Disposable design: the science of thin-walled packaging optimisation

1st April 2013

As our requirement for disposable food packaging continues to expand, more packaging producers are turning to computational simulation in order to satisfy the structural, commercial and environmental design requirements. Ronald van Dijk describes some of the issues in thin-walled packaging design, and how numerical simulation can be used produce an optimal solution to a complex problem.

Optimum packaging design has important economic and ecological consequences. Whilst a design must have sufficient material to be inherently functional, there is considerable ongoing discussion regarding the amount of packaging waste generated, and the obvious implications for the environment.

However, reducing both the required materials, and the amount of wastage also has a substantial economic driver, with small savings per produced unit representing a potentially large cost benefit. Consequently, research into optimum packaging design seeks not only to serve the environment, but the manufacturer's budget as well.

The economic benefits of a successful design alone are significant. Over millions of produced packaging units, a relatively small material reduction resulting from optimisation can result in significant cost saving. For example, in recent research for Unilever, a mathematical model for calculating optimal wall thickness, delivered an annual saving of E200000 on one of its edible oils packaging solutions. Perhaps it is not surprising that packaging manufacturer's are increasingly turning to numerical simulation in order to ensure that their packaging solutions are structurally functional whilst using the minimum material physically possible.

Packaging is a complex application. The processes which influence both the packaging and its contents are of a physical, chemical and mechanical nature, and although each process has been subjected to research individually, very little is known on their interaction. When designing new packaging, or redesigning an existing format, it is important that all of these processes should to be taken into account. Only by assessing the relevance of each effect is it possible to formulate the proper quantified packaging requirements. Once all of packaging issues are quantified and the design objectives established, the packaging can then be optimised for structural performance.

The physical and chemical aspects are the solution of gasses inside the packed product and the permeability of gasses through the packaging itself. During their shelf life, almost all products oxidise. In some cases all of the oxygen present in the packaging oxidises with its contents, while in other cases there is question of oxidation releasing residual gasses (such as with cheese). This has consequences for the structural mechanics of the design, which in addition to withstanding typical forces induced by filling, transportation, and stacking, must also be able to react positively to the progressive internal partial-vacuum which these processes can create. The mechanical process is further complicated by the nature of the structure itself. The slenderness of the walls means that progressive deformation of the packaging under loading alters its stiffness, and the changing orientation of geometry and loadings must be taken into account in order to achieve a physically meaningful representation. In simulation terms, the analysis performed must be geometrically non-linear, requiring a continual update of the structural configuration and the loadings as the deformation progresses.

In South-Africa, Unilever used 10million 750ml bottles per year to pack edible oils. Edible oils tend to oxidise easily depending on the time, temperature, pureness and oxygen pressure. This oxidation process has various effects. In addition to rendering the oil unfit for sale in the long run, in the shorter term, as the oxygen in the top of the bottle reacts with its contents, the oxidation of the oil creates an underpressure (vacuum) inside the packaging. This vacuum, causes both structural deformation and in turn, further permeation of oxygen from outside the bottle to the oil. This newly entered oxygen can then be consumed by the oil further reducing its pureness. The increasing vacuum causes the packaging to dent, both reducing its structural integrity and warning the consumer that there is something wrong with the product. It is therefore evident that any valid mathematical treatment of packaging applications, must couple both the structure itself, and a model of how the structure reacts with the oxygen and nitrogen in oxidising oil.

There are two ways to determine the reaction of packaging to an internal vacuum. For an existing packaging solution the information could be obtained from experiments. However, for new packaging designs, numerical simulation is increasingly becoming the preferred option. Numerical simulations are relatively simple, cost effective and ultimately variable. Using these techniques manufacturers are able to predict behaviour of packaging under a variety of vacuum conditions, which may be hard to achieve or quantify in practice. They also offer the advantage of enabling virtual changes to the packaging ­ for example alternative design, segmentation of the material, or a range of material contents. While structural simulations of this type are not new, it is the coupling of this behaviour to the oil oxidation formulae and the general data on gas which makes these types of simulation unusual.

With the view to the formulation of the packaging requirements, a common objective is to prevent denting resulting from the formation of the internal vacuum. In order to achieve this there are two design solutions. At one extreme, it is possible to make the bottle so strong that it is able to resist the created vacuum. However, such a solution would need an unnecessary amount of material and would not represent a satisfactory solution from either an economic or environmental perspective. The second option is to design a bottle which camouflages the created vacuum by deforming in such as way that it is not recognised by the consumer. This is possible by allowing the bottle to slightly compress in a vertical direction. Numerical simulation is an ideal technique to determine exactly the amount and shape of the deformation. Such computational models are not only suitable for oil bottles, but can be applied to any packaging solution involving one or more of the processes described. Through numerical simulation, a computerised optimisation is able to determine the optimal form and wall thickness segmentation. Optimisation studies using fixed parameters (ie bottle radii) for the packaging form will result in an optimal ribbed bottle. However, similar optimisation studies can be performed without parametric restrictions, allowing the computational model to find the optimal bottle form itself. The model can also be extended to include the manufacturing process of the packaging as well. In these cases, simulation of the astretch-blow' forms of the bottle use the pre-form thickness segmentation as a design parameter, and the bottle strength as a design requirement. One the finished form of the bottle is parametrically describe, the computational solution, automatically searches for the optimum configuration of the design parameters.

Ronald van Dijk is a member of the Process Consulting Group at MSC.Software in Gouda, Netherlands. or and





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