The processes and industry behind the manufacturing of sand cores is constantly evolving to drive more environmental friendly solutions while also improving the cost and cycle times. Various well-known companies are specialized to offer solutions to modern foundries from sand preparation, core shooting, core hardening and core removal.
For prototyping and small series the additive manufacturing known as 3D-printing is leading the way. For large series the cold box process and inorganic core manufacturing are both evolving while inorganic processes are constantly gaining market share due to raising environmental standards around the globe.
The key concept behind inorganic binders is that heat enables hardening of sand cores. Unfortunately sand and binder compositions have a limited heat conductivity. Therefore the heat is applied externally and transferred as close as possible to the sand cores via potentially complex core boxes. The heat transfer from the core box to the sand cores is supported by good heat conductivity from aluminum to sand. Low heat conductivity within sand are causing shell formations during the curing process as the heat can´t be transferred as fast from the outer core to the inner sand core. Financial aspects do not always justify a 100% curing of the sand cores due to the required times.
The project “ACS - Advanced Core Solutions”, developed by Soplain GmbH, is therefore focusing especially on the heat generation directly inside the sand cores by the application of electrical currents. This underlying principle of heat generation due to electrical resistance is well known and now applied to sand core manufacturing processes. The benefits are not only faster sand core hardening but also better energy efficiency and reduced maintenance effort as external heat generation outside the core box could be eliminated.
The concept of generating heat by electricity is not new. Initial patents have been filed back in 1970 while they never succeeded. The main reason was that core boxes are usually made from aluminum or steel. Any application of electricity will therefore result in short circuit once electricity is applied. The newly patented processes tackles this issue by adjusting the core box material to align the electrical resistance of the core box material with the electrical resistance of the sand binder mixture.
As a consequence the electrical resistance between 2 electrodes is almost the same at any given point and therefore a homogenous flow of electrical current through the core box and sand core is ensured. The positive effect is that heat is directly generated inside the sand core and each part is equally heated up. This results in better sand core quality due to homogenous hardening and allows an immediate use of sand cores afterwards.
The team behind ACS - Advanced Core Solutions has demonstrated the process on a lab scale and is currently preparing a validation on regular industry core shooters.
1. Experimental approach
Dozens of existing inorganic sand binder mixtures have been tested to measure the electrical resistance temperature curve. For this approach a sample size of various sand –binder mixtures were heated up constantly by applying electrical current and the electrical conductivity and temperature was measured. With this approach the optimal electrical resistance has been identified to apply the maximum energy level into the sample.
Picture 2 shows the typical curve which can be measured.
There are usually 3 main phases when applying electrical current to sand-binder mixtures:
Phase 1: Initial resistance. After applying the electrical current the initial resistance needs to be overcome. Once the sand-binder mixture will heat up this resistance will drop as with temperature increase more electric charge carriers will be available.
Phase 2: Constant reduction of electrical resistance till the sand-binder mixture reach about 100°C and the electric charge carriers evaporate. At the end of this phase the maximum energy can be applied to the samples. The electrical conductivity in this phase also defines the ideal conductivity (red dot in picture above) of the core box material to ensure the resistance is nearly identical between the sand-binder mixture and the core box material.
Phase 3: Resistance start to increase as the electric charge carriers have been removed from the core. The temperature (depending the pressure) is now above 110 °C and with higher temperature the resistance constantly increase further.
During tests the inorganic binder used required with conventional core shooters about 150°-180°C. This defined temperature is usually defined to ensure no damage to the binders due to overheating while at the same time the temperature is chosen as high as possible to ensure a better transfer of the heat into the sand cores. With the newly patented process these working temperatures could be reduced as it is not necessary to run the core box with higher temperatures as the required heat are directly achieved inside the sand cores.
The electrical conductivity of the core box material was based on the temperature resistance curve of the sand-binder mixture. If the electrical resistance core box material differs massively from the sand-binder-mixture then the electrical current will either flow only through the core box or through the shortest path across the sand core. In both cases the sand core will not be equally heated as demonstrated in picture 3.
2. Advantages of newly patented process:
Various advantages have been identified during the lab trials and during preparations for the pilot project.
The main benefits are expected for large sand cores with a high annual volume. This is related to the effect that especially in larger sand cores the equally generated inside the sand core will demonstrate the highest quality and efficiency gains. The results should be larger sand cores without shell formation which can be instantly used after the curing time. Furthermore can this process be applied also to larger family core boxes as each sand core cavity could be equipped with an individual electrode pair and consequently controlled according to the sand core thickness.
Any external heat generation can be eliminated which will not only positively influence the energy consumption per core but also the maintenance, space and capital requirements.
The barrier for application in foundries has been drastically reduced as existing inorganic binder compositions can be applied without any required reformulation. Also existing core shooters can be reworked to adapt to the new core box and controlling unit.
All in all a significant cost and time reduction of up to 33% is foreseen versus conventional core hardening processes. This is mainly driven by 3 aspects: Energy consumption, cycle time and maintenance cost. All benefits are currently explored during lab trials and will be confirmed in identified projects in the near future.
At the moment the new technology promises a high potential to improve the efficiency of sand core manufacturing while reducing the energy consumption. The foundational feasibility studies and lab trials are almost completed. At the moment the preparations for a scale-up to industry equipment is underway with identified pilot partners to demonstrate the benefits during a fully automated manufacturing process. Further results will be shared in the near future.
We are constantly looking for interested partners to apply this technology and quantify the benefits in a continuous production environment. Contact us to evaluate how this technology can help to improve your production.