Increasing demands on casting quality and the trend to thin-walled structures for high performance components have led to stricter requirements on the quality, and at the same time, greater geometrical complexity of sand cores. Simulation helps optimize the design of core boxes and establishes robust process conditions for shooting, gassing and curing of organic and inorganic binder systems for cold and hot core boxes.
Simulation of sand core making processes
In the shooting process a blow head filled with sand is pressurized with air, leading to the fluidization of the sand which results in a “fluid” consisting of an air/sand/binder mixture. This fluid flows from the blow head through shooting nozzles into the core box, repelling the air out of the box through venting nozzles. The goal of the shooting is to achieve a density distribution of the sand in the core box as high as possible and at the same time as uniform as possible. Process parameters that can be varied are the shooting pressure and the number and position of shooting and venting nozzles. In order to save time and money it is desirable to use as few nozzles as possible without sacrificing the quality of the core.
Different configurations of shooting and venting nozzles and their impact on the resulting sand density distribution can be analyzed using simulation. Predicted velocities and shear stresses allow the engineers to draw conclusions on the wear and hence the lifetime of the core box.
In organic binder systems the sand is coated with an organic resin. The hardening of this resin is accomplished by a gaseous agent, usually amine, which is typically injected through the nozzles which were used for the shooting. This gassing needs to be sufficiently long that the gas reaches every part of the core in order to ensure that the core is hardened everywhere. On the other hand, the gassing should not be longer than necessary in order to save the poisonous gas.
The simulation predicts the amine concentration distribution in the core over time which is equivalent to the hardness of the core. This allows the engineers to decide on a sensible time-scale for the gassing process.
For an increasing number of castings, inorganic, water-based binder systems are used instead of poisonous, organic systems. Besides the advantage of an emission-free core production process these systems reduce the core gas production during the casting process, improving the casting quality.
For the hardening of the sand core the water must be removed from the core which is typically accomplished by the injection of hot air. For these binder systems the residual moisture in the core is a measure for the hardness. The simulation has to model not only the air flow through the core but also the evaporation and condensation of the water or vapour and the transport of the vapour with the hot air.
The image below shows the correlation of predicted residual moisture and the strength (or the damage) of a real core.
Tempering of core boxes
In certain core manufacturing processes, such as hot box and Croning, the hardening of the core is accomplished through a thermal reaction of the binder in a heated core box. The heating of the box is carried out with heating channels and electrical heating elements. A uniform temperature distribution in the core box is desirable for a good core quality. The simulation predicts the temperature distribution over time for a certain configuration of heating elements and provides an indication of the uniformity of the heating and the time required to reach the desired temperature.
Validation of the core blowing model
Experiments and simulations for a water jacket core
Core shooting experiments were carried out at TU Munich’s Foundry Institute. Process parameters such as shooting time and pressure, number of inlets and vents were varied and their influence on the core quality analysed. The occurrence of defects in the real cores correlated well with the areas of low sand density in the simulation (see the picture below).
Application of the core blowing model
Improving casting quality for a rear axle housing
Quality assurance detected casting defects in a rear axle housing (see the image below). The defects seemed to be a consequence of the core’s surface defects. Simulations were carried out to support this hypothesis and to advise measures to improve the core surface quality. Finally the core and hence the casting quality could be improved by a different configuration (number and position) of core box vents.
Research project: Prediction of the lifetime of core boxes
Core boxes are mostly made of aluminium with a polyurethane resin coating. The erosion of the core box surface by the sand during the shooting process is the limiting factor for the lifetime of a core box. The project goals were the analysis of erosion processes, understanding the influence of the surface treatment on the lifetime and the development of a computational model that allows the prediction of the erosion caused by a multitude of shots in a single simulation.
A generic core box (see below) was built with inserts of different shapes.
The numerical model derives a quantity for the erosion based on the spatial and temporal integration of pressure and shear forces at the core box walls. The erosion predicted by the model was in good agreement with the experimental values (see the picture below).
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