The role of gas generators in the production process of foundry cores

In the Cold-Box process, a different catalyst is used depending on the type of process adopted for the reaction between sand and resins.
A fundamental equipment for all Cold-Box processes is the gas generator, which varies in components depending on the catalyst used.
This plant is equipped with devices to transform the catalyst into vapor and, in some cases, to heat the washing air when necessary.
In addition, it has tools to monitor the quantities of catalyst and washing air required.

Gas generators are available in various types, including:

• Bubbling with the use of inert gas
• Mechanical injection or pressure injection of inert gas
• Timed injection

Gas generators with differential pressures are preferred.

Gassing Pressures

Ideal conditions are achieved when the core box is properly designed:

• The maximum pressure during amine introduction should be 0.15-1 bar
• The maximum pressure during washing should be 1-2 bar, avoiding amine leakage from the core box

The choice of gas pressure is important because:

• The catalyst/transport gas mixture will be richer in amine if the initial gas pressure is low
• The speed at which the catalyst/air mixture passes through the core will increase if the washing pressure is high

The use of dry air is essential because moisture

• Reduces the mechanical strength of the Part B resin
• Reduces the fluidity of the mixture, compromising the compactness of the core

It is recommended to be careful of moisture, which can come from various sources such as blow air, gasification/washing air, and sand transport air.

Amine Gas Generator

The gas generator, which is the steam generator for amines in the ISOCURE process, must be suitable for the type of amine used.
The higher the boiling point of the amine, the greater the power of the gas generator needed to reach and maintain the necessary temperature consistently.

• In the case of vertical core boxes equipped with well-positioned filters, the percentage of amines to be used, calculated based on the weight of the isocyanic resin (part 2), is 22-26% for TEA, 17-20% for DMEA, and 20-24% for DMIPA. Instead, with horizontal core boxes, the percentage increases due to the distribution of filters, which often causes preferential exits
• The heater inside the gas generator must maintain a constant air temperature for the entire processing cycle, as this is critical to the distribution of amine in the core box and to reduce the emission of unpleasant odors
• An excessively high air temperature can cause the resin to adhere to the core box and filters, causing continuous production interruptions for cleaning. Conversely, if the temperature is too low, the amine delivery times must be increased, which requires a higher quantity of amine. For metal core boxes, the recommended temperature is 40/100°C, with a possible increase if the sand temperature is below 15/18°C. However, for the correct temperature setting, it is important to consider the shape and size of the cores, in addition to the type of catalyst used, as already mentioned
• Excessive use of amine can have a negative effect on the resin, as it can cause easy disintegration of the sand in areas of higher concentration, which manifests as white spots on the core. These white spots can cause a casting defect known as “veining”
• During the gassing and washing process, it is important to maintain a minimum pressure of 0.2-0.3 bar inside the core box to ensure even distribution of the amine. Too high of a gassing and washing pressure could instead cause separation of the overly sticky mixture in the core box
• The tubes used for gassing and washing should be as short as possible and protected from the cold, without increasing their diameter towards the gassing head. A slight suction is suggested where the freshly produced cores rest
• Materials compatible with amines include nylon, Teflon TFE and FEP, Viton rubber, and stainless steel

The phenolic-isocyanate process uses different amines as catalysts, including:

• Triethylamine (TEA): it has a very high boiling point of 80/90°C and requires a long reaction time, but it has low cost and low odor emissions
• Dimethylethylamine (DMEA): it has a boiling point of 34/36°C, which makes it suitable for evaporation at lower temperatures, allowing for a faster distribution in the sand-resin mixture using only hot air, and it has a medium cost
• Dimethylisopropylamine (DMIPA): it has a medium boiling point of 64/65°C and has intermediate characteristics between TEA and DMEA, but it has a high cost
• Dimethylpropylamine (DMPA): it has a boiling point of 65°C and is an alternative to DMIPA, with an adequate reaction time, low odors, but also a high cost

Gas Generator for processes using CO2 as a catalyst

The phenolic-alkaline system uses heated carbon dioxide (CO2) as a catalyst.
To use CO2 in a cylinder, it is necessary to have a heater at the cylinder’s valve outlet to prevent freezing.
The outlet pressure must be regulated using a pressure regulator set at 4/5 bar that feeds a tank maintained at a temperature of 40-50 degrees.
The size of the tank depends on the consumption of CO2, and the pressure inside the tank must always be higher than the output pressure.
A large pressure reducer (1 inch) must be connected to the outlet of the storage tank via a short tube, possibly of the same diameter, up to the gasification plate. However, the tube could accumulate condensation, so it is advisable to position it inclined towards the gasification head or the source tank to prevent problems during the gasification phase.
It is important to keep in mind that:

• To gas the compound, it is advisable to use a pressure of 0.8-1 bar and maintain the same pressure during gasification.
  to obtain the best hardening, small diameter holes (1-2 mm) can be drilled on the gasification plate, positioned at maximum distances of 8-10 cm.
  during gasification, many exhaust filters are not necessary; only the filter corresponding to the gasification hole can be plugged
• The hardening time is fast, about 10 seconds for every 10 cm of depth. For large thickness cores and moulds, it is recommended to place tubes at the gasification head to allow gas passage inside the mould. In this case, it is not necessary for the gasification plate to cover the entire surface of the mould; a disk of any material inserted into the tube that comes into contact with the sand surface is sufficient

Characteristics and operation of the sulfur dioxide (SO2) gas generator

Sulfur dioxide is supplied in liquid form through cylinders or tanks, heated to 40°C to create a pressure of 4 bar that will convey the liquid to the steam generator through appropriate pipes.
The gas generator must be equipped with devices capable of controlling the emission of sulfur dioxide, transforming it into steam and maintaining the set pressure constant.
After the introduction of sulfur dioxide, it is necessary to wait a few seconds before introducing the washing air, which must be kept at a constant temperature during the entire process (55/60°C).
The duration of the operation depends on the operator’s needs since the hardening of the core occurs over time, thanks to the sole introduction of sulfur dioxide.

To prevent the formation of condensation in the pipes, the connection tube from the gasification head to the steam generator must be as short as possible and without diameter increases or depressions.
To obtain a good formation of the core during gasification, the filters applied to the core box are sufficient for this purpose.
The required amount of SO2 for 1000 kg of sand varies from 3 to 7 kg; if the consumption exceeds this amount, a careful check of the entire system is necessary.
The plant that supports the use of SO2 must be built with the utmost attention, using fittings that ensure maximum sealing; although a minimal gas leakage is not dangerous for the surrounding people, it may be necessary to temporarily move away from the place.
The use of SO2 would be impossible with inadequate aspiration and scrubbing systems.

Before being released into the atmosphere, the vapors produced by these systems must be treated after being suctioned.
Typically, the abatement systems consist of one or more towers, through which the amine-rich air from the core shooting machines is forced to pass through a flow of acidic solution that, kept in circulation by a pump, neutralizes the amine and the purified air is discharged into the atmosphere.
Usually, aqueous solutions of phosphoric acid are used.
When the system is made of suitable synthetic material, such as polypropylene and/or polyethylene, the use of aqueous solutions of sulfuric acid is more convenient.

During the operation of the system, the lost water from the neutralizing solution undergoes continuous evaporation and must be regularly replenished with the addition of clean water.
Its efficiency is verified by continuously detecting the pH of the neutralizing solution: when the pH exceeds the value prescribed by the system supplier, the solution must be replaced and disposed of properly.

To achieve optimal operation of the system, it is necessary to follow some guidelines:

• Use a section that is at least three times larger than the entry line
• Limit the number of bends and connections as much as possible
• Maintain a slight positive or negative pressure in the vent line during shooting and gassing operations
• Install a dust trap before the system for reducing amine vapors

It is also important to keep in mind that:

• Excessive pressure inside the vent line can slow down hardening
• Excessive negative pressure during hardening can unbalance the amine flows inside the core box and promote the deposition of resin on the filters

To further improve the operation of the plant, it is advisable to use an extraction hood next to the shooting plate, connected to the exhaust line that goes to the abatement system.
Another precaution is to place the shot blasting machines in a closed cabin.
During the moulding process, some problems may arise:

• Long hardening times
• Incomplete hardening of the cores after gassing
• Too frequent cleaning of the core box
• High consumption of catalyst
• Excessive amine odor
• High rejection of cores.

After completing the assembly of the core box, the operator must perform the following checks:

• Ensure that the installed core box on the machine is clean and that the air passage filters are not clogged with sand residues, if necessary using products to facilitate their removal
• Check that all the holes on the gassing plate are open so that the gas produced by the generator can enter the core box through the shooting hole
• Adjust the cycle times of the gas generator according to the type of core to be produced
• Ensure that the agglomerated sand feeding hopper is loaded and perform a first manual loading cycle to fill the shooting tank
• Verify the correct functioning of the suction or abatement system
• Activate the heating of the gas generator and wait for the temperature to reach the set point. Before starting production, it is advisable to perform some manual gassing cycles in empty mode to partially heat the gassing piping
• Start the automatic production cycle of the machine while respecting the baseline position

Long curing times

The sum of the catalyst entry phases, curing reaction, and washing defines the curing time.
However, in the case of medium and large weight cores, the total curing time should not exceed 25% of the total cycle time of a core shooter machine.
Sometimes, a too long curing time is observed with high-weight cores, which can be caused by an inadequate gas generator capacity or insufficient sizing of the amine transport line.
Cores up to 35 cm thick must cure at the same rate as medium cores if there is sufficient amine volume. Increasing the amount of amine and adjusting the pressures is sometimes sufficient to reduce curing time. It is advisable to install a pressure gauge on the gas plate to set the most suitable pressures for curing, between 0.15 and 0.35 bar for low pressure and between 1 and 2 bar for high pressure.
Sometimes it is necessary to modify the amine flows inside the core box by acting on the position and section of the filters.
The total section of the filters in the core box has the following typical values to consider:

• 3.5 cm2/kg of mixture for entry filters
• 2.8 cm2/kg of mixture for exhaust filters

Cores that are not completely hardened after gasification

After gasification, problems with incomplete hardening of the cores may occur.
In these cases, the operator must prolong the hardening time to compensate for the unhardened areas.
The failure to harden some parts of the core may indicate that the catalyst did not reach the affected areas during the cycle.
The unhardened sand tends to remain inside the core box.
In such situations, the problem is solved by increasing the hardening time and then making the modifications indicated in the previous point.

Cleaning the core box too often

Proper equipment design and adequate sizing of the gasification plant are necessary to ensure that a well-designed and well-utilized core box can work for at least two shifts without requiring significant cleaning operations. Some core boxes for high productions work continuously for three months with only a quick cleaning during lunch breaks.

“Sticking to the core box”
This term is improper: it refers to superficial zones that are not well adhered to the core at the time of extraction from the core box. When the problem occurs in the upper part of the core, it is actually a crack, while in the lower part of the core, resin deposits occur. This type of sticking cannot be eliminated by increasing the release agent.

• The localized sticking defect is more pronounced in areas of the core that are not well compacted, such as the upper part of a porous core. An excess/type of resin in the mixture reduces the sliding ability and leads to resin deposits, aggravating this defect
• Sticking due to the intervention of high pressure is very similar to the previous one and can occur at the entrance filters. In this case, the first hardened layer of sand, not yet thick enough, yields to the premature impact of high pressure. The collapse occurs precisely at the filters and the surface crust detaches from the rest of the core. The phenomenon can occur with both horizontal and vertical split core boxes

Excessive catalyst consumption

Core boxes, when used with well-designed and calibrated equipment and properly positioned filters, can operate with an average DMPA consumption (8/10% of the total Cold-Box A+B resin).
Good = 1 cc/kg
Typical = 2 cc/kg
Excessive = 3 cc/kg

Excessive catalyst consumption can be caused by:

• Incorrect filter placement in the core box, which allows the amine to escape through the thin sections of the core
• Dirty filters, which are often neglected and require more attention to cleaning, even with appropriate cleaners that soften the hardened grains and facilitate their removal
• Low gas pressure, which prevents the amine from being evenly distributed within the gas chamber
• Overall vent filter section larger than the inlet filter section: amine vapors prefer areas of the core with less resistance. This defect is particularly common in horizontally divided core boxes. The problem is less frequent in cases where the amine flow must pass through a thicker layer of sand, which creates enough counter-pressure to promote the diffusion of the amine flow towards the sides
• Excessive depression in the downstream vent line of the core box, which reduces the resistance of the sand to the amine flow and prevents its diffusion towards the sides. During high-pressure gassing, the pressure gauge on the gassing plate should indicate values between +/- 0.07 bar
• Losses along the parting line of the core box, or between the gas plate and the core box, cause the escaping amine not to contribute to sand hardening

Excessive amine odor

The following causes are mainly responsible for the annoying ammonia odor around plants that use the Cold-Box urethane process:

• Leaks of liquid amine from the gasifier: the perfect sealing of the gasifier and all its accessory parts (pipes, gaskets, valves) must be verified to eliminate the odor caused by liquid amine leaks; it is preferable to maintain everything in slight depression
• Leaks of amine vapors throughout the plant: amine losses in the plant, in addition to causing long hardening times and high amine consumption, are annoying for the operator; counterpressures in the lines should be avoided, and the plant’s tightness should be carefully checked
• Residue of amine at the core box exit: a barely noticeable amine odor at the time of demolding is characteristic of a properly hardened and washed core. Improper positioning of filters in the core box or insufficient washing is usually the cause of excessive odor; this can be temporarily solved by prolonging the washing times while waiting to make changes to the equipment

High scrap rate of cores

The Cold-Box urethane process allows obtaining cores with immediate strength higher than that achievable with other methods. Therefore, a high quantity of scrapped cores is not due to the nature of the Cold-Box urethane process, but to different causes that can arise during production.
The main ones are:

• Wrong handling of the core extraction device from the core box
• Poorly compacted core, which breaks in an area not adequately pressed: in this case, the problem can be solved by adjusting the pouring pressure and/or moving the filters; even if the cold-box mixture has good fluidity, the plant must be carefully designed
• Aged mixture: as soon as the part A and part B resins are mixed with the sand, a slow spontaneous hardening process begins. The mixture must be used before the hardening proceeds beyond a certain limit to avoid producing defective cores; this limit mainly depends on the characteristics of the sand and its temperature
• Use of humid sand or compressed air
• Poor mixing due to too low temperature of the components
• Insufficient quantity, for the type of sand used, of one or both components
• Imbalance between the quantities of the two components
• Excessive residual amine inside the core