use recycled sand or large-sized cores should be careful in using resins of the same family or evaluating their compatibility. It is recommended to keep the supplies separate; this to facilitate product controls and for any conformity disputes.

The Shell Moulding process uses phenolic resin-coated sands, obtained through a coating process. The reaction conditions, such as the type of catalyst, temperature, and phenol-to-formaldehyde ratio, determine the type of phenolic resin used. The main types of phenolic resin used in foundries are novolacs and resoles.

In the sand coating process, it is possible to incorporate additional substances with the aim of modifying the technical properties of the coated sands.

Typically used additives are:

• Accelerating agents: for faster and more uniform phenolic resin polymerization.
• Anticresting agents: to reduce the formation of crestings during casting; iron oxides are black (magnetite) or red.
• Plasticizers: maintain the necessary elasticity during the bonding phase and reduce thermal and mechanical shocks during casting.

They are widely used in the production of shells but also for cores.

Shell Moulding is a thermal process that uses a refractory coated sand. During exposure to high temperatures above 150°C, formaldehyde is rapidly released and reacts with the liquid-phase novolac phenolic resin (at that temperature), transforming it into a resin called bakelite. In this way, the original resin that coated the sand transforms into a heat-resistant resin that is also stable at high temperatures.

To ensure the quality of the material during the transportation and storage of the sand, it is important to follow specific rules. Firstly, the transporter must use containers dedicated exclusively to the transportation of sand, avoiding any possibility of contamination with other materials, such as cement. In otherwise, special attention must be paid to the risk of contamination of the container content.

Additionally, during the discharge of sand using pneumatic systems, it is important to use the lowest possible pressure and maintain a minimum distance from the storage silo to avoid breaking the sand grains and altering the grain size distribution. Furthermore, in the case of sand used for the Shell Moulding process, care must be taken during discharge as the previously resin-coated and dried grain may lose its cohesive power during the operational process.

To ensure the quality of the material during transportation and storage of sand, it is important to follow specific rules:

• Firstly, it is necessary to avoid contamination of the sand with other materials, such as cement. Make sure that the transporter uses containers dedicated exclusively to the transport of sand.
• Furthermore, if pneumatic systems are used during sand discharge, it is important to use the lowest possible pressure and maintain a minimum distance from the storage silo to avoid breaking sand grains and altering the grain size. In the case of sand used for the Shell Moulding process, it is important to pay extra attention during discharge as the grain, previously coated with resin and dried, may lose its cohesive power during the operational process.
• Ensure that the bottom of the storage silo is equipped with a grid made of a flat iron 8/10 cm high, arranged in squares of about 15 cm, to ensure uniform distribution of the sand. Avoid the formation of preferential lanes in the descent of sand into the silo, caused by different grain size, which could cause dimensional separations of the grains and problems with the processing cycle and the castings themselves.
• Bags and hoppers containing sand must be stored in a dry environment, away from sources of moisture and other materials. Verify that the sand from the last shipment has been fully used before using that from the next shipment.
• Position the silo in a location accessible for replenishment and protected from extreme temperatures.