Approach of simulating the mechanical compression-processes in modern moulding machines 1

Künkel-Wagner is one of the global leading companies offering complete moulding plants, sand preparations and pouring machines for green sand casting applications. It can look back on over a hundred years of history, development, innovation and tradition. In the case of modern markets, customers’ requirements concerning process reliability are still increasing. A further advancement of products with classical development-tools is hardly realized. Furthermore, in the past, putting a moulding line into operation or changing the product to be produced, has entailed a pilot production including a high rate of failures until the required parameter has been found.

Therefore, the ambition is to use simulation tools to extend the engineering possibilities, especially for main processes of the manufacturing chain. In some fields there are already experiences in simulating process steps, such as the pouring process and its theoretical description with commercial software.
Künkel-Wagner is now adopting the acknowledgements to the case of compacting mechanically moulding sand to reduce the effort and amount of pilot productions. Based on a well known simulation package, which is using the finite element methods, a theoretical description of the mechanical behaviour of moulding sand is developed and parameterised. Main component of this simulation is the choice of the material model.

In this case, a model to describe soils under different loads is being used. Major loads during a compaction process are hydrostatic pressure states, which are accounted by a defined load curve. This load curve can be measured previously by logging a force-strain-curve in a cylindrical standard test piece. According to Mohr-Coulomb, the deviatoric (shear) behaviour is moreover characterised by the inner friction angle and a specific cohesion. This data is commonly well-known and can be applied easily to the material model.
The first application is the widely-used multi-piston-compression. To illustrate the mechanisms of sand motions resulting from the penetration of the individual pistons, the simulation is reduced to a two-dimensional slice of a pattern, flask and sand arrangement. Frictional losses during the compression due to relative velocities between sand and pattern/flask are also considered.

The results are shown in image 1. At the beginning of the compression, the distribution of the velocities sheds light on the internal motions of sand particles. A tangential component is particularly visible at the pattern shoulders, the sand “flows” past the pattern contour. At the end of the process, the plastic strain is displayed and can be interpreted as a measurement of the achieved compaction.

As a further development, the existing model is being modified to simulate the second mechanical compression process – the TWINPRESS-procedure. It is characterised by a relative motion between the inner part of the pattern plate and the flask. The aim is to induce the compaction energy where it is needed  at the parting line of the mould.

The results of the reached compaction are especially in comparison with those of the multi-piston-compression. Image 2 shows curves of the strain directly under the outer, respectively left, piston and at the bottom of the deepest mould section. After completing the compaction with the multi-piston-compression, the strain at the bottom is approx. 32% lower than the strain under the piston. The losses follow from inner frictional effects and reaction forces with the surrounding surfaces of the pattern and the flask.

With TWINPRESS, the losses are merely about 19%, as shown in the upper image. Accordingly, this procedure works significantly more effective and can help save energy.
The considered simulations are just a small forecast to what modern simulation tools can provide. First of all, it helps to deepen the theoretical comprehension of physical relations during the single compression procedures. Further it allows, after validating the material model, to predict approximate mould strengths in regions of interest. Thus it is possible to detect potential problematic mould regions and to take precautions.

Another important issue is the parameterization of moulding machines. The foundryman has got an abundant set of parameters like hydraulic pressures for the several pressure circuits, delay times for specific machine motions and many others. Additionally, it is sometimes difficult to avoid that the output fluctuates due to perturbations at the melting furnace, sand preparation or other possible reasons. Therefore, it is ineluctable that the moulding properties of the sand, which are available directly at the moulding machine, also fluctuate. The main cause is evaporation effects of the internal sand moisture. The mechanical behaviour of the moulding however depends predominantly on the moisture rate. Hence the primal perturbations of the production cause inevitable fluctuations of the mould qualities and therefore of the produced casting.

When the whole process chain is well understood and the relations between the moulding sand properties and the achieved mould qualities are clearer, a quicker and more effective reaction on perturbations is possible. For the short term, the parameter set of the moulding machine can be adapted to compensate the changing sand quality and hold the mould quality on a good level. For the medium term, the sand preparation can be readjusted to get back to the right sand properties, which are needed at the moulding machine.
In summarizing it can be stated that the simulation of compacting procedures provides the theoretical comprehension of the process’ physics. This can be used to predict the effects of parameter sets on the mould quality and to react properly to changing boundary conditions within the production. That is why the simulation helps to significantly increase the process reliability of moulding lines.

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