In this work, modelling of physical foaming extrusion of LDPE is carried out in order to achieve a better understanding of the mechanisms involved in foam manufacturing. Foaming by extrusion is a four-step process. First, the pellets are introduced and molten in an extruder. Gas is then injected under pressure and dissolved in the polymer matrix. The mixture is then significantly cooled to give more strength to the material while maintaining a certain level of pressure. Finally, foam expansion occurs at the die exit. At this location, the dissolved gas undergoes a strong decompression leading to the nucleation and growth of bubbles. The objective of this study is to better understand the origin of the limitation of foaming based on the combination of an experimental analysis of the foaming process and the prediction of a model. The modelling tools are focused on the expansion occurring at the die exit, in order to quantify all the important parameters for the control of the foaming structure. The model, labelled as “cell model”, is based on previous works [1,2] and considers the growth of a single bubble in a mixture of polymer matrix and dissolved gas (blowing agent) [3]. In order to take into consideration the viscoelastic character of the polymer, the rheological behaviour is represented by a multi-Maxwell model. The foaming behaviour of two LDPE commercial grades provided by Total Research & Technology Feluy (Belgium) is compared for two different foaming agents (isobutane and carbon dioxide). The extrudate expansion at the die exit is analysed experimentally for different conditions (mainly temperature and gas concentration). An analysis of the extrusion parameters is performed to determine the quantity of dissolved gas which is effectively used for the foaming process. In order to compare with the experimental results, the cell model considers the gas concentration and the relaxation spectrum of each LDPE grade. As a consequence, particular attention is devoted to the determination of the solubility and the diffusivity of the blowing agent in the molten polymer based on literature data. The main effort concerns the analysis of the influence of the rheological properties of the two LDPE grades and the properties of the blowing agent on the size and stability of the cells. The modelling predictions are compared with the foam expansion and the foam density, revealing that the use of the cell model provides an accurate estimation of the final properties of the foam in the case of isobutane. The difference in final foam density is used to make hypotheses on the physical phenomena which can limit the foam expansion. Indeed, gas loss or polymer crystallization can limit the foam expansion and this is related to the temperature at the die exit [4]. Nevertheless, there is an open question on the role of strain hardening behaviour on foam expansion [3,5]. These different hypotheses will be discussed. REFERENCES [1] Reglero Ruiz, J.A., Vincent, M., Agassant, J.‐F., Claverie, A. and Huck, S. (2015), Morphological analysis of microcellular PP produced in a core‐back injection process using chemical blowing agents and gas counter pressure. Polym Eng Sci, 55: 2465-2473. [2] Reglero Ruiz, J.A., Vincent, M. and Agassant, J.‐F. (2016). Numerical Modeling of Bubble Growth in Microcellular Polypropylene Produced in a Core-Back Injection Process Using Chemical Blowing Agents, Int. Polym. Proc., 31: 26-36. [3] Otsuki, Y. and Kanai, T. (2005), Numerical simulation of bubble growth in viscoelastic fluid with diffusion of dissolved foaming agent. Polym Eng Sci, 45: 1277-1287. [4] Naguib, H.E., Park, C.B. and Reichelt, N. (2004), Fundamental foaming mechanisms governing the volume expansion of extruded polypropylene foams. J. Appl. Polym. Sci., 91: 2661-2668. [5] Weingart, N., Raps, D., Lu, M., Endner, L. and Altstädt, V. (2020). Comparison of the Foamability of Linear and Long-Chain Branched Polypropylene—The Legend of Strain-Hardening as a Requirement for Good Foamability. Polymers, 12(3):725.