Mould design Improving cooling inserts with evolutionary theory

Editor: Eric Culp

In a project to run until the end of next year, algorithms taken from nature may provide the key to designing thermal channels in plastic injection moulds, according to researchers Christian Hopmann and Philipp Nikoleizig.

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The Institute for Plastics Processing (IKV) has access to its own injection moulding machines to check the study results.
The Institute for Plastics Processing (IKV) has access to its own injection moulding machines to check the study results.
(Source: IKV)

Injection moulding is one of the most-widespread processes for making plastic products. Within this process, the mould is accountable for several complex functions during the moulding cycle, which is mainly characterised by the cooling of the melt, the freezing and the demoulding point. Heat removal is carried out to a large extent via an integrated tempering system inside the mould, with other heat transport mechanisms like convective flux or heat conduction playing minor roles. An economic process can particularly be achieved through fast heat removal. In addition, the cooling of the melt also influences the attributes of the part such as the surface quality or the warpage. The latter can result from unbalanced cooling and thereby a different shrinkage of individual sections of the part.

Tempering channels are usually created with boreholes inside the mould. Their course is then defined via stoppers at required positions. However, additional elements can allow heat removal in hard-to-access areas, for example, ascending pipes, high conductivity brads or other elements. Innovative approaches propose additive manufacturing of the mould insert. The procedure creates a layer-by-layer build-up of the insert, which allows for almost any shape and course of the tempering channel.

Moulds, like the world, are increasing in complexity

Growing functional integration and increased customer demands have resulted in increasingly complex mould technology. Consequently, the available space for the tempering system is limited by numerous elements like slide bars, splits, ejector systems and additional functional groups.

As a result, the location of a suitable course for the tempering channels is often not evident, and their determination can require much of the time in mould development. Furthermore, a proposed course of the tempering channels should be validated through injection moulding simulation during the thermal mould design. Using simulation in the design stage can save a lot of money because this method can anticipate problems occurring later and be remedied before hand, such as the exact course of the cooling channels. On the other hand, iterative mould design with several development loops requires a lot of time, which can lead to competitive disadvantage and a lower profitability.

The Institute of Plastics Processing (IKV) at the RWTH Aachen University has therefore been examining possible ways to shorten thermal mould design and speed up mould development in general. An interesting concept is the use of evolutionary algorithms to automatically generate the course of the tempering channels. These algorithms were developed in parallel in the United States and Germany in the late fifties and mid-sixties with a slightly different approach.

Letting natural selection spawn the best solution

They are based on the idea of an optimisation using nature as the ideal example. Every life form evolves from its inception and strives towards an improved state with every generation. Therefore, a given optimisation problem can be improved over several generations of development. Based on a start population, the relevant parameters were randomly varied and combined. Afterwards, the capability of those recombined products of the population are evaluated via a target function. The best solution serves as the start population for the next generation, depending on the design of the algorithm. This sequence repeats until a suitable solution is found or a stop criterion is reached.

Cooling channel geometry evolves with each step

By transferring this method to the tempering channels of an injection mould, their exact course shall be evolutionarily varied and optimised. This leads to a channel course automatically generated by objective criteria. For example, one starting point is the variation of a single vertex of the tempering channel. This procedure can be extended to every other vertex and concludes in a different global tempering situation of the mould. However, criteria other than the positions of the vertices have to be included in the variation, for instance, the channel diameter or the number of vertices. The development of a proper target function therefore has special importance for finding and benchmarking a good solution. Every influencing variable that interacts with the tempering system has to be included in the parameters and mathematically described. Also, single factors have to be weighted for the target function. The precise weight will be calculated in this project phase in interplay with injection moulding simulations by first using simple geometry. The simulation will help to determine the effect of the variation of single factors. The quality of a specific course of the tempering channel can then be qualified by a single number through the target function. This method makes possible the ability to recognise good channel sections and reuse them in the following generations.

On to the next level

One of the hopes of the project is that the algorithms will be transferred to more complex geometries to analyse the effect for real tempering problems. The weighted factors have to be redefined during this stage, if necessary, so the algorithm will improve during the tests.

Using the algorithm in software will enable users to enhance mould design and save time in optimising the course of the tempering channels.