CAE software can be viewed as stand-alone system, where structural analysis provides correct information about the stiffness and reliability of a molded part or mold, and flow analysis tells you a lot about how the cavity is filled. But what are the actual boundary conditions of the injection molding process? Are all the assumptions about injection and shape of molding cavity correct? Answers to many of these questions can be obtained by combining analysis software in such a way that they determine the conditions themselves, allowing different programs to interact.
In 2001, at the Cadmould User Meeting in Aachen, Germany, Simcon presented the basics and possibilities of coupled analysis in injection mold design.
In the first approach, funded by COSMOS (ESPRIT project), the coupling was programmed and conducted between Cadmould and MARC. In this partnership, Cadmould provided the analysis of the filling process, which it normally does. However, the coupled simulation with the structural analysis software generates additional information about how the cavity surface is locally loaded by the forces induced by the injected melt. This can be a major problem when complex molded shapes require multi-cavity molds, with sliders or thin cores. Mold components that are not sufficiently rigid can temporarily or permanently deflect or even break due to such forces.
In addition to mechanical stress in the mold, which can cause structural damage, feedback of mold deformation with cavity filling can occur. If mold body and thin wall deformations become large due to high injection pressures, this can cause partial or complete throttling of flow paths. This is even more important because it is a self-coupled process, occurring as follow:
- reduced flow cross-section slows down the flow
- the cross-section is blocked earlier and the flow is more unbalanced
- off-center forces increase; also, a displaced core will increase the flow gap on one side and decrease it on the opposite side
- the flow will be easier on the first side and more difficult on the other side
- the pressurized area will move further and further away from the original position.
The phenomena of “excessive mold deformation” and “flow path obstruction” are caused by asymmetric loads on mold walls, sliders, and cores. They are expected to occur during cavity filling. In many cases, peak loads will occur near the end of the filling process when most areas of the cavity have been filled, as injection pressure increases rapidly to fill the remaining volume of the cavity. However, there may be cases where the melt must pass critical areas of the mold earlier in the filling process. Also, the shape of the cavity may be too complex to predict the time of maximum load or deformation in advance, in which case the exact value of this critical time should be part of the simulation results.
The aforementioned discussion illustrates the need to closely link the two simulations with a constant exchange of information between them.
The coupled simulation process is completely controlled by Cadmould. It requires no user interaction with the structural analysis software. However, it may be useful to have a basic understanding of the post-processing capabilities of the structural analysis software in order to evaluate all aspects of the results.
The structural analysis is based on a solid tetrahedral FEM mesh.
In this article, we will describe two examples where coupled analysis was able to predict problems with fragile inserts and flaccid cores, and helped to find the right solution relatively quickly.