Applying Software-based Optimisation Methods to Develop Heavy-duty Lightweight Structures
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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.
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.
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.
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.
Initial topology optimisation can be performed once material, construction space, load cases and bending constraints have been defined.
▸ 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.
The heavy-duty pallet developed has a structure as light as 11 kg, far less than comparable pallets available on the market.
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.
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.
▸ Maximum and minimum wall thickness
▸ Minimum draft angles
▸ Low mass accumulations
▸ Avoidance of wall thickness flaws and
▸ Favourable wall-to-rib thickness ratios.
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
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.