## 1 Introduction  
Expanded polystyrene (EPS) is manufactured from expandable polystyrene beads that are fused and molded using dry saturated steam. The final product is low-density EPS foam, which has a closed-cell structure, wherein 95% of the volume is occupied by air. EPS is ultra-lightweight and has excellent insulation, cushioning, waterproofing, and soundproofing properties; thus, it is used for various purposes in various industrial fields. The biggest advantage of using EPS as a cushioning packaging material for products is the ability to mass-produce uniform-quality standardized products. The EPS cushion curve is essential for designing EPS-based cushioning systems; appropriate amounts of cushioning materials are determined based on this curve [1]. However, because EPS is a viscoelastic material, it exhibits time-dependent creep; therefore, the fixing of the packaged product becomes loosened and unexpected frictional motion may occur frequently, partially damaging the product and/or destroying the outer package. In addition, when packaged products are loaded in multiple stages in a logistics warehouse for a long time, the accumulated amount of the EPS creep deformation in each package causes deflection and inclination of the unitized load, which ultimately affects stability. This phenomenon becomes more severe for local loads, when the product's center of gravity becomes biased. Therefore, for optimizing EPS-based cushioning systems, the creep behavior, which is the viscoelastic property of EPS, should be analyzed and accounted for. In addition, the creep behavior should be validated, for ensuring the FEA reliability and convergence, when used for designing EPS-based cushioning systems. In general, the compressive creep behavior of a material proceeds in three steps, as shown in Figure 1: (1) primary (transient creep), (2) secondary (steady-state creep), and (3) tertiary (accelerating creep). In the primary step, the strain increases with decreasing strain rate. In the second step, the strain increases linearly with time, yielding a constant strain rate. The tertiary step is characterized by a rapid strain increase, which rapidly leads to failure. However, the actual creep behavior of a material varies significantly, depending on the applied stress: stabilizing after transient creep, leading to destruction after rapidly progressing through the three creep steps, or fully proceeding with the three creep steps. The creep behavior, from instantaneous elastic strain to steady-state creep after loading, can be described by the Burgers model, given in Equation (1). The constants of this model allow a quantitative understanding of the characteristics of the different creep phases. In the Burgers model, the Maxwell and Kelvin elements are connected in series. The spring of the Maxwell element represents instantaneous elastic strain while the dashpot represents a non-reversible viscous flow. The Kelvin element describes delayed elasticity. Many thermoplastics exhibit continuously decreasing creep rates, with zero strain-rate asymptotes [2]. Nevertheless, the creep behavior of such plastics may still be described by the Burgers model over a finite time span. \[\varepsilon(t)=\sigma_{o}/E_{o}+\sigma_{o}/E_{r}\{1-exp(-t/\tau_{r})\}+(\sigma _{o}/\eta_{v})t \tag{1}\]  
where, \(E_{o}\) is the instantaneous elastic modulus (Pa), \(E_{r}\) is the retarded elastic modulus (Pa), \(\eta_{v}\) and \(\eta_{r}\) are the viscosity coefficient of the dashpot [Pa-s], and \(\tau_{r}\) is the retardation time (\(=\eta_{r}/E_{r}\)) (s). The short- and long-term dimensional stability of plastic parts at elevated temperatures is an essential design requirement for many applications, including electrical connectors and various automotive applications. Because characterizing the creep performance of plastics is of primary importance in these applications, significant effort has focused on predicting and analyzing the plastics' creep behavior.