Inorganic process: advantages and sustainability of producing inorganic foundry cores

In a foundry context, the binding systems used for cores formation can be divided into two categories based on chemical properties:

• Organic binders
• Inorganic binders

Organic binding systems are based on polymer resins that, through a polymerization process, give the mold the necessary strength for use in casting.
Among the most widely used systems for the production of cores are:

• Polyurethane Cold-Box
• Shell Moulding
• Hot-Box and Warm-Box

These systems allow for the formation of geometrically complex cores and the production of equally complex castings.
However, they have significant limitations, including: emissions of dangerous substances, bad odors and fumes during moulding, combustion and thermal cracking problems during casting, resulting in the loss of mechanical properties and the development of gases that can compromise the quality of the casting causing porosity or blowholes.

On the other hand, inorganic binding systems are made up of products that do not undergo thermal degradation and are not susceptible to “burning”. In the past, the only inorganic system used for the production of cores was Sodium Silicate, hardened through a carbonation process with CO2. This process is odorless both during moulding and casting and does not emit fumes, odors, or hazardous substances, but it does produce a certain amount of gas due to the crystallization water and the thermal decomposition of the sodium carbonate that forms during hardening.

Inorganic cores

Foundries are constantly attentive to design requirements and strict environmental and safety standards (UNI EN 13725:2004 and European Parliament Directive 2010/75/UE).
In the production process of foundry cores, the greatest difficulties are represented by the combustion and thermal cracking of binders during casting, resulting in the loss of mechanical properties and the formation of gas, which are potential sources of porosity and cavities within the cast.

The emissions of dangerous substances, fumes and odors are not limited to the casting phase, but also apply to moulding. Despite efforts and improvements in the overall process, attention is shifting towards the use of alternative binders, in particular, inorganic binders.
Inorganic binders use only water as a solvent and their binding matrix is made up of a mixture of silicates, phosphates, and borates, in varying percentages depending on the desired properties.
Inorganic cores are widely used in foundries for the automotive industry in Germany, as no combustion products are produced during moulding and casting (fumes, BTX, BTEX, amines, odors).

Currently, in terms of total volumes, inorganic binders constitute a relatively low share of binders used globally, but their use in foundries has been increasing in recent years.

As a binder, a modified sodium silicate aqueous solution is used: the main difference is probably due to the additive used, the composition of which is not specified.

Sodium silicate is a binder obtained by fusing high-purity silica sand and sodium (or potassium) carbonate at temperatures above 1300°C, following the reaction:
SiO2 + n Na2CO3 -> SiO2 * n NaO + CO2.

The product obtained is a soluble glass, slightly coloured green and blue due to impurities (mainly iron oxides) present in less than 1%.
Available in liquid form, it is obtained by dissolving sodium silicate in an autoclave with steam at 2-3 bar.
The higher the silica content, the more difficult the solubilization, which in some cases takes place directly during production.
Sodium silicate is mainly classified based on two parameters: the modulus (ratio of silica to soda) and the concentration, and in foundry also the viscosity.

Depending on how they are modified, silicates can undergo four different chemical reactions:

• Hydration/dehydration: hydration involves adding water, while dehydration is the removal of water. Only silicates with a SiO2 proportion of less than 1Na2O:4SiO2 can be dissolved in water. Usually, a modulus of between 2 and 3 and water in the percentage of about 50% is used. Commercial value soluble sodium silicates include orthosilicates, metasilicates, disilicates, and other silicates
• Reaction with metal ions: Soluble silica reacts with all metal ions to form the corresponding insoluble, long-term stable silicate
• Precipitation/gelation: Liquid silicate solutions are destabilized when the solution pH is brought below 10.7 with the addition of an acid. The result is a colloid or gel, depending on the concentration
• Surface charge modification: This phenomenon occurs only with dissolved silica, which has an anionic charge of 2. Silica can donate its charge to other materials and negatively charge them, causing dispersion and deflocculation effects

For the agglomeration, some of the described chemical reactions can be used. The amount of binder required varies based on the particle surface area, moisture percentage, chemical composition, and hardening method, and is typically between 1% and 4%. The silicate coats and binds the particles through dehydration or gelation. The bonds formed by dehydration are glassy and strong and can be dissolved in water, unless post-treated at temperatures above 250°C.
The bonds formed by dehydration are strong but soluble in water, while those formed by gelation are weaker but less sensitive to water. Sodium silicate is environmentally friendly and does not emit odors during the forming and casting phase, however, the difficulty of decoring and smoothing during casting can be solved by adding additives.

The choice of the type of silicate to use depends on its modulus, which influences solubility, viscosity, pH and other properties.

Physical Hardening Processes

The physical drying processes of cores with inorganic binders, such as Hot-Box and microwave, improve the mechanical strength by 60-100% compared to the previously mentioned chemical processes.
During these processes, the binder becomes a glass film that holds together the particles.
Depending on the technology used, the reaction speed varies and can therefore influence the robustness of the bonds formed.

Physical hardening processes are characterized by a low amount of binder, short periods of time (for example, the Hot-Box cycle time is between 20-70 s at 210°C), and the partial reversibility of the drying, which allows for easy removal, such as by absorption in water.
However, absorption causes problems in storage in environments with high relative humidity, for which additives such as reducing monosaccharide and organ silane products are used that increase the long-term stability of the cores (96 hours) even with 81% relative humidity, as well as resistance to flexion up to 25%.

The importance of the amount and distribution of water in the binder system is a crucial element. Sodium silicates are micellar colloidal systems and the water molecules inside are not equal.

We can distinguish three types of water based on the amount of energy to be removed:

• Free: it is easily released during heating above 100°C
• Weakly bound: like free water, above 100°C it is 100°C
• Tightly bound: it is found in the adsorbed layer of micelles and can only be considered completely dissociated after firing at 900°C

The content of free water in solution increases with the reduction of the Na+ ion concentration. The distribution of water is determined through DTA and TG analysis, but these techniques are not able to distinguish between weakly bound and free water.

With respect to the choice of the binder, there is a relationship between the flexural strength (in MPa) and the amount of binder and the modulus of the silicate.
The flexural strength increases with the increase in modulus.
This can be explained in that the modulus has an impact on the size of the colloidal particles and the structure of the silicates:

• The acid silicate gel is composed of a combination of very small particles and larger ones, to achieve the maximum coordination number of micelles
• With the increase in the number of small particles, the degree of polycondensation decreases
• Low residual tension in the gel
• Absence of reaction products and a reduction in the content of free water
• After dehydration at temperatures above 200°c, the content of bound water is around 2%

Additives

Regarding additives, it is important to know that the solidification of inorganic binder has a certain reversibility. This means that, in the presence of rehydration, bond destruction occurs.
However, this effect can be prevented by using additives, which can be of organic or inorganic nature.
The quality of the inorganic binder produced by a company mainly depends on the additive used, and often precise information is not available.

The lack of additives causes a reversible dehydration that reflects in a rapid deterioration of the bending resistance of cores in stock.
The loss of resistance during storage can be expressed with a simple formula that considers the bending resistance after 2 hours (σ2h) and after 96 hours of storage (σ96h): S(96) = [(σ2h – σ96h) /σ2h] *100 (%)

Thermal stability and surface of castings

Regarding stability and surface of castings, having a high-quality surface in castings is an essential requirement in the production of cores. Organic binders have a positive effect in this regard as their surface layer of anthracite forms a protective barrier between the metal and the core, and their cushioning action that counteracts metal static pressure prevents metal penetration between the sand grains.
This is not possible with inorganic cores, as there are no combustion products and the volume of generated gas (water and air) is much smaller.
Consequently, for inorganic cores, another solution must be adopted using additives that reduce the wettability of the core by the metal and increase the degree of compaction, making it more difficult for the metal to penetrate the surface.

Emissions

During casting production, foundries must deal with not only the issue of reducing harmful emissions, which can be solved by using water as a solvent, but also odor emissions.
To assess the behavior of cores created with different processes after casting, a practical evaluation method has been developed to measure odor emissions, in a project funded by the EU in collaboration with the Institute for Foundry Practice (IFG).

In summary, the use of inorganic binders has many advantages:

• Increased environmental awareness due to the absence of emissions from combustion and thermal cracking products
• Reduced gas porosity within castings due to the reduced amount of gas emitted
• Excellent surface quality of castings, similar to or even superior to that obtained with organic processes
• Reduction in costs for air cleaning
• Reduction in maintenance/replacement costs as no carbon residue is formed
• Absence of odors during formation and casting, leading to improved working conditions

However, there is no doubt that the system also has serious problems: due to their natural properties, inorganic cores tend to accumulate moisture from the air. This material seeks to balance its moisture with that of the environment and/or temperature. In high humidity and temperature conditions, this means that polymerization is partially reversible, with cores losing their mechanical strength and releasing large amounts of water vapor during casting.

Humidity and temperature must not be evaluated singly: with constant relative humidity, lower temperatures favor longer storage times. An option may be to store cores in air-conditioned rooms, with controlled humidity between 30% and 50%.

In short, there are several options available, but the control of environmental conditions is not always guaranteed. These may include:

• Air conditioning throughout the foundry
• Separate and air-conditioned warehouses
  surveillance of critical areas
• Adaptation of the production process to climatic conditions
• Minimum storage through in-line production
• Optimization of binder systems
• Control of the thickness of the outer shell

In addition to the selection of binders and additives, there are other factors that can influence the storage stability of the cores. An important factor is the thickness of the baked outer shell: the thicker it is, the more stable the core becomes during storage. The formation of the shell, which is crucial for its final strength, can be controlled through temperature and time in the box. The choice of sand also has an impact on storage stability, as too fine sand indicates a higher specific surface area that favors water absorption.

Regarding inorganic moulding, there are various types of refractory sands available for use, including:

• Chromite sand
• Silica sand
• Kerphalite
• Olivine sand
• Synthetic ceramic sand CERABEADS
• Zircon sand

Silica sand is the most commonly used, however, due to issues related to thermal expansion, it is often replaced with pure synthetic ceramic sand or a blend with silica sand.

Grain size composition of sands for inorganic castings

As the binder of the casting system does not generate gases during the process, the cores do not need high permeability, which means that very fine sands with wide grain size distributions can be used.
The choice of sand grain size should be based on these factors:

• Desired surface roughness of the casting
• Ease of filling during the shoot (sands that are too fine or too coarse can be difficult to shoot)
• Ease of gassing (good permeability allows for quicker drying during the gassing phase)
• Type of casting system used (gravity shell, low pressure, or lost foam)

Very fine sands with wide grain size distributions can be used as a specific permeability is not required.

Core shooter machine

The inorganic system has a fluidity similar to Hot-Box, although slightly lower than that of polyurethane Cold-Box.
The shooting pressure must be at least 5 ÷ 6 atmospheres with a plate that has a high number of bushes and a core chamber that extends horizontally.

It is recommended to pay attention to any premature drying, which can be avoided by using humidity-saturated compressed air or a water nebulizer inside the shooting head.
For small cores, it is recommended to use a sand shooting head for 3 ÷ 4 cycles.

As previously mentioned, this process can be considered a Hot-Box system, so it is necessary that the core boxes are made of metal, steel or cast iron, and have an adequate heating system.
The necessary temperatures vary from around 160°C for cores with thin sections, up to 200°C for easy cores with high thickness areas.
The design of the ventilation system is fundamental, as it must facilitate the passage of air.

Core box

The core box can be heated in different ways through:

• The use of burners
• Electrical energy
• Thermal oil
• Water vapor

The use of burners for heating generates carbon dioxide that interacts with the liquid binder and facilitates the hardening of the composite within the shooting bushes.

It is crucial that the temperature distribution is uniform across the entire surface of the core box.

Gassing

To complete the moulding process, a hot air gassing system is required. A high thermal and volumetric capacity hot air generator (gas generator) must therefore be in place. The blowing pressure should be between 2 and 4 bar, with a significant amount of hot air being fed into the core box. The temperature of the blowing air should be between 160 and 200°C.

Removing the cores

The inorganic system is known for its exceptional rigidity, becoming similar to glass once it has dried.
Its mechanical properties are solid, with bending resistance of around 250 N/cm2 when extracted and 400 N/cm2 when cold.
However, being hard and brittle, it does not allow the moulding to absorb any deformations during extraction.
To avoid any breakage, the core boxes and extractors must be perfectly aligned and synchronized. Additionally, special attention must be paid to the draft angles for the same reasons.

Post-curing

To achieve high thicknesses, very long gassing periods are required.
The recommended procedure during forming is to only form the outer surface and then heat it in a furnace at a temperature between 100 and 130°C to complete the drying process of the more substantial parts. Fully drying the inside of the product is crucial to prevent moisture migration and preserve its mechanical properties, avoiding the risk of losses.

In summary, the material used for the production of inorganic cores is a liquid binder combined with a powder additive. This binder is supplied in an aqueous solution that requires storage at temperatures above 5°C and has a low viscosity and a density between 1.1 and 1.4 g/cm3.

To start the core forming process, a homogeneous mixture of powders and liquid binder must be prepared. The amount of powder and binder is regulated by a screw feeder with a weighing cell and liquid metering pumps.
The recommended proportions for the binder and additive are:

• 1.8-2.5% by weight for the liquid binder
• 0.9-1.2% by weight for the powder additive

To avoid the binder solution being influenced by the cold, the temperature of the feeding containers must be kept above 5°C.
Using sand at too low temperatures could reduce the effectiveness of the binder system and increase cycle times. The ideal temperature for the sand should be between 15°C and 25°C.

The machine that creates the cores (sparaanime) and the mixer are interconnected through a hopper that must be kept at a controlled temperature and humidity. Climatic conditions can cause variations in the temperature and humidity of the hopper, as verified by a thermohygrometer located near the mixer.
To prevent the mixture from hardening too soon inside the hopper, it may be necessary to install a nebulizer to keep the air inside the hopper saturated with water.
Additionally, a cooling system at the base may be necessary to keep the nozzles clean, which may otherwise become clogged due to premature hardening of the mixture.
The area near the heated core casing presents the risk of a new premature hardening of the mixture.

The mixture is shot into the mould, the core casing through the core shooter machine.
The required shot pressure is 4-5 atm, and the shot plate must have many sockets to ensure adequate flow.
The temperature of the upper and lower casings varies between 140°C and 200°C depending on the granulometry of the core and must be uniform on the surface of the mould. When the mixture comes into contact with the walls of the mould, an outer shell is formed that maintains the shape and improves resistance to moisture.

The hardening process is supported and accelerated by hot air blowing (120°C – 200°C) at a pressure of 2-4 bar.
The core casing must have an adequate venting system for the passage and discharge of hot air.
After about 40 seconds, the hardening process is complete and the core casing can be extracted manually or by robot.
Since the binder is inorganic, there are no condensate residues on the core casing and cleaning is not necessary every cycle. If necessary, a release agent can be used.
The moulding parameters are specific to each core, as the surface/volume ratio varies greatly.
For example, cores with greater thickness (such as voids) require longer blowing times and
higher temperatures.

Procedure for Mixing

To achieve the desired result, the inorganic system consists of two main parts:

• A liquid binder
• A powder additive

The preparation of the mixture requires a specific procedure that includes:

• Measuring the refractory sand into the mixer
• Adding the powder additive
• Mixing for 10-15 seconds to distribute the additive throughout the sand
• Measuring the liquid part
• Mixing until a complete homogenization is achieved

Adding the powder simultaneously or after the liquid part is not recommended, as this can cause the formation of clumps that are difficult to uniform.
The amount of liquid binder to be used must be determined based on factors such as the type of binder, the type of refractory sand (silica, chromite, CERABEADS, etc.), the grain size of the refractory sand, the geometry of the core to be formed and the difficulty of the mixture to be shot.

Storage of inorganic cores

Hydrosoluble inorganic salts can be rehydrated for cores that are to be stored in low relative humidity environments.
The addition of powder additives reduces the rehydration sensitivity by 60% at 20°C for a maximum of about 10 days (traditional recipe).
If the relative humidity is lower, the storage time increases and, in the absence of humidity, the cores remain stable for years.
Insufficient drying and inappropriate storage can cause:

• Decreased mechanical properties
• Gas formation during casting

The salts can be recovered through a drying cycle in the oven (with caution).
Optimal conditions:

• Temperature: 15-25°C
• Humidity: 30%

The storage temperature also has a strong impact on shelf life: at the same relative humidity of the environment, low temperatures favor longer storage.

Coating of the cores

The coating of the cores produced with the inorganic system is possible for several reasons, such as:

• Increasing the visual presentation of the castings
• Applying the low-pressure castings
• Preventing reactions between the metal and the mould

Currently, the use is limited to alcoholic-based refractory coatings, as the binder is not sensitive to the paint solvent.
New systems are also being explored to allow the use of water-based refractory coatings.

Sand Core in IOB in HPDC

The reason why the inorganic method is used in the HPDC process:

• Cores are produced with a reliable form and process
• Core production is environmentally friendly, avoiding dangerous materials and emitting no odors or gases
• Cores have high resistance
• Cores can be easily painted with water or alcohol-based products
• They are easy to extract from casting with high-pressure water jet
• Sand cores can be regenerated and reused.

Critical aspects of the HPDC system:

• Cores must be made from a suitable mixture of silica sands
  a specific binder is required to achieve adequate resistance
• A water or alcohol-based refractory coating is essential
• Core design requires extra support in the model, complicating the process
• The casting process must be adapted, including the casting systems, filling times, reduced piston speed and appropriate pressure
• It is still a work in progress

The inorganic casting system

The casting behavior of the inorganic system evaluates the following parameters in particular:

• Thermal stability
• Development of fumes and gases
• Cooling power

Thermal stability in casting

The high-temperature resistant inorganic formation, up to around 900°C, can’t suffer from damage such as combustion or degradation as it is made of inorganic materials. This ensures high stability in casting production, preventing problems such as erosion, penetration, or geometric deformations.

Gas and smoke emission in casting

As it is an inorganic system, it is not subject to thermal combustion or degradation, so it does not produce fumes and gases during casting. As a result, the following advantages are obtained:

• There are no porosities or protrusions in the castings
• There is no odor or dangerous emission to the environment
• There are no deposits of carbon or pitch in the shells
• Reducing the heights of the moulding materials necessary to prevent the formation of gas in the core

The cooling power in molding

The system has high thermal conductivity.
In organic mixtures, the polymer that coats the refractory sand grains of the core tends to have an insulating effect on thermal conductivity and prevents the transfer of heat inside the moulding.
On the contrary, the system has the characteristic of conducting heat very well, cooling the surfaces of the casting that come into contact with the moulding more quickly.

The castings created with inorganic cores have the following characteristics:

• Greater density
• Finer and more compact metal crystal lattices
• Reduced formation of fins caused by the expansion of silica sand
• Cooling rate comparable to that of the shell with lower residual stresses after cooling

A faster cooling rate allows for a shorter cycle and a reduction in the volume of the feed system.

Mechanical decoring

One of the tasks of the powder additive is to react with the liquid binder at high temperatures (T > 450°C).
This interaction between the two components produces a binder that is even more “glassy” than what is obtained during casting.
During mechanical cleaning, the casting is very fragile and efficiently absorbs the vibrations generated by the cleaning plant.
Cleaning times can be reduced by 1/5 to 1/10 compared to organic systems.
Only thermal cleaning does not produce results.