RTM-Worx User Manual
Philosophy

1.3 RTM-Worx Philosophy

1.3.1  Application of RTM Simulation
1.3.2  Maximizing result vs. cost ratio
1.3.3  Accuracy and permeability data

It is important to understand the RTM-Worx philosophy if you want to exploit the capabilities of the simulation code and take full advantage of the potential that simulation technology has to offer.

1.3.1 Application of RTM Simulation

The most important question is of course, when flow simulations are needed and what the criteria are to make this decision. It is also the most difficult question, and we believe that the reason that simulation technology is not really exploited at it's full potential is largely due to the fact that doing calculations with classical FEM applications is too expensive because the initial investments are high: not just the cost of the software, but especially the necessary training of the specialist needed to prepare and run the calculations while each simulation project is very time-consuming. RTM-Worx has changed this. How that has been achieved is discussed in the next paragraph, for now we're able to give the answer the question we started with: Use simulation on each product unless you can afford to waste your time and money:

• In order to exploit simulation results, you have to be able to translate the results of the calculation to the real world situation. There is always a difference because the simulation is done in a simplified and idealized world. The only way to learn this is by experience, which is why you need a very good reason not to do simulations if you have access to RTM-Worx.
• There are of course products that can be optimized and produced efficiently without using simulation calculations. If your company limits itself to making this class of products, do not invest in simulation software. For all remaining organizations, those products are ideally suited to gain experience in the translation of simulation results to real-world applications and vice versa. This knowledge can be exploited in new projects and complex products to minimize risks and optimize production.
• Doing simulations forces you to collect all relevant data in a very early stage, which can prevent unforeseen risks and problems at a later stage where it might even delay production.

These statements are, of course, only true when the simulation calculations are not too expensive. Otherwise the benefits of simulation don't outweigh the costs of doing the calculations. That is what the next paragraph is about.

1.3.2 Maximizing result vs. cost ratio

This has been the most important design goal in the development of RTM-Worx. While a lot of research groups are paying much attention to new features and increasing accuracy to improve simulation software, PolyWorx has decided to invest most of the development time and effort in the interface and the reliability and robustness of the numerical implementation of the flow calculations. This has significantly reduced the skills needed to work with the software and eliminates a lot of unnecessary repetitive tasks which are now fully automated in the integrated environment. An additional benefit to you is that the interface makes it much easier for us to test new features very thoroughly which results in a more robust software package.
Maximizing the results from the simulations does not necessarily mean that the simulation needs to be more accurate or must be able to produce more results. It is simply not possible to realize a simulation code that produces realistic results at a cost that is lower than the cost of doing experiments. The reason that simulations can be cheaper than real-world experiments is inherently related to the fact that the calculations are done in an idealized, simplified virtual environment.
The main benefits of simulations are that the calculations produce much more data than you will ever be able to measure and that it is very easy to change the product geometry, fabric and resin property or process parameters. This only makes sense when you can rely on the fact that when you choose the best option out of two alternatives based on simulation calculations will also lead to the best solution in practice. Basically this defines the minimum accuracy required from the simulation. It is nice to have a more accurate calculation but the additional accuracy does not increase your return of investment. In addition, the accuracy of resin and permeability data is generally much lower than the numerical inaccuracy.
The conclusion is that the degree of usefulness of the simulation software depends much more on the reliability of the calculations. In other words, that the predictability of the accuracy of the calculations is most important, because that is the basic requirement for comparison of alternatives. This has been achieved by a high degree of consistency in the numerical model and omitting any 'fudge factors'.
Maximizing the results means achieving your goals in the shortest amount of time. The combination of calculation speed and reliability allows you to run a lot of calculations in a short time to develop a feeling for the process. You will be able to minimize the risks, identify bottlenecks, find possibilities to improve the process by alterations in the injection strategy and the design of the part itself.

1.3.3 Accuracy and permeability data

The accuracy of isothermal simulation calculations done with RTM-Worx is very high and depends on the number of elements and the way the part is injected. Generally, the numerical error in the total filling time is below 1% if you use about 1000 elements (triangles and runners) in your model. Effectively, the accuracy of the calculation depends mainly on the accuracy of resin viscosity and fabric permeability's.
Often, the situation is even worse: permeability data is not available and there is no time and/or money to do the measurements. Although this means that you cannot make a reliable prediction of the total injection time, the magnitude of the permeability fortunately does not influence the shape of the flow fronts.
Generally, you will have to do predictions based on inaccurate or missing permeability data. RTM-Worx has been designed to help you out: you can develop an injection strategy as long as you know the relative difference of permeability for the reinforcement and in order to deal with the uncertainties in the material properties, the speed of the calculation allows you to include reinforcement permeability's as additional variables in a parameter study. Part of the results you generate will be lower and upper bounds for the reinforcement permeability's that will guarantee successful injection of the part within a given timeframe!

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