Rafael Sterzer and Johannes Pladt
Mechanical Engineering Experts
Fresenius Medical Care AG & Co. KGaA
Fresenius Medical Care is the world's leading provider of products and services for people with kidney disease and is also the leading provider of dialysis products such as dialysis machines and dialysis filters, that have to withstand various mechanical loads.
For health reasons a breakdown or malfunction is not an option. Using PART Engineering Software helps to analyze the parts strength and to increase the confidence in the machines.
The strength assessment of plastic components is often a problem in practice. It is important for the beneficial application of simulation to achieve a good trade-off between the simplest possible modeling approaches (material model, strength assessment) and sufficient fidelity of the simulation results. If non-standardized simulation and assessment approaches are used, or experience-based, subjective procedures are employed, uncertainties follow and the comparability of different analyses is often not possible.
In order to assess a plastic component correctly, there is more than just the FE analysis results. For example multiaxiality, orientation, weld lines, aging, condition, temperature, etc. These effects need to be taken into account adequately. That is why we, at Fresenius Medical Care, use the PART Software Products Converse and S-Life Plastics in order to increase the accuracy of such simulation at an early design stage.This way we create a balanced ratio of effort and accuracy, so that a plausible and reproducible assessment of plastic components is integrated in our CAE process using existing tools.
For our dialysis machine parts a very important aspect is to ensure that there is no leakage in the circular flow. The shown example (Figure 1) shows a spout and during the assembly a connector is pressed into it. In usage, a constant inner pressure is applied. This leads to a long-term load the material needs to withstand. Right at the beginning the results do not show any problem, but after approx. 100 hours significant areas exceed the max. allowed utilization ratio and that is exactly what had happened in the prototype test. The connection worked correctly for several days and then started leaking. This problem can only be foreseen with a procedure taking care of the long-term strength of the plastic material.
Another special assessment is to take care of the anisotropic material properties that come along with fiber reinforced plastics. Here an injection molding simulation needs to be conducted before the mechanical simulation in order to consider a holistic approach. The problem that occurred for this part (Figure 2) was that the load direction was perpendicular to the fiber orientation which leads to reduced strength in that area. Either changing the position of the injection gate or changing the part design would be possible options in this case. The design was slightly modified and the strength assessment showed that the limit was no longer exceeded. Such results are very important to know in advance, because later in the development process this would lead to time-consuming and costly, production-interrupting mold changes that could have been avoided.
Simulation of plastics can be challenging, but S-Life Plastics the simplified procedures in S-Life Plastics help to reduce the work load and increases the confidence in simulation results. The standardized procedure avoids mistakes and assures that reproducibility and comparability of the results is always given, even for occasional users and therefore the results are easy to communicate and to justify.