## 2 Materials
### Comparison with Other Materials  
The selected material must not only enable the fabrication of intricate geometries but also adhere to stringent standards of safety, durability, hygiene, and environmental sustainability. Unlike single-curvature geometries such as cylinders or cones, which can be unrolled onto planar surfaces without significant distortion, double-curvature geometries present inherent challenges due to their non-developable nature. These surfaces cannot be flattened into a two-dimensional plane without introducing deviations or tolerances, often necessitating material adjustments such as stretching, warping, or localized deformation to achieve the desired form. This characteristic places additional demands on the starting material, which must exhibit the flexibility and mechanical properties required to accommodate these deformations while maintaining structural integrity. Traditional materials such as wood, metal, glass, and conventional plastics offer distinct advantages but are often constrained by their inability to meet these complex manufacturing requirements within this highly specialized context. * **Wood:** This is a common material for playground equipment, valued for its natural aesthetic and ease of fabrication. Techniques such as steam bending and laminating allow some flexibility in shaping wood for curved applications. Nonetheless, it is prone to environmental degradation, including rot, warping, and insect infestation, which can compromise both safety and durability. Its porous surface also presents hygiene challenges, as it can harbor bacteria and fungi, posing potential health risks in environments designed for children [16]. * **Metal:** Metals provide exceptional strength and durability, making them popular choices for playground equipment. On the other hand, their application in \begin{table}
\begin{tabular}{l c c} \hline
**Property** & **Value/Range** & **Unit** \\ \hline Thermal Expansion Coefficient & \(3.5\times 10^{-5}\) & mm/m\({}^{\circ}\)C \\ \hline Water Absorption & 0.03 & \% \\ \hline UV Resistance & \(\Delta E<1.5\) (after 10 years) & — \\ \hline Fire Classification & B s1 d0 & Euroclass \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._complex, double-curved geometries is limited due to the need for specialized and costly fabrication techniques, such as metal stamping, incremental sheet forming, or hydroforming [17]. Metal surfaces also present safety risks in outdoor environments, as they can become excessively hot or cold, potentially causing burns or discomfort. Additionally, susceptibility to corrosion requires regular maintenance to ensure structural integrity and prevent failures [17]. * **Glass:** Although it can be molded into complex forms using techniques like slumping or bending, glass is generally unsuitable for playground equipment due to its inherent brittleness and the risk of breakage, which presents significant safety concerns. Even tempered or laminated glass, while stronger and less prone to fragmentation, can still shatter under impact, making it an impractical choice for environments designed for children [18]. * **Conventional plastics and composites:** Widely used in playgrounds due to their moldability and lower cost, conventional plastics can be shaped into intricate designs through methods such as thermoforming or injection molding. Nevertheless, they often fall short in terms of mechanical strength and UV resistance, limiting their durability in outdoor applications. Prolonged exposure to sunlight can lead to degradation, resulting in brittleness and potential breakage. Environmental concerns also arise from the non-recyclable nature of certain plastics and the release of microplastics into the environment [19]. Fiber-reinforced composites offer enhanced strength and durability, but challenges such as wrinkling or residual stresses during fabrication can adversely impact both structural integrity and aesthetic quality [20].