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Casting Simulation as a Tool in Concurrent Engineering
Arno Louvo, M.Sc CT-Castech Inc. Oy Presented at Casting 1997- International ADI and Simulation Conference May 28-30, 1997, Espoo, Finland
Casting process simulation has become an industry standard. No foundry that produces high quality castings can consider simulation as unnecessary. Today's dynamic world requires quick responses to customer needs. Also, accurate and "right" costs need to be defined to each work.
Simulation can prove to be a decisive factor in getting the order. It can help in calculating the real costs of the job, and it can be used as a tool in the negotiations in getting the quality right. Similarly, the need to "rig it on the safe side" will become unnecessary.
With the evolution of 3D CAD-programs, the modelling has been made easy for the designer. This means that modelling does not take much more time than just the making of the drawings- but allowing much more applications to the model. Simulation is just one application of 3D-modelling.
It has been estimated that about 90% of the defects in components are due to mistakes in design and only 10% are due to manufacturing problems. It has been calculated that the costs to change the design increase ten-fold in every step of the design and manufacturing process. Hence, all the methods and tools used to ensure the success of the design will significantly affect the total manufacturing costs. Casting process simulation and castability analysis based on casting simulation are these kind of productivity tools.
Casting Simulation in Foundries
Finland is an example of a country where casting simulation has been adopted very extensively as an important part of the casting process design and the quality system of the foundry. Today, the foundries which cover around 90% of the production of the cast machine components, use casting simulation as an everyday tool. Over 90% of those foundries use CastCAE simulation program. There are also many known cases where the foundry and the machine shop have worked together in optimising the component weight, utilising simulation as a tool. In those cases, either the foundry or the consulting organisation has made the simulations with the help of a designer from the foundry and the machine shop. This, however, indicates that there is a need for tools to optimise cast components- in early stages of the design- to be used by machine designers. Further, casting simulation has helped foundries to point out the factors that have a significant effect on the price of the casting.
Simulation has been a very effective tool in many Finnish foundries. Some foundries have found out it is worthwhile to simulate every new job before the start of the production, to avoid surprises. As an example, the foundries that use CastCAE simulation program report the following benefits (source: CADDET report, Result 195 and CADDET report, Result 266.):
- Energy savings of 3-6%
- Improved product quality
- Less remelting and refinishing
- Shortened lead time, increased production
- Payback time of 1-2 years
- 84% wastage reduction
- 5% weight reduction
The aim of the foundry is to produce profitable components and, at the same time, secure that the pieces fulfill the quality standards set for the components. Still, in many cases foundries encounter difficulties to reach both of these goals, because of bad design or inappropriate quality requirements. This is one of the major reasons why the castability of the component should be checked in the early stages of the design- in machine shops- and then further optimised, together with the foundry engineers.
It is quite common that the foundry will contact the customer only after the first unsuccessful test castings, and proposes changes to the design, to be able to produce defect-free castings profitably. Quite often it is a question of minor changes e.g. in machining allowances, which will then be agreed. However, because of the unsuccessful test castings already done, the costs of producing the castings- and the lead time- have already increased.
If there are no simulation results of the component, it might become difficult to make a quote. The price formation does not necessarily correspond to the real costs, and might prove to be unprofitable, either to the foundry or to the customer. A long and confidential customer relationship necessitates mutual openness and "proper" price. If the customer has made the castability analysis, the foundry engineer can use it to estimate how complicated a casting system is needed, which is the basis for the price. Probably the problem areas can also be shown and a proposal of the changes can be made.
A New Tool for Component Design
Very often foundries need to design gating and risering systems for components that were not originally designed to be cast. However, if the castability were checked and optimised already in the designing stage, a lot of useless work would be avoided, as well as time and money would be saved both in machine shops and foundries.
It is a generally accepted fact that the later the changes are made in the design and production process, the more expensive they will be. Every step in the process chain multiplies the cost of changes roughly by a factor of ten.
The problem has been that the designers have not known enough about casting requirements. They are used to designing components and adding machining stocks, drafts, etc., but the castability has still remained somewhat a mystery. Efforts to overcome this problem have been made. One example of these efforts is the program CastCHECK. It is based on numerical simulation.
The idea of CastCHECK is to make a fast castability analysis, without the complexity of the regular casting simulation programs and the need to design the complete casting system with risers, sleeves, gates, chills, cores and mould. Very little information about casting process is needed to use CastCHECK. The designer needs to get the stereolithography file (STL) from any 3D-solid modeling program. Also, if the casting method and the material are known, the results will be more trendsetting. The minimum wall thickness of the component needs to be given to the program, to define how many calculation elements will be needed in the analysis. All information program needs is given in one dialog. The analysis results are shown as 3D "X-ray pictures" which describe the potential defect areas in the component.
CastCHECK runs on Windows NT/95, MacOS and on most Unix platforms.
3D Design to Key Position - Also in Machine Shops
A big obstacle to more extensive co-operation between the foundry and the machine shop has been the lack of 3D CAD-systems or their incompatibility. 2D CAD-systems are quite common in most machine shops, but quite few of them utilise 3D design. In Finland, the usage of 3D CAD has increased significantly during the last three years, partly because of NC-machining of patterns and partly because of the introduction of simulation. All the foundries having simulation facilities use 3D CAD on a daily basis.
The problems with data transfer are still obvious. The implementation of the so-called standardised data transfer formats (e.g. IGES) are not consistent between different CAD systems. This means that the data transfer quite often fails. Perhaps the most consistent way to transfer data today is the simple STL format, which can be used- not only for rapid prototyping- but for both simulation and patternmaking. However, the 3D-design with the present CAD-systems is quite simple- and pre-eminently when creating assemblies- it is almost invaluable. If the assembly includes cast components, the advantage of 3D design increases further.
3D-model potentiates making of different analysis in machine shops, and further, using the same model in the foundry in making casting simulation. As a result of this procedure, the design mistakes and imperfections can be avoided and the turn-around time will decrease. Both of them can be measured directly in money. It is quite possible that the simulation program can be paid back during just one prototype series.
With the development of electronic data transfer, the possibility of utilising concurrent engineering has become apparent. There has been a lot of talk of it for many years, but only few have adopted the techniques. Concurrent engineering is a very difficult subject, because it requires openness about all factors, like the costs and profits. This can be achieved with partneship, but it is not always necessary.
In producing castings, concurrent engineering is not just another fancy term, but a hard fact, what comes to competitiveness.
Fig.1 shows a flowchart of a traditional procedure of designing components and casting systems without the help of numerical simulation and 3D CAD-programs.
Fig.2 shows a flowchart of a design process, in which 3D CAD and simulation are used by the foundry. In Finland this is the most typical situation today. In this procedure, simulation results are used when foundry and machine shop engineers negotiate about construction and other design changes, including the cost of alternative design options.
Fig.3 shows a flowchart, in which 3D CAD and simulation tools are utilised both by the machine shop and the foundry. This is the procedure which is expected to be reality in most cases of designing new castings in Finland by the end of this year.