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Lightweight Design worldwide

, Volume 10, Issue 3, pp 18–23 | Cite as

Applying Software-based Optimisation Methods to Develop Heavy-duty Lightweight Structures

  • Felix Dehmel
  • Steve Kuhnen
  • Birgit Paul
  • Kai Steinbach
Construction Example of a heavy-dututy palletet
  • 188 Downloads

Incorporating a range of optimisation methods into the development process paves the way to significantly reduce the mass of complex components subject to high stresses. Using various approaches in the development steps expedites the emergence of solutions that meet specifications and are technological needs; as exemplified by a heavy- duty pallet with a lightweight design.

Pallets — particularly the heavy-duty variant — are typically used day in day out without a second thought, despite the serious consequences if they fail. In the transportation and logistics sector, pallets safeguard loads and allow economic loading and unloading times. Where dealing with heavy static loads that exceed 1000 kg, a compromise has to be sought between the pallet dead weight and its maximum safe working load. In the extremely price-sensitive logistics sector, what counts is not only the fact that the lighter the pallet, the easier it is to handle, but also — above all — that the pallet can be purchased at the same or even lower price. All the more reason why injection moulding has established itself as the technology of choice for plastic pallets.

Unlike standard pallets, heavy-duty pallets are preferred for static loads and primarily deployed to store products rather than in-house transport. While load applications still centre on raising pallets via forklift, rack storage is just as important for processes involving heavy loads. This load process is particularly critical, since the pallet generally rests on its edges and may then sag.

Using cutting-edge computational and optimisation methods and leveraging process and material expertise, Leichtbau-Zentrum Sachsen GmbH is a venue for projects to develop market-focused lightweight solutions that are often infeasible with traditional development processes. As shown by the study described below, these methods were successfully applied to develop a lightweight heavy-duty pallet and include significant potential for reducing mass.

Material selection

Most of the plastic pallets now on the market are produced in an injection moulding process using high-density polyethylene (HDPE). This material is economical and exhibits good mould-filling behaviour, but lacks rigidity and strength. Furthermore, like all thermal plastics, HDPE tends to suffer from creep (cold flow). With continuous loads like those experienced in rack storage, this results in sagging and long-term plastic deformation of the pallet. To counteract this effect, most plastic heavy-duty pallets can only achieve full load capacity for rack storage with additional metal supports. This makes it costlier for users, as well as limiting the usability of the pallets. A starting priority thus means focusing on materials that are suitable for injection moulding, feature better mechanical properties and are less prone than HDPE to creep. The polymers considered in the course of this study were restricted to inexpensive polyolefins suitable for injection moulding, to allow production of the lightweight pallet being developed via tried-and-tested technology.

A comparison of plastics that are typically suited to injection moulding in terms of mechanical properties and material price, Figure 1, clearly shows that glass fibre-reinforced polypropylenes (PP-GF) can significantly boost mechanical properties, enhancing the potential for lightweight design. This group of materials, however, has known limitations with viscosity in the injection moulding process as well as toughness. This can result in a relatively fragile pallet, unable to withstand the impacts of everyday transportation and logistics. The proportion of fibres was therefore limited to 20 %, despite the availability of granulates with higher fibre content.
Figure 1

Comparison of selected injection moulding materials with regard to their mechanical properties normalised to their purchase price (© Leichtbau-Zentrum Sachsen GmbH)

One further factor to be taken into consideration was the structural design, meaning that favourable mould-filling behaviour had to be achieved by using contours that minimised branching and featured moderate wall thickness. Given these constraints, however, it is recommended to select PP-GF for further consideration, based on their significantly higher mechanical properties.

Methodology

The heavy-duty pallets in use today are the fruit of several decades of optimisation, alongside a parallel increase in requirements imposed by users and standards. Imposing standardised geometry alongside the process-related technical requirement for inexpensive injection-moulding production make it virtually impossible to achieve significant improvements using traditional load analysis and structural design methods. They only elicit marginal improvement to what is already an excellent technological standard. To exploit all the potential of lightweight design in the study presented here, methods of numerical structure and topology optimisation were applied.

Load cases relevant for designing heavy-duty pallets are described in Figure 2. The outcome of optimisation should satisfy all load cases. First, standardised permissible bending must be complied with for each load case (based on ASTM D1185-2009). Secondly, a safety factor of S=2 against fracturing is applied as a strength criterion. Any special load cases were disregarded for optimisation in the early design phase.
Figure 2

Load cases relevant for pallet design (© Leichtbau-Zentrum Sachsen GmbH)

Initial topology optimisation can be performed once material, construction space, load cases and bending constraints have been defined.

Topology Optimisation

The construction space usable for a pallet supporting structure results from applicable standards and is illustrated as usable structure volume in Figure 3. No further restrictions were imposed, based on hygiene compatibility requirements for example. For this study symmetrical load cases were considered. Moreover, since also the pallet has vertically symmetrical section, a quarter model could be used for structural optimisation.
Figure 3

FE volume model as a quarter model of the available construction space (© Leichtbau-Zentrum Sachsen GmbH)

Topology optimisation was performed using the following parameters:
  • ▸ Design variable FE element concentration

  • ▸ Permissible stress = σmax ≤ 41 MPa

  • ▸ Permissible bending = umax, “racked” ≤ 21 mm, umax, “forklift” ≤ 21 mm, umax, “bending” ≤ 3 mm

  • ▸ Target size = minimum volume.

While the deformation boundary conditions are assigned to individual FE nodes or nodal groups and one load case each, the stress boundary condition σmax ≤ 41 MPa applies to all elements in the FE network (tensile strength of PP-GF-30 considering a safety factor of S=2).
The “raw model” resulting from topology optimisation, Figure 4, on the one hand illustrates the maximum lightweight construction potential in an extreme case, but remains, conversely, far from a final geometric model. The structure illustrated satisfies all the use cases considered for optimisation in equal measure, Figure 2. A colour scale next to the generated geometries indicates the areas most exposed to stress. Orange plotted lines indicate the main load paths. Although these optimisation findings cannot obviously function as the direct basis for a detailed design, they do provide crucial clues for further development and allow important design specifications to be derived for subsequent tasks.
Figure 4

Result plot of basic topology optimisation (left) and derivation of design information (right) (© Leichtbau-Zentrum Sachsen GmbH)

The heavy-duty pallet developed has a structure as light as 11 kg, far less than comparable pallets available on the market.

This design information is used to prepare a CAD shell model of the pallet. Contours and ribbing are based on the topology plot results, where, for example, the main load paths are reinforced with thick-walled longitudinal ribs, Figure 5.
Figure 5

CAD quarter model using the design information from topology optimisation (© Leichtbau-Zentrum Sachsen GmbH)

The resulting pallet model has walls approx. 4 mm thick in many areas and a structural mass of approx. 16 kg, while slightly exceeding the strength criterion.

Optimisation Fine-tuning

Details on manufacturing and application restrictions still need to be added to the CAD model derived from topology optimisation. At the same time, there is still room to reduce mass further. For example, the result of topology optimisation hints at over-dimensioned global wall thickness in many areas and considerable remaining potential to reduce mass. This is why the creation of the basic CAD model is followed by optimisation fine-tuning.

Fine-tuning refers to all the procedures used to modify material thickness and contour details, but not the overall geometrical structure. Free size optimisation, where optimisation software determines the optimum shell thickness for each element in the FE shell model, is one useful way to perform detailed wall optimisation. Further boundary conditions of free size optimisation correspond to the parameters listed above for topology optimisation. However, since some aspects of manufacturing technology can now be included in the development process, a maximum wall thickness of 5 mm is also specified.

The results of free size optimisation, Figure 6, can subsequently be translated into a functional specification for wall thickness distribution that is usable for production. Subdividing the results into structural groups facilitates their use in further optimisation steps. Here however, taking manufacturing and process constraints into account is a must. For example, for the planned implementation of the pallet using injection moulding, these would include:
  • ▸ Maximum and minimum wall thickness

  • ▸ Minimum draft angles

  • ▸ Low mass accumulations

  • ▸ Avoidance of wall thickness flaws and

  • ▸ Favourable wall-to-rib thickness ratios.

Following optimisation fine-tuning, the estimated structural mass of the pallet is 11 kg. Subsequent detailed FE analysis shows that the maximum stress limit of 41 MPa has been exceeded in a few places. The final places exhibiting excess stress are subjected to local (free) shape optimisation to bring them within the specified stress limits. In this process, the optimisation software can shift the coordinates of a node set to the edge of the FE network in line with the designer’s instructions, to smooth local stress exaggerations. As Figure 7 demonstrates, adjusting the contours of a highly stressed rib slightly can significantly lower tension.
Figure 6

Visualisation of various wall thicknesses of the quarter model as result of free size optimisation (© Leichtbau-Zentrum Sachsen GmbH)

Figure 7

Before and after comparison (left and right) of the tension level of the pallet radius range using free shape optimisation (centre) (© Leichtbau-Zentrum Sachsen GmbH)

Clearly, strength is the key driver behind the heavy-duty pallet design, while rigidity requirements, with less than 50% of the permissible bending limits utilised, remain well within permissible limits. The safety margin of the vertical ribs and walls against rigidity failure is around 300%.

Deriving a Production-oriented Design

Further steps are necessary to translate the draft pallet produced in various optimisation steps into a production-ready design. The draft design was revised to take account of the boundary conditions listed above for the injection moulding process. Although the intricate, high resolution structure is basically demouldable, an extremely complex moulding tool is required. For economic reason also dictate that the element may be horizontally separated during subsequent product implementation, adding an extra joining process. Nevertheless, it is still possible to test the general suitability of the geometry for injection moulding in this phase of development via a filling simulation, Figure 8.
Figure 8

Pressure distribution during mould filling (left) and temperature at the flow front (right) during injection moulding of the pallet structure (© Leichtbau-Zentrum Sachsen GmbH)

The latter shows that the cavity fills completely within 7 s using an injection pressure of 300 bar, which is moderate for the volume of the element. The low pressure in the corner areas must be counteracted by numerous injection points to avoid flaws. As Figure 9 shows, the fibres (with PP-GF) align themselves within the highly stressed ribs to resist the loads. Swirling is limited to the extreme corner areas, resulting in quasi-isotropic material properties.
Figure 9

Formation of fibre orientation during the injection moulding process (© Leichtbau-Zentrum Sachsen GmbH)

Conclusion

The design of a mass-optimised heavy-duty pallet, Figure 10, developed in the course of this study achieves a structural weight of around 11 kg, which is distinctly below the average weight of comparable pallets available on the market.
Figure 10

Design model of the optimised heavy-duty pallet (© Leichtbau-Zentrum Sachsen GmbH)

The boundary conditions selected for development mean that an actual pallet based on this design offers the user further benefits in addition to a reduction in mass, such as eliminating additional rack storage supports. As a follow-up to this study, the additional functional and structural requirements for heavy-duty pallets not considered here must be incorporated into the component design. Besides critical special load cases (for example shocks while manoeuvring, being dropped onto corners and edges or transport over uneven surfaces), this also involves constraints arising from the production process itself.

The mass optimisation of a heavy-duty pallet, as presented here, clearly shows that software-based optimisation methods can be invaluable for the development process and that using them spawns solutions barely feasible using traditional development methods. This helps cut development costs and boost the degree of lightweight construction in the solution, particularly with complex geometries and many load cases.

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Felix Dehmel
    • 1
  • Steve Kuhnen
    • 1
  • Birgit Paul
    • 1
  • Kai Steinbach
    • 1
  1. 1.Leichtbau-Zentrum Sachsen GmbHLeichtbauDeutschland

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