The technology of cores and moulds is adapting to increasingly rigorous environmental challenges, through the adoption of new materials and low-emission binders.
The aim is to produce high-quality components without compromising environmental sustainability.


In metal casting, cores are components used to create cavities within the mould, where the molten metal should not enter. These cavities can be designed for functional purposes, such as the cylinders within engine blocks, or to reduce the overall weight of the component.

The cores are positioned within the mould before it is closed, using the seats created by the core prints of the pattern. However, depending on the casting technology used, the core can also be fully integrated into the mould.

In order to ensure the mechanical strength of the sand core, binders, or chemical compounds that adhere to the sand grains, are used to provide mechanical strength. The amount of binder varies from 1 to 3%, while the remaining 97-99% is sand.

In recent years, environmental standards for the metal casting industry have become increasingly strict (UNI EN 13725:2004; European Parliament Directive 2010/75/UE). Specifically, the most common binders used for core formation, despite their reduced percentage content, emit up to 70% of volatile organic compounds (VOCs). Therefore, the current goal is to replace them with low-emission binders that provide comparable physical and mechanical properties at high temperatures.

Foundry cores: what they are, characteristics, and types


Foundry cores are special components made of refractory material, typically silica or quartz sand with the addition of binders, which are chemical mixtures that help hold the grains together and provide mechanical strength to the core itself.

These cores are used to obtain holes, cavities or special shapes in castings such as grooves or recesses.

Their use is essential for the formation of the internal parts of castings, but they can also be assembled to avoid many mechanical operations.

In some types of castings, the use of cores allows for the creation of undercuts which would otherwise not be possible. Cores are essential for the production of numerous components found in the market.

Foundry core properties:

  • Low or zero gas evolution during casting
  • High temperature resistance
  • Mechanical strength
  • Erosion resistance during casting
  • Dimensional stability in hot and cold conditions
  • No cracking on the surface prior to metal cooling
  • Easy evacuation of the cast
  • Plasticity during the metal cooling phase
  • Compatibility with resins already used in foundries
  • Easy storage (hydrophobic)
  • Low cost.

Sand-made cores must possess essential properties to perform their task efficiently, including:

  • Structural strength, necessary to avoid damage during:

1. Movement from supplier to foundry

2. Placement in casting molds (ramming)

3. The casting itself: despite contact with the high-temperature liquid metal, it must maintain its geometry without bending or breaking.

  • In case the strength provided by the binder is not sufficient, to increase robustness, it is necessary to add iron reinforcements to the core.
  • Permeability: the core produces gas due to the decomposition of resins during contact with liquid metal; these gaseous substances must flow out rapidly to avoid being trapped in the casting, thus causing defects such as blowholes, which would increase the amount of scrap pieces.
  • Flexibility: during metal solidification, the core must avoid tensions in the casting that can cause deformation or breakage.
  • Crumbliness: when the casting becomes solid, the core must be easily removed mechanically or thermally during shakeout (operation before cutting and deburring phases of the casting).
  • Ability to resist to the moisture.


Foundry cores: production



Foundry cores are created using a product final shape to which sand, previously mixed with resin and/or reactant, is added.

The amount of binder is from 1 to 5% while the remaining 95-99% is sand.

The sand and the percentage of resin are defined based on the final product to be obtained and on the foundry processing performed.

Foundry cores can be made in two ways: manually or through the use of automatic machines called core shooter machines.

The latter allows for a faster production especially for medium/large quantities of cores.

In the case of small-sized cores in limited quantities or large-sized or heavy cores that cannot be produced with the core shooter machine, manual moulding is used as the machine’s setup times do not justify its use.

The core shooter machine automatically performs the moulding, gassing, and single or continuous extraction of the cores, reducing production times and maintaining the same production quality with every cycle.

The core consists of the negative (hollow part) of the shape to be produced.

A core container, the core box, is necessary to produce it. 

Foundry cores: types

The types of cores differ based on the hardening process, the category of binder used, and the machines or equipment used for the production of cores and moulds.


Foundry cores


The technologies used for creating foundry cores vary depending on the hardening method used and the machinery employed.

Foundry cores consist of a mixture of sand, generally of a siliceous type, a binder (which can be organic or inorganic), and additives to improve the properties of the molding material.

For the production of foundry cores, steel moulds, known as core boxes, are used, which can be filled with the mixture through automated or manual processes. For example, core shooter machines use compressed air to inject the sand into the core box in a matter of seconds, ensuring high dimensional accuracy.

The core boxes are made of steel for hot-working (UNI X40CrMoV5-1-1KU or H13 hardened), while the structural parts of the mould that are not in contact with the mixture are made of cast iron; with the presence of gaseous catalysts, its halves of the core box are sealed with rubber lining.


Foundry moulds



Moulds are a mixture of sand and additives used to shape and support the contact with the melted metal during casting production. The moulds contains and shapes the external part of the casting and must be strong enough to not deform or deteriorate before the metal solidifies.

There are various types of resins that can be used for moulds production, each with specific characteristics, such as high-temperature resistance, mechanical strength, and curing speed. The choice of resin depends on the material to be produced, such as iron or steel.

The curing of the moulds usually occurs at room temperature through a chemical reaction between the resin and a catalyst added during the mixing phase. For small or high-volume moulds, gas-curing facilities can be used to speed up production.

The mould can be produced above a pattern contained in a box, which is removed after the mixture hardens. To reduce costs and increase strength, it can be produced within an iron bracket, which is removed after the casting has cooled.

The shell is different from the moulds due to its irregular external shape (corresponding to the shape of the pattern) and its nearly uniform thickness, previously defined to ensure the required mechanical strength and reduce costs. It is usually made of pre-coated sand at high temperatures and requires the use of metal models, making the process more expensive and longer, also because of the long production times.

It is strongly indicated for small castings that require dimensional accuracy and good surface quality.

Characteristics of foundry cores

The properties of cores depend on the metal alloy used and the casting technique. However, they must meet strict requirements, the main ones are:

  • High mechanical strength until solidification is complete
  • High heat resistance
  • Resistance to moisture
  • Gas permeability
  • Ease of removal (decoring)

Achieving high mechanical strength on thin sections is essential to ensure casting conditions, however, this can complicate the process of removing the cores from the casting, increasing the required time.

The cores used in foundry must have specific properties to ensure a perfect casting of the melted metal. Firstly, it is essential that they have a high mechanical resistance to avoid damage during the transport and crumbling phase and also to resist the pressure of the melted metal during casting.

In addition, they must be refractory, that is, able to withstand high temperatures without undergoing chemical damage or deterioration of mechanical properties. Permeability is another crucial factor as the core must allow the free passage of gases and vapours to avoid defects in the castings. Permeability is influenced by the grain size and shape of the sand used: spherical and homogeneous grains give the best performance. To ensure lower permeability, greater compaction and smaller intergranular spaces are necessary; this is given by grains as small as possible.

To ensure the free escape of gases during casting in the case of poorly permeable cores, several techniques can be adopted:

  • Hole tubes, called lanterns, can be used and also used as reinforcements for the structure.
  • In the case of large cores, pieces of coke can be inserted inside, which generate porous areas through which gases can escape.
  • The insertion of vent channels through a needle or tapers that create an exit for the gases without compromising the structure of the core.

Moisture tolerance is closely related to the amount of gaseous substances emitted during casting and the robustness of the cores: excessive moisture absorption can result in a deterioration of mechanical properties and a reduction in material properties. Moisture resistance is one of the main problems related to inorganic moulding.

Complete removal of the cores is a crucial requirement for the cores, which can be achieved through mechanical or thermal methods. Organic resins tend to decompose easily under thermal action, ensuring good removal.

In recent years, the automotive sector has seen strong growth in the use of aluminium due to its high specific strength and low weight. The production of these components requires the use of cores to create cavities or lighten the structure. In these cases, the cores must meet the previously mentioned requirements, with particular attention to permeability. In aluminium foundry, refractoriness is not as important as the temperatures involved are relatively low.


Cores or molds: molding materials

For the production of cores, molds or casting shells, the main materials used are sands bound with chemical resins.

Foundry sand


The sands used for moulding cores, moulds, or casting shells come from various sources such as quarries, lakes, or recovery (mechanical, thermal, and industrial washing). They can be Silica, Zirconium, Olivine, or Chromite and have different chemical properties and costs.

Zirconium sand and Chromite sand are mainly used as additives to eliminate defects in castings, while Olivine is less used due to its difficult to use chemical properties with binders.

The choice of sand will depend on the needs and type of foundry production; on the market, there are different sands with different origin and price characteristics.

Generally, sand is supplied in bags, metal hoppers, or by trucks equipped with tanks with pneumatic discharge.

Foundry cores production proces

Foundry cores with Cold-Box process


The Cold-Box process is a reaction between a mixture of sand and resins at room temperature, accelerated by a catalyst in the form of vapor or gas, which passing through the mixture causes a rapid hardening.

The processes are:

• CO2


Foundry cores with Shell Moulding process



The Shell Moulding process, also referred to as the “hot” process, uses pre-coated sand and a hot mould to produce cores, moulds, or shells. The mould, or core box, is typically made of steel. The sand used in this process is referred to as pre-coated, meaning each grain is coated with resins. When in contact with the heat of the mould, the sand softens and the sand grains bond together.

The cores produced through this method are hollow, mechanically strong, and have a smooth and compact surface, suitable for producing medium to small-sized pieces with weights ranging from a few grams to 20 kg.


Foundry cores with Inorganic process


The inorganic process uses a silicate (e.g., liquid glass) as a binder for the sand that is poured into a heated metal mould once mixed.

The sand hardening process is quite long and takes place through the evaporation of the water-based solvent contained in the silicate.

This process does not produce any pollution in the air inside or outside the plant, therefore having a zero environmental impact.


Foundry cores with Hot-Box process


Moulding materials, either loose or shootable, wet, and organic-bound, used in Hot-Box process in the cores production are hardened in heated metal moulds through specific machines, known as core shooter machines.

The heat accumulated during this moulding process of cores leads to a complete hardening of the core itself. This method allows for the creation of cores with high complexity and precision as fluidity and shaping are very high.


Foundry cores with Warm-Box process


Compared to the Hot-Box, it only differs in the type of catalyst used. It is a less common method.


Foundry cores with Thermoshock

In this process, Cold-Box machines are used to fill the core boxes. After closure, the core box continues its journey into the heated oven and is then extracted at the output after reaching the baking point. This method is commonly used by foundries that produce boilers and radiators.

The cast iron cores can be produced through various techniques, depending on the type of binder used and how it is activated.

The most commonly used binders are:

  • Inorganic: Silicates and refractory cements
  • Organic: Thermally hardening polymers or thermoplastics, with one or more components


Main processes for the production of foundry cores


The cast iron cores can be produced through various techniques, depending on the type of binder used and how it is activated.

The most commonly used binders are:

  • Inorganic: Silicates and refractory cements
  • Organic: Thermally hardening polymers or thermoplastics, with one or more components


Main casting processes can be summarized as follows:

  • At room temperature:
    • Gaseous catalyst
    • Cold-Box Polyurethane (polyurethane resins gassed with tertiary amines)
    • ISOSET (acrylic/epoxy resins, gas = SO2)
    • Polyurethane (catalysed with high boiling amines)
    • Furan (catalysed with acids)
    • Phenolic (catalysed with acids)
    • Alkaline-Phenolic (catalysed with organic esters)
    • BETASET (alkaline phenolic resins, gas = Methyl Formate)
    • Sodium Silicates (sodium silicates, gas = CO2)
    • No-Bake (self-hardening)
  • Other processes:
    • Refractory cement
    • Drying oils
    • Silicates and esters
    • Ethyl silicate


Strengths and Weaknesses of Various Processes

The final quality of the core depends on the type of binder used and the way it is set. These factors influence the final results in terms of productivity, costs, cold mechanical strength, etc.

Some of the most important parameters of a moulding process are:

  • Forming time: the time required to achieve the hardening of the binder, once the “crosslinking” reaction has been activated (in the case of polymers it occurs with high temperature and/or catalysts)
  • Decoring and thermal stability: quality parameters that summarize the characteristics of the binder in use
  • Binder percentage: the weight percentage of the binder present in the mixture has consequences on the final cost of the core, on the environmental impact of the process and on the recycling of the sands
  • Bench life: the time from the moment of mixing to the loss of workability


Foundry cores with No-Bake process


The “self-hardening” No-Bake process, in which sand is bonded with a resin and an acidic catalyst through a continuous mixer, is used in resin sand moulding.

The No-Bake process involves premixing the return sand with a fixed percentage of virgin sand, which is then introduced into the mixer along with the catalyst (acid), the binder (resin), and the sand.

Each sand grain is thus wet with the binder; before it hardens, thanks to the effect of the catalyst, the mixture is poured into moulds for the creation of shapes: the pressure of the mixture then takes place manually.

Other processes exist on the market, but they are used less frequently or only for certain production needs.

Foundry cores: the most common defects that cause issues in castings

For castings, the surface quality required is high and this is a fundamental factor in the production of cores. Organic binders have positive effects in this sense, as the surface layer of anthracite forms a protective barrier between the metal and the core. In addition, the gas cushion that opposes metallostatic pressure prevents metal penetration between the sand grains. This is not possible with inorganic cores, as there are no combustion products and the amount of generated gas (water and air) is much lower compared to the previous one.


Possible sources of defects and solutions


Blowholes: Typically found under the casting surface or skin, they can vary in size and be grouped or separated. They appear as round cavities with smooth, oxidized walls.

The defect is usually caused by:

  • Low permeability of the casting sand
  • Insufficient or absent vent holes
  • Excessive amount of resin in the core (too much recovery sand from discarded castings may have been added)
  • Excessive moisture in the sand (mold or core)
  • Excessive release agent on the patterns
  • Paint not fully dry


Pinholes or Pinspots: these are small blowouts, isolated or grouped under the skin.

The defect is usually caused by:

  • Excessive nitrogen and/or hydrogen in the resin, in the recycled sands, or in the paint
  • Contaminated inoculants or paint


Bright Carbon: the metal surface appears shiny and dark in color. The defect can be corrected by using an appropriate paint or iron oxide paint.

The defect is usually caused by:

  • Type of resin used
  • Excessive resin
  • Poorly compressible sand
  • Insufficient ventilation


Burrs Caused by Cores:

Burrs are observed on the outside of the casting. The issue is typically caused by:

  • Beads in the core runners
  • Check the dimensions of the runners in the pattern and core box
  • Imperfect closing of the cope


Burrs are observed in the casting, between the core and the casting.

  • Excessive liquid release agent present in the core box
  • If present on the top of the core, it is caused by air infiltration during the shot
  • If located laterally to the casting and have an irregular shape, they are caused by sand losses during the shot or gassing


Metal infiltration/erosion: metal drags sand into the mold or casting.

The defect is usually caused by:

  • Coarse sand
  • Low mechanical strength of the molds or cores
  • Insufficient refractoriness
  • Stale molding sand mixture
  • Insufficient compaction of the mixture
  • Resins not suitable for metal temperature
  • Insufficient amount of resin
  • Paint not completely dry or improperly used
  • Moisture absorption during storage


Cracks: castings that show cracks.

The defect is usually caused by:

  • Sand used too fine
  • Excessive resin rigidity
  • Overuse of resin in core production


Mouse tails or finners: These are normally irregular drosses. If not resolved, use an iron oxide patina or additives.

The defect is usually caused by:

  • Excessive rigidity of the resin
  • Use of too coarse and round sand for the production of molds or cores


Rough surface: Orange-peel shaped irregularity.

The defect is usually caused by:

  • Presence of oxides in the sand that make it contaminated
  • Excess of catalyst


Casting Defects:

  • Rounded metal/sand clumps
    • Insufficient mixing of resins/sand
    • Core box breakage
    • Excessive gassing
  • Flat, thin and clumpy formations of sand/metal that form on the top of the casting. The defect is usually caused by:
    • Separation of a portion of the top surface of the core
    • Excessive use of resin
    • Paint not fully dry or not appropriate
    • Porosity of paint and sand
    • Resin with high gas development
    • Too fine sand
    • Moisture

C.f. e P.i. 02160640245
REA n° 210595 Vicenza
Cap. Soc. Euro 60.000

Privacy Policy

Primafond - Thiene (VI)

viale del Lavoro 36/38
36016 Thiene (VI) – Italy
tel. +39 0445 361.759