from Dipl.-Ing. Rudolf Wintgens, Schopfheim
For about 10 years we are expieriencing a renaissance of inorganic core sand binders due to environmental issues and the actual advantages in the foundry process.
Modern inorganic binder systems have compared to conventional CO2 hardening systems a different hardness reaction.
The Hardness reaction takes place mainly through removing the moisture. Hence there are two main requirements from the core shooter and the tooling: The core shooter and core box must provide enough heat for the purge air to remove the moisture from the core box. There are several different heating methods each with their own individual advantages and disadvantages
Along with the two main requirements core box heating and heated purge air, other devices are recommended to ensure a constant high quality and economic production.
The additional devices should stop premature sand hardening and ensure the sand stays flowable.
Since about 10 years inorganic binder is experiencing a renaissance due to environmental aspects and the advantages in the actual foundry process.
While manufacturing cores and while casting no (or very little) organic substances are released. The positive effects for the working area and the environment are therefore economically complemented; due to the fact that no (or very little) extraction is needed which in turn means no emission control system is necessary.
Especially with alloy die casting molds where the contamination of the cast alloy dies through combustible products and binder condensates reduces the efficiency.
Using inorganic binder increases the up time due to a considerably lower cleaning time.
Some inorganic binders can be adjusted to the water solubility needed. With some castings where it is difficult to decore it is possible to wash out the core until the casting is completely free of sand. The Hardness reaction takes place mainly through removing the moisture. Along with the two main requirements, core box heating and heated purge air, other devices are recommended to ensure a constant high quality and economic production. The additional devices should stop premature sand hardening and ensure that the sand stays flow able.
Sand mix and distribution system
Starting with the sand preparation the special characteristics and requirements of inorganic sand mix have to be taken into account.
The binder systems exist mainly out of two liquid and one powder component. For the liquid dosing conventional dosing units can be used. In comparison with the powder additive feeders such as saw dust and iron oxide a lot less additive is needed in the inorganic process therefore the dosing accuracy is a lot higher.
The inorganic sand mix reacts from the loss of moisture with premature curing. This means some effort is needed for the process,
The sand should have a temperature that is close to that of the mixer. It is recommended that a heat cool sifter is used.
The mixer should be a closed unit so that no moisture is lost to the surrounding area. When using one mixer for several machines the distribution bowl and the sand funnel of each machine should have a cover. Even with covers over the containers it is possible for dry sand to form. The dry sand crust should be removed at regular intervals otherwise blow tubes may block and/or the dry sand may cause imperfections in the core. Premature curing, sand crusting and cleaning can be reduced when the moisture loss is compensated. Humidifying via spray nozzles has proven non effective due to water droplets washing binder from the sand corns. A solution is a humidifying system that cold vaporized water be added to the sand mixes hence no condensation, no droplets of water meaning the binder is not washed from the sand.
The cold vaporized water is produced via ultra sound generators (patented by Laempe & Mössner GmbH).
The amount of sand changes due to sand crusting can be reduced because each machine has its own mixer directly above the shoot head. It is also possible to position the mixer directly on the shoot unit so that the mixer base makes the shoot pressure proof. With this system the filling operation is not applicable plus there is no sand deposit on the funnel.
The normal temperature of a core box when curing with heat is approximately 150°-220°C. These temperatures can be reached using electricity, thermo oil or steam.
The thermal expansion should be taken into account when designing the core box. The basic needs for short cycle times are a sufficient amount of vents to allow safe and speedy exhausting of moisture from all areas of the core box.
The two main requirements, core box heating and heated purge air have to be provided by the machine
The different methods of heating the core box have specific disadvantages and advantages:
When working with heated purge air the heater should be installed as near as possible to the gassing plate / gassing hood, to avoid any heat loss in the pipe work. If that is not possible the pipe work should be insulated, or even better trace heated. If the machine has steam heating then besides using the conventional electric heaters it is possible to use a steam heated heat exchanger.
Due to radiation and contact heat, from the heated core box the machine also warms up. The funnel, shoot head shoot, plate and shoot nozzles (if used) should be cooled to avoid premature hardening of the sand mix.
Some of the inorganic sand systems are partly heat active. Cooling the shoot head and shoot plate prevent premature hardening of the sand mix and significantly reduce the cleaning time.
When cooling it is necessary to pay attention that the temperature difference between the cooled components and the ambient temperature is not too high this could cause condensation which could wash the binder from the sand. To achieve production quality and repeatability it is possible to control the temperature of each Individual water cooled circuit
According to which binder system is used the core box temperatures are similar to that of warm box and the hot box process. The machine due to heat expansion has to be designed accordingly. Floating centering points must compensate for any expansion, so that the locating of core box and machine are correct independent of the temperature.
It is essential that the core shooter prevents premature hardening of the sand mix. Direct air contact is made in the sand funnel and at the shoot tubes, vaporizing water in both areas effectively stops the sand from drying. Furthermore the sand mix that is still in the shoot unit comes in contact with air after shooting and shoot exhaust. The loss of moisture in the shoot unit can be compensated with adding vaporized water. With machines where the shoot unit moves after shooting to the fill position should have moisture added when filling and then covered. The same as with the sand distribution it is necessary that the vaporization take place with no water drops.
According to the flow ability of the sand and the position of the shoot tubes it is possible that sand channels or sand bridging takes place in the shoot unit. The sand mix will then only flow in certain areas. The rest sand is then unusable in the system; it dries and can lead to poor core quality. Sand channels or sand bridging can be effectively stopped using a bridge breaker (patented by Laempe & Mössner GmbH) By just turning during shoot and shoot exhaust the bridge breaker breaks down the channeling, at the same time while turning, the sand amount can be calculated by measuring the torque this enables the controls to automatically fill the shoot unit
The mixed sand is generally alkaline and shows traces of various salts. For this reason parts that come in contact with the sand mix should be manufactured out of corrosion resistant.
Handling and storing cores
In comparison with cores made with organic binders inorganic cores do not have the same strength immediately after manufacturing, the core strength improves after drying and cooling down. The core strength has to be taken into account when handling e.g. core removal ,fettling, assembling. The speed and the clamping pressure of the handling in both manual and automatic have to be adjusted accordingly. As in all warm/hot box process the defining should be done at a constant temperature normally after cooling down. Defining at different temperatures can lead to core damage. The cores should be stored in a room with a controlled low humidity to avoid lower quality
Summary and Forecast
When the characteristics of inorganic core production is observed and the process parameters of the binder system can be kept within tight limits, this type of core production can be seen not only as an environmental point of view but as a real alternative for core production.
The necessity to use hot tooling (at the moment) is the main restriction, at this time inorganic serial core production is the only economical method. Due to high investment for the tooling, small batch or single core production is not economical.
So far some of the challenges in the reclaimable area have been solved, mix ability with sand sorts and usability with high melt materials. It is foreseeable that in the near future binder systems will be available, which will through skilful mixing and new machine technology solve the problems.
Due to process optimization in the foundry technology an environmental improvement will take place.