Discover all the secrets of producing foundry cores using the Cold-Box process: learn about the usage of sand and additives, the different types of processes, and most importantly, understand the impact this process has on the environment.

Introduction

Cold-Box technology is a series casting sand core production process based on a reaction between sand and resins at room temperature, accelerated by a catalyst.
It has been used for over 50 years and continues to be the main method of producing sand cores due to its wide range of applications, efficiency and economic convenience.
Cold-Box cores have exceptional strength and dimensional accuracy properties, satisfying the needs of modern castings.
In addition, technical specialties such as elasticity, thermal stability, and reduced gas generation are constantly improving.

Cold-Box binder systems allow for:

  • Best quality castings
  • Thin wall sections
  • Extremely complex casting structures
  • Processing of various alloys
  • Highly automated production

The benefits for foundries are:

  • Established and robust process
  • No need for heated core box
  • Quick cycle times
  • High flexibility in the choice of equipment materials (plastics, wood, metal)
  • Excellent core storage life
  • Excellent sand reusability
  • Minimum binder addition required

The method involves the chemical transformation at room temperature of a combination of silica sand and a two-component binding agent. The mixture is shot into the mould and passed through a catalyst.
In particular, for the Cold-Box polyurethane technology, a binding agent composed of two resins is used: a benzylphenol resin in organic solvents and a polyisocyanate in solvent.
The components are mixed with the sand in a discontinuous machine and then shot into the casting machine. After injection into the mould, a tertiary amine (DMEA, DMPA, TEA, DMIPA) is added and the catalyst is removed through the air. The catalyst is a reaction accelerator, but it is not consumed.

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The procedure involves the cold polymerization of a mixture of quartz sand and organic bicomponent binder. The catalyst extracted during the gasification phase and subsequent washing cannot be discharged into the environment as the impact would be too strong. To reduce emissions, the air used for gasification and washing is collected and sent to a scrubber where an aqueous solution with acid is used. The binder content varies between 0.9 and 2%, depending on the fineness of the sand used and the type of casting. Using fine-grained sands can improve the surface characteristics of the cast, but would require the use of a larger amount of resin and more difficulty in venting gases.

The cold casting process has many advantages such as low costs and lower energy consumption compared to the Hot-Box, good mechanical strength at room temperature, high productivity due to rapid hardening, optimal stetting and smooth surfaces. This process allows for the creation of large, uniform cores without the possibility of core breakage; in hot processes, core breakages might occur. However, cores created in the cold process are less thermally stable and emit an ammonia odor during casting.

The molding plant consists of three essential elements:

The Cold-Box production process of cores is a system that does not involve heating the core boxes during production. For over fifty years it has been a widely used technology thanks to its high productivity and ability to create large cores. This process is the preferred choice for foundries, as it holds a market share of over 80%.

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The material preparation for molding is carried out through discontinuous or continuous mixers. Different types of sands, such as quartz, zirconium or chromite, can be used as long as they do not contain alkaline components, especially basic metallic oxides. The sand used should not exceed 30°C and the processing time should not exceed two hours. After casting, the core is hardened in the core box through a concentrated ammonia mist.

The method of core production with reduced catalyst needs and short gassing times is crucial for economic efficiency and environmental impact. The gassers must ensure precise dosing of the catalyst, generally liquids at room temperature, which is transported to the core areas via a carrier such as air, so that it hardens uniformly and quickly. The air used for transportation should be substantially dry.

The binder material consists of a two-component organic system, namely a phenolic resin (component 1) and a polyisocyanate (component 2). This system hardens quickly at room temperature in the presence of a catalyst to ensure the core has sufficient durability for further handling.
Component 1 is a 55% catalytic solution of phenol-formaldehyde resin, where the solvent serves to dilute the highly viscous resin and influence properties such as reactivity, moisture resistance, and sand durability. Component 2 is primarily composed of difenylmethane diisocyanate or MDI for short (about 80%) and is diluted with solvents to achieve specific system properties.

The liquids used as diluents include aromatic (naphtha) and aliphatic compounds, polar carboxylic acid esters, plasticizers, fatty acid esters, and organic silicates.

Recently, efforts have been made to limit emissions such as odor or BTX substances by selecting specific solvents. These have been banned from many Cold-Box forms due to their potential for BTX release. However, it should be noted that the majority of BTX substances come from the phenol-formaldehyde resin and isocyanate, which make up about 70% of a Cold-Box formula. Therefore, BTX production is inevitable due to the basic chemical nature of a Cold-Box system, regardless of the solvent composition. Furthermore, the origin of BTX substances is controversial. High temperatures and reduction effects, such as in cast iron, can lead to organic substances that recombine into benzene derivatives via intermediates.
To improve sustainable and economic efficiency, an effective measure is to improve the efficiency of the binder by increasing reactivity and reducing the total amount of binder used.

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Through innovative resin synthesis processes and targeted solvent combinations, it is possible to create binders with exceptionally high performance compared to conventional systems. The mechanical properties of sand cores made with these systems are comparable to those of standard products, even with a 25% reduction in binder. The increased reactivity leads to a significant increase in immediate strength. This initial strength is often a limiting factor in core production. After their creation, cores must be strong enough and fully hardened in all their parts to be treated in further processing stages such as handling by robots, joining or coating.
In the core production process, a reduction in binder also has a significant potential for saving on ammonia. This not only brings benefits in terms of odor but also in terms of material costs. In addition, reducing ammine in the core improves moisture resistance. Increasing reactivity is therefore an efficient way to optimize the entire core production process, for example, by reducing gassing times or the tendency to stick.
However, it is important to consider that an increase in ammine core can have a negative impact on moisture resistance.
Ultimately, increased reactivity allows for optimization of the entire processing chain.

Sand characteristic

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The sandy material used can be new or recycled, with rounded or angular shapes and different granulometries, but it is essential that it is dry and free of dust.

  • A rounded shape of sand particles guarantees greater mechanical strength with the same amount of resin
  • On the other hand, the angular shape requires a greater amount of resin due to the greater surface area. Greater fineness of sand particles or the presence of dust results in an increase in the required amount of resin
  • Moisture represents a critical factor and can be present in the sand, in the air during shooting, in the washing air, or during core storage. The presence of moisture can cause defects in casting and reduce storage time. Conversely, the absence of moisture is essential for obtaining cores with good mechanical strength and a reasonable storage time
  • The optimal temperature for sand use is between 20 and 25°C, but it can be acceptable between 10 and 30°C following the appropriate precautions during the production process
  • With cold sand, the reactivity of the mixture is slower and less fluid
  • On the other hand, if the sand is too hot during the mixing phase, evaporation of solvents present in the resins can occur, compromising the mechanical strength of the core and causing resin adhesion to the core box
  • The sand and resin mixture should be used as soon as possible, unless it is a Cold-Box process – epoxy/acrylic.If the mixer serves multiple machines, it is important that it is programmed for the quantity and type of mixture required by each one
  • The sand mixture should not be left in the mixer or on the transport belts, as it may harden, accumulate moisture, or be contaminated by dust, thus compromising the bench life
  • When the sand and resin mixture enters the bench life loss phase, it is no longer usable, and the produced cores, even if apparently acceptable, will not have the required properties

It is essential to pay attention to the preservation of the materials used. The most common source of contamination is water. Water represents a critical issue because it interacts with the B component resin, generating CO2. This reaction causes the formation of crusts and the hardening of the product. To prevent contamination, it is necessary to store the components away from the weather.

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Cold-Box Phenolic – Isocyanate

In the 70s, self-hardening resins were modified to initiate the production of cold-formed small and large series moulds. Fast production, the ability to use wooden or resin mould boxes, and low costs favoured their spread. Later, ongoing research by manufacturers brought further modifications, contributing to the further expansion of the process that involves using phenolic and isocyanate resins.
In the production of moulds suitable for all types of metals, this method is widely used and has been the most popular for a long time.
The sand and resin mixture can be prepared with any type of mixer.
Once the mixing is completed, it is transported to a hopper located on the moulding machine, ready to be introduced into the cartridge or shooting head (these are essential for proper mould filling and are generally patented by machine builders).
However, with the evolution of machines, the use of the cartridge has been reduced as the tube that contains it is treated with special paints that prevent the accumulation of sand on the walls and do not affect the functionality of the shot.

  • Regarding the shooting phase, it refers to the application of pressurized air to a sand mixture through the sudden opening of a valve calibrated for volume of the cartridge or shooting head. The shooting head is the part that is placed before shooting on the mould to obtain its filling
  • The shooting plate, which supports the mixture ready to be introduced into the mould, must be equipped with appropriate holes or flanges of specific sizes and configurations for each mould or moulding to be produced
  • Usually, the shooting plate has a thickness of 10 mm. In this case, the hole or flange must have a conical shape for 8/9 mm from the side in contact with the mixture, thus avoiding the presence of the latter on the mould at the time of lifting the shooting head after its filling
  • The positioning of the holes or slots on the plate for the passage of the mixture must be made in correspondence with the least subject to wear areas of the mould or moulding, and without being directed towards the model figure or towards any air evacuation filters, as this could cause the breakage or immediate closure of the latter
  • For the Cold-Box procedure, it is important to maintain moderate shooting pressures. It is recommended to place filters in the points where the mixture is not well pressed. These filters will also be useful during the gasification phase
  • If the mixture is not well compacted, residual dust may form inside the mould, which will require frequent cleaning. If the compacting of the mixture is not adequate, metal inclusion may occur during the casting phase.

Cold-Box Phenolic-Alkaline

These binders are recommended for making cores and moulds with low odor emission.
They are suitable for all types of metals, but particularly recommended for steel.
These binders are not recommended for complex cores, as they are strong during casting but fragile during handling.
They are suitable for manual core and mould making, but not very flowable during automatic formation.
For mixture preparation, a mixer with radial blades and volumetric feeders is recommended.
Heated carbon dioxide is used as a catalyst.

Regarding use with steel, these binders have the following advantages:

  • Reduced odor during shaping
  • High thermal resistance during casting
  • Excellent stetting

However, if used with metals other than steel, they may have some drawbacks such as:

  • Difficulty in cleaning the casting
  • Limited shelf life of the mixture
  • Long production time

Cold-Box Silicate

One of the first sand binders, which is still in use today.
Mixing is difficult due to high density and usage percentage.
This process has been surpassed by more advanced technologies despite being simple to implement.
However, it is still used in some steel foundries.
Uses CO2 as a catalyst.

Advantages of this binder include:

  • Long-term storage of the mixture
  • Reduced system cost
  • High thermal resistance during casting
  • Ability to recycle the mixture
  • Zero environmental impact

On the other hand, disadvantages include:

  • Reduced fluidity of the mixture in molding
  • Difficulty in cleaning the cast
  • Limited sand recovery
  • Short life of the mixture if exposed to air
  • Gas development during casting as with other resins
  • Absorption of moisture by the molds, making it difficult to use if not in short periods

The casting mixture consists of silica-based sand bound by a solution of sodium silicates (2-3%) and hardened by carbon dioxide. Additives in the form of resins improve the compaction and disintegration process. The casting process involves blowing carbon dioxide into the formed mixture.

The characteristics of the produced castings are:
Variable weight: from small parts to 1000 kg
Finished surface superior to earth casting

Suitable for mass production with short realization times and can be automated
Regenerable at 40%.

Cold-Box Epoxy – Acrylic

Suitable for the production of large and complex cores, it is an innovative process.
It consists of a mixture of epoxy-acrylic resins with the addition of peroxide and catalysed by sulfur dioxide.
The speed of solidification of the core and the initial mechanical resistance allow for immediate use, which, if automated, allows for parallel work with an automatic moulding plant, also thanks to the low cleaning required for the core box.
The sand-resin mixture has an unlimited lifespan if protected from moisture and can be used even years later, if carefully stored in a container.
Sulfur dioxide (SO2) is used as a catalyst.
The process is flexible and allows for the production of small and medium series of any size.

If the core production is carried out with semi-automatic machines, the process will be slowed down due to the emission of sulfur dioxide (SO2) odor from the freshly produced core.
The operator will then have to wait the necessary time, based on the size of the core, before being able to extract it from the core box.
However, if the production is carried out with automatic machines, the process is very efficient.
It is important to use a mixer with scraping blades and volumetric dosers to mix the resins, as these are sensitive to temperature variations.

In addition, the peroxide used in the process must in no way come into contact with copper materials. It can be mixed with the resin or concentrated.

This process has many advantages, including:

  • Minimum production times
  • Compatibility with all types of metals
  • Unlimited life of the mixture
  • High fluidity of the mixture
  • No cleaning of plants (unhardened mixture can be reused)
  • Minimal cleaning of the casting box (if nonexistent)
  • No cleaning time required at the end or beginning of the shift
  • No wear of the mixing plants and almost none for the casting box
  • High cost reduction for medium and large series, of small and medium size produced with automatic machines
    Possibility of 100% thermal or mechanical recovery of sand
  • Compared to other processes, a reduction of 90% of waste castings

Disadvantages of the process:

  • High amount of sulfur dioxide (varies based on regional regulations), and also the handling and connections of the tanks require specific training of the personnel in charge
  • Presence of a sulfur dioxide odor during the handling of the moulds by the operator
  • Continuous ventilation near the operator is recommended during storage
  • Presence of harmful catalysts.
    Environmental and safety issues for operators

The exhaust system must be installed in front or from below relative to the operator to avoid any inconvenience.

The mixture consisting of silica sand and a two-component binder is shot onto the mould.
The polymerization of various synthetic resins, activated by a gaseous catalyst at room temperature, allows the creation of cold forms and cores.
This method allows the creation of large-sized shapes without the need for heating, but the blown air can be heated to accelerate the hardening process.

Shot weight: from 1 to 100 Kg
Final surface quality: good

Environmental properties

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To improve the environmental performance of the Cold-Box process, approaches are being taken to:

  • Replace harmful components with more sustainable alternatives
  • Reduce the amount of necessary additives by optimizing current technology
  • Integrate inorganic elements into organic binders
  • Use additives that neutralize produced contaminants

In the last 20 years, alternative environmentally-friendly production processes for cores have been introduced to the market, which have allowed a decrease in odorous emissions during production and a significant reduction in BTX (benzene, toluene, xylene) values after casting. Methyl ester-based solvents derived from renewable and vegetable sources have been used, replacing aromatic solvents and reducing CO2 emissions.

There are also universal products that dissolve the two components, phenolic resin and polyisocyanate, in aromatic solvents or Cold-Box systems with silicate-based solvents, which contain silicon-compound molecules instead of hydrocarbons and therefore have a lower carbon content, reducing BTX, BTEX, and CO2 emissions.

In the final stages of evolution, many companies have adopted the latest generation of Cold-Box, which has focused on the organic reduction path. In addition to the silicate component in the solvent, the resin molecule has also been modified, partially replacing conventional OH groups with silicate elements.
The use of urethane Cold-Box systems requires a rigorous evaluation of various factors that affect health, safety and working environment.
Critical substances, exposure levels and precautions to be taken are indicated in the 16 points of the safety data sheet of the three components.

Additives

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Additives are essential for obtaining the desired properties in the casting results and for preventing any defects.
They can be categorized as organic, inorganic or a combination of both.

The use of inorganic additives can have a significant impact on reducing emissions in the foundry (such as CO2, pollutants, odors and fumes).

Advantages of the latest generation of additives:

  • Low usage required
  • Minimal gas formation
  • High resistance
  • Almost no tendency to erode
  • Excellent anti-sagging effect
  • In many cases, they allow to avoid painting the cores

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